A salicylic acid carboxyl methyltransferase-like gene LcSAMT from Lycium chinense, negatively regulates the drought response in transgenic tobacco

Abstract

Drought is the main environmental factor which limits growth, photosynthesis and yield of agricultural crops worldwide. Salicylic acid (SA) is a phenolic compound involved in plant growth and development. Salicylic acid carboxyl methyltransferase (SAMT) can catalyze the methylation of SA with S-adenosyl-L-methionine as the methyl donor to form methyl salicylate (MeSA). However, the relations between SAMT and drought tolerance in plants are largely unknown. In this study, a SAMT gene, LcSAMT, was isolated from Lycium chinense and characterized. Escherichia coli expressed LcSAMT was found to function as salicylic acid methyltransferase using in vitro enzyme assays. The expression of LcSAMT gene was observed to be elevated in L. chinense under drought stress treatment. The overexpression of LcSAMT gene markedly enhanced the MeSA content and reduced the accumulation of SA in the transgenic tobacco plants. The conversion of MeSA from SA led to the depletion of the free SA pool. The overexpression of LcSAMT gene in tobacco significantly increased sensitivity of transgenic plant to drought stress, as measured by morphological and physiological factors such as leaf photosynthetic rate and chlorophyll content, which might be due to the decreased SA accumulation. The increased accumulation of ROS, elevated MDA levels, reduced proline contents and lowered expression of APX, CAT and SOD genes were also observed in the LcSAMT transgenic tobacco plants under drought stress, which means that the LcSAMT-overexpressing transgenic tobacco plants had decreased resistance to oxidative stress in comparison with control plants under drought stress. These results thus suggested that LcSAMT responded to drought stress might through the regulation of ROS accumulation in plants. Furthermore, LcSAMT-overexpressing transgenic tobacco plants displayed decreased ABA accumulation and reduced transcript expression of NtNCED1 and NtRD22 genes. Therefore, the increased sensitivity of transgenic plants overexpressing LcSAMT gene to drought stress might also through an ABA-dependent pathway.