Metabolic link between auxin production and specialized metabolites in Sorghum bicolor

Abstract

Aldoximes are amino acid derivatives that serve as intermediates for numerous specialized metabolites including cyanogenic glycosides, glucosinolates, and auxins. Aldoxime formation is mainly catalyzed by cytochrome P450 monooxygenases of the 79 family (CYP79s) that can have broad or narrow substrate specificity. Except SbCYP79A1, aldoxime biosynthetic enzymes in the cereal sorghum (Sorghum bicolor), have not been characterized. This study identified nine CYP79-encoding genes in the genome of sorghum. A phylogenetic analysis of CYP79 showed that SbCYP79A61 formed a subclade with maize ZmCYP79A61, previously characterized to be involved in aldoxime biosynthesis. Functional characterization of this sorghum enzyme using transient expression in Nicotiana benthamiana and stable overexpression in Arabidopsis thaliana revealed that SbCYP79A61 catalyzes the production of phenylacetaldoxime (PAOx) from phenylalanine, but unlike the maize enzyme, displays no detectable activity against tryptophan. Additionally, targeted metabolite analysis after stable isotope feeding assays revealed that PAOx can serve as a precursor of phenylacetic acid (PAA) in sorghum and identified benzyl cyanide as an intermediate of PAOx-derived PAA biosynthesis in both sorghum and maize. Taken together, our results demonstrate that SbCYP79A61 produces PAOx in sorghum and may serve in the biosynthesis of other nitrogen-containing phenylalanine-derived metabolites involved in mediating biotic and abiotic stresses.

Occurrence, Function, and Biosynthesis of the Natural Auxin Phenylacetic Acid (PAA) in Plants

“…However, the PAA content in Tropaeolum majus (<16 pmol/gFW) is lower than IAA (>70 pmol/gFW) [ 13 ]. Monocots and non-vascular plants accumulate 300 to 5000 pmol/gFW of PAA, similar to other dicots [ 5 , 7 , 11 , 12 , 14 , 15 ] ( Table 1 ). Taken together, PAA is widely distributed in the plant kingdom, and overall, PAA accumulates more than IAA in most plants.…”

“…Several studies have shown that IAA can be made from IAOx in Brassicales and monocots [ 11 , 73 , 74 , 75 ] ( Figure 1 a). Similarly, it was shown that PAOx is a precursor of PAA in Arabidopsis and monocots [ 8 , 11 , 15 ] ( Figure 1 b). Arabidopsis plants overproducing PAOx increase PAA and display altered morphology, such as epinasty leaves and elongated hypocotyls, similar to those shown in plants with increased IAA [ 8 , 11 , 42 ].…”

“…Another PAA biosynthesis pathway is the aldoxime pathway, using phenylalanine-derived aldoxime, phenylacetaldoxime (PAOx), as an intermediate [ 8 , 10 , 11 , 15 , 66 ]. Aldoximes, such as indole-3-acetaldoxime (IAOx) and PAOx, as well as the aldoximes derived from other amino acids, are well characterized as precursors of various specialized metabolites, such as glucosinolates, cyanogenic glycosides, and camalexin as well as nitrogenous volatiles [ 31 , 33 , 67 , 68 , 69 , 70 , 71 , 72 ].…”

“…Arabidopsis plants overproducing PAOx increase PAA and display altered morphology, such as epinasty leaves and elongated hypocotyls, similar to those shown in plants with increased IAA [ 8 , 11 , 42 ]. Maize and sorghum plants fed with labeled PAOx produce labeled PAA [ 11 , 15 ]. These findings indicate a wider distribution of the aldoxime-derived auxin biosynthesis pathway throughout the plant kingdom rather than being limited to Brassicales.…”

Heterologous expression ofPtAAS1reveals the metabolic potential of the common plant metabolite phenylacetaldehyde for auxin synthesisin planta

“…Several studies revealed that the expression of key biosynthetic enzymes that initiate the biosynthesis of aromatic amino acid-derived specialized metabolites resulted in altered auxin phenotypes and chemotypes (Bak and Feyereisen, 2001; Bak et al, 2001; Irmisch et al, 2015; Günther et al, 2018; Perez et al, 2021; Perez et al, 2022). In Arabidopsis, another link between specialized metabolism and auxin signaling might be established by indole glucosinolate hydrolysis products with high affinity to the major auxin receptor Transport Inhibitor Response 1 (TIR1), which could result in competitive binding and thereby feed into the auxin signaling cascade (Vik et al, 2018).…”