ABI5 promotes heat stress-induced chlorophyll degradation by modulating the stability of MYB44 in cucumber

Abstract

The yellowing of leaves caused by the decomposition of chlorophyll (Chl) is a characteristic event during senescence, which can be induced by various environmental stresses. However, the molecular mechanisms of high temperature-induced Chl degradation in horticultural plants remain poorly understood. Here, we found that heat stress induced Chl degradation and the expression of <i>ABI5</i> and <i>MYB44</i> in cucumber. Silencing of <i>ABI5</i> compromised heat stress-induced Chl degradation, and the transcription of pheophytinase (<i>PPH</i>) and pheophorbide <i>a</i> oxygenase (<i>PAO</i>), two key genes in Chl catabolic pathway, but silencing of <i>MYB44</i> exhibited the opposite results. Furthermore, ABI5 interacted with MYB44 <i>in vitro</i> and <i>in vivo</i>. ABI5 positively regulated heat stress-induced Chl degradation through two pathways. ABI5 directly bound to <i>PPH</i> and <i>PAO</i> promoters to promote their expression, leading to accelerating Chl degradation. On the other hand, the interaction between ABI5 and MYB44 reduced the binding of MYB44 to <i>PPH</i> and <i>PAO</i> promoters and led to the ubiquitination-depended protein degradation of MYB44, thereby alleviating the transcription inhibitory effect of MYB44 on <i>PPH</i> and <i>PAO</i>. Taken together, our findings propose a new regulatory network for ABI5 in regulating heat stress-induced Chl degradation.