The Sorghum (Sorghum bicolor) Brown Midrib 30 Gene Encodes a Chalcone Isomerase Required for Cell Wall Lignification

Abstract

In sorghum (Sorghum bicolor) and other C4 grasses, brown midrib (bmr) mutants have long been associated with plants impaired in their ability to synthesize lignin. The brown midrib 30 (Bmr30) gene, identified using a bulk segregant analysis and next-generation sequencing, was determined to encode a chalcone isomerase (CHI). Two independent mutations within this gene confirmed that loss of its function was responsible for the brown leaf midrib phenotype and reduced lignin concentration. Loss of the Bmr30 gene function, as shown by histochemical staining of leaf midrib and stalk sections, resulted in altered cell wall composition. In the bmr30 mutants, CHI activity was drastically reduced, and the accumulation of total flavonoids and total anthocyanins was impaired, which is consistent with its function in flavonoid biosynthesis. The level of the flavone lignin monomer tricin was reduced 20-fold in the stem relative to wild type, and to undetectable levels in the leaf tissue of the mutants. The bmr30 mutant, therefore, harbors a mutation in a phenylpropanoid biosynthetic gene that is key to the interconnection between flavonoids and monolignols, both of which are utilized for lignin synthesis in the grasses.

Response of sorghum lines carrying recently identified brown midrib (bmr) mutations to stalk rot pathogens and water deficit

“…The recently characterized Bmr30 was demonstrated to encode CHI; bmr30 mutants were shown to have reduced levels of total flavonoids and total anthocyanins up to 40% and 60%, respectively, in nutrient‐deprived seedlings (Tetreault et al, 2021). Although 4‐coumaroyl‐CoA is a product of 4CL (Bmr2) and the initial substrate of flavonoid metabolism, bmr30 mutants are not entirely devoid of flavonoid biosynthesis.…”

“…Recently, six additional bmr mutations were identified visually from an ethyl methane sulfonate (EMS)‐mutagenized population of BTx623, and their allelism was determined through complementation tests (Saballos et al, 2012; Sattler et al, 2012, 2014; Scully et al, 2016). Two of the genes, Bmr19 and Bmr30 , have been identified, and mutations in these genes result in significantly reduced lignin concentrations (Adeyanju et al, 2021; Saballos et al, 2008; Sattler et al, 2014; Tetreault et al, 2021). However, neither of the genes encode enzymes in monolignol biosynthesis.…”

“…However, neither of the genes encode enzymes in monolignol biosynthesis. The Bmr30 gene encodes a chalcone isomerase (CHI), an enzyme in flavonoid synthesis, and therefore, bmr30 mutants are also impaired in flavonoid biosynthesis (Tetreault et al, 2021). The Bmr19 gene encodes a folylpolyglutamate synthase (FPGS) (Adeyanju et al, 2021), which is required for 1‐carbon metabolism to regenerate the cofactor (coenzyme) S‐adenosylmethionine (SAM).…”

“…However, neither of the genes encode enzymes in monolignol biosynthesis. The Bmr30 gene encodes a chalcone isomerase (CHI), an enzyme in flavonoid synthesis, and therefore, bmr30 mutants are also impaired in flavonoid biosynthesis (Tetreault et al, 2021).…”

Dynamic regulation of phenylpropanoid pathway metabolites in modulating sorghum defense against fall armyworm

“…The following compounds were included in the assay: apigenin, catechin, chalconaringenin, cyanidin, daidzein, delphinidin, epicatechin, genistein, hesperetin, kaempferol, luteolin, naringenin, phloretin, proanthocyanindin A2, procyanidin B2, quercetin, quercetin-3-galactoside, quercetin-3-glucoside, resveratrol, and rutin. Total flavonoids was estimated using spectrophotometer as described previously (Tetreault et al, 2021).…”