Ultraviolet-B Stress Increases Epidermal UV-Screening Effectiveness and Alters Growth and Cell-Wall Constituents of the Brown Midrib bmr6 and bmr12 Mutants of Sorghum bicolor

The brown midrib bmr6 and bmr12 mutants of sorghum (Sorghum bicolor) have alterations to the phenylpropanoid pathway impairing the activity of cinnamyl alcohol dehydrogenase (CAD) and/or caffeate5/hydroxyferulate O-methyl transferase (COMT) enzymes, which inhibit lignin synthesis. Interestingly, these phenylpropanoids can also act as sunscreen compounds in plants and potentially attenuate ultraviolet radiation. We examined the effects of ultraviolet-B (UV-B; 280–320 nm) exclusion on growth, cell-wall constituents and UV-screening abilities of bmr6, bmr12, a double mutant (bmr6 bmr12; dm) and wild-type (WT) genotypes of S. bicolor. Plants were grown in a UV-transparent greenhouse under filters that either transmitted 2.8% (Mylar) or 90% (Aclar) of UV-B. The greenhouse experiment was a 2 × 4 (UV treatment × genotype) complete factorial design. Sorghum grown under reduced UV were 23% taller and had 22% fewer leaves. Among genotypes, the WT plants were 5%–12% taller than the bmr6, bmr12 and dm mutants. The near-ambient UV-B treatment group was more effective at UV screening and had a 16% higher UV-screening effectiveness than those under reduced UV-B. Sorghum plants with the bmr6 and dm genotypes had 8%–19% higher UV-shield than the bmr12 and WT. Plants grown under the reduced UV-B treatment had 5% less hemicellulose and 6% more cellulose in their cell walls. There were no overall treatment effects on bulk soluble phenolics, chlorophyll fluorescence (F_v/F_m) or lignin concentrations. These results are a possible indication that the bmr mutants of S. bicolor have a varied response to UV-B exclusion due to alterations in the phenylpropanoid pathway leading to redistribution of metabolites.