**2.2 Starchy endosperm**

Starchy endosperms, which accumulate starch and storage proteins, encoded by transcripts that are expressed differentially in these cells, make up the largest body of cell in the endosperm [21]. There are two types of starchy endosperm present in the cereal crop. The first, and most important, is the inner cells of cell files that remain after endosperm cellularization is complete. The second source of starchy endosperm cells is the inner daughter cells of aleurone cells that divide periclinally. These cells redifferentiate to become starchy endosperm cells and likely are the source of the so-called subaleurone cells found adjacent to the aleurone layer in the starchy endosperm in all cereals. Several collections of mutants such as *dek* (defective kernel) [22], and *Dee-D1* (DEFECTIVE ENDOSPERM-D1) [23], physically located on the long arm of chromosome 1D involved in the genetic control of endosperm development in wheat. The absence of *Dee-D1* in the genome of hexaploid wheat leads to a decrease in the number of grains and thousand grain weights. Similarly, DWARF AND RUNTISH SPIKELET1 (DRUS1) and DRUS2 [24] and EMBRYONIC FLOWER2a (OsEMF2a), a zinc-finger containing component of polycomb repressive complex 2 impaired endosperm development in rice [25].

### **2.3 Aleurone**

The aleurone layer covers the perimeter of the endosperm with the exception of the transfer cell region. Wheat have one layer of aleurone cells, while rice has one to several layers, functions in seed germination by mobilizing starch and storage protein reserves in the starchy endosperm through the production of hydrolases (α-amylase), glucanases, and proteinases after hormone (gibberellic acid) stimulation from the embryo [26]. In the mature grain of cereals, the aleurone layer consists of an estimated 250,000 aleurone cells derived by an estimated 17 rounds of anticlinal divisions. Toward the end of seed maturation, a specialized developmental program confers desiccation tolerance to the aleurone cells, allowing them to survive the maturation process. *Several mutants such as, Crinkly4* (*Cr4*), a receptor like kinase protein [27], *Supernumerary aleurone layer1* (*Sal1*), homolog of the human *Charged vesicular body protein/Chromatin modulating protein1* gene [28], Defective seed5 (des5), bZIP zinc finger transcription factor RISBZ1 and the DOF zinc finger transcription factor RPBF regulate both storage protein biosynthesis and the differentiation of the aleurone [29].
