Decrease in cell size and cell wall components under ectopic expression of the CaRLK1 gene

Abstract

<jats:title>Abstract</jats:title> <jats:p>BackgroundThe Capsicum annuum receptor-like kinase 1 (CaRLK1) gene encodes a transmembrane protein with a cytoplasmic kinase domain and an extracellular domain. It functions as a negative regulator of plant cell death. Ectopic expression of CaRLK1 showed the hypoxia-resistance and enhanced cell division and proliferation. In this study, it was investigated which genes were controlled by ectopic expression of CaRLK1 because it decreased its average cell size of transgenic RLK1ox cells compared with that of wild-type cells (BY-2). ResultsThe average diameter (AD) of the protoplast of RLK1ox cells was about 10 µm shorter than the AD of BY-2 cells. The diameter distribution of the circular protoplasts is mainly from 20 to 40 μm (71.7%) in RLK1ox cells, whereas from 30 to 50 μm (73.0%) in BY-2 cells. Furthermore, cell volume of RLK1ox cells is also 2.5-times smaller than that of BY-2 cells. Smaller cell size of the RLK1ox cells may be related with the inhibited cell expansion because expressions of 12 expansin A genes, 6 expansin-like B genes, 2 ACC synthase genes, and 5 ACC oxidase genes were suppressed, but expressions of 7 gibberellin 2-oxidase genes were induced. The cell walls between two RLK1ox cells are approximately 138.83±4.12 nm thick on average, while those between two BY-2 cells are approximately 156.58±4.54 nm thick. Furthermore, the total content of neutral sugars in the cell wall of RLK1ox cells is less than that of BY-2 cells (about 25%). The RLK1ox cells contained 30% less glucose content than did the BY-2 cells. Thinner cell wall of the RLK1ox cells is related with the decreased cellulose biosynthesis and hemicellulose biosynthesis. Expressions of 7 CESA genes, 5 sucrose synthase 1 genes, 3 mannose-1-phosphate guanyltransferase genes, and 6 glucomannan-4-beta-mannosyltransferase genes were suppressed. The suppressed expressions of 14 polygalacturonase genes may also contribute to make the cell wall of RLK1ox cell thinner than that of BY-2 cell. ConclusionOf special emphasis is its impact of CaRLK1 gene on cell size control and cell wall thickness. Smaller cell size of the RLK1ox cells correlates with the inhibited cell expansion.</jats:p>