*3.1.2. ABA and drought tolerance*

Drought is lack of water in the soil. Drought stress in plants arises due to water deficit conditions and results in removal of water from the cell membranes, disrupting the lipid bilayer structure. In addition, protein denaturation and accumulation of cellular electrolytes results in disruption of cellular metabolism [63]. Therefore, drought causes osmotic stress, and osmotic stress causes dehydration and inhibition of water uptake in plants. ABA accumulates under osmotic stress conditions and plays an important role in the stress response and tolerance of plants. In addition to autoactivation of SnRK2s by inhibition of PP2Cs in the ABA signaling cascade, hyperosmotic stress activates SnRK2s [64]. SnRK2 kinases are a major component of the osmotic stress signaling pathway. The Arabidopsis triple mutant *snrk2.2, snrk2.3, snrk2.6* shows severe drought intolerance and ABA-insensitivity [42]. Also ABF2, ABF3, and ABF4 act as transcriptional activators in mediating ABRE-dependent ABA signal‐ ing, which confers drought tolerance in vegetative tissues [40].

ABA induces expression of many transcription factors as well as genes that encode enzymes in the synthesis of osmoprotectants [65]. Osmolytes are compatible solutes such as amino acids (proline), sugar alcohols (mannitol, pinitol), and other sugars that accumulate without disrupting the function of proteins. Osmolytes make an osmotic adjustment facilitating a favorable water potential gradient and promote stress tolerance [66].

Dehydrins and LEA-like proteins act as cellular chaperones that protect cellular membranes and macromolecules in the cell [2]. During seed maturation seeds undergo dehydration stress. LEA proteins accumulate in the embryo as a result of osmotic stress and their functions include protection of enzymes, lipids, and mRNAs from dehydration. LEA proteins have been found to protect mitochondrial membranes from damage. LEA proteins are produced in an ABAdependent and ABA-independent manner under osmotic stress [47,58].

Under moderate water stress conditions plant root growth has to be maintained in order to keep the plants alive. ABA accumulates under moderate water stress and mediates auxin transport in the root tip, which enhances the proton pumps in the plasma membrane. Proton secretions in the root tip play an important role in primary root growth and root hair devel‐ opment under moderate drought stress [67].

Based on the critical water level, drought tolerance is considered to be mechanisms that confer tolerance to moderate dehydration. Further dehydration requires desiccation tolerance mechanisms in order to restore the ability of cells to rehydrate successfully [68].
