Cloning,characterization, and expression of the BjEXPA1 gene and its promoter region from Brassica juncea L.

To explore the regulation of expansin expression under heavy metal stresses, a novel expansin gene, BjEXPA1, and its upstream region were obtained from Indian mustard (Brassica juncea L.). BjEXPA1 encodes a cell wall protein of the α-subgroup of the expansin family. The expression of BjEXPA1 is regulated by exogenous phytohormones, NaCl, and heavy metals. Sequence analysis of its upstream region revealed the presence of several putative cis-acting elements, including phytohormone response, abiotic stress response, and tissue-specific elements. GUS activity under the control of the BjEXPA1 promoter allowed observation of BjEXPA1 spatial and temporal expression patterns. The reporter construct indicated that BjEXPA1 is induced by exogenous gibberellin and auxin, and inhibited by abscisic acid, CdCl₂ and PEG6000. Transgenic tobacco seedlings overexpressing BjEXPA1 showed an altered phenotype and enhanced sensitivity to cadmium. Higher H₂O₂ accumulation and extensive plasma membrane damage were detected in the leaves of transgenic seedlings when exposed to Cd, compared with wild-type plants. Our findings support the conclusion that overexpression of BjEXPA1 in tobacco renders plants susceptible to Cd stress, probably due to disruption of the growth process and/or the breakdown of cell walls.