The pepper MAP4K kinase CaMAP4K3 negatively regulates drought resistance

Mitogen-activated protein kinase (MAPK) cascades are crucial in plant responses to various stresses. While the positioning of MAP4Ks within the canonical MAPK signaling module in plants remains controversial, emerging research continues to shed light on their functional roles. However, information on the MAP4K gene family in pepper (Capsicum annuum) is still limited. In this study, seven putative MAP4K genes (designated as CaMAP4K1–CaMAP4K7) were identified from the pepper genome, each containing a conserved serine/threonine kinase domain. These genes were differentially expressed across various pepper organs, with CaMAP4K3 exhibiting consistently high expression in all organs and significant induction under drought stress. Kinase assays revealed that CaMAP4K3 is an active kinase whose activity is enhanced by drought and salt stress. Functional studies showed that silencing CaMAP4K3 enhanced drought resistance in pepper plants, reducing transpirational water loss and increasing leaf temperatures. Conversely, CaMAP4K3 overexpression in tobacco and Arabidopsis reduced drought tolerance, as evidenced by increased wilting and transpirational water loss. Additionally, CaMAP4K3-overexpressing Arabidopsis plants exhibited reduced sensitivity to abscisic acid (ABA) during seed germination and seedling growth. Collectively, these results suggest that CaMAP4K3 impairs drought resistance in pepper plants and potentially affects seed germination and seedling growth through the regulation of ABA signaling.