Permissive action of H2O2 mediated ClUGT75 expression for auxin glycosylation and Al3+- tolerance in watermelon.

Abstract Although Al3+-toxicity is one of the limiting factors for crop production in acidic soils, little is known about the Al3+-tolerance mechanism in watermelon, a fairly acid-tolerant crop. This work aimed to identify the interaction between the H2O2 scavenging pathway and auxin glycosylation relevant to watermelon Al3+-tolerance. By analyzing expressions of hormone-related ClUGTs and antioxidant enzyme genes in Al3+-tolerant (ZJ) and Al3+-sensitive (NBT) cultivars, we identified ClUGT75s (B1, B2, and D1) and ClSOD1-2-ClCAT as crucial components associated with Al3+-tolerance. Al3+-stress significantly increased H2O2 content by 92.7% in NBT and 42.3% in ZJ, accompanied by less Al3+-, auxin (IAA and IBA), and MDA contents in ZJ than NBT. These findings coincided with significant ClSOD1-2 expression and stable dismutation activity in NBT than ZJ. Hence, higher H2O2 content in the root apex of NBT than ZJ correlated with a significant increase in auxin content and ClSOD1-2 up-regulation. Moreover, Al3+-activated ClUGT75D1 and ClUGT75B2 in ZJ coincided with no considerable change in IBA content, suggesting that glycosylation-mediated changes in IBA content might be relevant to Al3+-tolerance in watermelon. Furthermore, exogenous H2O2 and IBA indicated ClUGT75D1 modulating IBA is likely dependent on H2O2 background. We hypothesize that a higher H2O2 level in NBT represses ClUGT75, resulting in increased auxin than those in ZJ roots. Thus, excess in both H2O2 and auxin aggravated the inhibition of root elongation under Al3+-stress. Our findings provide insights on the permissive action of H2O2 in the mediation of auxin glycosylation by ClUGT75 in root apex for Al3+-tolerance in watermelon.