**2. Developmental process and final structure of cereals seed**

Biologically, seed is a mature fertilized ovule that consists primarily three parts: the embryo, endosperm, and seed coat (integuments) [3, 4]. The development of seed begins with double fertilization, in which one of the male gamete fertilizes with haploid egg cell to form an embryo and the other male gamete fertilizes the megagametophyte's diploid central cell to form the triploid nuclear endosperm [5]. The event of seed development, which described below can be divided into three phases: a morphogenesis and cell divisions for endosperm development (0–7 Day post anthesis, DPA), embryo development (7–15 DPA), and maturation (14 to 28 DPA), which includes embryo growth at the expense of endosperm, seed desiccation and storage materials accumulation [6].

#### **2.1 Endosperm development**

The nuclear type of endosperm development is the most common in monocot plants, particularly cereals, where initial endosperm nucleus divides repeatedly without cell wall formation, resulting in a characteristic coenocyte-stage endosperm [7, 8]. The morphogenetic event of the early stages of endosperm development was observed in wheat [9] and rice [10, 11]. The first division of the triploid endosperm nucleus, in which the daughter nuclei are separated in the central cell, without cell wall formation in subsequent mitotic divisions, results in a 256 to 512 multinucleate cell (the endosperm coenocyte) [8, 12]. The nuclei enter a 2-day mitotic hiatus, lead to the formation of interzonal phragmoplast, occurs 3 days after pollination. While much information about the regulation of phragmoplast formation and expansion remain unknown, recent evidence suggests that the mitogen-activated protein kinase cascade plays a key role in this process [13]. The development of cellularization in the coenocytic endosperm then begins with the formation of radial microtubule on all nuclear surfaces. Soon after, the microtubules from the adjacent nuclei meet, creating interzones where callosebased wall material is deposited. Further, radial microtubules that encase each nucleus undergo reorganization, anchoring the nuclei to the central cell wall while extending toward the central vacuole in a canopy of microtubules. In cereals, the endosperms become fully cellular during 6 to 8 days after pollination if this process is repeated four to five times [14, 15].

The fully developed cereal endosperm consists of four main cell types: the aleurone layer, transfer cells, starchy endosperm, and cells of the embryo-surrounding region [16]. The former two cells, i.e. Aleurone layer, transfer cells remain alive at the end of cereal seed development, while later two including starchy endosperm, and cells of the embryo-surrounding have undergone programmed cell death (PCD) with characteristic DNA laddering and organelle degradation [17].

The cereal endosperm has attracted attention from researchers because of its economic importance, and much insight has accumulated about the genes underlying the accumulation of storage products such as proteins and starch. Additionally, the endosperm protects the embryo from atmospheric oxygen that eventually leads to the formation of hydroperoxides and cell death [18] and critical cross-talk between abscisic acid (ABA) and gibberellin (GA) regulating seed development, size, dormancy or storage breakdown during germination are also the results of endosperm—embryo interactions [19, 20]. Considerably less is known about the genes that regulate the developmental biology of these cell types, which is the topic of this section. Cell fate specification in cereal endosperm is believed to occur by positional signaling at an early developmental stage [12]. For simplicity, each cell type is described separately below, although cell fate specification occurs

simultaneously with the cellularization process described above. How this integration occurs is unknown, but elucidation of the molecular controls for each of the four cell types should lay the foundation for understanding the genetic specification of the entire endosperm body plan.
