CHRK1, a chitinase-related receptor-like kinase, plays a role in plant development and cytokinin homeostasis in tobacco

CHRK1 encodes a receptor-like kinase that contains a chitinase-related sequence in the extracellular domain in Nicotiana tabacum. In this study, we showed that CHRK1 is mainly expressed in the shoot apex region including leaf primordia and young leaves, and germinating seedlings and vascular tissues, based on GUS activity of transgenic tobacco plants carrying the CHRK1 promoter-GUS fusion gene. Transgenic tobacco plants in which CHRK1 expression was suppressed exhibited pleiotrophic developmental abnormality, including formation of proliferating shooty calli from emerging seedlings and severely altered seedling development. At the cellular level, ectopic cell proliferation, reduced cell specificity, and aberrant chloroplast development were observed. The transgenic lines contained 3-fold higher level of cytokinin than the wild-type plants. Consistently, the transgenic seedlings exhibited a typical cytokinin response in the absence of hormone, such as deetiolation under the dark. Based on these results, we propose that CHRK1 is involved in a developmental signaling pathway regulating cell proliferation/differentiation and the endogenous cytokinin levels in tobacco.