Division of Plant Biology, Bose Institute, P1/12 C. I. T. Scheme VIIM, Kolkata–700054, West Bengal, India.
A putative family 3 glycosyl hydrolase (GH) gene showed significant differential expression in resistant Sinapis alba, compared with the susceptible Brassica juncea, as part of the initial responses during interaction with the necrotroph Alternaria brassicicola. To understand the mechanism of induction, the promoter was isolated and deletion analysis carried out. All the promoter fragments were fused with the β-glucuronidase gene and the expressions were studied in stable B. juncea transgenics and transiently transformed Nicotiana tabacum. Analysis of the expression of the promoter showed the presence of functional abscisic acid (ABA)-, jasmonic acid (JA)-, and salicylic acid (SA)-responsive cis elements. Interestingly, the promoter was found to be induced in both S. alba and B. juncea upon challenge with A. brassicicola but, in S. alba, SA had an inhibitory effect on the pathogen-induced expression of the gene whereas, in B. juncea, SA did not have any negative effect. Therefore, the SA-mediated inhibition in S. alba indicates that the induction is probably through JA or ABA signaling. The difference in the mechanism of induction of the same promoter in the resistant and susceptible plants is probably due to the differential hormonal responses initiated upon challenge with A. brassicicola.