The MdMEK2-MdMPK6-MdWRKY17 pathway stabilizes chlorophyll levels by directly regulating MdSUFB in apple under drought stress.

Drought stress severely limits plant growth and production in apple (Malus domestica Borkh.). To breed water-deficit-tolerant apple cultivars that maintain high yields under slight or moderate drought stress, it is important to uncover the mechanisms underlying the transcriptional regulation of chlorophyll metabolism in apple. To explore this mechanism, we generated transgenic 'Gala3' apple plants with overexpression or knockdown of MdWRKY17, which encodes a transcription factor whose expression is significantly induced by water deficit. Under moderate drought stress, we observed significantly higher chlorophyll contents and photosynthesis rates in overexpression transgenic plants than in controls, whereas these were dramatically lower in the knockdown lines. MdWRKY17 directly regulates MdSUFB expression, as demonstrated by in vitro and in vivo experiments. MdSUFB, a key component of the sulfur mobilization (SUF) system that assembles Fe-S clusters, is essential for inhibiting chlorophyll degradation and stabilizing electron transport during photosynthesis, leading to higher chlorophyll levels in transgenic apple plants overexpressing MdWRKY17. The activated MdMEK2-MdMPK6 cascade by water-deficit stress fine-tunes the MdWRKY17-MdSUFB pathway by phosphorylating MdWRKY17 under water-deficit stress. This fine-tuning of the MdWRKY17-MdSUFB regulatory pathway is important for balancing plant survival and yield losses (chlorophyll degradation and reduced photosynthesis) under slight or moderate drought stress. The phosphorylation by MdMEK2-MdMPK6 activates the MdWRKY17-MdSUFB pathway at S66 (identified by LC-MS), as demonstrated by in vitro and in vivo experiments. Our findings reveal that the MdMEK2-MdMPK6-MdWRKY17-MdSUFB pathway stabilizes chlorophyll levels under moderate drought stress, which could facilitate the breeding of apple varieties that maintain high yields under drought stress.