Genomics of drought

Foreseeable global environmental changes combine with a growing world population to increase pressure on water resources. In many regions, diminishing water availability for irrigation is leading to major constrains in crop productivity, posing a real threat and a challenge for food security in the next few years. Plants have evolved a myriad of adaptive mechanisms to cope with water scarcity. We can identify several different tolerance and resistance strategies, of multigenic nature and with different molecular backgrounds that may occur in the same or in different individuals. Therefore, in order to improve plant productivity under low water availability, we must understand the basic mechanisms of perception and molecular response to water deficit conditions. In this chapter, we revise the most relevant morphophysiological adaptations of plants, from roots to shoots, to resist water deficit, and we focus in more depth in the underneath molecular mechanisms. We discuss the perception and signal transduction mechanisms related to abscisic acid (ABA), a key phytohormone modulating several water deficit responses, as well as ABA-independent mechanisms. We also cover the molecular aspects leading to the accumulation of compatible solutes, aimed to reduce water loss or minimize its damaging effects, and analyze the impact of water scarcity on photosynthesis and regulation of stomatal gas exchanges. An overview of the epigenetic regulation of drought stress response is also provided. Although drought tolerance is a difficult trait to breed for, especially if based on a single-gene approach, we also highlight some genes described as positively affecting crop performance and productivity under water deficit regimes and that may aid in future breeding programs. Given the large extent of work already developed to address the drought effect on plants, this review is necessarily incomplete. However, it was designed to elucidate the main drought response strategies and to uncover the major findings and indicate ways for future progress.