Plant Physiol. (1998) 117: 43-53
Expression Pattern of the Carrot EP3
Endochitinase Genes in Suspension
Cultures and in Developing Seeds1
Arjon J. van Hengel2,
Flavia Guzzo3,
Ab van Kammen, and
Sacco C. de Vries*
Agricultural University Wageningen, Department of Molecular
Biology, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands
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ABSTRACT |
Carrot (Daucus carota)
extracellular protein 3 (EP3) class IV endochitinases were previously
identified based on their ability to rescue somatic embryos of the
temperature-sensitive cell line ts11. Whole-mount in situ hybridization
revealed that a subset of the morphologically distinguishable cell
types in embryogenic and nonembryogenic suspension cultures, including
ts11, express EP3 genes. No expression was found in
somatic embryos. In carrot plants EP3 genes are
expressed in the inner integumentary cells of young fruits and in a
specific subset of cells located in the middle of the endosperm of
mature seeds. No expression was found in zygotic embryos. These results
support the hypothesis that the EP3 endochitinase has a "nursing"
function during zygotic embryogenesis and that this function can be
mimicked by suspension cells during somatic embryogenesis.
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INTRODUCTION |
Carrot (Daucus carota) cell cultures secrete many
different proteins into the medium, a process that contributes to the
conditioning of the medium. Conditioned media are reported to have a
promoting effect on the initiation of somatic embryogenesis (Hari,
1980
; Smith and Sung, 1985). The initiation of somatic embryogenesis results in major changes of the extracellular protein pattern compared
with that of intracellular proteins (De Vries et al., 1988). Some of
the secreted proteins are thought to be related to the formation of
embryogenic cells and somatic embryos. To investigate whether a causal
relationship between certain secreted proteins and embryogenic
potential exists, secreted proteins that were differentially expressed
under different culture conditions were analyzed and their localization
was studied.
The extracellular protein EP1 is secreted only by nonembryogenic cells
(Van Engelen et al., 1991), whereas the extracellular protein EP2,
identified as a lipid-transfer protein, is secreted only by embryogenic
cells and somatic embryos (Sterk et al., 1991). These results showed
that different cell types contribute to the total pattern of proteins
secreted into the culture medium (Van Engelen and De Vries, 1993).
Another way of studying a causal relationship between secreted proteins
and embryogenic potential led to the identification of extracellular
protein 3 (EP3), which was identified as a chitinase.
EP3 was originally purified as a protein capable of rescuing somatic
embryos in the mutant carrot cell line ts11 at the nonpermissive temperature (De Jong et al., 1992). The acidic endochitinase EP3 was
found to be a member of a small family of class IV chitinase genes
(Kragh et al., 1996). These highly homologous isoenzymes are encoded by
at least four EP3 genes. Two of these proteins, EP3-1 and
EP3-3, were purified and shown to have subtly different effects on the
formation of somatic embryos in newly initiated ts11 embryo cultures
(Kragh et al., 1996).
Since the effect of the chitinases was mimicked by Rhizobium
sp.-produced nodulation (Nod) factors (De Jong et al., 1993), it was
proposed that the chitinases are involved in the generation of signal
molecules essential for embryogenesis in ts11 (De Jong et al., 1993).
The EP3 proteins produced by ts11 at the permissive and nonpermissive
temperatures did not show any difference in biochemical characteristics
compared with the ones produced in wild-type cultures and were capable
of rescuing ts11 somatic embryos. It was also shown that the
sensitivity of ts11 to chitinases coincided with a transient decrease
in the amount present of this otherwise functional set of proteins (De
Jong et al., 1995).
The roots of leguminous plants are known to produce chitinases, and
during the interaction with Rhizobium sp., these
plant-produced chitinases have been suggested to control the biological
activity of Nod factors by cleaving and inactivating them. In this way, chitinases are proposed to have the potential to control plant morphogenesis and cell division (Staehelin et al., 1994).
Many chitinase genes are induced upon infection, wounding, and
treatment with elicitors. Infection can lead to a 600-fold induction of
chitinase activity (Métraux and Boller, 1986). The molecular
mechanisms and signaling pathways responsible for this induction remain
largely unclear (Graham and Sticklen, 1994). The response of a plant to
infection involves nonspecific responses, because challenge by
different pathogens can induce production of the same set of
pathogen-response proteins (Meins and Ahl, 1989). The induction is not
restricted to the infection zone, since uninfected areas of infected
leaves and even uninfected second leaves show an increase in chitinase
activity (Métraux and Boller, 1986).
We aimed to establish in carrot, first, which cells in wild-type and
ts11 suspension cultures express the EP3 genes, and second, whether (and if so, where) the same chitinases are expressed during zygotic embryogenesis. The results show that the carrot EP3
class IV endochitinases are expressed in a subset of (most likely)
nonembryogenic suspension cells and in carrot plants in the integument
cells and endosperm during zygotic embryogenesis.
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MATERIALS AND METHODS |
Plant Material and Culture Conditions
Cell-suspension cultures of carrot (Daucus carota cv
Autumn King/Trophy) and ts11 were initiated and maintained as described previously (De Vries et al., 1988). One-week-old suspension cultures were used for gene-expression analysis and protein-localization experiments. Hypocotyl sections were prepared as described by Guzzo et
al. (1994). The hypocotyls of 1-week-old plantlets were divided into
segments of 3 to 5 mm and incubated for 10 d in B5 medium with 2 µm 2,4-D and then returned to B5 medium without 2,4-D.
One-week-old cell-suspension cultures of an embryogenic cell line were
grown in the presence of Phytophthora infestans or Botrytis cinerea or supplemented with 0.002 or 0.02 mg
mL
1 chitosan. After 2 d of incubation the
cells and fungi were removed by filtration and the resulting cell-free
conditioned media were assayed for the presence of EP3 by western
analysis, as described by Sterk et al. (1991).
Cell-Suspension Fractionation Analysis
Suspension cultures were sieved to obtain cells and cell clusters
between 50 and 125 µm. The cells were loaded onto a discontinuous gradient of 10, 20, 30, and 40% Percoll (Van Engelen et al., 1995). After centrifugation the cells at the interfaces between the different densities of Percoll were recovered with a syringe, washed in B5 medium
containing 2 µm 2,4-D, and allowed to grow in B5 medium with 2 µm 2,4-D. After 5 d the proteins in the
medium were analyzed.
Partial Purification of Seed Proteins
Dry, mature seeds were ground in liquid nitrogen with a mortar and
pestle. Ten milliliters of 25 mm Tris-HCl, pH 8.5, was added to 2 g of ground seeds. After incubation for 15 min on a rotary shaker at 4°C, the resulting slurry was filtrated through Whatman filter paper and supplemented with 0.5 mL of DEAE-Sepharose FF
resin, equilibrated in 25 mm Tris-HCl, pH 8.5. After
incubation on a rotary shaker for 1 h at 4°C a column of the
DEAE-Sepharose FF resin was poured and washed twice with 25 mm Tris-HCl, pH 8.5, and the bound proteins were eluted
with 0.2 m KCl in 25 mm Tris-HCl, pH 8.5. The
protein sample was desalted with dialysis.
Chitinase Activity Determination
Chitinase activity was determined essentially as described by
Trudel and Asselin (1989).
In Situ Hybridization
For whole-mount in situ hybridizations, 1-week-old suspension
cultures grown in B5 medium in the presence of 2 µm 2,4-D
were washed in B5 medium, run through nylon sieves with pore
sizes of 125 or 30 µm, and allowed to grow for another week in the
absence of 2,4-D. The cells were concentrated by centrifugation and
immobilized onto poly-l-Lys-coated glass slides by mixing
0.5 mL of cell suspension with an equal volume of fixation buffer (130 mm NaCl, 10 mm NaPO4 buffer, pH 6.4 [PBS] with 0.1% Tween 20 [PBT], 70 mm
EGTA, 4% paraformaldehyde, 0.25% glutaraldehyde, and 10% DMSO). For
fixation the slides were placed on a heated plate (30°C) for 30 min
and then washed with methanol and then ethanol. Postfixations were performed after xylene and proteinase K treatments. Fixed cells and
embryos were prehybridized in a solution containing PBT, 50% deionized
formamide, 0.33 m NaCl, and 50 µg/mL heparin.
Single-stranded RNA probes 230 nucleotides in length were transcribed
from pAJ41, a plasmid containing the NruI-StyI
fragment from EP3B cDNA (Kragh et al., 1996). Digoxygenin
sense and antisense probes were synthesized (Boehringer Mannheim). For
each slide 100 ng of RNA probe was denatured in a solution containing
15 µg of yeast tRNA and 50 µg of poly(A+)
RNA, and this was mixed with prehybridization solution and applied to
the slides. The hybridization was for 16 h at 42°C. The slides were washed and incubated with anti-digoxigenin fragment,
antigen-binding alkaline phosphatase diluted 1:2500 in PBT containing
50 µg of BSA and 25 µg of plant-protein extract to reduce
nonspecific staining. The nonbound antibodies were removed by several
washes with PBT, and the cells were stained in 100 mm NaCl,
5 mm MgCl2, 100 mm Tris-HCl, pH 9.5, and 0.1% Tween 20 supplemented with 9 µL/mL nitroblue tetrazolium and 7 µL/mL 5-bromo-4-chloro-3-indolyl
phosphate. Slides were mounted in PBS and 0.8% glutaraldehyde and
analyzed using Nomarski optics.
In situ hybridization on sections was carried out essentially according
to the protocol of Cox and Goldberg (1988) except for the use of
digoxigenin-labeled RNA probes.
Immunolocalization
Immunofluorescence labeling on suspension-cultured cells was
carried out as described previously (Van Engelen et al., 1991). The
anti-EP3 serum was obtained by immunizing a rabbit with the purified
protein (De Jong et al., 1995). Both fluorescein
isothiocyanate- and alkaline phosphatase-conjugated antibodies were
used. For the latter, the staining was performed as described above. In control experiments preimmune serum was used.
Tissue Printing
Fresh plant material was cut and pressed onto PVDF membranes
(Millipore). These membranes were washed in methanol and incubated in
PBS. They were either directly stained in an amido black solution (Gershoni and Palade, 1982) or treated as immunoblots and incubated with rabbit EP3 polyclonal antibodies.
RT-PCR
Total RNA was isolated from different plant tissues as
previously described (De Vries et al., 1982). From each sample 2 µg was used for reverse transcription. A mixture of RNA, 20 units of RNA
Guard (Pharmacia), 1 mm deoxyribonucleotide triphosphates, 5 mm MgCl2, 1× RT buffer (10 mm Tris, pH 8.8, 50 mm KCl, and 0.1% Triton
X-100), and 50 ng of oligo(dT)12-18 in a volume
of 20 µL was incubated at 83°C for 3 min. The mixture was
subsequently incubated at 42°C for 10 min, after which time 4 units
of avian myeloblastosis virus RT was added and the incubation was
continued for 1 h. The samples were denaturated at 95°C for 5 min and diluted to a final volume of 100 µL. A PCR reaction for the
amplification of the ubiquitin cDNA was carried out, using 5 µL from
the RT mixture, 1× Taq polymerase buffer (Boehringer
Mannheim), 100 µm deoxyribonucleotide triphosphates,
1 unit of Taq polymerase, 100 ng of downstream
ubiquitin primer
(5
-TATGGATCCACCACCACGG/AAGACGGAG-3),
and 100 ng of upstream ubiquitin primer
(5-TAGAAGCTTATGCAGATC/TTTTGTGAAGAC-3)
(Horvath et al., 1993) in a total volume of 50 µL.
After denaturation for 30 s at 94°C the samples were subjected
to 15, 20, or 30 cycles of 30 s at 94°C, 30 s at 48°C,
and 2 min at 72°C. The PCR products were run on a 1% agarose gel and blotted on Nytran+ membranes (Schleicher & Schuell). For Southern analysis, a full-length ubiquitin probe was
used. The concentration of cDNA was standardized by comparing the
signals on the Southern blot. Equal amounts of cDNA were used for
a control PCR reaction with ubiquitin primers, and one was used with
EP3 primers.
Both PCR reactions were done simultaneously, using the same PCR
conditions. In database searches the downstream primer
(5-ATGGCACGGATGGTTGCCCCGAAACCTTG-3) for EP3 showed
homology only to class IV chitinases. Therefore, amplification of class
I chitinase cDNAs cannot occur, despite their high homology
to EP3s. A PCR product with a length of 201 nucleotides was
amplified using this primer and an upstream primer (5-GTATTTTGGCCGCGGCCCTCTTCAGC-3). The probe used for the detection of
the EP3 PCR product was the 230-nucleotide insert from
pAJ-41. All Southern blots were washed three times in 0.1% SSC and 1% SDS at 65°C before being exposed to radiographic film.
| |
RESULTS |
Cell-Specific Expression of the EP3 Genes and
Localization of the Encoded Proteins in Suspension Cultures
To identify the suspension cells that express the EP3
genes, whole-mount in situ mRNA localization was used on entire,
immobilized suspension cultures. Several cell lines differing in
embryogenic potential were used to obtain a reliable indication of cell
specificity in EP3 expression. It was not possible to distinguish
between the different members of the family of EP3 genes
because of their very high homology (Kragh et al., 1996). For in situ
hybridization EP3 mRNA probes spanning the main class IV
chitinase-specific deletions were used. This makes it unlikely that
related class I chitinases were also detected. Several other probes
were used to show that cell-specific expression patterns in
plant-tissue cultures can be obtained by whole-mount in situ
hybridization (Schmidt et al., 1996, 1997).
Figure 1A shows a single
EP3-expressing suspension cell from an embryogenic
suspension culture in the midst of cells that show no staining above
background. This representative sample suggests that EP3
mRNAs could be detected only in a subpopulation of the total
embryogenic culture. Nonembryogenic cultures showed essentially the
same EP3 expression pattern (data not shown). Counting the
number of stained cells in several cultures revealed that between 4 and
6% of the total number of cells in an embryogenic culture express the
EP3 genes. The number of embryo-forming cells in a
comparable culture does not exceed 1% (Toonen et al., 1994), suggesting that there is no quantitative relation to
EP3-expressing cells.
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| Figure 1.
EP3 gene expression in suspension cultures. A
to D, Examples of EP3-expressing cells in an embryogenic
cell culture. E and F, Cell clusters present in an embryogenic cell
culture. G, Cell cluster present in an embryogenic cell culture
hybridized with an EP3 sense probe. H, Heart-stage
somatic embryo; the arrow points to an attached
EP3-expressing single cell. I, Torpedo-stage somatic embryo devoid of EP3 expression. J, Cells present at the
periphery of the proliferated cell mass on a section of a hypocotyl
explant that was cultured in the presence of 2,4-D for 10 d; the
arrow points to a single Ep3-expressing cell. K, An
elongated single cell released from a hypocotyl explant that was
cultured in the presence of 2,4-D for 20 d. L, Cell clusters
present in an embryogenic cell suspension subjected to immersion
immunofluorescence; the presence of the EP3 protein is indicated by the
green fluorescence. M, Cell cluster in a suspension of the mutant cell
line ts11; the presence of the EP3 protein is indicated by a purple
precipitate. Plant material was analyzed by whole-mount in situ
hybridization with antisense EP3 RNA probes (A-F and
H-K) or with a sense EP3 RNA probe (G). Light
microscopy (coupled with Nomarski optics for A-E and M) was used for
visualization of the purple precipitate (the use of Nomarski optics
resulted in a change from purple to brown in A-F and J). In H,
EP3 gene expression is visible as a purple precipitate
in an individual cell. Immunolocalization of EP3 proteins in
suspension cultures (L and M) was done by immersion immunofluorescence
using either fluorescein isothiocyanate (L) or alkaline phosphatase as
a second antibody. Bars = 50 µm.
{/ANNT;152064n;3872n;center;704n}
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The highest concentration of EP3 mRNA in both embryogenic
and nonembryogenic cultures was found in single cells that were elongated and often strongly curved or coiled (Fig. 1B) or in single,
vacuolated cells that were elongated (Fig. 1C) or rounded (Fig. 1D).
Staining was also often seen in small clusters of cells (Fig. 1E),
consisting of no more than about 30 cells. Cells that were loosely
attached to large clusters present in embryogenic cultures
occasionally showed intense staining (Fig. 1F), whereas the sense
control hybridizations were always negative (Fig. 1G).
Figure 1H shows a single EP3-expressing cell attached
to a heart-stage somatic embryo completely devoid of
purple staining. Whole-mount in situ localization on
globular-, heart-, or torpedo-shaped somatic embryos did not reveal
EP3 mRNAs (Fig. 1I). To determine when the first
EP3-expressing cells appeared during embryogenic cell
formation, hypocotyl explants were treated with 2,4-D for 10 d,
during which time embryogenic cell formation occurs (Guzzo et al.,
1994). In this system cell division was reinitiated in cells of the
vascular tissue and generated a mass of rapidly proliferating cells.
Only after 10 d were a very small number of cells at the periphery
of the proliferating mass found to contain EP3 mRNA (Fig.
1J). This is 3 d later than the appearance of the first cells
competent to form somatic embryos in this system (Schmidt et al.,
1997).
Subsequently, hypocotyls were transferred to hormone-free medium and
after another 5 d, some of the peripheral cells started to
enlarge. The EP3-positive cells were in some cases
elongated. In a population of single cells released from these explants
after 20 d, elongated, curved cells were the only cells that
stained positively (Fig. 1K). Single cells shown to be competent to
produce embryogenic cells never have this morphology, as was determined by cell tracking (Schmidt et al., 1997). We conclude, therefore, that
EP3 gene expression is not directly correlated with
embryogenic cell formation or somatic embryo development.
It was found previously that EP3-encoded chitinases occur in
the medium (De Jong et al., 1992). To investigate whether all of these
chitinases are secreted in the medium, whether some remain in the cell
walls of the suspension cells that produced them, or whether they are
present in somatic embryos, immersion immunofluorescence was used (Van
Engelen et al., 1991).
In embryogenic suspension cultures a relatively high amount of the
protein could be detected on the surface of a subset of cells. The
chitinase was identified on single cells as well as on small clusters
(Fig. 1L). Because EP3 chitinases are secreted proteins and contain a
signal sequence (Kragh et al., 1996), we assume that the EP3 chitinases
(Fig. 1L) are present in a component of the extracellular matrix. The
protein could not be detected on the surface of somatic embryos, but
cells that were loosely connected with somatic embryos often contained
EP3 (data not shown). EP3 proteins were also observed on cells of the
mutant cell line ts11 (Fig. 1M), which is also capable of
producing EP3 proteins (De Jong et al., 1995). The localization of the
EP3 proteins therefore appears to correspond to the pattern of
expression of their encoding genes, suggesting that EP3 proteins are
retained in the extracellular matrix of the cells that produced them,
even though a substantial amount is secreted into the medium.
The production of chitinases can be induced by pathogens and elicitors,
and in carrot suspension cultures many chitinases are induced after
treatment with fungal wall components (Kurosaki et al., 1990).
Suspension-cultured cells were exposed to chitosan as an elicitor or to
P. infestans or B. cinerea. The medium of these
cultures was isolated and the amount of EP3 produced was determined
with western analysis. Scanning of the absorbance of the bands revealed
that the constitutive production of the protein was not affected by the
fungi or the elicitor (Fig.
2).
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| Figure 2.
Presence of EP3 chitinases in media of cultures
treated with fungi and elicitors. Media of suspension cultures grown in
the presence of: lane 1, 0.002 mg/mL chitosan; lane 2, 0.02 mg/mL chitosan; lane 3, P. infestans; lane 4, B. cinerea; and lane 5, control suspension culture. Proteins were
separated by denaturing SDS-PAGE and detected by
immunoblotting.
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Expression of the EP3 Isoenzymes in Suspension Cultures
With the methods used so far it was not possible to discriminate
between different EP3 isoenzymes or the mRNAs belonging to them.
Five different EP3 chitinase isoenzymes have been detected in the
medium of suspension-cultured cells of different cell lines by using
native PAGE (Kragh et al., 1996). Given the subtle differences of the
isoenzymes EP3-1 and EP3-3 in their biological effect on ts11 embryo
formation, it was of interest to determine whether the different
isoenzymes were produced by the same or different cell types. No
correlation was found between the embryogenic capacity of a given cell
line and the EP3 isoenzymes produced (Fig. 3A). However, fractionation of suspension
cultures showed that different cell types produced different relative
amounts of isoenzymes. A population of cells <50 µm
consisted almost solely of single cells. A western blot of the proteins
in the medium showed that only the isoenzymes EP3-1 and EP3-3 were
produced by this cell population (Fig. 3B). Cells that were aggregated
in large clusters did not pass through the 125-µm sieve. This
fraction produced the isoenzymes EP3-1, EP3-3, EP3-4, and EP3-5.
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| Figure 3.
Presence of EP3 chitinases in media of
nonembryogenic, embryogenic, and density-fractionated embryogenic
cultures. A, Detection of EP3 isoenzymes in the medium of several
different suspension cultures. Lanes 1 and 2, Nonembryogenic suspension
cultures; and lanes 3 to 6, embryogenic suspension cultures. B,
Detection of EP3 isoenzymes in the medium of a Percoll-fractionated
suspension culture: lane 1, cells < 50 µm; lane 2, 0 to 10%
Percoll; lane 3, 10 to 20% Percoll; lane 4, 20 to 30% Percoll; lane
5, 30 to 40% Percoll; and lane 6, cells > 125 µm. The proteins
were separated by nondenaturing PAGE and detected by immunoblotting.
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Density fractionation of a 50- to 125-µm cell fraction on a Percoll
block gradient resulted in four subfractions of cells with different
densities. The subfractions with relatively high densities contained
more clusters compared with the low-density subfractions, which
contained mainly single cells. After the cells were cultured for
another 5 d the original difference in cell types was retained.
Comparable amounts of EP3 proteins were produced in all cell
populations; however, a shift in the relative amounts of the different
isoenzymes was observed. The pattern of isoenzymes produced by the
cells with the lowest density resembled that of the cell population
that passed through the 50-µm sieve. A relative increase of the
isoenzymes EP3-2 and EP3-5 was observed in cell populations with
higher densities. This is in agreement with the pattern of isoenzymes
produced by other cell lines (Fig. 3A). Cell lines that produce
isoenzymes EP3-2 and EP3-5 always contain clusters of small, usually
cytoplasm-rich cells. Presumably, EP3 chitinase isoenzymes 2 and 5 are
produced by these cell clusters, whereas single cells are
preferentially responsible for the production of EP3 chitinase
isoenzymes 1 and 3.
Localization of EP3 Expression in Plants
The more sensitive method of RT-PCR was used instead of northern
analysis to study EP3 gene expression. The primers allowed us to discriminate between EP3 and chitinases of class I, since
the downstream primer has homology with only class IV chitinases, and
the primers span a region that contains deletions characteristic of
class IV chitinases. However, discrimination of the different
EP3 isoenzymes was not possible because of their very high
homology (Kragh et al., 1996).
In leaves, stems, storage roots, and normal roots (Fig.
4A, lanes 1-4), very little expression
of EP3 could be detected. EP3 mRNA was found in
developing seeds at 3, 7, 10, and 20 DAP (Fig. 4A, lanes 6-9). Only
very low levels of EP3 gene expression occurred prior to
pollination (Fig. 4A, lane 5). The highest expression was found in
seeds 10 DAP, which roughly corresponds to the early globular-stage
embryo. In mature, dry seeds the amount of EP3 mRNA had
decreased (Fig. 4A, lane 10), whereas during germination EP3
mRNA was hardly detectable (Fig. 4A, lanes 11 and 12). The presence of
EP3 mRNA in developing seeds at 3, 7, 10, and 20 DAP could
be confirmed by hybridizing a northern blot with the end-labeled downstream primer that had been designed for the RT-PCR. The length of
the EP3 transcripts was approximately 950 nucleotides (data not shown).
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| Figure 4.
Expression of EP3 genes in carrot
plants in the leaf (lane 1), stem (lane 2), storage root (lane 3), root
(lane 4), flower (lane 5), fruit harvested 3 DAP (lane 6), fruit
harvested 7 DAP (lane 7), fruit harvested 10 DAP (lane 8), fruit
harvested 20 DAP (lane 9), mature seed (lane 10), seed 12 h after
imbibition (lane 11), seed 60 h after imbibition (lane 12),
genomic DNA (lane 13), and water control (lane 14). B, Ubiquitin
control. Gene expression was determined by RT-PCR, followed by transfer
to membranes and Southern hybridizaton.
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In situ mRNA localization using sections of different plant tissues
such as flowers, roots, root tips, hypocotyls, cotyledons, shoot
apices, and shoot meristems confirmed that EP3 mRNA was not
detectable in these tissues (data not shown). Only in developing and
mature seeds could EP3 mRNAs be found. In the early stages of seed development (approximately 5-6 DAP), EP3 mRNA was
detected in the inner integument cells lining the surface of the embryo sac in which the zygote or the early embryo is located (Fig.
5, A and B).
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| Figure 5.
EP3 gene expression in seeds. A,
Cross-section of a fruit 7 DAP. B, Longitudinal section of a fruit 7 DAP. C, Cross-section of a fruit 20 DAP. D, Longitudinal section of a
mature seed. E, Longitudinal section of a mature seed. F, Zygotic
embryo in a longitudinal section of a mature seed. G, Transverse
section of a seed that soaked in water for 60 h. H, Transverse
section of a seed that soaked in water for 60 h. In the
immunolocalizations, the presence of EP3 proteins is visible as a dark
precipitate. Bar = 100 µm. I, Control tissue print of a mature
seed stained with amido black. J, Immunolocalization of EP3 on a tissue
print of a fruit containing two developing seeds 20 DAP. K,
Immunolocalization of EP3 on a tissue print of a mature seed (a drawing
of the printed seed is superimposed on the picture). L,
Immunolocalization of EP3 on a tissue print of a seed that soaked in
water for 60 h. Plant material was analyzed by in situ
hybridization on sectioned carrot seeds with antisense
EP3 RNA probes (A-H). Light microscopy (coupled to
Nomarski optics for A-E) was used for visualization of the purple
precipitate (the use of Nomarski optics resulted in a change from
purple to brown in D). Immunolocalization of the EP3 protein was done
by tissue printing followed by immunostaining (I-L). i, Integuments
surrounding the developing embryo and endosperm; SC, seed coat; e,
endosperm; em, embryo. Bar = 50 µm.
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Since the highest amount of EP3 mRNAs was found 10 DAP, and
the in situ hybridizations showed their presence in the integument cells, we conclude that the EP3 chitinases are produced by the integument cells shortly before they are degraded. At 20 DAP the degradation of the integuments is almost completed (data not shown). During the time that the number of integument cells declines and the
volume of endosperm cells increases (until 35 d after anthesis), a
decline in the level of seed EP3 mRNAs was observed. In the developing endosperm at 20 DAP, EP3 mRNA could not yet be
detected (Fig. 5C). In the endosperm of mature seeds, EP3
mRNA was restricted to a narrow zone of endosperm cells starting at the
cavity in which the embryo is located up to almost the opposite end of
the endosperm (Fig. 5, D and E). Transverse sections showed that the EP3-expressing zone has a width of two to three cells and is
located in the middle of the endosperm. In zygotic embryos no
EP3 mRNA could be detected (Fig. 5F).
During seed development, cellularization of the endosperm starts with
the outer layers of endosperm and is completed in the middle of the
endosperm, which implies that EP3 genes are expressed in a
place where the cellularization of the endosperm has been completed.
The cells that contained the EP3 mRNAs also have protein bodies, which could be made visible using Nomarski optics (data not
shown).
When carrot seeds are soaked, germination is initiated, the embryo
starts to develop, and the central cavity is enlarged because of
degradation of the endosperm. The cells in which EP3 mRNA is seen are thought to be the first ones to be degraded. The expression of
the EP3 genes then shifts outward to the next cell layer
that now lines the central cavity, as shown by in situ localization of
EP3 mRNA in germinating seeds (Fig. 5, G and H). Because EP3 chitinases are secreted proteins, it was of interest to localize these
enzymes during the development of seeds and in germinating seeds. For
this purpose tissue printing of cut seeds onto nitrocellulose was
applied. Protein transfer was ensured by staining with amido black,
revealing a protein-distribution pattern in which the major seed
tissues could be clearly observed (Fig. 5I). In the tissue prints the
EP3 chitinase protein was localized using EP3 antibodies. The
antibodies strongly react with all EP3 class IV chitinases but have a
weak affinity for the 34-kD class I chitinase from carrot (Kragh et
al., 1996).
Control experiments were performed using preimmune serum or antibodies
raised against the related barley class I chitinase K that recognizes
only the 34-kD class I chitinase and not the EP3 chitinases. No seed
proteins present on the tissue prints were recognized by the barley
class I chitinase K-reacting serum or by the preimmune serum (data not
shown). We therefore conclude that the tissue prints almost exclusively
show EP3 chitinases.
The EP3 protein was uniformly spread in the integuments at 10 DAP and
in the developing endosperm at 20 DAP (Fig. 5J). The presence of the
chitinase proteins in the developing seeds is therefore only partially
in agreement with the EP3 gene-expression data. This points
to transport of the EP3 proteins from the maternal integument tissues
toward the endosperm. However, we cannot completely rule out that a
very low level of EP3 gene expression that was below
detection limits occurred in the peripheral endosperm cells. In mature
seeds the protein was restricted to the inner cell layer of the cavity
that surrounds the embryo and to a zone of cells that starts at this
cavity and ends almost at the opposite side of the endosperm (Fig. 5K),
which corresponds precisely with the expression pattern of the
EP3 genes as determined by in situ hybridization (Fig.
5D).
In seeds in which imbibition had occurred the protein was again found
to be spread uniformly in the endosperm. This pattern was observed in
seeds soaked for 16 h as well as in seeds soaked for 60 h
(Fig. 5L). In the 60-h-soaked seeds, enlargement of the embryo and the
beginning of the degradation of the endosperm were already visible.
Seeds soaked in water containing 1 mm cycloheximide did not
show any enlargement of the embryo or endosperm degradation. Tissue
prints of the cycloheximide-treated seeds showed that the localization
of the EP3 chitinase protein was the same as in the seeds soaked in
water only (data not shown). This suggests that there is no de novo
synthesis of EP3 in the outer layers of the endosperm upon imbibition
but that the water uptake is responsible for the observed outward
spread and results in a uniform presence of EP3 protein in the
endosperm.
The EP3 genes are expressed in the maternal integument
cells. The EP3 chitinases, however, were detected in the developing endosperm at a time that EP3 chitinase gene products cannot
be detected in the endosperm and as a consequence they must be largely of maternal origin. In mature seeds the integuments are completely degraded and then the EP3 proteins are produced in a subset of the
endosperm cells. During germination the secreted EP3 chitinases diffuse
in the water that is present in the cell walls to the outer region of
the endosperm and around the embryo, in contrast to the inward-directed
transport that occurs in developing seeds.
A western blot of partially purified seed proteins showed that
isoenzymes EP3-2, EP3-3, EP3-4, and EP3-5 were present in dry, mature seeds (Fig. 6). EP3-1 could not
be detected in seeds because of its low affinity for the DEAE-Sepharose
FF resin that was used in the purification method. Isoenzymes EP3-2 to
EP3-5 were all capable of degrading chitin (data not shown), as
determined using glycol-chitin overlay gels. Except for isoenzyme
EP3-1, all EP3 isoenzymes found in suspension cultures were present in
developing seeds but at different relative amounts (Fig. 3). In Figure
6, both in the conditioned-medium sample and in the seed extract, additional bands are visible between EP3-2 and EP3-3. These bands are
in a position corresponding to that of the carrot class I chitinases
(Kragh et al., 1996). In both seed and conditioned-medium samples they
represent a minor component of the total amount of chitinase present.
View larger version (111K):
[in this window]
[in a new window]
| Figure 6.
Comparison of EP3 chitinases in suspension culture
and in seeds. Lane 1, Proteins obtained from the conditioned medium of a suspension culture; and lane 2, proteins obtained from mature, dry
carrot seeds. The proteins were separated by nondenaturing PAGE, and
the EP3 chitinases were detected by immunoblotting.
|
|
| |
DISCUSSION |
In this study carrot suspension cells that produce EP3 chitinases
were identified. Using whole-mount in situ hybridization, we found that
a subset of the cells present in an embryogenic culture contains
EP3 mRNA. On the basis of the number, cell type, and presence of EP3 mRNA-producing cells in embryogenic and
nonembryogenic cultures, we found that there was no correlation with
the ability to produce somatic embryos and the presence of
EP3 mRNA. No EP3 gene expression in somatic
embryos was found. The occurrence of the EP3 chitinase protein in the
walls of suspension-cultured cells corresponds to the presence of the
EP3 mRNA in the cells.
In plants EP3 mRNA was found in the inner integument,
whereas EP3 proteins were found in the endosperm. Later, cells in the center of the endosperm do express the EP3 genes and this is
likely to be responsible for the presence of EP3 chitinases during
imbibition and germination. No EP3 gene expression was found
in zygotic embryos. Together with the absence of a pathogen- or
elicitor-induced response in EP3 gene expression, the
results support the earlier conclusion that the carrot EP3 class IV
chitinases are primarily involved in embryogenesis (De Jong et al.,
1992).
The EP3 chitinases that are produced by single cells were identified as
isoenzymes EP3-1 and EP3-3, whereas isoenzymes EP3-2 and EP3-5 were
predominantly produced by clusters. Purified EP3-1 and EP3-3
chitinases are both able to rescue the carrot somatic embryo variant
ts11 (Kragh et al., 1996); other EP3 isoenzymes were not tested for
this capability. The arrest in somatic embryo development in this
carrot variant ts11 has been shown to be due to a transient reduction
in secreted EP3 (De Jong et al., 1995). On the basis of the expression
pattern of the EP3 genes it is likely that cells that
produce EP3 do not develop into embryos themselves. Thus, it appears
that EP3 chitinases, or products of their enzymatic activity, diffuse
via the conditioned medium to cells that are able to help somatic
embryos develop and in this way play a "nursing" role in the
process of somatic embryogenesis.
Since EP3 expression was also found in developing carrot
seeds, it could be expected that the chitinases play a similar role in
zygotic embryogenesis. In carrot, flowers are pollinated directly after
anthesis and endosperm development starts soon after fertilization. Division of the primary endosperm nucleus leads to a large number of
endosperm nuclei before the first division of the zygote has taken
place. These nuclei are located in the upper part of the embryo sac,
around the zygote, and at the periphery of the embryo sac
(Borthwick, 1931). Cell wall formation in the carrot endosperm starts
7 d after anthesis (Gray et al., 1984), at about the time that the
embryo is in the two-cell stage (Borthwick, 1931), and is complete
21 d after anthesis. The integuments that surround the embryo sac
start to break down 6 to 7 DAP, at the moment that the endosperm
becomes cellular (Lackie and Yeung, 1996; E.C. Yeung, personal
communication). Maximum endosperm volume is reached about 35 d
after anthesis. Mature seeds can be harvested about 80 d after
anthesis.
The endosperm of mature carrot seeds consists of two different cell
types, peripheral cell layers containing calcium oxalate crystals and
more centrally located cells containing protein bodies with globular
inclusions (Menon and Dave, 1988). The outermost cell layer of the
endosperm has a high content of ER, whereas the central cells contain
large protein bodies and lipid droplets or only lipid droplets. The
endosperm cells that directly surround the embryo have very thick cell
walls, contain lipid droplets, and have a large vacuole (Timmers,
1993).
The EP3 chitinase proteins found in the endosperm of 20 DAP seeds are
most likely produced by integument cells. This indicates a maternal
contribution to the proteins that are present in the endosperm.
Recent evidence for a role of maternal tissues in endosperm formation
comes from the analysis of gametophytic mutations in Arabidopsis. In
this species, fie
(fertilization-independent endosperm) mutations (Ohad et al., 1996) that are female gametophytic and specifically affect endosperm formation are found. These data demonstrate a maternal component in endosperm formation.
FIE/fie integuments that surround a mutant fie
female gametophyte are degraded during development and give rise to the
seed coat in the absence of fertilization, suggesting that Arabidopsis
integument and seed coat development are initiated in response to a
signal produced by the female gametophyte (Ohad et al., 1996).
In mature, dry seeds the EP3 mRNAs are located in the middle
of the endosperm. In several other plant species, chitinases have been
found in seeds. For example, in cucumber an endochitinase was found
that is present only in mature seeds and in seeds during the early
stages of germination (up to 2 d; Majeau et al., 1990). In barley
a class I (Chi26) and a probable class II chitinase were found
primarily in endosperm and aleurone tissues (Swegle et al., 1992).
Chitinase mRNAs accumulated in barley endosperm and aleurone during
seed development and were present from 15 d after anthesis until
the end of germination (Leah et al., 1991; Swegle et al., 1992). For
Chi26 an enhancer sequence has been identified that directs the
aleurone-specific expression of this gene (Leah et al., 1994).
Cells of the starchy endosperm of cereals are dead at maturity and
incapable of synthetic processes (Bewley and Black, 1994); therefore,
the synthetic processes in the aleurone layer may resemble those that
take place in the endosperm in species that do not contain aleurone
tissue but rather live endosperm cells. In maize two 28-kD chitinases
have been found that show high levels of expression in soaked seeds.
These chitinases were reported as class I chitinases, but based on
current definitions they should be reclassified as class IV chitinases
(Huynh et al., 1992). Our results show that the localization of
EP3-producing cells is restricted to the inner tissues of both the
young fruits and the mature seeds. This pattern of localization
suggests that the EP3 chitinases do not function primarily as enzymes
that protect seeds against pathogens, as is commonly assumed for seed
chitinases (Graham and Sticklen, 1994).
EP3 mRNAs are found in integuments and in endosperm cells that surround
the central space in which the embryo is located. Both tissues are
destined for degradation. In barley, apoptosis in the aleurone layer
has been demonstrated during seed germination (M. Wang et al., 1996b),
and fungal infection has been shown to induce apoptosis (H. Wang et
al., 1996a). Since pathogen-induced chitinases are also induced prior
to cell death (Kurosaki et al., 1987), this may suggest a more general
role for chitinases in apoptosis.
The EP3 gene-expression pattern in seeds supports the
hypothesis that chitinases are not only essential for the ts11 mutant but play a more general role in plant embryogenesis. Another, indirect
indication for the importance of chitinases during seed development
comes from a recent study of tobacco plants with low chitinase levels
because of silencing caused by transgene expression of a tobacco class
I chitinase. The silenced chitinases were transiently reset to a
high-expressing state 8 to 11 DAP (Kunz et al., 1996). An explanation
for the resetting of the silenced chitinases was not given, but it
cannot be excluded that the requirement for chitinases during this
period in seed development is a driving force for the resetting.
The finding that, in addition to the EP3 chitinases,
Rhizobium sp.-produced Nod factors are capable of rescuing
embryo formation in ts11 (De Jong et al., 1993) has led to the
hypothesis that the presence of EP3s may result in the generation of
GlcNAc-containing molecules that have a positive effect on the
development of somatic embryos, although the correlation between EP3s
and the Rhizobium sp. Nod factors remains to be elucidated
(De Jong et al., 1995). Chitinases are capable of hydrolyzing Nod
factors (Staehelin et al., 1994), whereas labeling studies indicate
that plants may produce molecules that are analogous to Nod factors
(Spaink et al., 1993).
Several examples in different systems appear to support the hypothesis
that GlcNAc-containing oligosaccharides are important in development
(Ioffe and Stanley, 1994). During Xenopus sp. embryonic development the DG42 gene is found to be expressed only between the
late midblastula and neurulation stages (Rosa et al., 1988). This gene
has some similarity with fungal chitin synthases and an even stronger
homology with the Rhizobium sp. NodC gene (Bulawa and Wasco,
1991). The DG42 protein has been shown to catalyze the synthesis of
short chitin oligosaccharides in vitro. The array of chitin
oligosaccharides formed bears a striking resemblance to the
oligosaccharides produced by NodC (Semino and Robbins, 1995). Homologs
of this Xenopus sp. DG42 are present in zebrafish and mouse
(Semino et al., 1996).
Regarding the biological function of the EP3 chitinases in plant
embryogenesis, we propose that they are involved in reinitiating cell
division in embryogenic cells and embryos as part of a nursing cell
system that is required for but not restricted to embryogenesis. This
hypothesis is based on the following observations: (a) the EP3
chitinases promote embryogenic cell formation and the number of somatic
embryos as well as their progression in development when added to
ts11 cultures (De Jong et al., 1992); (b) the expression of
EP3 genes in cells that do not develop into embryos in
culture and an absence of expression in somatic embryos; (c) the
expression of EP3 genes in maternal tissue and the
subsequent secretion of the encoded chitinase proteins, resulting in
the presence of EP3 in the extracellular matrix of the endosperm
surrounding globular-stage zygotic embryos; and (d) the absence of
expression in zygotic embryos and the expression in endosperm cells
prior to and during germination. The role of EP3 could be direct and
involve a structural property such as the chitin-binding domain of the
protein. It appears more likely that its function involves the
catalytic properties of the enzyme through the release or modification
of a GlcNAc-containing signal molecule, but this awaits the
identification of such a molecule in plants. Whether such molecules are
themselves soluble or attached to the extracellular matrix will then
help to further unravel the role of chitin-based signaling molecules in
plant embryogenesis.
| |
FOOTNOTES |
1
This work was supported by The Netherlands
Organization for Scientific Research (A.J.v.H.) and by the European
Commission Biotechnology Program PTP-Biotech (F.G.).
2
Present address: Department of Cell Biology, John Innes
Centre, Colney Lane, Norwich NR4 7UH, UK.
3
Present address: Istituto Policattedra, Universita di
Verona, Verona 37134, Italy.
*
Corresponding author; e-mail sacco.devries{at}mac.mb.wau.nl; fax
31-317-483584.
Received June 25, 1997;
accepted February 12, 1998.
| |
ABBREVIATIONS |
Abbreviations:
DAP, days after pollination.
RT, reverse
transcriptase.
| |
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Abstract
Introduction
