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This article is a Plant Cell Advance Online Publication. The date of its first appearance online is the official date of publication. The article has been edited and the authors have corrected proofs, but minor changes could be made before the final version is published. Posting this version online reduces the time to publication by several weeks. 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. 1 of 1 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 Permissions https://www.copyright.com/ccc/openurl.do?sid=pd_hw1532298X&issn=1532298X&WT.mc_id=pd_hw1532298X eTOCs Sign up for eTOCs at: http://www.plantcell.org/cgi/alerts/ctmain CiteTrack Alerts Sign up for CiteTrack Alerts at: http://www.plantcell.org/cgi/alerts/ctmain Subscription Information Subscription Information for The Plant Cell and Plant Physiology is available at: http://www.aspb.org/publications/subscriptions.cfm © American Society of Plant Biologists ADVANCING THE SCIENCE OF PLANT BIOLOGY