Overexpression of GmNAC085 enhances drought tolerance in Arabidopsis by regulating glutathione biosynthesis, redox balance and glutathione-dependent detoxification of reactive oxygen species and methylglyoxal

Glutathione (GSH)-dependent detoxification mechanisms are crucial for protecting plants from the adverse effects of abiotic stresses, including drought. Plant tolerance to drought is regulated by a set of transcription factors that are considered as an integral part of signaling networks in plants. Recently, there have been excellent appreciations on the drought tolerance roles of NAC (NAM, ATAF1/2, and CUC2) transcription factors in plants. In the current study, we sought the mechanisms on how the soybean GmNAC085 functions in the regulation of GSH biosynthesis and GSH-dependent detoxifications of reactive oxygen species (ROS) and methylglyoxal (MG) in order to promote drought tolerance in Arabidopsis thaliana. Our results showed that overexpression of GmNAC085 led to a marked increase in the level of reduced GSH, while decreasing oxidized GSH (GSSG) with a concomitant enhancement of the level of GSH/GSSG ratio in the leaves of Arabidopsis 35S:GmNAC085 transgenic plants. This GSH-based redox balance in transgenic lines was likely due to the increased expression of GSH-biosynthesis genes GSH1 and GSH2, as well as enhanced recycling of GSSG to GSH by glutathione reductase. The significant inductions of the activities of GSH-dependent enzymes glutathione peroxidase and glutathione-S-transferase were also evident in the 35S:GmNAC085 plants, revealing higher protection of transgenic lines against oxidative stress. We also observed that 35S:GmNAC085 plants efficiently removed toxic aldehyde MG by enhancing the activities of glyoxalase (Gly) I and Gly II, which is coincided with the increased transcript levels of Gly I and Gly II genes in the leaves. These results demonstrate a previously unknown function of GmNAC085-guided GSH-dependent detoxifications of ROS and MG, thereby providing a new insight into the mechanisms associated with GmNAC085-mediated drought tolerance. Additionally, expression of GmNAC085 under drought-inducible Arabidopsis RD29A promoter also enhanced drought tolerance without growth retardation, suggesting GmNAC085 as a potential genetic tool for developing plants with improved drought tolerance capacity.