**2.4 Transfer cells**

Transfer cells develop in the basal endosperm over the main vascular tissue of the maternal plant, where they facilitate solute (mainly of amino acids, sucrose, and monosaccharides), transfer across the plasmalemma between the symplastic (maternal plant) and apoplastic (endosperm) compartments [30]. However, sucrose is not delivered in this form to transfer cell; instead, it is converted into monosaccharide glucose and fructose through the major activity of cell-wall invertase, offering a mechanism for controlling cell division and even cell differentiation in developing kernels [31].

In cereals, the *miniature1 (mn1)* mutant and *GRAIN INCOMPLETE FILLING1* (*GIF1*) which encodes a cell-wall invertase, exclusively expressed in transfer cells, have significant smaller grains, implying that invertase contributes to create a sucrose concentration gradient in the apoplastic gap between the pedicel and the endosperm by hydrolyzing sucrose to glucose and fructose [32, 33]. Furthermore, in developing *mn1* kernels, the abundance of auxin and transcript of YUCCA genes was drastically reduced, implying that sugar level influence auxin level in seed, which in turn regulates specific aspects of seed developments [34].

Several groups of transcripts, for instance, OsPR602 and OsPR9a in rice and Endosperm 1 (*END1*) in barley have been shown to be expressed preferentially in endosperm transfer cells during the early stages of grain filling [35]. Further, an orthologues gene from *Triticum durum* (TdPR60) and *Triticum aestivum* (TaPR60), a small cysteine-rich protein with a hydrophobic signal peptide, predicted to interact with several protein, which are involved in the regulation of regulation of secretion and degradation of signal peptides in other organisms [36].

#### **2.5 The embryo-surrounding region**

The embryo-surrounding region (ESR) lines the cavity of the endosperm in which the embryo develops and has been studied most extensively in maize. The exact role of the ESR is unknown, but possible functions include a role in embryo nutrition, the establishment of a physical barrier between the embryo and the endosperm during seed development, and providing a zone for communication between the embryo and the endosperm. The ESR development is under the control of CLAVATA3, a peptide hormone with the conserved domain composed on 12 to 14 amino acids, regulates embryo and endosperm development, cotyledon establishment, and pollen wall formation in Arabidopsis [37], while root and stem development in wheat plants [38].

#### **2.6 Seed coat development**

The seed coat (also known as testa) is made up of two structures covering the nucellus [39], while the single integuments ovules can be found in members of certain families. The seed coat provides a mechanical shield protecting the embryo and the endosperm from the environment, but it also regulates phloem unloading of assimilates in growing seeds [40], fluid and gas exchanges with the environment, and seed dormancy and germination [41]. Generally, seed coat development and maturation precede that of filial tissues. In cereals, after an initial phase of cell division during the first two days after flowering (DAF), pericarp differentiation involves cell elongation along the longitudinal axis between 3 and 10 DAF coupled to PCD, and it coincides with the cellularization of the endosperm [42]. PCD in the pericarp may contribute to redistribution of nutrients, relaxation of physical constraints of the maternal tissue to allow inner growth of the filial tissue, and the re-activation, together with PCD in the nucellus and the nucellar projections, of post-phloem transport functions to allow passage of solutes [42]. Crosstalk among embryo, endosperm, and seed coat appears to be complex, but gene networks that coordinate development of these three seed compartments are being elucidated [41, 43].

### **3. Genetic regulation of seed development for improved yield**

Seed yield is a quantitative trait that is influenced by the genetics and environment. It is usually determined by plant height, number of primary and secondary

#### *Molecular and Transcriptional Regulation of Seed Development in Cereals: Present Status… DOI: http://dx.doi.org/10.5772/intechopen.99318*

branches, plant density, date of flowering, number of panicle per plant, number of seed per panicle, seed size including seed length and seed width, and finally seed weight [44, 45]. The last two traits, i.e. seed number and weight, were found to be trade-off [46], but recent evidence from studies in wheat suggests that increasing one yield component without reducing the other is possible [47]. The grain number has maintained higher phenotypic plasticity throughout domestication events when compared with grain weight, which enables crop to effectively respond to resource availability during early reproductive stages [46]. The critical periods for determination of grain number and weight are also generally considered separated by the developmental stage of anthesis (flowering), although Ugarte et al. [48] found that grain weight was affected by pre-anthesis environmental conditions in other cereals including wheat. The genotype × environment interaction for grain yield is likely strong in winter wheat [49] and rice [50].

To explore candidate genes underlying yield related traits, GWAS were conducted to identify underlying loci for each phenotype. Association mapping has been used to successfully discover significant marker–trait associations in cereal crops including rice [51–54] and wheat [55–58]. A large number of wellcharacterized QTLs such as GW2, GIF1, qSW5, GS3 and qGL7 in rice [59–63] and more than 40 QTL including TaGW2 [64–66] associated with kernel morphological traits such as kernel length, kernel width, kernel thickness, kernel length/width ratio, kernel length/thickness ratio, kernel width/thickness ratio, flag leaf width, length and area have been recently identified and mapped in wheat [67–70]. A variety of QTLs regulating seed size have been identified in other crop species, but they have yet to be functionally characterized [47, 71]. The additional genetic approaches on key agronomic traits for improved yield are presented in **Table 1**.


#### **Table 1.**

*Genetic approaches for improved seed yield in cereal crops.*
