Permissive action of HO mediated ClUGT75 expression for auxin glycosylation and Al- tolerance in watermelon.

Although Al-toxicity is one of the limiting factors for crop production in acidic soils, little is known about the Al-tolerance mechanism in watermelon, a fairly acid-tolerant crop. This work aimed to identify the interaction between the HO scavenging pathway and auxin glycosylation relevant to watermelon Al-tolerance. By analyzing expressions of hormone-related ClUGTs and antioxidant enzyme genes in Al-tolerant (ZJ) and Al-sensitive (NBT) cultivars, we identified ClUGT75s (B1, B2, and D1) and ClSOD1-2-ClCAT as crucial components associated with Al-tolerance. Al-stress significantly increased HO content by 92.7% in NBT and 42.3% in ZJ, accompanied by less Al-, 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 HO content in the root apex of NBT than ZJ correlated with a significant increase in auxin content and ClSOD1-2 up-regulation. Moreover, Al-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 Al-tolerance in watermelon. Furthermore, exogenous HO and IBA indicated ClUGT75D1 modulating IBA is likely dependent on HO background. We hypothesize that a higher HO level in NBT represses ClUGT75, resulting in increased auxin than those in ZJ roots. Thus, excess in both HO and auxin aggravated the inhibition of root elongation under Al-stress. Our findings provide insights on the permissive action of HO in the mediation of auxin glycosylation by ClUGT75 in root apex for Al-tolerance in watermelon.