SbXTH7, acting downstream of transcription factor SbHY5, regulates aluminum tolerance by modulating cell wall hemicellulose content and aluminum accumulation in sweet sorghum [Sorghum bicolor (L.)]

Aluminum (Al) toxicity is considered a critical abiotic stress, limiting crop yields in acid soil. Sweet sorghum (Sorghum bicolor L.) is a promising non-food crop for the manufacturing of industrial products such as sugar, alcohol and biofuel. It mainly grows in Al toxicity widespread tropics and subtropics. Previous studies have demonstrated that LONG HYPOCOTYL5 of sweet sorghum confers transgenic Arabidopsis Al tolerance by modulating the expressions of some typical Al-responsive genes. Here, RNA sequencing analysis revealed that SbHY5 is involved in regulating cell components and cell wall modification, especially the xyloglucan metabolic process (GO: 0010411). Furthermore, all 34 xyloglucan endotransglucosylases/hydrolases (SbXTHs) in sweet sorghum were subjected to bioinformatics analysis and named according to their chromosomal location. Multiple experimental analyses demonstrated that SbHY5 could directly bind to SbXTH7 promoter and downregulate its expression in response to Al toxicity. SbXTH7 expression in root apices and elongation zone significantly downregulated after Al treatment. In addition, SbXTH7 overexpression reduced Arabidopsis Al tolerance by increasing cell wall hemicellulose content and enhancing Al accumulation in roots. In conclusion, we showed the presence of a SbHY5-SbXTH7 axis-mediated hemicellulose-dependent regulatory pathway underlying plant Al tolerance.