The MsSCL28.5-MdMYB87 module enhances drought tolerance by promoting adventitious root development in apple rootstocks

Drought stress severely limits apple cultivation. Malus sieversii, a wild ancestor of domesticated apple, provides a valuable genetic resource for improving drought tolerance in cultivated apple rootstocks. Although plant-specific GRAS [GIBBERELLIC-ACID INSENSITIVE (GAI), REPRESSOR of GAI, and SCARECROW] transcription factors (TFs) are known to regulate development and stress responses, their precise functions and mechanisms in apple remain largely unexplored. Here, we demonstrated that overexpression of MsSCL28.5, a GRAS TF from M. sieversii, substantially enhances drought tolerance of rootstock 'M26' (Malus domestica). Transgenic plants exhibited increased adventitious root (AR) growth and improved reactive oxygen species scavenging capacity. MsSCL28.5 directly binds to the promoters of MdLBD16a and MdLBD21b, key genes regulating AR growth. We further identified the TF MdMYB87 as an interacting partner of MsSCL28.5. The resulting MsSCL28.5-MdMYB87 complex synergistically activates the transcription of MdLBD16a/21b. Additionally, grafting experiments confirmed that MsSCL28.5-overexpressing rootstocks systemically enhance the drought resistance of non-transgenic scions. Furthermore, we discovered that comprising reduced methylation of the SCL28.5 promoter coupled with a TATA-box insertion in the MYB87 promoter collectively contributes to the superior drought tolerance observed in M. sieversii. Our findings present molecular insights into how transcriptional regulation of SCL28.5-MYB87 module are integrated into drought response of different apple rootstocks.