Overexpression of malate dehydrogenase in transgenic tobacco leaves: enhanced malate synthesis and augmented Al-resistance
Numerous studies with transgenic plants have demonstrated that overexpression of enzymes related to organic acid metabolism under the control of CaMV 35S promoter increased organic acid exudation and Al-resistance. The synthesis of organic acids requires a large carbon skeleton supply from leaf photosynthesis. Thus, we produced transgenic tobacco overexpressing cytosolic malate dehydrogenase (MDH) cDNA from Arabidopsis thaliana (amdh) and the MDH gene from Escherichia coli (emdh), respectively, under the control of a leaf-specific light-inducible promoter (Rubisco small subunit promoter, PrbcS) in the present study. Our data indicated that an increase (120–130%) in MDH-specific activity in leaves led to an increase in malate content in the transgenic tobacco leaves and roots as well as a significant increase in root malate exudation compared with the WT plants under the acidic (pH 4.5) conditions irrespective of 300 μM Al3+ stress absence or presence. After being exposed to 25 μM Al3+ in a hydroponic solution, the transgenic plants exhibited stronger Al-tolerance than WT plants and the degree of A1 tolerance in the transgenic plants corresponded with the amount of malate secretion. When grown in an Al-stress perlite medium, the transgenic tobacco lines showed better growth than the WT plants. The results suggested that overexpression of MDH driven by the PrbcS promoter in transgenic plant leaves enhanced malate synthesis and improved Al-resistance.