Benzoxazinoid Metabolites Regulate Innate Immunity against Aphids and Fungi in Maize    

Abstract

Benzoxazinoids (BXs), such as 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA), are secondary metabolites in grasses. The first step in BX biosynthesis converts indole-3-glycerol phosphate into indole. In maize (Zea mays), this reaction is catalyzed by either BENZOXAZINELESS1 (BX1) or INDOLE GLYCEROL PHOSPHATE LYASE (IGL). The Bx1 gene is under developmental control and is mainly responsible for BX production, whereas the Igl gene is inducible by stress signals, such as wounding, herbivory, or jasmonates. To determine the role of BXs in defense against aphids and fungi, we compared basal resistance between Bx1 wild-type and bx1 mutant lines in the igl mutant background, thereby preventing BX production from IGL. Compared to Bx1 wild-type plants, BX-deficient bx1 mutant plants allowed better development of the cereal aphid Rhopalosiphum padi, and were affected in penetration resistance against the fungus Setosphaeria turtica. At stages preceding major tissue disruption, R. padi and S. turtica elicited increased accumulation of DIMBOA-glucoside, DIMBOA, and 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one-glucoside (HDMBOA-glc), which was most pronounced in apoplastic leaf extracts. Treatment with the defense elicitor chitosan similarly enhanced apoplastic accumulation of DIMBOA and HDMBOA-glc, but repressed transcription of genes controlling BX biosynthesis downstream of BX1. This repression was also obtained after treatment with the BX precursor indole and DIMBOA, but not with HDMBOA-glc. Furthermore, BX-deficient bx1 mutant lines deposited less chitosan-induced callose than Bx1 wild-type lines, whereas apoplast infiltration with DIMBOA, but not HDMBOA-glc, mimicked chitosan-induced callose. Hence, DIMBOA functions as a defense regulatory signal in maize innate immunity, which acts in addition to its well-characterized activity as a biocidal defense metabolite.

Influence of drought on plant performance through changes in belowground tritrophic interactions

“…In addition, to what extent the increased concentrations are the result of reduced root water contents is currently unclear. As BXDs have an important role in plant defense and protection against herbivores and pathogens (Ahmad et al., 2011; Glauser et al., 2011; Maag et al., 2016), the drought‐induced concentration increase may also alter plant interactions with other plant‐associated organisms (Erb et al., 2011; Robert et al., 2012). …”

Dynamic maize responses to aphid feeding are revealed by a time series of transcriptomic and metabolomic assays

“…6A), would be predicted to make the maize benzoxazinoid profile more like that of rye or wild barley. In addition to having direct toxic effects, DIMBOA can also lead to callose deposition as an antiaphid defense response in maize (Ahmad et al, 2011;Betsiashvili et al, 2015). It is not known whether DIBOA-Glc or its breakdown products can lead to the induction of callose formation in maize.…”

“…5A and 6A), a class of specialized metabolites in grasses that provide defense against insect herbivory (Frey et al, 2009;Ahmad et al, 2011). Ten enzymes (Bx1-Bx9 and Igl1) catalyze the formation of DIMBOA-Glc from indole-3-glycerol phosphate (Frey et al, 2009;Niemeyer, 2009; This is followed by methylation into HDMBOA-Glc through the activity of three Bx10 enzymes Mijares et al, 2013).…”

“…Aphid feeding does not increase overall DIMBOA-Glc or HDMBOA-Glc accumulation in tested maize cultivars (Cambier et al, 2001;Meihls et al, 2013). However, recent studies demonstrated that DIMBOA-Glc, but not HDMBOA-Glc, is secreted into the apoplast upon aphid infestation (Ahmad et al, 2011) and may regulate the accumulation of callose as a defense against aphid feeding Betsiashvili et al, 2015).…”

“…The predominant benzoxazinoids in maize foliar tissue are 4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one glucoside (DIMBOA-Glc) and 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one glucoside (HDMBOA-Glc; Frey et al, 1997Frey et al, , 2009). These compounds inhibit the growth of fungi, arrest insect herbivore growth, and cause allelopathic effects (Niemeyer, 2009;Ahmad et al, 2011). Benzoxazinoids are stored in a glucoside form that has reduced toxicity compared with the aglycones, which are produced upon cell disruption during pathogen or herbivore attack (Frey et al, 2009).…”