Fig. 3.

The function of ABA in seed germination and seedling establishment. Left, Seed completes germination successfully through degradation of active ABA into PA (phaseic acid) and DPA (dihydrophaseic acid)/DPAG with CYP707As regulated by REF6 and phaseic acid reductase (ABH2 and GT) respectively. During germination and seedling establishment, the core ABA signaling component SnRK2s and downstream ABI3/4/5 are activated or repressed by many factors directly or indirectly to promote seed germination and seedling establishment. For example, RAV1 and BASS2 bind to the ABI4 promoter to inhibit ABI4 expression, while MYB96 promotes ABI4 expression through binding to its promoter. A Casein Kinase 2 promotes ABI4 expression indirectly. Furthermore, ANAC060 transcription is activated directly by ABI4 through binding to its promoter to enhance post-germination. Several BTB-A2 proteins can impair SnRK2.3 stability to act as negative regulators of ABA signaling. NRT1.2 is identified as an ABA transporter to regulate downstream factors ABI1-ABI5, RAB18, etc. positively to mediate germination and seedling development. Further, some negative factors such as SUN24, UGT74E2, FOF2, and VQs regulate seed germination and seedling development through repressing ABI3-, ABI5-mediated ABA pathway. Activated and repressive effects are shown by arrows and bars, respectively.