Classification of Receptor-Like Cytoplasmic Kinases in Maize and Functional Analysis of ZmBLK1

Receptor-like cytoplasmic kinases (RLCKs) form a large family of proteins in plants. RLCKs have been found in different plant species, regulating plant immunity to different bacterial and fungal pathogens. Previous studies implicated Arabidopsis botrytis induced kinase1 (BIK1) and tomato protein kinase 1b (TPK1b) in plant resistance to Pseudomonas syringae and Botrytis cinerea. In this study, we classified 195 putative maize RLCKs into ten subfamilies. Based on the amino acid sequence similarity to BIK1 and TPK1b, a novel maize RLCK, zea mays bik1-like kinase 1 (ZmBLK1) was identified. Enzyme assays with cloned ZmBLK1 revealed a functional kinase when expressed in planta. The recombinant protein located to the plasma membrane. Expression of ZmBLK1 is highest in maize leaves compared to other structures at silking stage. Expression of the recombinant ZmBLK1 significantly reduced the rate of lesion spread in maize leaves inoculated with the Goss’s wilt pathogen. In maize kernels, expression of ZmBLK1 increases during kernel maturation. Kernels from transgenic maize overexpressing ZmBLK1 were not resistant to Aspergillus flavus or to aflatoxin contamination. In addition, mutations were made in ZmBLK1 that were hypothesized to create a constitutively active kinase. However, resulting proteins had similar activity to the wild-type ZmBLK1 and transgenic plants showed similar responses to the Goss’s wilt and Aspergillus ear rot pathogens. Overall, this research established the first characterization of RLCKs in maize and described a potential contribution of ZmBLK1 to maize immune responses.