**4. Molecular Network of Gibberellins and Abscisic Acid in Germination/ Dormancy**

As mentioned earlier, germination and dormancy release of seeds are governed by the GAs and ABA. According to previous reports, although there are many factors involved in seed germination, GA and ABA biosyntheses have been considered as an important internal factor for the release as well as the induction of dormancy [9]. Metabolisms of ABA and GA is always interrelated [2]. Seo et al. reported that *Arabidopsis aba2-2* (ABA inducer) mutant showed a higher level of GA biosynthesis as compared to the wild type. Contrastingly in the *gai* (GA insensitive) mutant, i.e., GA repressor, synthesis of ABA was higher, and also the degradation of ABA was lesser as than the wild type [11]. Antagonistic effect of GA and ABA is also crosslinked with other phytohormones such as ethylene, brassinosteriods, and auxins [9].

#### **4.1. Dormancy breakage**

the ROS is mobile toward the seed coats, aleurone layers, and embryonic axis, the attack of microorganism is prevented by the induction of SAR and PCD [19, 61]. Briefly, the plasma

infection. Subsequently, H2O2 induces a hypersensitive reaction which leads to PCD of the infected cell. Eventually, H2O2 can also directly affect the pathogens [62, 63]. The main categories of genes involved in the H2O2 induction are related to defense, transcription, signaling (e.g., phosphatases, kinases), and importantly ROS synthesis and degradation. Perturbation of endosperm for the radicle emergence leads to the induction of defense-related genes. It helps to protect the newly germinating seeds from the pathogens [64]. During the seed germination process, lower concentrations of the ROS are involved in the cell signaling process, whereas higher concentrations trigger the PCD to facilitate the radical protrusion [65].

Proteolytic cleavage of cell wall polymers is induced by the hydrolases such as mannose, glucanase, and cellulose. The scission of polysaccharides is a vital step to determine the radicle emergence. According to Muller et al., •OH accumulation is the main factor, which influences endosperm weakening by the breakdown of H-bonds in the cell wall-polysaccharide required for the radicle protrusion. Generally, •OH is extremely reactive and is considered as the most aggressive form of oxygenated derivatives [26]. Uncontrolled ROS accumulation affects the

**Figure 1.** Schematic representation of the involvement of the reactive oxygen species (ROS) in the quiescent seed for the nondormancy and dormancy conditions. Dots represent the accumulation of ROS, blue for superoxide (O2

for hydrogen peroxide (H2O2), and purple for hydroxyl radical (•OH). The LEA, late embryogenesis-abundant pro‐ teins; SAR, systemic acquired resistance; PCD, programmed cell death; LOO•, lipid peroxyl radical; LOOH, lipid hy‐


, which is converted into H2O2 by SOD during the

−

82 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

membrane NADPH oxidase produces O2

**3.9. Endosperm weakening**

droperoxide.

Gibberellins are majorly responsible for the breakdown of dormancy [54]. However, in the later phase of embryogenesis, i.e., during the maturation drying, GA production must be reduced and ABA synthesis is upregulated for the proper maturation and to preserve seed vigor. During the imbibition stage, the endogenous GA1 is released from the viable nondorm‐ ant embryo to its endosperm. It increases the activities of several enzymes, such as amylase, proteases, ribonucleases, and β-glucanase, which induce the hydrolytic cleavage in the aleurone layer [66]. Moreover, hydrolytic enzymes are also involved in the transcription, transportation of metabolites, and PCD. Along with the GA biosynthesis, genes encoding for various functions are either overlapped or attributed toward the germination, and this process is controlled by the GA [66, 67]. Moreover, genes encoding gibberellin 3-oxidase (*GA3ox*) (GA biosynthesis) and the soluble GA receptor (*GA-insensitive dwarf*, *GID*) are vital for the induction of seed germination. During the germination, the *GA3ox2* is involved in the fast phase of GA synthesis by catalyzing the conversion of GA from an inactive to a bioactive form. In the GA synthesis, genes, such as *gibberellin 3-β-dioxygenase 1* (*GA3ox1*) and *gibberellin 20-β-dioxygenase 1* (*GA20ox1*), play as the positive regulators and *GA 2-oxidase*, especially *GA2ox1*, is involved in the negative regulation [67]. The *GA2ox1* gene is engaged with the catabolism of GA. The *Leafy cotyledon 1 (LEC1)* encodes for the katanin p60 subunit, which promotes the cell elongation in the microfibrill. Kroj et al. reported that the *lec2* and *fusca3* (*fus3*) directly influence the expression of the *GA3ox2* by binding on its *2S3* promoter region, particularly on the RY *cis*- element motif. The RY motif also regulates the expression of genes encoding seed storage proteins (*SSP*) such as 2S albumins and 12S globulins. Four genes, such as *abi3* (ABA insensi‐ tive), *lec1*, *lec2* and *fus3*, are responsible for the conditional dormancy of embryos and are regulated by the ABA for proper maturation [68].

Higher expressions of cell wall-remodeling enzymes (CWRE) is associated with the radicle protrusion. Endo-β-mannanase, β-1,3-glucanase, expansin, xyloglucan endo-transglycosylase, pectin methylesterase, polygalacturonase, and galactanase are the notable major enzymes involved in the cell-wall modification [12]. In *Arabidopsis*, the application of GA induced the expression of the *extension-like gene 1* (*epr1*). The *epr1* is involved in the strengthening of micropylar endosperm cell wall to elongate the radicle of seeds [69]. Cell-wall-associated gene expression under the imbibition is preferentially linked with ABA and GAs in the endosperm weakening. During the rehydration, increased oil bodies and protein storage vacuole packed in cells start to mobilize from the endosperm cells to radicle tip. Penfeild et al. reported that lipids and proteins produced in the endosperm following the imbibition are higher than those produced in the embryo [70]. Consequently, the proteolytic activities are also at higher rates. The GA suppresses the activation of DELLA proteins to enhance the process of seed germi‐ nation [66, 67, 71]. The E3 ubiquitin ligase SCF (skp1-cullin-F-box-Rbx) complex was reported to be involved in the degradation of DELLA proteins via the ubiquitin-26S proteasome pathway [72]. Perception of GA and its signal transduction determines various other mecha‐ nisms in the plant along with the seed germination (**Figure 2**).

**Figure 2.** Simplified framework of the dormancy release mediated by GA in concordant with the ROS. Interactions be‐ tween the genes are described in the text.
