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

maturation, and facilitates late seed maturation processes, synthesis of storage proteins to prevent seed abortion, induce primary dormancy and as well as allows successful germination of the successive seedling (62). Kanno et al (63) demonstrated that ABA synthesized in both maternal and zygotic tissues during seed development, and maternal ABA can be translocated to the embryos and induced seed dormancy. ABA deficient mutants of maize ( Zee Mays ), Arabidopsis and tomato ( Solanum lycopersicum ), rice ( Oryza sativa ) and Nicotiana tobacco lost their dormancy potential and resulted in precocious seed germination and viviparity (64-67). Liu, et al (66) further demonstrated that exogenous application of ABA in three rice cultivars positively correlated with their seed dormancy. Similarly, results of analysis of endogenous ABA content in vegetive reproductive organs have revealed that ABA plays key role in bulbs, root and tuber dormancy induction and maintenance (37, 44, 68). For instance, combined analysis of transcriptome and targeted metabolome has revealed that in lily bulbs, AB13 and AB15 which are both necessary precursors for ABA induced AtWRKY2 expression which reduced dormancy duration, while AtWRKY2 knockout mutant bulbs exhibited increased dormancy duration under ABA high content (69). The AtWRKY2 expression induction by AB13 and AB15 which lead to bulbs dormancy duration reduction may be as a result of rate limiting feedback mechanism of these ABA precursors that might negatively regulated some of signal pathways which were corrected by AtWRKY2 knockout and exogenous ABA treatment. ABA has also been implicated in dormancy induction and maintenance in barley seed (70). Analysis of ABA deficient or insensitive mutants of various barley species that exhibit short dormancy duration or pre-harvest sprouting has provided strong evidence that ABA is involve in dormancy initiation during barley seed development (71, 72). The growth inhibitory activity of ABA has also been reported in standard ABA bioassays of crops such as the Avena cepa, wheat coleoptile and lettuce hypocotyl (73, 74). Other studies have shown that NAC family is involved in regulating multiple hormones signaling pathways some of which negatively influence ABA dormancy induction. It has been reported that GhNAC83 affects the dormancy of gladiolus bulbs by negatively regulating ABA signal transduction and cytokinin biosynthesis (75). Also, Kim et al, (76) reported that ABA controls dormancy and bulb formation in lily plants, whereas, fluridone (ABA inhibitor) prevents dormancy induction when both of them were separately applied exogenously. Similarly, it has been demonstrated that ABA controls dormancy induction in onion bulbs, but not involve in onion bulb formation as decreased level of ABA by fluridone application did not affect the formation bulbs scales (46), but reduced dormancy duration. Furthermore, Alamar, et al, (44) concluded that ABA and its metabolites (phasic acid) induced and prolonged onion bulb dormancy under ethylene supplementation. In potato tuber, ABA has been reported to be involved in regulation of dormancy induction and wound healing (77, 78). ABA content was observed to be highest immediately after harvest when meristem dormancy is deepest, and gradually fall during storage as dormancy weakens (77). Similarly, it has been demonstrated that ABA play key role in mediating potato dormancy which has been well characterized in meristematic tissue and it is shown that ABA accumulation reaches maximum during tuber and dormancy induction and declining of ABA content was shown to be the determinant factor in potato tuber dormancy breaking (79-81). Recently, Tosetti et al,(43) also demonstrated that parenchymatic tissue ABA content reached maximum at onset of vine senescing which coincides with the onset of dormancy induction, but was rapidly decreased by continuous ethylene treatment which led to earlier dormancy breaking. Implying that ethylene antagonistically prevents the ABA dormancy induction in potato tuber. Most of the studies on yam tuber hormonal control of dormancy have concentrated mainly on abscisic acid (ABA) related analogues compounds such as phenolic growth inhibitors and in particular batatasins. Batatasins belong to the phenolic class stillbenoids. They occur naturally in many plant species exhibiting dormancy. In Dioscorea, they have been isolated in D. alata, D. cayenensis, and D. opposite (73, 82). They are more concentrated in the peel, (the region closest to the meristematic layer where sprouts originate) than in the pulp. By isolating these compounds over time, it has been revealed that the concentration of batatasins increased from 150 days after planting, attaining a maximum at tuber maturity, when tubers are declared dormant (83), then declined gradually until sprouting (73, 82, 83). Exogenous application of batatasins I, II, III, IV, and V have inhibited the growth of shoot and buds in potato and other plants, delayed the appearance of shoot and buds in some yam spp., for example , D. alata, D. cayenensis, and D. esculenta by about 15 days (83-85). In D.alata , the true dormant period (endodormancy) has been estimated to be about 220 days which begins from the onset of tuber induction to appearance of tuber germination meristems (TGM) and is not affected by PGRs. But in D. alata specie, seed tuber duration of this long endodormancy period has been drastically shortened by fluridone (an ABA biosynthesis inhibitor) to extent that still growing tuber started showing anatomical signs of germination (86). Also, Hamadina and Craufurd (87) reported that the level of phenolic compounds increased in D. rotundata is higher in developing tubers than at tuber maturity/ vine senescence, implying that this compound is also involved in the tuber development. © 2022 Global Journals 1 Year 2022 56 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