Transcription factor MdLBD41 negatively regulates resistance to Glomerella leaf spot in apple by suppressing ROS production

Glomerella leaf spot (GLS) is a devastating disease of apple. The defense mechanisms of apple plants against GLS remain incompletely understood. In the present study, we report a GLS resistance regulatory module in apple involving MdLBD41, MdbHLH144, MdTPL1, and the target genes MdRBOHD1/2, which are responsible for reactive oxygen species (ROS) generation during infection by Colletotrichum fructicola, a causal agent of GLS. Compared with the resistant cultivar, the susceptible cultivar exhibited a higher expression level of MdLBD41 in response to C. fructicola infection, suggesting the involvement of MdLBD41 in the defense responses of apple plants against GLS. This differential expression of MdLBD41 was associated with distinct levels of histone acetylation in the MdLBD41 promoter region between susceptible and resistant cultivars. Functioning as a transcriptional repressor, MdLBD41 suppressed MdRBOHD1/2 expression and reduced ROS generation by recruiting the TOPLESS corepressor MdTPL1. This repression could be relieved by MdbHLH144, which interacted with MdLBD41 and inhibited its binding to the MdRBOHD1/2 promoters. Thus, we identified a previously uncharacterized regulatory mechanism governing MdRBOHD1/2 expression and ROS generation during C. fructicola infection. This mechanism represents a finely tuned regulatory strategy that balances effective disease resistance with protection against cellular damage in apple plants.