Overexpression of a Mitogen-Activated Protein Kinase SlMAPK3 Positively Regulates Tomato Tolerance to Cadmium and Drought Stress

Abstract

Mitogen-activated protein kinases (MAPKs) activation is a common defense response of plants to a range of abiotic stressors. <i>SlMPK3</i>, a serine-threonine protein kinase, has been reported as an important member of protein kinase cascade that also functions on plant stress tolerance. In this study, we cloned <i>SlMPK3</i> from tomato and studied its role in cadmium (Cd²⁺) and drought tolerance. The results showed that transcripts of <i>SlMAPK3</i> differentially accumulated in various plant tissues and were remarkably induced by different abiotic stressors and exogenous hormone treatments. Overexpression of <i>SlMAPK3</i> increased tolerance to Cd²⁺ and drought as reflected by an increased germination rate and improved seedling growth. Furthermore, transgenic plants overexpressing <i>SlMAPK3</i> showed an increased leaf chlorophyll content, root biomass accumulation and root activity under Cd²⁺ stress. Chlorophyll fluorescence analysis revealed that transgenic plants demonstrated an increased photosynthetic activity as well as contents of chlorophyll, proline, and sugar under drought stress. Notably, cadmium- and drought-induced oxidative stress was substantially attenuated in <i>SlMAPK3</i> overexpressing plants as evidenced by lower malondialdehyde and hydrogen peroxide accumulation, and increased activity and transcript abundance of enzymatic antioxidants under stress conditions compared to that of wild-type. Our findings provide solid evidence that overexpression of <i>SlMAPK3</i> gene in tomato positively regulates tolerance to Cd²⁺ and drought stress, which may have strengthen the molecular understanding of <i>SlMAPK3</i> gene to improve abiotic stress tolerance.