Superoxide Serves As a Putative Signal Molecule for Plant Cell Division: Overexpression of CaRLK1 Promotes the Plant Cell Cycle Via Accumulation of O and Decrease in H O

Reactive oxygen species (ROS) exert both positive and negative effects on plant growth and development and therefore receive a great deal of attention in current research. A hot pepper, Capsicum annuum receptor-like kinase 1 (CaRLK1) was ectopically expressed in Nicotiana tabacum BY-2 cell and Nicotiana benthamiana plants. This ectopic expression of CaRLK1 enhanced cell division and proliferation in both heterologous systems. Apparently, CaRLK1 is involved in controlling the cell cycle, possibly by inducing expressions of cyclin B1, cyclin D3, cyclin-dependent protein kinase 3, condensin complex subunit 2 and anaphase-promoting complex subunit 11 genes. CaRLK1 overexpression also increased transcript accumulation of NADPH oxidase genes, generation of O and catalase (CAT) activity/protein levels. In parallel, it decreased cellular H O levels and cell size. Treatment with Tiron or diphenyleneiodonium (DPI) both decreased the cell division rate and O concentrations, but increased cellular H O levels. Tobacco BY-2 cells overexpressing CaRLK1 were more sensitive to amino-1,2,4-triazole (3-AT), a CAT inhibitor, than control cells, suggesting that the increased H O levels may not function as a signal for cell division and proliferation. Overexpression of CaRLK1 stimulated progression of the cell cycle from G /G phase into the S phase. It is concluded that the CaRLK1 protein plays a pivotal role in controlling the level of O as signaling molecule which promotes cell division, concomitant with a reduction in H O by the induction of CAT activity/protein.