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Plant Physiol. (1998) 116: 599-604
Floral Scent Production in Clarkia
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ABSTRACT |
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The fragrance of Clarkia breweri (Onagraceae), a California annual plant, includes three benzenoid esters: benzylacetate, benzylbenzoate, and methylsalicylate. Here we report that petal tissue was responsible for the benzylacetate and methylsalicylate emission, whereas the pistil was the main source of benzylbenzoate. The activities of two novel enzymes, acetyl-coenzyme A:benzylalcohol acetyltransferase (BEAT), which catalyzes the acetyl esterification of benzylalcohol, and S-adenosyl-l-methionine:salicylic acid carboxyl methyltransferase, which catalyzes the methyl esterification of salicylic acid, were also highest in petal tissue and absent in leaves. In addition, the activity of both enzymes in the various floral organs was developmentally and differentially regulated. S-Adenosyl-l-methionine:salicylic acid carboxyl methyltransferase activity in petals peaked in mature buds and declined during the next few days after anthesis, and it showed a strong, positive correlation with the emission of methylsalicylate. The levels of BEAT activity and benzylacetate emission in petals also increased in parallel as the buds matured and the flowers opened, but as emission began to decline on the 2nd d after anthesis, BEAT activity continued to increase and remained high until the end of the lifespan of the flower.
Flowers of Clarkia breweri ([Gray] Greene;
Onagraceae), an annual plant native to California, emit a strong, sweet
fragrance consisting of 8 to 12 different volatiles. These volatiles
are derived from two biochemical pathways, one leading to
monoterpenoids, the other to phenylpropanoids-benzenoids (Raguso
and Pichersky, 1995 Volatile esters are common in floral scents, where they may contribute
substantially to the total scent output. In C. breweri, for
example, benzylacetate constitutes 20 to 40% (w/w) of the total scent
output (depending on the particular C. breweri line), whereas the other two esters each constitute about 5% of the scent (Raguso and Pichersky, 1995 Here we report on the emission of the three ester constituents of the
C. breweri scent: benzylacetate, benzylbenzoate, and methylsalicylate. We have also developed enzymatic assays to test for
the activities of the biosynthetic enzymes that catalyze the formation
of methylsalicylate and benzylacetate: SAMT and BEAT, respectively. The
activity of benzoyl-CoA:benzylalcohol benzoyltransferase, the
hypothetical enzyme that catalyzes the formation of benzylbenzoate (Gross, 1981 Plant Material, Growth Conditions, Headspace Collection, and
GC-MS Analysis
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INTRODUCTION
Top
Abstract
Introduction
Methods
Results & Discussion
References
). In the last group three are the esters
benzylacetate, benzylbenzoate, and methylsalicylate (Raguso and
Pichersky, 1995
).
). In addition to acting as attractants for
pollinators, volatile esters such as methylsalicylate and methyljasmonate have been implicated in cell-to-cell signaling (Farmer
and Ryan, 1990
; Shulaev et al., 1997
). To date, however, little is
known about the enzymes that catalyze the condensation of the alcohol
and acid moieties of such volatile esters in plants.
; Croteau and Karp, 1991
), was not tested because of the
lack of a labeled substrate with a suitably high specific activity. The
activities of SAMT and BEAT throughout the lifespan of the flower
follow complex patterns. These patterns are compared with those of two
previously described enzymes involved in scent biosynthesis in C. breweri flowers, LIS and IEMT (Pichersky et al., 1994
; Dudareva et
al., 1996
; Wang et al., 1997
). Overall, the data show that scent
production in C. breweri is a complex process that involves
spatial and temporal patterns of regulation that are not necessarily
identical for all of the enzymes involved.
![]()
MATERIALS AND METHODS
Top
Abstract
Introduction
Methods
Results & Discussion
References
. All headspace collections were performed in a growth chamber (Conviron, Pembina, ND) under a 12-h light/12-h dark photoperiod. Temperature was set at 22°C during the light period
and at 18°C during the dark period. In all experiments headspace
collections from ambient air and from vegetative tissues were used as
controls.
Time Course of Ester Production
Volatile production of esters in individual flowers of four separate plants was monitored over a 6-d period beginning on the day before anthesis and continuing until floral abscission. Headspace volatiles were collected as described by Raguso and Pichersky (1995)Localization and Quantitation of Emission of Esters in Floral Parts
The specific floral parts responsible for scent emission were determined, and the emission levels were quantified by headspace collection, essentially as described by Raguso and Pichersky (1995)SAMT and BEAT Enzyme Extraction and Assay
Enzyme Extraction
A crude protein extract was prepared as previously described (Wang et al., 1997SAMT Enzyme Assays and Product Analysis
Assay samples were prepared by adding the following to a 1.5-mL microcentrifuge tube: 10 µL of crude extract, 10 µL of 5× assay buffer (250 mm Tris-HCl [pH 7.5] and 14 mm 2-mercaptoethanol), 1 µL of 50 mm salicylic acid, 1.0 µL (2 × 10
5 mCi) of 0.34 mm
S-[methyl-14C]adenosyl-l-Met
(NEN Research Products), and 28 µL of water to bring the assay volume
to 50 µL. Assay samples were incubated at 30°C for 30 min in a
heating block. The radioactively labeled methylated product was
extracted by the addition of 100 µL of ethyl acetate, and 20 µL of
the organic phase (on top and clear in color) was transferred to a
scintillation vial with 2 mL of nonaqueous scintillation fluid
(Bio-Safe NA, Research Products International, Mount Prospect, IL) and
counted in a liquid-scintillation counter (model 2S6800, Beckman). The
raw data (counts per minute) were converted to picokatals (picomoles of
product produced per second) based on the specific activity of the
substrate and using the appropriate correction factors for counting
efficiency. Controls (for SAMT as well as for BEAT) included assays
with boiled crude extracts and with buffers only, and background
radioactivity produced in such assays was subtracted from all of the
results. The specificity of SAMT was tested with several related
substrates such as benzoic acid. No activity was detected with
substrates other than salicylic acid.
). Products were also analyzed by GC-MS after organic extraction from scaled-up reactions of 1 mL of total volume, with both substrates nonradioactive and at a final concentration of 1 mm each. Because esters
are hydrolyzed in concentrated acid solutions, the enzymatic assays were not stopped by the addition of concentrated acid, as is often done, but HCl hydrolysis (at a final concentration of 0.3 m) was also carried out to distinguish between methylation
of the carboxyl OH group versus methylation of the 2-OH group on the
benzyl ring. An enzymatic activity of the latter type was not found in
the crude extracts of C. breweri flowers.
BEAT Enzyme Assays and Product Analysis
Assay samples were prepared by adding the following to a 1.5-mL microcentrifuge tube: 10 µL of crude extract, 10 µL of assay buffer (250 mm Tris-HCl [pH 7.5] and 14 mm 2-mercaptoethanol), 1 µL of 50 mm benzylalcohol, 0.4 µL (2 × 10
5 mCi) of 1 mm
[14C]acetyl-CoA (Amersham), and 28.6 µL of
water to bring the assay volume to 50 µL. Assay samples were
incubated at 30°C for 15 min in a heating block. The radioactively
labeled product was extracted by the addition of 100 µL of hexane.
The product extracted into the organic phase was analyzed as described
above for SAMT.
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RESULTS AND DISCUSSION |
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Temporal Variation in Emission of Benzenoid Esters
The strong, sweet floral scent of C. breweri is unique in its genus and is correlated with pollination by moths, a mode of reproduction that is novel among Clarkia spp. (McSwain et al., 1973). We have previously shown (Raguso and Pichersky, 1995
Localization and Quantification of Benzenoid Ester Emission from
the Different Parts of the Flower
BEAT and SAMT Activities in Flowers
BEAT and SAMT Activity in Flower Parts
1 12 h
1.
Benzylbenzoate and methylsalicylate were emitted at much lower levels,
peaking at 2.7 and 1.9 µg flower
1 12 h
1, respectively.

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Figure 1.
Emission of benzenoid esters from C. breweri flowers as measured by headspace collection at 12-h
intervals and GC-MS analysis. A, Emission of benzylacetate; B, emission
of benzylbenzoate; and C, emission of methylsalicylate. Data are
means ± se (n
3).

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[in a new window]
Figure 2.
Emission of benzenoid esters from C. breweri flowers and flower parts on d 1 of anthesis as measured
by headspace collection at 24-h intervals and GC-MS analysis. Black
bars, Benzylacetate; shaded bars, benzylbenzoate; and open bars,
methylsalicylate. Data are means ± se
(n = 5-8).
). Such results could mean
that organs other than the emitting one are involved in controlling the
flux of the pathway. However, the most likely explanation is that such
decreases and increases were brought about by the injury sustained by
the flowers in these experiments. It is noteworthy that emission of
methylsalicylate, a compound known to be involved (together with
salicylate) in the response of plant vegetative tissue to pathogen
damage (Shulaev et al., 1997
), increased in the injured flowers.
Because salicylate is derived from the benzoic acid pathway (Yalpani et
al., 1993
), it is perhaps not surprising that increased synthesis of
salicylate (as an intermediate in the synthesis of methylsalicylate,
and possibly as an end product) resulted in the concomitant decrease in
the biosynthesis of benzylacetate and benzylbenzoate.
). Specifically, benzylacetate, benzylbenzoate, and methylsalicylate are
found in the scent of many moth-pollinated flowers (Kaiser, 1993
;
Knudsen and Tollsten, 1993
). In addition, methylsalicylate is also
reported to be important in plant defense and communication (Dicke et
al., 1990
; Shulaev et al., 1997
). However, no enzymatic activities
capable of forming these products have been reported, and the pathways
leading to benzoate, benzylalcohol, and salicylic acid have only been
partially elucidated. For example, it is known that the benzene ring is
derived from trans-cinnamic acid (Yalpani et al., 1993
; Lee
et al., 1995
) and that benzoic acid is converted to salicylic acid by
benzoic acid 2-hydroxylase (Leon et al., 1993
).
Temporal Variation in BEAT and SAMT Activities
Received July 10, 1997;
accepted October 22, 1997.
Abbreviations:
BEAT, acetyl-CoA:benzylalcohol
acetyltransferase.
IEMT, S-adenosyl-l-Met:(iso)eugenol
O-methyltransferase.
LIS, linalool synthase.
SAM, S-adenosyl-l-Met.
SAMT, SAM:salicylic acid
carboxyl methyltransferase.
We thank John D'Auria for his help with the GC-MS analyses.
Clarke S
(1992)
Protein isoprenylation and methylation at carboxyl-terminal cysteine residues.
Annu Rev Biochem
61:
355-386
[Web of Science][Medline]
Croteau R, Karp F (1991) Origin of natural odorants. In
PM Muller, D Lamparsky, eds, Perfumes: Art, Science and Technology.
Elsevier Applied Science, New York, pp 101-126
Dicke M,
Van Beek TA,
Posthumus MA,
Ben Dam N,
Van Bokhoven H,
de Groot A
(1990)
Isolation and identification of volatile kairomone that affects acarine predator-prey interactions.
J Chem Ecol
16:
381-396
[CrossRef][Web of Science]
Dudareva N,
Cseke L,
Blanc VM,
Pichersky E
(1996)
Evolution of floral scent in Clarkia: novel patterns of S-linalool synthase gene expression in the C. breweri flower.
Plant Cell
8:
1137-1148
[Abstract]
Farmer EE,
Ryan CA
(1990)
Interplant communication: airborne methyl jasmonate induces synthesis of proteinase inhibitors in plant leaves.
Proc Natl Acad Sci USA
87:
7713-7716
Gross G (1981) Phenolic acids. In EE Conn, ed, The
Biochemistry of Plants: A Comprehensive Treatise, Vol 7. Academic
Press, New York, pp 301-316
Kaiser R (1993) The Scent of Orchids. Elsevier Editiones Roche,
Basel, Switzerland
Knudsen JT,
Tollsten L
(1993)
Trends in floral scent chemistry in pollination syndromes: floral scent composition in moth-pollinated taxa.
Bot J Linn Soc
113:
263-284
[CrossRef]
Knudsen JT,
Tollsten L,
Bergstrom G
(1993)
Floral scents: a check-list of volatile compounds isolated by head-space techniques.
Phytochemistry
33:
253-280
[CrossRef][Web of Science]
Lee H,
Leon J,
Raskin I
(1995)
Biosynthesis and metabolism of salicylic acid.
Proc Natl Acad Sci USA
92:
4076-4079
Leon J,
Yalpani N,
Raskin I,
Lawton MA
(1993)
Induction of benzoic acid 2-hydroxylase in virus-inoculated tobacco.
Plant Physiol
103:
323-328
[Abstract]
MacSwain J,
Raven P,
Thorp R
(1973)
Comparative behavior of bees and Onagraceae. IV. Clarkia bees of the western United States.
Univ Calif Publ Entomol
70:
1-80
Pichersky E,
Raguso RA,
Lewinsohn E,
Croteau R
(1994)
Floral scent production in Clarkia (Onagraceae). I. Localization and developmental modulation of monoterpene emission and linalool synthase activity.
Plant Physiol
106:
1533-1540
[Abstract]
Raguso RA,
Pichersky E
(1995)
Floral volatiles from Clarkia breweri and C. concinna (Onagraceae): recent evolution of floral scent and moth pollination.
Plant Syst Evol
194:
55-67
[CrossRef]
Shulaev V,
Silverman P,
Raskin I
(1997)
Airborne signalling by methyl salicylate in plant pathogen resistance.
Nature
385:
718-721
Wang J,
Dudareva N,
Bhakta S,
Raguso RA,
Pichersky E
(1997)
Floral scent production in Clarkia breweri (Onagraceae). II. Localization and developmental modulation of the novel enzyme S-adenosyl-l-methionine:(iso)eugenol O-methyltransferase and phenylpropanoid emission.
Plant Physiol
114:
213-221
[Abstract]
Yalpani N,
Leon J,
Lawton MA,
Raskin I
(1993)
Pathway of salicylic acid biosynthesis in healthy and virus-inoculated tobacco.
Plant Physiol
103:
315-321
[Abstract]

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[in a new window]
Figure 3.
The reactions catalyzed by BEAT and SAMT.
).
Therefore, we developed an enzymatic assay to test for SAMT activity
using salicylic acid and [14C]SAM.
View this table:
Table I.
BEAT and SAMT activities in C. breweri on d 1 of
anthesis
Values are averages of three independent measurements. Protein
concentrations in the different organs are given in "Materials and
Methods," and can be used to calculate specific activities per
milligram of protein. The total weight of each class of organs in the
flowers is from Pichersky et al. (1994)
.
). Because petals possess the highest levels of BEAT- and SAMT-specific activities per
milligram fresh weight among all floral organs, and because they
constitute slightly more than one-half of the total mass of the flower,
it is not surprising that 90% of the total BEAT activity and 80% of
the total SAMT activity in the flower are found in the petals.
; Wang et al., 1997
). Because no
BEAT or SAMT activities were found in vegetative tissue, it is unlikely
that this tissue makes a significant contribution to the final step of
synthesis of benzylacetate and methylsalicylate, although earlier
precursors may come from such parts of the plant.

View larger version (15K):
[in a new window]
Figure 4.
Levels of different BEAT and SAMT activities in
different parts of the flower during the lifespan of the flowers. A,
BEAT activity; and B, SAMT activity. For each data point, flowers from three different plants were combined for each assay, and two to three
enzyme assays were conducted and the mean was obtained. se
values for data points on d 1 of anthesis are given in Table I;
se values for other time points are similar.
, Petals;
, stigma;
, style;
, stamens; and
, sepals. pkat, Picomole
of product per second.
). However, the temporal variation in levels of
BEAT activity, which shows little or no decline at the end of the
lifespan of the flower (although emission of benzylacetate
does decline), is similar to that observed for IEMT, except that IEMT
levels peaked on d 1 of anthesis and stayed stable afterward (Wang et
al., 1997
), whereas BEAT activity did not peak until the 4th d after
anthesis (Fig. 4).
; Dudareva et
al., 1996
; Wang et al., 1997
) these data show the existence of at least
two types of patterns for enzymes involved in scent production in
C. breweri flowers. The activities of the first group
of enzymes, such as LIS and SAMT, increase in young flowers and decline
in old flowers, whereas the activities of the second group of enzymes,
such as IEMT and BEAT, increase gradually during the lifespan of the
flower and remain high in old flowers.
), the causes and
consequences of high levels of activity of biosynthetic enzymes in old
flowers, without concomitant emission of the volatile products, are not
known. Although it is possible that the biosynthetic pathways in which
these enzymes participate are blocked elsewhere, another possibility
that remains to be investigated is that the products produced in the
reactions catalyzed by these enzymes are required for additional
processes in the flowers other than scent emission. A third possibility
is that as the flower ages, substrates may be diverted to other
compartments and are not accessible to the scent biosynthetic enzymes.
1
This research was funded by National Science
Foundation grant no. IBN-9417582 to E.P. R.A.R. was supported in
part by a National Institutes of Health/Genetics Training Grant
fellowship.
![]()
FOOTNOTES
2
Present address: Horticulture Department, Purdue
University, West Lafayette, IN 47907.
3
Present address: Arizona Research Laboratories,
Division of Neurobiology, P.O. Box 210077, University of Arizona,
Tucson AZ 85721-0077.
*
Corresponding author; e-mail lelx{at}umich.edu; fax
1-734- 647-0884.
![]()
ABBREVIATIONS
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ACKNOWLEDGMENT
![]()
LITERATURE CITED
Top
Abstract
Introduction
Methods
Results & Discussion
References
Copyright Clearance Center: 0032-0889/98/116/0599/06
© 1998 American Society of Plant Physiologists
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