Enhanced exudation of malate in the rhizosphere due to AtALMT1 overexpression in blackgram (Vigna mungo L.) renders increased aluminum tolerance

Worldwide 50% of soil is acidic, which induces aluminum (Al) toxicity in plants; as the phyto-availability of Al(3+) increases in acidic soil. Plants responds to Al(3+) toxicity by exuding organic acids into the rhizosphere. Organic acid responsible for Al(3+) stress response varies from species to species, which in case of blackgram (Vigna mungo L.) is citrate. In blackgram, an Arabidopsis malate transporter, AtALMT1 was overexpressed with the motive to induce enhanced exudation of malate. Transgenics were generated using cotyledonary node explants through Agrobacterium tumefaciens mediated transformation. The putative transgenics were initially screened by AtALMT1 specific genomic DNA PCR followed by quantitative PCR. Two independent transgenic events were identified and functionally characterized in T3 generation. The transgenic lines, Line1 and 2 showed better root growth, relative water content and chlorophyll content under Al(3+) stress. Both the lines also accounted for less oxidative damage, due to lesser accumulation of ROS molecules. Photosynthetic efficiency, as measured in terms of Fv/Fm, NPQ and Y(II); was more when compared to wild type. Relative expression of genes (VmSTOP1, VmALS3, VmMATE) responsible for Al(3+) stress response in blackgram showed that, overexpression of a malate transporter didn't have any effect on their expression. Malate exudation increased whereas citrate exudation didn't show any divergence from WT. Pot stress assay exhibited that the transgenics showed better adaptability to acidic soil. This report demonstrates that the overexpression of a malate transporter in a non-malate exuding species renders it with better adaptability to Al(3+) toxicity in acidic soil without effecting its stress response mechanism.