activation factor (ATAF), and cup-shaped cotyledon (CUC). Much of the molecular work to understand the regulation of drought tolerance has been done in Arabidopsis, helping elucidate the basic processes below. DREB TFs DREB1/CBF TFs DREB1A, DREB 1B, and DREB 1C are plant specific TFs which bind to drought responsive elements (DREs) in promoters responsive to drought, high salinity and low temperature in Arabidopsis. Overexpression of these genes enhance the tolerance of drought, high salinity, and low temperature in transgenic lines from Arabidopsis, rice, and tobacco. DEAR1/DREB and EAR motif protein 1 is a TF with an entirely different purpose nothing to do with drought stress. Tsutsui et al 2009 found Arabidopsis DEAR1 (At3g50260) to respond to pathogen infection, chitin, and oligomers of chitin. DREB2 TFs DREB proteins are involved in a variety of functions related to drought tolerance. For example, DREB proteins including DREB2A cooperate with AREB/ABF proteins in gene expression, specifically in the DREB2A gene under osmotic stress conditions. DREB2 also induces the expression of heat-related genes, such as heat shock protein. Overexpression of DREB2Aca enhances drought and heat stress tolerance levels in Arabidopsis. AREB/ABF TFs AREB/ABFs are ABA-responsive bZIP-type TFs which bind to ABA-responsive elements (ABREs) in stress-responsive promoters and activate gene expression. AREB1, AREB2, ABF3, and ABF1 have important roles in ABA signalling in the vegetative stage, as ABA controls the expression of genes associated with drought response and tolerance. The native form of AREB1 cannot target drought stress genes like RD29B in Arabidopsis, so modification is necessary for transcriptional activation. AREB/ABFs are positively regulated by SnRK2s, controlling the activity of target proteins via phosphorylation. This regulation also functions in the control of drought tolerance in the vegetative stage as well as the seed maturation and germination. Other TFs TFs such as NAC (composed of NAM, ATAF, and CUC), are also related to drought response in Arabidopsis and rice. Overexpression in the aforementioned plants improves stress and drought tolerance. They also may be related to root growth and senescence, two physiological traits related to drought tolerance. Natural drought tolerance adaptations Plants in naturally arid conditions retain large amounts of biomass due to drought tolerance and can be classified into 4 categories of adaptation: Drought-escaping plants: annuals that germinate and grow only during times of sufficient times of moisture to complete their life cycle. Drought-evading plants: non-succulent perennials which restrict their growth only to periods of moisture availability.