The maize CR4 receptor-like kinase mediates a growth factor-like differentiation response

The maize crinkly4 (cr4) gene encodes a predicted receptor kinase that is required for the normal differentiation of leaf epidermis (Becraft et al., 1996). Here we confirm that the presumptive cytoplasmic domain is capable of autophosphorylation on serine and threonine residues. The cr4 transcript is most abundant in young developing tissues of the shoot but was not detected in the root. Phenotypic analysis of strong mutant alleles indicates that cr4 functions throughout shoot development to control the differentiation of internal as well as epidermal tissues. However, epidermal cells appear most sensitive to perturbations in CR4 signaling because epidermal defects are more pronounced than internal cell defects. Epidermal cells show irregularities in shape, cell wall thickness and structure, cuticle formation, and vesicle trafficking. Some plants show tumor-like proliferations of cells on the leaves. Thus CR4 regulates an array of developmental responses including cell proliferation, fate, pattern, and differentiation suggesting a function analogous to growth factor responses in animals.