Reactive oxygen species (ROS) are produced in plant cells primarily as by-products of aerobic energy metabolism. They are also generated during plant adaptation responses to environmental stresses, such as drought and high salinity. Therefore, plants have evolved ROS-detoxifying enzymes and antioxidants to cope with ROS accumulation. However, if stress conditions are prolonged, the level of ROS will surpass the capacity of the detoxifying machinery, causing oxidative damage to cellular constituents. It is known that ROS act in abscisic acid-mediated stress responses to sustain plant survival under adverse growth conditions. However, it is largely unknown how ROS metabolism is linked to stress responses. Here, we show that a drought-responsive NAC transcription factor NTL4 promotes ROS production by binding directly to the promoters of genes encoding ROS biosynthetic enzymes during drought-induced leaf senescence. Leaf senescence was accelerated in 35S:4ΔC transgenic plants over-expressing an active form of NTL4 under drought conditions. The 35S:4ΔC transgenic plants were hypersensitive to drought, and ROS accumulated in the leaves. In contrast, ROS levels were reduced in NTL4-deficient ntl4 mutants, which exhibited delayed leaf senescence and enhanced drought resistance. These observations indicate that NTL4 acts as a molecular switch that couples ROS metabolism to drought-induced leaf senescence in Arabidopsis.
Keywords: abscisic acid, Arabidopsis, drought stress, leaf senescence, NAM/ATAF1/2/CUC2 (NAC), reactiveoxygen species