Domestication-Selected Promoter Insertion in WRKY17 Increases Cadmium Sensitivity in Apple

With increasing industrialisation and human activities, heavy metal pollution has become a serious environmental concern, particularly cadmium (Cd) contamination. This study reveals significant differences in Cd tolerance between wild apple (Malus spp.) and cultivated apple (Malus domestica). Through pan-genome analysis, we identified the transcription factor WRKY17 as a key regulator of Cd stress response, with a 3355-bp insertion (P-INS) in its promoter region being the primary genetic basis for this differential tolerance. In cultivated apples, P-INS suppresses WRKY17 expression, leading to reduced Cd tolerance. In contrast, wild apples lacking P-INS exhibit activated WRKY17 expression. Further investigation demonstrated that WRKY17 enhances Cd tolerance by inducing the expression of long non-coding RNA lncRNA400. Mechanistically, lncRNA400 forms an R-loop structure that recruits the histone demethylase JMJD5 to remove H3K27me3 marks from the promoter of the Plant Cadmium Resistance gene PCR2, thereby activating PCR2 expression. Notably, WRKY17 activation also accelerates leaf senescence, explaining why P-INS was retained during apple domestication-its suppression of WRKY17 maintains better agronomic traits despite reduced Cd tolerance. In apple cultivation, grafting wild apple rootstocks with cultivated scions effectively combines the Cd-tolerant traits of wild varieties with the delayed leaf senescence characteristics of cultivated cultivars, providing a practical solution for the apple industry to address Cd contamination.