**5. The emerging role of auxin, jasmonates, brassinosteroids, and ethylene in seed dormancy regulation**

ABA and GA are not the only phytohormonal regulators of seed dormancy establishment and release. Their action is modulated by other phytohormones, such as auxin, jasmonates (JA), brassinosteroids (BR), and ethylene.

#### **5.1. Action of auxin pathway components in seeds**

Auxin promotes seed dormancy release and germination. Constitutive induction of auxin biosynthesis in *iaaM‐OX* line inhibits precocious germination in Arabidopsis. Contrarily, the switched off activity of *auxin response factor 10* (*ARF10*) and *ARF16*, auxin‐dependent tran‐ scription factors, in *arf10/arf16* double mutant, causes faster precocious germination than in the wild type. The role of auxin in the control of seed dormancy includes the action of *ABI3*. The double mutants, *abi3‐1/iaaM‐OX* and *abi3‐1/99999mARF16* (line resistant to miR160), show the reduced dormancy phenotype. Therefore, the activation of auxin signaling pro‐ motes *ARF10* and *ARF16*, which in turn induces *ABI3* and seed dormancy (**Table 1**) [93]. Analysis of after‐ripened wheat grains showed increased expression of *TaIAA‐alanine resis‐ tant 3* (*TaIAR3*) encoding hydrolase releasing IAA from conjugates. It was observed in paral‐ lel with the higher IAA level in seeds during imbibition. Probably, seed dormancy release may be associated with the increased auxin content in seeds of monocot plants. Furthermore, *TaAuxin‐resistant 1* (*TaAXR1*), *TaUbiquitin‐related protein 1* (*TaRUB1*), and *TaARF2* were also upregulated in after‐ripened wheat grains. TaAXR1 is associated with AUX/IAA proteasome‐ mediated degradation, whereas TaRUB1 is related to ubiquitin action. The higher expression of *TaAXR1* and *TaRUB1* can exert a negative impact on auxin signaling (**Table 1**) [72].

#### **5.2. Dual role of jasmonic acid in seed dormancy regulation**

The role of JA (Jasmonic Acid) in seed dormancy is ambiguous. The increased JA content was detected in nondormant Arabidopsis seeds. Probably, the decrease of JA content during imbi‐ bition in nondormant seeds is associated with germination promotion [94]. Application of JA precursor, 12‐oxo‐phytodienoic acid (OPDA) promotes the expression of *ABA1*, *ABI5*, and *RGL2* in after‐ripened seeds and inhibits seed germination. OPDA also exerts a regulatory action on the crucial seed dormancy component, *MFT* [95]. The opposite effect of JA on seed Seed Dormancy: The Complex Process Regulated by Abscisic Acid, Gibberellins, and Other Phytohormones... http://dx.doi.org/10.5772/intechopen.68735 89


The role of DOG1, the GA‐related regulator of seed dormancy, was also described in ABA signaling in seeds. *ABI5* is positively promoted by DOG1, which in turn leads to the regulation of many *late embryogenesis abundant* (*LEA*) and *heat shock protein* (*HSP*) genes. Moreover, the double‐mutant *abi3‐1/dog1‐1* shows the lower sensitivity to ABA than *abi3‐1*, and in control condition, it produces mature dry green seeds. It suggests the positive relationship between *DOG1* and *ABI3; therefore,* DOG1 may be responsible for ABA‐GA interactions in seeds

ABA and GA are not the only phytohormonal regulators of seed dormancy establishment and release. Their action is modulated by other phytohormones, such as auxin, jasmonates (JA),

Auxin promotes seed dormancy release and germination. Constitutive induction of auxin biosynthesis in *iaaM‐OX* line inhibits precocious germination in Arabidopsis. Contrarily, the switched off activity of *auxin response factor 10* (*ARF10*) and *ARF16*, auxin‐dependent tran‐ scription factors, in *arf10/arf16* double mutant, causes faster precocious germination than in the wild type. The role of auxin in the control of seed dormancy includes the action of *ABI3*. The double mutants, *abi3‐1/iaaM‐OX* and *abi3‐1/99999mARF16* (line resistant to miR160), show the reduced dormancy phenotype. Therefore, the activation of auxin signaling pro‐ motes *ARF10* and *ARF16*, which in turn induces *ABI3* and seed dormancy (**Table 1**) [93]. Analysis of after‐ripened wheat grains showed increased expression of *TaIAA‐alanine resis‐ tant 3* (*TaIAR3*) encoding hydrolase releasing IAA from conjugates. It was observed in paral‐ lel with the higher IAA level in seeds during imbibition. Probably, seed dormancy release may be associated with the increased auxin content in seeds of monocot plants. Furthermore, *TaAuxin‐resistant 1* (*TaAXR1*), *TaUbiquitin‐related protein 1* (*TaRUB1*), and *TaARF2* were also upregulated in after‐ripened wheat grains. TaAXR1 is associated with AUX/IAA proteasome‐ mediated degradation, whereas TaRUB1 is related to ubiquitin action. The higher expression

of *TaAXR1* and *TaRUB1* can exert a negative impact on auxin signaling (**Table 1**) [72].

The role of JA (Jasmonic Acid) in seed dormancy is ambiguous. The increased JA content was detected in nondormant Arabidopsis seeds. Probably, the decrease of JA content during imbi‐ bition in nondormant seeds is associated with germination promotion [94]. Application of JA precursor, 12‐oxo‐phytodienoic acid (OPDA) promotes the expression of *ABA1*, *ABI5*, and *RGL2* in after‐ripened seeds and inhibits seed germination. OPDA also exerts a regulatory action on the crucial seed dormancy component, *MFT* [95]. The opposite effect of JA on seed

**5.2. Dual role of jasmonic acid in seed dormancy regulation**

**5. The emerging role of auxin, jasmonates, brassinosteroids, and** 

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

**ethylene in seed dormancy regulation**

**5.1. Action of auxin pathway components in seeds**

brassinosteroids (BR), and ethylene.

(**Figure 2**) [54].

Note: auxin response factor (ARF), IAA‐alanine resistant 3 (IAR3), auxin‐resistabt 1 (AXR1), ubiquitin‐related protein 1 (RUB1), allene oxide synthase (AOS), 3‐ketoacyl coenzyme A (KAT3), lipoxygenase 5 (LOX5), allene oxide cyclase (AOC), brassinosteroid insensitive 2 (BIN2), de‐etiolated 2 (DET2), DWARF 4 (DWF4), br signaling kinase 2 (BSK2), 1‐aminocyclopropane‐1‐carboxylic acid oxidase (ACO), ethylene triple response 1 (ETR1), ethylene insensitive 2 (EIN2).

**Table 1.** Regulators of auxin, jasmonic acid, brassinosteroid, and ethylene pathways in seed dormancy promotion or release.

dormancy exists in wheat. JA was shown to reduce the promoting effect of blue light on seed dormancy in a nitrate‐dependent way [96]. Additionally, after ripening promotes expression of JA biosynthesis genes: *TaAllene oxide synthase* (*TaAOS*), *Ta3‐ketoacyl coenzyme A* (*TaKAT3*) and *TaLipoxygenase 5* (*TaLOX5*) in wheat grains. However, the level of JA decreases during imbibition (**Table 1**) [72]. The cold‐induced release of seed is associated with the increase in JA endogenous content. Cold stratification process promotes the expression of *TaAOS* and *TaAllene oxide cyclase* (*TaAOC*). Furthermore, JA positively regulates *TaNCED1* and *TaNCED2* activity and thus enables seed germination through ABA biosynthesis repression in wheat (**Table 1**) [96, 97].

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

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

#### **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 content in seeds [105].

## **6. Conclusions**

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. The ABA‐GA crosstalk ensures the precise seed response according to developmental stage, environmental factors, and seasons. Many components of the ABA and GA pathway, for exam‐ ple ABI3, ABI4, ABI5, RGL2, MFT, and DOG1, are responsible for the proper regulation of seed dormancy. Additionally, auxin, jasmonic acid, brassinosteroids, and ethylene modulate the ABA pathway in seeds. Furthermore, epigenetic control of dormancy‐related components also occurs. Therefore, seed dormancy regulation appears to be a very elaborate process. In monocot plants, a part of the seed dormancy regulatory mechanism acts in a different manner. Action of MFT and JA pathway seems to be reverse in comparison to dicot plants. A better understand‐ ing of precise phytohormonal regulation of seed response of cereals can help in obtaining new varieties with the appropriate seed dormancy level.
