SlALKBH9B is involved in drought-induced flower drop by regulating ethylene production

Drought induces tomato (Solanum lycopersicum) flowers and fruits drop, which causes serious yield and economic losses in agriculture. However, the mechanism of action remains unclear. N6-methyladenosine (m6A) methylation is a prevalent epigenetic change integral to the growth, development, and adaptation of plants to abiotic stress factors. However, whether it participates in drought-induced abscission remains to be further studied. Here, we report that tomato demethylase alpha-ketoglutarate-dependent dioxygenase B (AlkB) homolog 9B (SlALKBH9B) exerts a detrimental influence on the regulation of drought-induced flower drop by mediating ethylene production. We found that drought markedly reduced the expression of SlALKBH9B, and knockout of SlALKBH9B enhanced flower drop, while overexpression of SlALKBH9B delayed the flower drop. Under drought conditions, the ethylene production of Slalkbh9b exhibited a considerably greater yield than that of the wild type (WT), while SlALKBH9B overexpression plants had lower ethylene production. Application of ethylene could abolish the delayed abscission effect of overexpression of SlALKBH9B. Further studies showed that drought downregulated SlALKBH9B expression, which specifically enhanced the methylation level of the 3′ untranslated region (UTR) of tomato ethylene excess producer 1 (SlETO1), leading to a decrease in the stability of SlETO1 mRNA and its protein translation efficiency. The loss of SlETO1 resulted in the accumulation of tomato 1-aminocyclopropane-1-carboxylic acid synthase 3 (SlACS3) and SlACS8 in the abscission zone (AZ) and then boosted ethylene production to accelerate abscission. Our results show that SlALKBH9B is an important inhibitor for drought-induced abscission and reveal a new mechanism through which drought-enhanced ethylene production leads to flower drop.