Global Journal of Science Frontier Research, G: Bio-Tech & Genetics, Volume 22 Issue 2

alata (35). (5) Central amyliferous parenchyma is a layer of vascular bundles of cells growing in size outwards, but often blocked by individual cell blocks in procambium layer above it. It contains calcium oxalate and tannin cells and also play roles in osmotic pressure balance (33), and modifies the respiratory quotient and detoxify the system. It is speculated to be linked with a high metabolic activity, which is indicated by the multiplication of raphides in the growing zones (36). Figure 3 above shows the schematic diagram of anatomic structure of dormant tuber, depicting the anatomical mechanism of dormancy induction in yam III. R egulation of D ormancy in Y am T ubers Yam tubers enter into dormancy enduring tuber bulking and vine senescing. The longevity of dormancy depends on levels of phytohormones, the crosstalk between them, intricate genetic regulatory networks, as well as environmental cues (37). For decades, some molecular and physiological surveys have revealed that different hormonal pathways regulate different aspect of tuber development (38-41). Though, these studies were conducted on other crops, however, an evolutionary survey has revealed strong similarities between Arabidopsis , tomato and potato in hormonal dynamism, crosstalk, signaling pathways and the networks regulating seed and tuber dormancy, indicating conserved evolutionary processes across a wide spectrum of plants (38). Since potato and yam tubers share very close physiological and morphogenetical communalities, it is believed that the molecular and physiological machineries regulating dormancy in the two crops will be similar, some specie specific different notwithstanding. Therefore, due to lack of information on molecular and physiological mechanisms regulating yam tuber dormancy, information on potato tuber will be adapted in discussing yam tuber dormancy here. It is now clear that abscisic acid (ABA) gibberellins (GAs), auxins, and to lesser extent cytokinins (CKs) and ethylene (ET), are the main phytohormones that play key roles in molecular and physiological regulation of dormancy in both conventional seeds, tubers, rhizomes and bulbs (37, 42-46) While crosstalk between the main stream regulatory hormones signal networks and some other phytohormones like strigolactones, brassino- steriods jasmonic acid salycylic acid also play some roles (47). There is also sugar metabolism and the signaling crosstalk with phytohormones in dormancy regulation in several crops(48-50). However, in this review due to want of space, discussion will be limited to three main phytohormones (ABA, Auxins, GAs) and Sugars metabolism and the crosstalk between sugar signaling pathways and hormones regulatory networks. a) Abscisic acid mediated regulation of dormancy In conventional seeds, at maturation the embryo is kept in a quiescent state in which all nutrients are stored without any mobilization and no cell division or elongation takes place. Hence, germination-promoting genes are not activated, this is because the radicle does not penetrate the testa and endosperm, where it can access sugar for energy and nutrients required to initiate growth processes (51). Similarly, in non-conventional seed like yam tuber, similar phenomenon also takes place, for instance in mature dormant tuber the anatomical structure presented in (fig 2) above revealed that at maturity; the procambium region which is responsible for growth, morphogenesis and tissues conductivity is separated from central amyliferous parenchyma layer (the food and nutrient warehouse of tuber) by a layer of cell blocks and as long as this block is maintained, dormancy is maintained and germination is blocked. Because for the processes of germination to be initiated the procambium cells must gain access to the amyliferous parenchyma layer to transport nutrients and sugar that will provide the required energy to initiate the processes at the upper region. Therefore, procambium, cell blocks and central amyliferous parenchyma can be likened to be radicle, tasta and endosperm of tuber seed. It has been demonstrated that the chromatin structure determines the expression of genes and thereby regulates several developmental processes (51). Many genes associated with chromatin remodeling have been reported to regulate also seed dormancy and germination (37, 52-54). Evidence indicates that abscisic acid (ABA) is involve in chromatin remodeling (55). For example, the histone methyltransferase gene KYP/SUVH4 is repressed by ABA(53), while histone acetyltransferase gene HvGNAT/MYST is induced by ABA (56), and as expected epigenetic regulating genes HUB1 and RDO2 are up- regulated during seed dormancy induction. This is because during dormancy the cell is not undergoing cell division and the chromosomes are tightly packed by histone proteins, therefore, activation of histone proteins will likely be repressed by any factor that positively influence dormancy induction and maintenance such as ABA and other phytohormones. ABA is derived from epoxycarotenoid cleavage and is one of the most important plant hormones, with most versatile roles in various physiological functions of plants such as; transpiration, dormancy induction, maintenance and germination and improved resistance to extreme environmental stress during plant development (57-59). Maternal ABA has been reported to play a key role in embryo morphogenesis and desiccation, stomatal movement, synthesis of stress proteins and metabolites and seed maturation in tobacco and Arabidopsis (41, 60, 61). However, ABA is also de novo synthesized in embryo and testa and accumulates during embryo development, seed 1 Year 2022 55 © 2022 Global Journals Global Journal of Science Frontier Research Volume XXII Issue ersion I VII ( G ) Physiological and Molecular basis of Dormancy in Yam Tuber: A Way Forward towards Genetic Manipulation of Dormancy in Yam Tubers