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IN BRIEF
Metabolic Crosstalk: Interactions between the Phenylpropanoid
and Glucosinolate Pathways in Arabidopsis
The phenylpropanoid pathway is big in
plants—particularly in trees, which can get
big in no small part because of the lignin
produced through this pathway. In addition
to the huge carbon sink represented by lignin
(reviewed in Eudes et al., 2014), the phenylpropanoid pathway also produces important
small molecules such as flavonoids. By
contrast, the glucosinolate pathway is small
potatoes—or rather, small Brussels sprouts,
as these sulfur- and nitrogen-containing compounds generally occur in Brassica species,
including Arabidopsis thaliana (reviewed in
Baskar et al., 2012). Glucosinolates participate in plant defenses against herbivores and
occur in broadly varying types; for example,
Arabidopsis Col-0 produces ;30 different
glucosinolates. Different glucosinolates form
from different precursor amino acids, including Trp (indole glucosinolates), Ala, Ile, Leu,
Met, or Val (aliphatic glucosinolates), and Phe
or Tyr (aromatic glucosinolates).
To examine secondary metabolism, Kim
et al. (2015) characterize the reduced epidermal fluorescence5 (ref5) mutant, which
has reduced levels of phenylpropanoids, resulting in decreased fluorescence under UV
light (see figure). Positional cloning showed
that REF5 encodes the cytochrome P450
monooxygenase CYP83B1, an enzyme involved in biosynthesis of indole glucosinolates. Indeed, although ref5 was originally
identified by its reduced levels of the phenylpropanoid compound sinapoylmalate,
the ref5 mutant also has reduced levels of
indole glucosinolates. Moreover, accumulation of the glucosinolate precursor indole3-acetaldoxime (IAOx) results in increased
production of indole-3-acetic acid, causing
high-auxin phenotypes, such as longer
hypocotyls, in the ref5 mutants.
In addition to high-auxin phenotypes, IAOx
accumulation changes phenylpropanoid metabolism. The authors showed this by examining triple mutants affecting CYP831B and the
upstream enzymes CYP79B2 and CYP79B3,
which produce IAOx. They found that the triple
mutants do not accumulate IAOx and also
www.plantcell.org/cgi/doi/10.1105/tpc.15.00360
The ref5 mutant phenotype. Three-week-old wild type and reduced epidermal fluorescence mutant ref5 under
visible or UV light. Wild-type plants show blue fluorescence from the phenylpropanoid sinapoylmalate; reduced
sinapoylmalate leads to red chlorophyll autofluorescence. (Reprinted from Kim et al. [2015], Figure 1A.)
show higher levels of sinapoylmalate, indicating
that the low-sinapoylmalate phenotype results
from high IAOx, not low indole glucosinolates.
Examination of phenylalanine, flavonoids, and
other intermediates in the phenylpropanoid
pathway in different mutants also showed
that the inhibitory effect of IAOx likely occurs
early in the pathway, possibly acting on the first
committed enzyme of the pathway, PHENYLALANINE AMMONIA LYASE2. Double mutants
of REF5 and CYP83A1/REF2, which encodes
another enzyme in the glucosinolate pathway,
showed a synergistic effect on the phenylpropanoid pathway, with a 96% reduction in
sinapoylmalate contents compared with
a 70% reduction in each single mutant.
Moreover, a screen for suppressors of ref5
identified a subunit of the Mediator complex,
indicating that changes in transcription involving the Mediator complex may cause the
repression of the phenylpropanoid pathway.
Identification of the crosstalk between glucosinolate and the phenylpropanoid pathway,
and of the role of the Mediator complex, helps
us understand secondary metabolism in plants
and has many potential applications. For ex-
ample, metabolic engineering efforts have targeted lignin for forage and biofuels feedstock
applications and have targeted glucosinolates
for their plant defense and human anticancer
applications. Future work may identify crosstalk with other metabolic pathways in the complex regulation of plant secondary metabolism.
Jennifer Mach
Science Editor
[email protected]
ORCID ID: 0000-0002-1141-6306
REFERENCES
Baskar, V., Gururani, M.A., Yu, J.W., and Park,
S.W. (2012). Engineering glucosinolates in
plants: current knowledge and potential uses.
Appl. Biochem. Biotechnol. 168: 1694–1717.
Eudes, A., Liang, Y., Mitra, P., and Loqué,
D. (2014). Lignin bioengineering. Curr. Opin.
Biotechnol. 26: 189–198.
Kim, J.I., Dolan, W.L., Anderson, N.A., and
Chapple, C. (2015). Indole glucosinolate biosynthesis limits phenylpropanoid accumulation in Arabidopsis thaliana. Plant Cell 27:
10.1105/tpc.15.00127.
The Plant Cell Preview, www.aspb.org ã 2015 American Society of Plant Biologists. All rights reserved.
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Metabolic Crosstalk: Interactions between the Phenylpropanoid and Glucosinolate Pathways in
Arabidopsis
Jennifer Mach
Plant Cell; originally published online May 8, 2015;
DOI 10.1105/tpc.15.00360
This information is current as of August 3, 2017
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