In the growth of cereal seeds
In the growth of cereal seeds there are two phases, development and germination, separated by dormancy. The developmental phase contains the three following stages: 1. early development including double fertilization which results in the formation of the diploid embryo (by fusion of the egg cell with one sperm cell nucleus) and the triploid endosperm (by fusion of two polar nuclei in the central cell with second sperm cell nucleus); 2. differentiation, in which different types of seed cells (embryo-surrounding cells, transfer cells, starchy endosperm and aleurone layer) are formed and storage reserves are accumulated in the endosperm; 3. maturation, which comprises desiccation and seed dormancy (Domínguez and Cejudo, 2014, Sabelli and Larkins, 2009).
The germination phase includes processes from uptake of water by dry seeds to the formation of the autotrophic seedling. There are four stages of the germinative phase, as follows: 1. uptake of water by seed (imbibition); 2. activation of enzymatic systems; 3. metabolism of storage reserves and their transport from endosperm to embryo; 4. the emergence of the radicle and growth of the seedling (Miransari and Smith, 2014).
Dormancy is a result of evolutionary AR 231453 of cereals to the difficult weather conditions as revealed by the inhibition of germination. Properties of seed dormancy are inherited but the length and depth depend largely on environmental factors during maturation, harvesting and storage of seeds (Gao and Ayele, 2014). For humans, seed dormancy is a potential advantage because it prevents seeds from preharvest sprouting (PHS) that leads to the premature degradation of the storage materials and reduces their quality and yield (Humphreys and Noll, 2002). However, cereal seeds in deep dormancy are characterized by low agronomic value due to incomplete biochemical and physiological maturity.
An integral feature of cereal seed growth (development and germination) is the programed cell death of its individual tissues. Maternal tissues, such as the nucellus and pericarp, undergo progressive degeneration via PCD during the early stages of seed development. This process allows mobilization of cellular contents to nourish new tissues, such as the embryo and the endosperm (Domínguez and Cejudo, 2014). Next, during seed maturation, the endosperm undergoes PCD but the contents of its cells are not mobilized until germination. Therefore, the only tissues that are alive when seed development is completed are those of the embryonic axis, scutellum and aleurone layer. During seed germination, the scutellum and aleurone layer are responsible for the production of the hydrolytic enzymes that allow mobilization of the storage materials of the starchy endosperm, which serve to support early seedling growth. After this process is completed, cells in the scutellum and aleurone layer undergo PCD and their contents are used to support the growth of the germinated embryo (Domínguez and Cejudo, 2014, Young and Gallie, 2000) (Fig. 1B).
The role of cysteine proteases in development and germination of cereal seeds Plant proteolysis is a complex process that involves many metabolic networks, different subcellular compartments, and various types of peptidases, mainly cysteine, serine, aspartic and metallo-proteases (Van der Hoorn, 2008). Among the proteases encoded by plants, approximately 140 are cysteine proteases that belong to 15 families distributed in five clans, as classified in the MEROPS peptidase database (Rawlings et al., 2012). Many research findings indicate that cysteine proteases may be the most abundant group of proteases responsible for degradation and mobilization of storage proteins (Grudkowska and Zagdańska, 2004, Martínez et al., 2009). Additionally, precursors of reserve proteins are processed into mature proteins via limited proteolysis in the storage tissue of developing seeds. Moreover, during seed growth, with the participation of cysteine proteases, all of the seed tissues undergo progressive degradation via PCD (Fig. 1B).