Indole-3-glycerolphosphate synthase, a branchpoint for the biosynthesis of tryptophan, indole, and benzoxazinoids in maize

Authors: RICHTER, ANNETT - Boyce Thompson Institute; POWELL, ADRIAN - Boyce Thompson Institute; MIRZAEI, MAHDIEH - Boyce Thompson Institute; WANG, LUCY - Boyce Thompson Institute; MOVAHED, NAVID - Boyce Thompson Institute; MILLER, JULIA - Cornell University; Pineros, Miguel; JANDER, GEORG - Boyce Thompson Institute

Submitted to: The Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2021
Publication Date: 1/18/2021
Citation: Richter, A., Powell, A.F., Mirzaei, M., Wang, L.J., Movahed, N., Miller, J.K., Pineros, M., Jander, G. 2021. Indole-3-glycerolphosphate synthase, a branchpoint for the biosynthesis of tryptophan, indole, and benzoxazinoids in maize. The Plant Journal. 106:245-257. https://doi.org/10.1111/tpj.15163.
DOI: https://doi.org/10.1111/tpj.15163

Interpretive Summary: Plants produce a wide variety of secondary metabolites to defend themselves again herbivores and pathogens. In plants, indole is not only the precursor of the essential amino acid Tryptophan, but also the precursor of benzoxazinoids, which defend plants from the attack of numerous arthropod pests. The synthesis of these indole-derivatives is mediated by various enzymes, while the formation of the indole precursor compound earlier in the synthesis pathways is catalyzed by the IGP enzyme. The maize genome contains three genes that encode IGPs. In this work, we report experiments that confirm the function of the three maize IGPs and determine whether they represent a branch point in the indole biosynthesis pathway. We demonstrate that IGPS1 and IGPS3 function mainly in primary metabolism (biosynthesis of tryptophan), whereas IGPS2 is mostly involved in specialized metabolism (the biosynthesis of defensive metabolites). This work provides new insight into the regulation of indole metabolism and how separate indole pools can be used for both primary and specialized metabolism within the same cellular compartment.

Technical Abstract: The maize (Zea mays) genome encodes three indole-3-glycerolphosphate synthase enzymes (IGPS1, 2, and 3) catalyzing the conversion of 1-(2-carboxyphenylamino)-l-deoxyribulose-5-phosphate to indole-3-glycerolphosphate. Three further maize enzymes (BX1, benzoxazinoneless 1; TSA, tryptophan synthase alpha subunit; and IGL, indole glycerolphosphate lyase) convert indole-3-glycerolphosphate to indole, which is released as a volatile defense signaling compound and also serves as a precursor for the biosynthesis of tryptophan and defense-related benzoxazinoids. Phylogenetic analyses showed that IGPS2 is similar to enzymes found in both monocots and dicots, whereas maize IGPS1 and IGPS3 are in monocot-specific clades. Fusions of yellow fluorescent protein with maize IGPS enzymes and indole-3-glycerolphosphate lyases were all localized in chloroplasts. In bimolecular fluorescence complementation assays, IGPS1 interacted strongly with BX1 and IGL, IGPS2 interacted primarily with TSA, and IGPS3 interacted equally with all three indole-3-glycerolphosphate lyases. Whereas IGPS1 and IGPS3 expression was induced by insect feeding, IGPS2 expression was not. Transposon insertions in IGPS1 and IGPS3 reduced the abundance of both benzoxazinoids and free indole. Spodoptera exigua (beet armyworm) larvae show improved growth on igps1mutant maize plants. Together, these results suggest that IGPS1 and IGPS3 function mainly in the biosynthesis of defensive metabolites, whereas IGPS2 may be involved in the biosynthesis of tryptophan. This metabolic channeling is similar to, though less exclusive than, that proposed for the three maize indole-3-glycerolphosphate lyases.