Figure 1.

A simplified model for abscisic acid generation, transport and perception in plants. ABA biosynthesis is induced by environmental cues in the vascular tissues. ABA can also be released at ER from ABA-GE that stored in the vacuole or transported from the vacuolar system by an unknown ABA-GE transporter (? stand for unknown a mechanism). Both sourced ABAs are transported to intercellular space by the ABA transporters, AtABCG25 which majorly located in vascular tissue and exports ABA outward the cells. The newly synthesized ABA is then transported to the cells of acting site for ABA-responses by passive and/or active transport. The ABA importers and exporters can efficiently and directly deliver ABA to the acting site. However, part of the ABA is catabolic inactivated, which also plays a pivotal in regulating the intensity of ABA signal. PYR/PYL/RCAR receptors percept ABA signal intracellularly then combine with the negative regulators of ABA signal, PP2Cs/ABI1, to form a ternary complex. Thus, the negative regulator is inactivated whereas the downstream targets of PP2Cs, SnRK2s, are allowed to be activated by phosphorylation. The activation of SnRK2s will thus initiate an ABA response. In cell without ABA signal, the SnRK2s is inactivated by PP2Cs.