Drought is the most important crop yield-limiting factor, and detailed knowledge of its impact on plant growth regulation is crucial. The maize (Zea mays) leaf growth zone offers unique possibilities for studying the spatiotemporal regulation of developmental processes by transcriptional analyses and methods that require more material, such as metabolite and enzyme activity measurements. By means of a kinematic analysis, we show that drought inhibits maize leaf growth by inhibiting cell division in the meristem and cell expansion in the elongation zone. Through a microarray study, we observed the downregulation of 32 of the 54 cell cycle genes, providing a basis for the inhibited cell division. We also found evidence for an upregulation of the photosynthetic machinery and the antioxidant and redox systems. This was confirmed by increased chlorophyll content in mature cells and increased activity of antioxidant enzymes and metabolite levels across the growth zone, respectively. We demonstrate the functional significance of the identified transcriptional reprogramming by showing that increasing the antioxidant capacity in the proliferation zone, by overexpression of the Arabidopsis (Arabidopsis thaliana) iron-superoxide dismutase gene, increases leaf growth rate by stimulating cell division. We also show that the increased photosynthetic capacity leads to enhanced photosynthesis upon rewatering, facilitating the often-observed growth compensation.Drought imposes a major limitation on crop productivity (Boyer, 1982). Currently, no less than 75% of the world's freshwater supplies are utilized in agriculture, and it is more than likely that the expanding world population and unfavorable climate conditions will decrease its availability in the near future (Wallace, 2000). For example, climate change trends toward increasing drought are predicted to reduce U.S. maize (Zea mays) yields between 15% and 30% (Lobell et al., 2014). Therefore, increasing crop productivity under conditions of limiting water availability is of major importance. To achieve this, a systems-level understanding of how plant growth adapts to drought is a scientific requirement.The inhibition of leaf growth is one of the earliest responses to limited water availability, leading to the reduction of transpiration and water conservation. This response can cost as much as 60% of the potential yield of a maize crop even in the absence of visual wilting symptoms (Ribaut et al., 1997).

The production and role of reactive oxygen species (ROS) in the expanding zone of maize (Zea mays) leaf blades were investigated. ROS release along the leaf blade was evaluated by embedding intact seedlings in 2Ј,7Ј-dichlorofluoresceincontaining agar and examining the distribution of 2Ј,7Ј-dichlorofluorescein fluorescence along leaf 4, which was exposed by removing the outer leaves before embedding the seedling. Fluorescence was high in the expanding region, becoming practically non-detectable beyond 65 mm from the ligule, indicating high ROS production in the expansion zone. Segments obtained from the elongation zone of leaf 4 were used to assess the role of ROS in leaf elongation. The distribution of cerium perhydroxide deposits in electron micrographs indicated hydrogen peroxide (H 2 O 2 ) presence in the apoplast. 2Ј,7Ј-Dichlorofluorescein fluorescence and apoplastic H 2 O 2 accumulation were inhibited with diphenyleneiodonium (DPI), which also inhibited O ⅐ 2 Ϫ generation, suggesting a flavin-containing enzyme activity such as NADPH oxidase was involved in ROS production. Segments from the elongation zone incubated in water grew 8% in 2 h. KI treatments, which scavenged H 2 O 2 but did not inhibit O ⅐ 2 Ϫ production, did not modify growth. DPI significantly inhibited segment elongation, and the addition of H 2 O 2 (50 or 500 m) to the incubation medium partially reverted the inhibition caused by DPI. These results indicate that a certain concentration of H 2 O 2 is necessary for leaf elongation, but it could not be distinguished whether H 2 O 2 , or other ROS, are the actual active agents.

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