**4. Role of phytohormones during seed filling**

Seed development is divided into two main phases: the cellular phase and maturation [128]. The cellular phase includes all the processes involved in the formation and development of the different parts of a seed. In this stage, storage reserves for the embryo are synthesized and seed filling takes place. Phytohormones regulate signaling between the embryo and the endosperm. Most studies on seed filling and development have used *Arabidopsis* and maize as model plants for dicotyledons and monocotyledons. Although monocotyledonous and dicotyledonous plants share common seed characteristics, seed filling and developmental processes differ significantly between the two groups. In developing seeds, precise coordination is required to organize cell distribution in tissues and organs, and to control seed filling. The cells in the seeds can control all these activities by producing and sensing signals. The synthesis and regulation of phytohormones in the process of seed filling is essential for seed development [129, 130]. Seed filling is a highly coordinated and complicated process involving hormonal control and constant exchange of signals between different parts of the embryo [128].

Many studies have shown that hormone levels change during seed development and filling. Phytohormones, such as ABA, GA, cytokinins, Indole-3-acetic acid (IAA), and ethylene regulate seed filling processes (**Figure 5**) [132, 133]. Phytohormone gradients are synthesized in distinct seed sections, and their ratio controls signals that activate or inhibit specific seed filling processes. Among the hormones, ABA plays a central role as it accumulates at high levels from fertilization to seed maturation. Therefore, ABA functions as a signaling molecule and is important for seed filling, seed growth, dormancy, and plant stress responses [134]. Seed filling rate was positively associated with the concentration of ABA, and higher concentration of ABA resulted in higher seed filling rate [135]. In maize, the concentrations of ABA were associated with seed filling rate and kernel weight [136, 137]. Seed filling in barley, wheat, rice, and sorghum is closely related to senescence and the senescence-related hormone ABA, which affects nutrient mobilization and grain filling time [138] and is involved in the expression of senescence-related genes in barley [139]. High ABA levels increase remobilization of previously stored carbon in grains and accelerate grain filling rate [140] and have significant effects on seed filling in upper and lower grains [141]. ABA also inhibits cell cycle while accelerating seed maturation by upregulating inhibitors of cyclin-dependent kinases, which are important regulators of the cell cycle [142, 143]. While ABA has a positive effect on stomatal activity, seed dormancy, and plant response to abiotic and biotic stresses, it has a negative effect on seed germination [144]. Other plant hormones, such as gibberellins, ethylene, cytokinins,

#### **Figure 5.**

*Schematic representation of phytohormone accumulation during seed development. (A), represents the stages from late seed development to seed maturity of a dicot plant. (B) Shows the changes in specific phytohormone levels from top to bottom. Longer bars indicate higher levels. Three types of endosperm are formed during maturation: unicellular stratified endosperm, micropylar endosperm, and chalazal endosperm. High concentrations of abscisic acid, present at all stages of seed development, are thought to play a key role in seed filling. Gibberellins are synthesized in the differentiation stage of the embryo to promote cell growth and expansion, and in the late maturation stage to activate proteolytic enzymes. Accumulation of abcisic acid inhibits all processes induced by gibberellins. The accumulation pattern of cytokinins is opposite to that of abscisic acid. Cytokinins play an important role in cell division and their levels gradually decrease during the cell enlargement phase. Ethylene production increases during the early stage of seed development. Auxin controls grain filling by regulating invertase activity. An increase in auxin levels improves sink capacity and nutrient uptake [131].*

brassinosteroids, and their antagonistic interactions with ABA could improve seed germination. ABA can stimulate sucrose storage in the seed coat and accelerate sucrose transport to the cotyledons during seed filling [145]. Gibberellins are also involved in cell differentiation and seed filling. Gibberellin concentration in seeds was not significantly related to seed filling rate or seed weight [146], but GA content had a negative effect on seed filling rate in rice seeds [140]. These studies showed that ABA and GA have antagonistic effects during seed filling [147]. The amount of bioactive GA decreased at the stages when ABA peaked with inactivation reactions to ensure normal seed filling and growth [148].

Cytokinins are involved in cell division, chloroplast formation, senescence, and stress tolerance in plants [149]. Cytokinins also play an important role in seed filling by inducing rapid cell division of endosperm cells [131]. In addition, zeatin (Z) and zeatin riboside (ZR) are biologically important cytokinins in higher plants [150]. Zeatin and ZR contents increase fertilization, kernel set, and endosperm growth in cereals [151]. High Z and ZR contents are necessary for seed filling, endosperm development, and cell division in wheat [151]. Higher Z and ZR contents in seeds can improve seed filling rate in the early and middle stages of seed filling and are associated with seed filling rate in rice and maize [152, 153]. Zeatin and ZR increase simultaneously with the peak of endosperm mitotic activity during seed filling [154]. Exogenous application of GA in maize improved the degree of grain filling by

#### *Seed Filling DOI: http://dx.doi.org/10.5772/intechopen.106843*

increasing the levels of auxin, GA, Z and ABA in the grains [155]. It was also found that auxin, GA and Z content in grains was positively associated with grain mass and filling degree of grains. Cytokinin oxidase decreases cytokinins content in the later stages of seed development [58]. Cytokinin content is related to flower development, grain filling and endosperm growth in rice [152]. Grain seeds also have high cytokinin contents during endosperm development, and cytokinins promote cell division in the early stages of seed filling [152]. In addition, cytokinins and GA have antagonistic effects on various processes of seed development [156]. The levels of Z, ZR, ABA, and IAA in maize were positively correlated with seed filling rate and negatively correlated with the GA levels [157]. In wheat, the levels of Z, ZR, ABA, and IAA were positively correlated with seed filling rate and seed mass, but the ethylene content was negatively correlated [158, 159]. In maize, ABA, Z, and ZR contents were also positively related to seed mass and seed filling rate, but GA content was not. Seed filling rate was dramatically increased when ABA and Z content were higher and GA content was lower [153]. ABA, IAA and ZR contents in maize seeds increased dramatically during the early stages of seed filling and decreased gradually until maturity [160]. Similarly, Z and ZR contents of maize gradually increased during the early stages of seed filling, while the GA content decreased [153]. Moreover, the fluctuations in ZR and IAA contents were similar, they briefly increased in the early stages of grain filling and then decreased in the kernels [155]. The contents of IAA and Z + ZR affect the seed filling rate of maize and are normally located in the endosperm, as they are required for cell division [161]. IAA has been proposed as a correlative signal from seeds that regulates the development of other organs [162]. In maize seeds, high IAA and low ethylene content were significantly associated with grain filling rate [129] and high IAA content in seeds increased cytokinins content [163]. These observations were confirmed by Ahmad [164] who reported that IAA and cytokinin content play an important role in grain filling of maize at early stages by regulating endosperm cell division and thereby increasing seed filling rate. In soybean, IAA concentration and seed filling rate were independent [145]. Seed filling in maize appears to be dependent on IAA synthesis and cell wall invertase activity [128]. The absence of endogenous auxin in the embryo could be lethal [128], indicating the critical functions of the phytohormones in seed development and germination. Invertase activity, together with auxin transport, is important in regulating pathways of carbon cleavage during early development. Sugar signaling is thought to increase phloem transport and sugar import into endosperm cells via invertase activity [165]. Ethylene is also involved in cell division and grain filling [112]. Higher ethylene concentration leads to lower cell division, grain filling, and starch concentration, and higher ethylene concentration leads to higher soluble sugar content in growing rice endosperm [166]. Since cytokinin is known to regulate cell number and cell division activity of rice endosperm, the deleterious effects of ethylene on grain filling and cell division could be mediated by its antagonistic effect with cytokinin [166]. Apart from rice, there are studies reporting that the effect of ethylene is antagonistic to cytokinin [167], because ethylene production in plant tissues promotes cytokinin inactivation [168]. The ratio of ABA to ethylene regulates grain filling rate in wheat [112].

These studies show the importance of phytohormones during seed development. Therefore, seed filling is determined by the content and interactions of various plant hormones that regulate different metabolic processes related to the synthesis and accumulation of seed reserves [169].
