**2.3. Environmental cues and epigenetic modifications in the regulation of the ABA pathway**

The expression of Arabidopsis ABA metabolism and signaling genes is regulated through environmental factors. The red (R) light pulse irradiation applied to the far‐red (FR) light pulse pretreated, dark‐imbibed seeds inhibits and induces the expression of *NCED6* and *CYP707A2,* respectively. It suggests that the ABA metabolism genes are under the control of PHYB (phytochrome B), which regulates germination in response to FR and R pulse light [60]. On the other hand, the blue light has a negative impact on the germination of dormant grains in cereals. The blue light–associated secondary dormancy induces *HvNCED1* and *HvNCED2* and weakly reduces *HvABA8'OH‐1* expression in grains [61]. The activation of *HvNCED1* is under the regulation of phytochrome photoreceptor, cryptochrome 1 (HvCRY1). It indi‐ cates that ABA biosynthesis and catabolism take part in blue light–dependent regulation of seed dormancy [62, 63]. The temperature and NO also exert an impact on ABA pathway in Arabidopsis seeds. The high temperature promotes the expression of ABA biosynthesis genes in imbibed seeds, whereas NO positively regulates ABA signaling during seed dormancy breaking [52, 64]. NO action may be associated with N‐end rule pathway, leading to degrada‐ tion of proteins with destabilizing amino acid residues. NO and oxygen are sensed by N‐end rule pathway with the participation of many protein regulators [65]. The components of N‐ end rule pathway, proteolysis 6 (PRT6) and arginyl‐tRNA:protein arginyltransferase (ATE), regulate after‐ripening, inhibit ABA signaling, and finally promote seed germination. PRT6 is E3 ligase promoting protein degradation via 26S proteasome. Some PRT6 substrates belong to the ABA pathway. As a result, ABA signaling is inhibited, and the activation of ABI3 and ABI5 is detained (**Figure 1**) [52].

The ABA metabolism and signaling genes are also regulated at epigenetic level during the establishment of seed dormancy. Kryptonite/SU(VAR) 3‐9 homolog 4 (KYP/SUVH4) is respon‐ sible for histone H3 lysine 9 dimethylation. Repression of *ABI3* by KYP/SUVH4 is required to release seed dormancy (**Figure 1**) [66]. Moreover, the expression of *ABA2*, *ABI3*, and *ABI5* is downregulated through the action of two histone demethylases, lysinespecific demethylases 1 and 2 (LDL1 and LDL2). Thus, the activity of LDL1 and LDL2 ensures decrease in primary seed dormancy via negative regulation of ABA response (**Figure 1**) [67].
