ZmWAKL21 is a maize wall-associated kinase-like protein that interacts with AvrRppC and contributes to southern corn rust resistance

Southern corn rust (SCR), caused by the fungal pathogen Puccinia polysora (P. polysora), is a major threat to maize production worldwide. To date, two major nucleotide-binding leucine-rich repeat (NLR) resistance genes, RppC and RppK, have been identified that confer protection against P. polysora. However, their durability is frequently undermined by the rapid evolution of the pathogen. While key components of NLR-mediated resistance have been characterized, the molecular interface between maize and P. polysora—particularly the roles of fungal effectors such as AvrRppC—remains largely unexplored. In particular, the early immune signaling events at the plasma membrane are still poorly defined in this pathosystem. In this study, an MBP-tagged AvrRppC pull-down MS (Mass spectrometry) screening system within the maize–P. polysora pathosystem was first established. Using this approach, we identified ZmWAKL21, which encodes a wall-associated receptor kinase-like protein. Molecular, biochemical, and cellular assays show that AvrRppC can bind ZmWAKL21 on the plasma membrane. During P. polysora infection, ZmWAKL21 positively increases the expression of ZmrbohD and ZmPR1, resulting in elevated production of reactive oxygen species (ROS). Kinase assays further revealed that ZmWAKL21 possesses intrinsic serine/threonine kinase activity, which is directly suppressed by AvrRppC, highlighting an arm race between host and pathogen. Despite this antagonistic interaction, transgenic maize plants overexpressing ZmWAKL21 exhibited significantly enhanced resistance to P. polysora compared with wild-type controls, whereas CRISPR-Cas9-generated knockout mutants were more susceptible. Together, our findings uncover an immune interface in the maize–P. polysora pathosystem, in which the fungal effector AvrRppC suppresses the kinase activity of ZmWAKL21, yet overexpression of ZmWAKL21 still enhances ROS-associated defense responses. This study provides new mechanistic insights into fungal effector–kinase interactions and the regulation of immune signaling in monocot plants.