OsBWMK1 mediates SA-dependent defense responses by activating the transcription factor OsWRKY33.

Mitogen-activated protein kinases (MAPKs) play important roles in responses to various environmental stresses. In a previous study, we demonstrated that OsBWMK1, which localizes in the nucleus, mediates PR gene expression by activating the OsEREBP1 transcription factor, and that the constitutive expression of OsBWMK1 also enhances resistance against pathogen infections [Y.H. Cheong, B.C. Moon, J.K. Kim, C.Y. Kim, M.C. Kim, I.H. Kim, C.Y. Park, J.C. Kim, B.O. Park, S.C. Koo, H.W. Yoon, W.S. Chung, C.O. Lim, S.Y. Lee, M.J. Cho, BWMK1, rice mitogen-activated protein kinase, locates in the nucleus and mediates pathogenesis-related gene expression by activation of a transcription factor, Plant Physiol. 132 (2003) 1961--1972]. Here, we report that OsBWMK1 phosphorylates OsWRKY33, which binds to the W-box element (TTGACCA) in several PR gene promoters, thereby enhancing DNA-binding activity of the factor to its in vitro cognate binding site. Transient coexpression of OsBWMK1 and OsWRKY33 in Arabidopsis protoplasts elevates SA-dependent expression of the GUS-reporter gene driven by the W-box element and the PR1 promoter. Furthermore, the levels of SA and H(2)O(2) are elevated in 35S-OsBWMK1 transgenic plants that show HR-like cell death. Altogether, OsBWMK1 may mediate SA-dependent defense responses by activating the WRKY transcription factor in plants.

Constitutive overexpression of the calcium sensor CBL5 confers osmotic or drought stress tolerance in Arabidopsis.

Calcium serves as a critical messenger in many adaptation and developmental processes. Cellular calcium signals are detected and transmitted by sensor molecules such as calcium-binding proteins. In plants, the calcineurin B-like protein (CBL) family represents a unique group of calcium sensors and plays a key role in decoding calcium transients by specifically interacting with and regulating a family of CBL-interacting protein kinases (CIPKs). In this study, we report the role of Arabidopsis CBL5 gene in high salt or drought tolerance. CBL5 gene is expressed significantly in green tissues, but not in roots. CBL5 was not induced by abiotic stress conditions such as high salt, drought or low temperature. To determine whether the CBL5 gene plays a role in stress response pathways, we ectopically expressed the CBL5 protein in transgenic Arabidopsis plants (35S-CBL5) and examined plant responses to abiotic stresses. CBL5-overexpressing plants displayed enhanced tolerance to high salt or drought stress. CBL5 overexpression also rendered plants more resistant to high salt or hyperosmotic stress during early development (i.e., seed germination) but did not alter their response to abiscisic acid (ABA). Furthermore, overexpression of CBL5 alters the gene expression of stress gene markers, such as RD29A, RD29B and Kin1 etc. These results suggest that CBL5 may function as a positive regulator of salt or drought responses in plants.