Mutation of Os4BGlu18, monolignol β-glucosidase, improves salinity insensitivity in a rice

Rice is a salt-sensitive crop, and its development and yield are greatly affected by salinity. Using a forward genetics approach, the salt-insensitive line, sitl2, was selected from a core population (M10) induced by gamma-ray irradiation. Under salt treatment, the sitl2 mutant showed greater fresh weight, chlorophyll content, and lower hydrogen peroxide (H2O2) levels than the wild type (WT). In addition, the sitl2 mutant showed a lower Na+ content in the shoots and roots under salinity stress. A single-base guanine deletion of Os4BGlu18, which hydrolyzes monolignol-glucoside by monolignol β-glucosidase, was found by whole genome re-sequencing. Subcellular localization of Os4BGlu18 and the mutant form (Os4BGlu18p.Gly115fs) GFP-tagged proteins were found in the cytosol of rice protoplasts. The expression levels of Os4BGlu18 and most UDP-glucosyltransferase genes examined were lower in the mutant than in the WT. Monolignols were extracted in higher amounts from WT shoots and roots than from the sitl2 mutant; however, monolignol glucoside was extracted in higher amounts in sitl2. Phloroglucinol-HCl staining showed that sitl2 had more intense and higher lignin content than WT. These results suggest that sitl2 might confer salt tolerance caused by the mutation of Os4BGlu18 and could be used as a new resource for breeding programs to enhance salt tolerance in rice.