Diversity of a wall-associated kinase gene in wild and cultivated barley

Phylogenetic Analysis of Wall-Associated Kinase Genes in Triticum Species and Characterization of TaWAK7 Involved in Wheat Powdery Mildew Resistance

Wall-associated kinases (WAKs), a group of receptor-like kinases, have been found to play important roles in defending against pathogens and in various developmental processes. However, the importance of this family in wheat remains largely unknown. Wheat powdery mildew is caused by Blumeria graminis f. sp. tritici (Bgt), which initiates infection on the cell surface and forms haustoria inside the cell; therefore, the defense to Bgt involves extracellular and subsequently intracellular signals. In this study, WAKs were identified genome-wide and analyzed phylogenetically, and then a transmembrane WAK gene that putatively participated in pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity to Bgt was functionally and evolutionarily investigated. In total, 1,193 WAKs were identified from wheat and its Gramineae relatives. Phylogenetic analysis indicated that WAKs expanded through tandem duplication or segment duplication. TaWAK7, from chromosome 2A, was identified as a Bgt-inducible gene both in susceptible and resistant materials, but it showed distinct responsive patterns. Functional analysis showed that TaWAK7 was involved in both the basal and resistance gene-mediated resistances. The specific gene structures and protein characteristics of TaWAK7, along with its orthologs, were characterized both in subgenomes of Triticum spp. and in the A genome of multiple wheat accessions, which revealed that TaWAK7 orthologs underwent complex evolution with frequent gene fusion and domain deletion. In addition, three cytoplasmic proteins interacting with TaWAK7 were indicated by yeast two-hybrid and bimolecular fluorescence complementation assays. Binding of TaWAK7 with these proteins could change its subcellular localization from the plasma membrane to the cytoplasm. This study provides a better understanding of the evolution of WAKs at the genomic level and TaWAK7 at the gene level and provides useful clues for further investigation of how WAKs transmit the extracellular signals to the cytoplasm to activate defense responses.

The Craterostigma plantagineum protein kinase CpWAK1 interacts with pectin and integrates different environmental signals in the cell wall

The cell wall protein CpWAK1 interacts with pectin, participates in decoding cell wall signals, and induces different downstream responses. Cell wall-associated protein kinases (WAKs) are transmembrane receptor kinases. In the desiccation-tolerant resurrection plant Craterostigma plantagineum, CpWAK1 has been shown to be involved in stress responses and cell expansion by forming a complex with the C. plantagineum glycine-rich protein1 (CpGRP1). This prompted us to extend the studies of WAK genes in C. plantagineum. The phylogenetic analyses of WAKs from C. plantagineum and from other species suggest that these genes have been duplicated after species divergence. Expression profiles indicate that CpWAKs are involved in various biological processes, including dehydration-induced responses and SA- and JA-related reactions to pathogens and wounding. CpWAK1 shows a high affinity for "egg-box" pectin structures. ELISA assays revealed that the binding of CpWAKs to pectins is modulated by CpGRP1 and it depends on the apoplastic pH. The formation of CpWAK multimers is the prerequisite for the CpWAK-pectin binding. Different pectin extracts lead to opposite trends of CpWAK-pectin binding in the presence of Ca²⁺ at pH 8. These observations demonstrate that CpWAKs can potentially discriminate and integrate cell wall signals generated by diverse stimuli, in concert with other elements, such as CpGRP1, pH_apo, Ca²⁺_[apo], and via the formation of CpWAK multimers.

Genome-wide analysis of wall associated kinase (WAK) gene family in barley

In plants, wall associated kinases (WAKs) form a unique subfamily of receptor like-kinases (RLKs). In Arabidopsis thaliana, WAK-RLKs are known to regulate biotic stress, cell expansion, and metal tolerance, but their detailed characterization in barley is lacking. In this study, we identified a total of 91 WAK genes in the barley genome and classified them into five groups. Evolutionary analysis of HvWAKs with AtWAKs revealed their species-specific expansion. The maximum number (19 to 20) of WAK genes were located on chromosomes 3, 5 and 6. WAK proteins exhibited similar types of motif distribution in their group. Characterization of a Ds transposon insertion mutant of the wak1 revealed differences in the root length. Further, HvSPL23 transcription factor was identified as a positive co-expressing gene with HvWAK1, suggesting its possible upstream regulator. Taken together, our study provides a base for the functional characterization of WAK family members in the future.