Brown midrib mutants in sorghum and their applications in renewable energy production

Lignin is a complicated phenolic polymer that is found in the secondary cell walls of plants. It is necessary for plant mechanical strength, vascular integrity, and stress tolerance. In sorghum (Sorghum bicolor L. Moench), the amount and composition of lignin greatly affect forage digestibility, biomass recalcitrance to bioenergy production and biochemical conversion process efficiency. Brown midrib (bmr) mutants, which exhibit reddish-brown pigmentation of the vasculature, have been a classic and potent tool for identifying lignin biosynthesis and engineering cell wall traits. Over the last 60 years, sorghum bmr mutants have been analyzed extensively and their characterization led to the identification of crucial genes associated with monolignol biosynthesis, such as bmr6 (cinnamyl alcohol dehydrogenase), bmr12 (caffeic acid O-methyltransferase), bmr2 (4-coumarate-CoA ligase), as well as having an indirect effect on lignification through one-carbon metabolism, i.e. bmr19. These changes modify lignin concentration, monomer composition, and interunit connections, leading to improved cell wall digestibility and decreased biomass recalcitrance. Lignin alteration involves physiological trade-offs that affect stem strength, lodging resistance, and disease susceptibility, and these are significantly influenced by genetic background and environmental factors. Recent advancements in genomics, systems biology, and genome-editing technologies have enabled precise manipulation of lignin pathways, uncovering compensatory metabolic networks that sustain plant fitness. This study consolidates existing knowledge on the molecular genetics, biochemical control, and physiological effects of brown midrib mutations in sorghum, focusing specifically on their implications in forage enhancement and renewable bioenergy generation. Through the amalgamation of traditional genetics and contemporary engineering methodologies, bmr sorghum serves as a paradigm C₄ grass for sustainable lignocellulosic bioenergy and climate-resilient agriculture.