OsCOMT expression in root regulates endodermal lignification and salt tolerance in rice

Lignin, a major hydrophobic biopolymer gets deposited from the outer exodermis to the inner stele in roots. This study analyzed the role of a lignified root system among 14 indica rice genotypes in elucidating the role of an apoplast barrier in mediating salt tolerance. Root cross-sections were microscopically examined for lignification patterns across the different cross-sectional layers using lignin-specific fluorescent dyes, and scores were assigned from 0 to 3 given the fluorescent intensities. Based on the lignification scores, the rice genotypes were classified into three anatomical groups, I, II, and III reflecting the lignification status. The group I genotypes were characteristic of an enhanced lignification of the endodermis, and metaxylem accompanied by a greater number of ‘U’ shaped lignified endodermal cells. The salt tolerant landrace Mundan, moderately salt tolerant landraces Arikayama and Cheriviruppu, and salt susceptible elite cultivars CO52 and ASD16 were found to occupy the groups I, II, and III, respectively. The expression level of a major lignin biosynthetic gene OsCOMT encoding the enzyme caffeic acid O-methyltransferase was significantly upregulated in Mundan under salt stress with a fold change of 6.88 compared to 0.97 for CO52. Furthermore, the concentration of sodium ions entering the metaxylem in the root was reduced by three-fold in Mundan compared to the salt susceptible cultivar ASD16, emphasizing the role of a lignified apoplastic barrier mechanism in reducing the influx of sodium ions into the xylem paving the way for salt tolerance.