**Author details**

**5.3. Brassinosteroids promote seed germination via repression of ABA signaling**

90 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

biosynthesis, whereas *TaBSK2* promotes BR signaling (**Table 1**) [100].

content in seeds [105].

**6. Conclusions**

**5.4. Ethylene represses ABA accumulation and promotes seed dormancy release**

Ethylene (ET) is positively related to seed dormancy release and germination promotion. In Arabidopsis, the expression of ET biosynthesis gene, *1‐aminocyclopropane‐1‐carboxylic acid oxi‐ dase* (*ACO*), is associated with imbibition; however, cold stratification reduces its expression (**Table 1**) [101]. Ethylene receptors, ethylene triple response 1 (ETR1) and ethylene insensitive 2 (EIN2) play a role in seed dormancy regulation. *etr1* and *ein2* mutants show the increased level of r seed dormancy associated with the increased level of seed ABA content [102, 103]. The higher expression of *NCED3* and lower activation of *CYP707A2* were observed in *ein2* and *etr1* mutants, respectively, compared to the wild type. It suggests a negative role of ethylene in the modulation of ABA pathway in seeds (**Table 1**) [104]. In wheat, after‐ripened grains express *TaACO* at a higher level than in dormant grains. Thus, the increased ET content in seeds is associated with dormancy loss also in wheat [100]. The role of ethylene in seed dor‐ mancy regulation includes regulation at epigenetic level. *SIN3‐like 1* (SNL1) and SNL2 reduce acetylation level of histone 3 lysine 9/18 and histone 3 lysine 14. The double mutant *snl1 snl2* shows reduced seed dormancy together with the increased expression of ethylene biosynthe‐ sis genes (*ACO1*, *ACO4*) and ABA catabolism genes (*CYP707A1*, *CYP707A2*). Therefore, SNL1 and SNL2 promote seed dormancy through simultaneous modulation of ethylene and ABA

Proper regulation of seed dormancy is crucial for appropriate timing of germination. Many environmental factors, including light and temperature, exert action on switch from dormancy to germination stage. Their action is mediated by phytohormones: ABA and GA. ABA is a master player for the entrance to and the establishment of seed dormancy. Many ABA‐related genes are necessary for the quiescent stage of seeds. Contrary to ABA, GA‐mediated pathway promotes germination under favorable conditions. Similar mechanism of seed dormancy regu‐ lation exists in monocot plants. The seed response is dependent on the ABA and GA balance.

Brassinosteroids (BR) act opposite to ABA signaling in the regulation of seed dormancy and germination. In Arabidopsis, the crucial regulator of seed dormancy, *MFT*, is under BR regu‐ lation in seeds. Therefore, MFT acts as a mediator of ABA and BR pathways in seeds [98]. Brassinosteroid insensitive 2 (BIN2) is a GSK3‐like kinase playing a negative role in BR sig‐ naling, and furthermore, it ensures the communication with ABA signaling. BIN2 interacts with ABI5 and phosphorylates it, which in turn promotes ABI5 activity during seed germina‐ tion [99]. *TaBIN2* activity is downregulated in the after‐ripened wheat seeds (**Table 1**) [100]. Expression analysis also showed the induction of genes encoding the positive components of BR pathway: *TaDE‐etiolated 2* (*TaDET2*), *TaDWARF 4* (*TaDWF4*), and *TaBR signaling kinase 2* (*TaBSK2*) in wheat after‐ripened grains. *TaDET2* and *TaDWF4* encode crucial enzymes for BR

Anna Skubacz and Agata Daszkowska‐Golec\*

\*Address all correspondence to: agata.daszkowska@us.edu.pl

Department of Genetics, Faculty of Biology and Environmental Protection, University of Silesia in Katowice, Katowice, Poland
