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Abstract

Maize is an important cereal crop that provides staple food to many populations. It is a major source of income for the farmers and is grown all over the world. Drought tolerance is the most important trait in maize, since limitation of water supply limits yield at most. The enhanced production of ROS during drought requires an increased GSH production for the efficient detoxification of ROS, thus the regulation of sulfur assimilation during drought is vital due to the dependency of GSH synthesis on the sulfur assimilation pathway. In this study I analysed the impact of drought on the sulfur assimilation pathway in maize. Maize seedlings exposed to drought for 10 and 12 days were severely affected in leaf and root biomass due to a decrease in plant water content and caused elevated levels of H2O2. The drought-induced increase in the ROS formation altered the redox state of GSH pool towards a more oxidized state and indicated oxidative stress in leaves and roots of drought-treated plants compared to control. Moreover, induction of GR transcription in leaves and roots and an increase in GR activity in leaves under drought imply an important role of GR in ROS detoxification and maintaining reduced GSH during drought. The lower steady state level of thiols in leaves is a consequence of decreased rate of GSH biosynthesis during drought. A decrease in the sulfate contents was observed indicating low availability of sulfur in the shoot during drought. Accordingly, the up-regulation in the Sultr1;1 and Sultr4;1 that is responsible for sulfate efflux from the vacuole and a decrease in the steady state levels of sulfate most likely indicate sulfur-starved situation in leaves during drought. Moreover, the transcriptional up-regulation of more than two-fold in ATPS isoforms and an increase in steady state level of APS reflects that ATPS is also rate limiting and regulated by sulfur status during drought. A reduction in the incorporation of 35S into cysteine and GSH suggests that drought limits the availability of sulfate to shoot, thus causing lower flux through the sulfur assimilation pathway into GSH. On the other hand in roots, thiols, sulfate, APS and sulfide were increased relative to the control. A reduction in the incorporation of 35S into cysteine and GSH during drought and the down-regulation of Sultr4;1 indicate the storage of sulfate in the vacuole that might contribute to reduced flux into cysteine and GSH. A strong reduction was observed in the transport of labeled sulfate in the stem of drought stressed plants. This clearly indicates that drought limits the availability of sulfate to shoot, thereby causing the down regulation of sulfur assimilation pathway and ultimately elevated levels of H2O2 in leaves.