Loss of COMT activity reduces lateral root formation and alters drought response in sorghum brown midrib (bmr) 12 mutant

Authors: SALUJA, MANNY - University Of Nebraska; ZHU, FEIYU - University Of Nebraska; YU, HONGFENG - University Of Nebraska; WALLA, HARKAMAL - University Of Nebraska; Sattler, Scott

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/2020
Publication Date: 3/1/2021
Citation: Saluja, M., Zhu, F., Yu, H., Walla, H., Sattler, S.E. 2021. Loss of COMT activity reduces lateral root formation and alters drought response in sorghum brown midrib (bmr) 12 mutant. New Phytologist. 229:2780-2794. https://doi.org/10.1111/nph.17051.
DOI: https://doi.org/10.1111/nph.17051

Interpretive Summary: In the US, sorghum biomass (stalks and leaves) serves as an important forage crop for livestock. In addition, sorghum is being developed as a bioenergy crop. Biomass is composed nearly, entirely of plant cell walls, and lignin, a cell wall component makes these cells resistant to breakdown into their constituents that can be converted to biofuels and green chemicals. Lignin also allows plants to transport water from their roots to their leaves and maintain cell wall integrity. USDA-ARS and its university collaborators examined the effects of the brown midrib 12 (bmr12) plants, which are impaired in their ability to synthesize lignin, under well-watered and drought conditions. The roots of bmr12 plants were altered as compared to normal sorghum. Gene expression analyses showed that bmr12 plants were primed to respond to drought even under well-watered conditions. Together, these findings showed that changing the plant cell wall to improve bioenergy conversion may alter plant responses to drought. This research provides a basis to further investigate the roles of cell walls in perceiving the environment, which is critical for developing forage and bioenergy crops in a changing climate.

Technical Abstract: Lignin is a key target for modifying lignocellulosic biomass for efficient biofuel production. Brown midrib 12 (bmr12) encodes the sorghum caffeic acid O-methyltransferase (COMT) and is one of the key enzymes in monolignol biosynthesis. Loss of function mutations in COMT reduces syringyl (S) lignin subunits and improves biofuel conversion rate. Although lignin plays an important role in maintaining cell wall integrity of xylem vessels, physiological and molecular consequences due to loss of COMT on root growth and adaptation to water deficit remain unexplored. We addressed this gap by evaluating the root morphology, anatomy and transcriptome of bmr12 mutant. The mutant had reduced lateral root density (LRD) and altered root anatomy and response to water limitation. The wild-type exhibits similar phenotypes under water stress, suggesting that bmr12 may be in a water deficit responsive state even in well-watered conditions. Bmr12 had increased transcript abundance of genes involved in (a)biotic stress response, gibberellic acid (GA) biosynthesis and signaling. We show that bmr12 is more sensitive to exogenous GA application and present evidence for the role of GA in regulating reduced LRD in bmr12. These findings elucidate the phenotypic and molecular consequences of COMT deficiency under optimal and water stress environments in grasses