CRINKLY4: Multifaceted Roles Beyond Epidermal Cell Differentiation in Plant Development?

Receptor-like kinases (RLKs) play essential roles in plant growth and development. CRINKLY4 (CR4), one of the first reported RLKs in plants, is a well-known regulator of epidermal cell differentiation during leaf and seed development in maize. Within the last four decades, the functional landscape of CR4 has emerged across diverse developmental contexts and species, including dicots (e.g., Arabidopsis Thaliana), monocots (e.g., Oryza sativa and Zea mays), and even moss (e.g., Physcomitrella patens). CR4 is currently considered to be a multifaced regulator of plant development beyond epidermal cell differentiation. In this review, we highlight the versatile roles of CR4 based on its expression and dynamic subcellular localisation patterns, we discuss the importance of its two extracellular domains and kinase activity for its functionality, and we summarise the CR4-mediated signalling pathways underlying columella stem cell differentiation, epidermal cell differentiation, and vascular bundle formation. Finally, we raise several open questions for future research.