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Peppermint Glands |
Monoterpenes are part of the biochemical arsenal that plants use
against herbivores and pathogens. They are also the principal constituents of the essential oils and resins that impart the characteristic flavors to herbs and spices, such as peppermint. During
peppermint leaf development, the total content of monoterpenes (e.g.
menthol) increases with age. The primary determinant of monoterpene
accumulation is biosynthesis: Catabolism and volatilization are
negligible. Monoterpenes are produced and stored in peltate glandular
trichomes, one of the three types of trichomes found on the peppermint
leaf. A storage compartment formed by the separation of the cuticle
from the apical walls of the disc cells (Fig.
1) surmounts the peltate glands.
In two companion papers in this issue, Turner et al. (pp.
655-663, 665-679) present an unprecedentedly detailed study of
the distribution and development of these monoterpene-producing glands. In their first contribution, the authors present an
ultrastructural study of the developing glands. Complementing
previous biochemical studies, these new ultrastructural findings
implicate the smooth endoplasmic reticulum and the leucoplasts
in monoterpene biosynthesis and export. In their second contribution,
Turner et al. report that peltate glands are continuously formed until
leaf expansion ceases. The complete development of a glandular
trichome, from initiation to filling, takes only 60 h, and only 20 to 30 h are required for the gland storage compartment to become
filled with essential oil
all the flavor it will ever contain.
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Molecular Biology of Dioecious Flowers |
Black cottonwood (Populus trichocarpa) and
related species are establishing themselves as the "Arabidopsis" of
arboreal species. The features that make Populus spp.
attractive to molecular biologists are their small genome size,
efficient transformation, relatively short flowering time (<5 years)
compared to other trees, and ease of propagation. It is unfortunate
that there is a relatively high potential for transgene escape from the
Populus sp. into wild populations because of the long
distance transport of its pollen and seeds and the ubiquity of its wild
relatives. Genetically engineered sterility may be the best strategy
for transgene containment. The isolation of a promoter specific to
reproductive tissue would be especially useful in this endeavor. In
this issue, Sheppard et al. (pp. 627-639) report on their
isolation of floral homeotic gene (PTD) from P. trichocarpa. PTD is not expressed in vegetative tissues and
its spatial and temporal expression patterns are sex specific (Fig.
2). This line of research will also
provide insights into the evolution of dioecy (the occurrence of male
or female flowers in different individuals). It will be interesting to
relate the development of the dioecious flowers of the
Populus sp. to the well-known ABC model of floral
development that has been so successful in explaining the development
of the four organs of the typical bisexual flower in terms of the
interactions of three classes of homeotic genes. Most floral homeotic
genes isolated to date belong to the MADS-box family of transcription
factors, and PTD is no exception. Phylogenetic analysis
indicates that PTD is homologous to previously described
floral homeotic transcription factors from herbaceous species.
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Figure 2.
Floral homeotic gene (yellow) is restricted to
reproductive tissue in the P. trichocarpa flower.
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Programmed Cell Death and Root Emergence |
During the normal development of deepwater rice, adventitious
root primordia form at the nodes. These adventitious roots functionally replace the basal roots, which are poorly suited for the anaerobic conditions associated with submergence. Root emergence depends upon
flooding and is mediated by ethylene (Fig.
3). The endogenous origin of adventitious
roots requires that the root primordia penetrate the nodal epidermis
and cuticle during emergence. In this issue, Mergemann and Sauter
(pp. 609-614) examine the question of whether penetration of the
epidermis is purely a mechanical process driven by the force of the
elongating root or does the programmed death of the overlying epidermal
cells precede and facilitate root penetration? The authors used Evans blue staining to observe the progress of cell death in the nodal epidermis following submergence. Induced cell death was apparent within
only 2 h and clearly preceded the growth of the underlying adventitious root. Cell death was inducible not only by submergence but also by the application of
1-aminocyclopropane-1-carboxylic acid, the natural precursor of
ethylene, or of ethephon, an ethylene-releasing compound (Fig. 3). The
ethylene receptor inhibitor 2,5-norbornadiene inhibited cell death. The
authors speculate that this programmed death response is selectively
advantageous because, in its absence, the plant, unable to distinguish
between attacks from outside versus within, would waste energy by
launching an unnecessary defense response.
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Cortical Microtubules and Gibberellin-Induced Growth
Anisotropy |
Cell elongation in plants is rarely uniaxial in the strict
sence. Although growth may be highly anisotropic, cell elongation is
almost always accompanied by significant, if minor, radial expansion.
The loss of transversely aligned cortical microtubules (CMTs), either
by natural or chemical means, enhances radial swelling and reduces
axial growth. Mutations or chemicals that block gibberellin (GA)
biosynthesis or signal transduction have the opposite effect: they
decrease axial growth and increase radial expansion. Previous researchers have proposed that GA may inhibit radial expansion by
causing CMTs to align transversely to the long axis of the growing
cells. In this issue, Wenzel et al. (pp. 813-822) present
data that call into question a role for CMTs in mediating GA's effects
on cell growth anisotropy. The authors take advantage of a dwarf barley
mutant whose short, wide blades, short elongation zone, and slow
elongation rate can be reversed by the application of GA. Unlike
wild-type plants, CMT orientation in the distal elongation zone of
untreated mutants was disordered. Treatment with GA enhances axial
expansion, suppresses radial expansion, and increases the transverse
orientation of CMTs in the distal elongation zone of the mutant. The
inhibition of radial expansion in GA-treated mutants, however, occurs
in the basal elongation zone where the CMTS are transversely arranged
in both GA-treated and -untreated mutants as well as in wild-type
plants. GA-induced changes in growth anisotropy, therefore,
precede GA-induced changes in CMT orientation.
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Nod Factor Signal Transduction |
The nitrogen-fixing nodules of legume roots result from
the infection of the roots by symbiotic rhizobial bacteria. During the
infection process, rhizobial bacteria secrete specific nodulation signals, called Nod factors (NFs), that have a complex biochemical structure. NFs are modified lipo-chitooligosaccharides, i.e. chitin oligomers that have a fatty acid replacing the N-acetyl
group on their non-reducing end. Structural substitutions at either the
reducing or non-reducing ends of NFs affect their host specificity, possibly by influencing their binding to specific plant receptors. Previous studies have revealed that an early step in NF-signal transduction is the opening of plasma membrane Ca2+
channels, leading to an increase in cytoplasmic Ca2+. In
this issue, Müller et al. (pp. 733-739) examine the efficacy of various modified NFs as well as chitin oligomers on cytoplasmic Ca2+ levels in transgenic soybean
cells expressing the Ca2+-sensitive photoprotein
aequorin. Their data indicate that soybean culture cells perceive
chitin oligomer elicitors and the structurally related NFs in a very
similar manner. The application of unmodified NF made the cells
refractory to chitin and vice versa. These results complement earlier
studies that found that NFs and chitins often activate some of the same
genes, although there are examples of differences in the responses of
plants to these two factors as well. Structurally modified NFs differ
in their ability to elicit the Ca2+ response.
Substitutions at either the reducing or non-reducing end, as well as
changes in the length of the oligomer backbone, influence the
Ca2+ response.
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Peppermint Glands
Molecular Biology of Dioecious...
