Abstract
During the growth and development of land plants, some specialized cells, such as tracheary elements, undergo secondary cell wall thickening. Secondary cell walls contain additional lignin, compared with primary cell walls, thus providing mechanical strength and potentially improving defenses against pathogens. However, the molecular mechanisms that initiate wall thickening are unknown. In this study, we identified an Arabidopsis (Arabidopsis thaliana) leucine-rich repeat receptor-like kinase, encoded by AtVRLK1 (Vascular-Related Receptor-Like Kinase1), that is expressed specifically in cells undergoing secondary cell wall thickening. Suppression of AtVRLK1 expression resulted in a range of phenotypes that included retarded early elongation of the inflorescence stem, shorter fibers, slower root growth, and shorter flower filaments. In contrast, up-regulation of AtVRLK1 led to longer fiber cells, reduced secondary cell wall thickening in fiber and vessel cells, and defects in anther dehiscence. Molecular and cellular analyses showed that down-regulation of AtVRLK1 promoted secondary cell wall thickening and up-regulation of AtVRLK1 enhanced cell elongation and inhibited secondary cell wall thickening. We propose that AtVRLK1 functions as a signaling component in coordinating cell elongation and cell wall thickening during growth and development.
Highlights
During the growth and development of land plants, some specialized cells, such as tracheary elements, undergo secondary cell wall thickening
An analysis of public microarray/RNA sequencing databases revealed that Arabidopsis At1g79620 is highly expressed in the vascular tissue (Supplemental Fig. S1), and it was named Vascular-Related Receptor-Like Kinase1 (AtVRLK1)
AtVRLK1 belongs to the leucine-rich repeat (LRR) VIII-1 Receptor-like kinase (RLK) subfamily, which has eight members in the Arabidopsis genome (Shiu and Bleecker, 2001)
Summary
During the growth and development of land plants, some specialized cells, such as tracheary elements, undergo secondary cell wall thickening. Development of the vascular system, which is composed of wall-thickened cells, starts with cambium cell division followed by differentiation into specialized cells, such as vessel elements and fiber cells These specialized cells form secondary walls inside of the primary wall upon completion of cell expansion. A number of RLKs that are expressed predominantly in the vascular tissue were identified by analysis of promoter:GUS transgenic plants (Wu et al, 2016) Their specific expression patterns suggest a function in vascular tissue development, including specification, differentiation, expansion, and secondary cell wall formation in specialized cells (De Rybel et al, 2016). We dissected the function of Arabidopsis AtVRLK1 and demonstrate its role in coordinating cell elongation and secondary cell wall thickening during growth and development
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