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

Annett Richter,Miguel A Piñeros,Navid Movahed,Mahdieh Mirzaei,Lucy J Wang,Julia K Miller,Adrian F Powell,Georg Jander

doi:10.1111/tpj.15163

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 igps1 mutant 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.

Highlights

Grasses, including maize (Zea mays), produce a wide variety of constitutive and inducible defense compounds to protect themselves against herbivores and pathogens
No interaction signal was detected for the IGPS1 IGPS2 combination. Both complementation of an E. coli trpC mutation and analysis of maize transposon insertion lines indicated that maize IGPS1, IGPS2, and IGPS3 encode functional indole-3glycerolphosphate synthases
IGPS1 and IGPS3 are in separate phylogenetic clades, consistent with their functions in defensive benzoxazinoid metabolism that is not found in Arabidopsis and other dicots that are included in the dendrogram

Summary

Introduction

Grasses, including maize (Zea mays), produce a wide variety of constitutive and inducible defense compounds to protect themselves against herbivores and pathogens. The core benzoxazinoid biosynthesis pathway is well-studied and involves seven BX enzymes (BX1-BX5, BX8, and BX9) that catalyze the formation of 2,4dihydroxy-1,4-benzoxazin-3-one glucoside (DIBOA-Glc) from indole-3-glycerol phosphate (IGP) (Figure 1; Frey et al, 1997, Niemeyer, 2009, Wouters et al, 2016). The first reaction in the pathway is catalyzed by BX1, an indole-producing indole-3-glycerolphosphate lyase. IGPS1 (GRMZM2G106950), a maize gene that is co-regulated with BX genes in maize inbred line B73, was predicted to encode indole-3-glycerolphosphate synthase (IGPS), which catalyzes the ring closure of 1-(2-carboxyphenylamino)-l-deoxyribulose-5-phosphate into IGP (Figure 1). Maize contains two additional predicted IGPS genes, IGPS2 (GRMZM2G169516) and IGPS3 (GRMZM2G145870), but the expression of these genes was not co-regulated with benzoxazinoid pathway genes (Wisecaver et al, 2017)

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