Influence of Ultraviolet Radiation on Cell Wall Constituents in Reduced-Lignin Mutants of Corn and Sorghum

Brown midrib mutants (BMR) of corn (Zea mays) and sorghum (Sorghum bicolor) have reduced lignin content and have recently gained popularity in Southern Minnesota as forage material for grazing animals. These mutants have lowered expression of cinnamyl alcohol dehydrogenase and caffeic O-methyltransferase enzymes in the phenylpropanoid pathway responsible for the construction of the cinnamyl alcohol subunits of lignin. Lignin is found in the secondary cell wall of higher plants, along with substantial amounts of cellulose and hemicellulose polysaccharides. Reduced lignin increases livestock digestibility of these plants, but may also increase susceptibility to environmental stress and lodging. Lignin is comprised of polymerized hydroxycinnamic acids which are also important in absorbing potentially damaging ultraviolet radiation (UV). We examined how sub-ambient levels of UV alter cell wall constituent concentrations over a 50-day experiment. Plants were grown in a UV-transparent greenhouse under filters that either reduce ("mylar") or transmit ("aclar") ambient UV. We harvested plants three times (days 20, 35 and 50) and examined concentrations of cellulose, hemicellulose and lignin using the filter-bag detergent technique in combination with a fiber analyzer (ANKOM Technology). In BMR corn, cellulose concentrations were consistently higher in plants growing under mylar than under aclar on all sampling dates (P