Light and brassinosteroids differentially modulate Arabidopsis seedling root growth in a largely independent manner

Brassinosteroids (BRs) are a class of steroid phytohormones that promote plant growth and tolerance to both biotic and abiotic stress. BAS1/CYP734A1, SOB7/CYP72C1, and BEN1 are three BR-inactivating enzymes characterized in Arabidopsis thaliana. The triple-null mutant bas1-2 sob7-1 ben1-3 exhibits strong BR-overproducing phenotypes. The CYP734A cytochrome P450 protein family is highly conserved across plant species. Ectopic overexpression of grapevine CYP734A15 in Arabidopsis causes severe BR-deficient phenotypes. BRs differentially modulate hypocotyl growth in photomorphogenic and skotomorphogenic seedlings. However, little is known about the potential involvement of light in BR-regulated seedling root growth. In this research, we grew Arabidopsis seedlings of the wild-type (WT) Col-0, the BR-overproducing mutant bas1-2 sob7-1 ben1-3, and two BR-deficient CYP734A15 overexpression mutants CYP734A15ox-3 and CYP734A15ox-4 in either continuous white light or constant darkness, with treatments of either brassinolide (BL, the most bioactive, naturally occurring BR) or the BR biosynthesis inhibitor brassinazole (BRZ) at a wide range of concentrations. Primary root length measurement results showed that light promoted seedling root growth regardless of endogenous BR levels. Both endogenous and exogenous BRs suppressed seedling root growth independently of light. BRZ treatments also suppressed root elongation in light-grown seedlings regardless of endogenous BR levels, probably due to its toxic effects. In the dark, BRZ still slightly suppressed root elongation in CYP734A15ox-3/4 seedlings but the effect switched to moderate promotion for roots growth in Col-0 and bas1-2 sob7-1 ben1-3. Collectively, our work suggests that light and BRs differentially modulate Arabidopsis seedling root growth in a largely independent manner.