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

with environmental signals (temperature and light) which are required to stimulate GA biosynthesis (Fig4b). This provides an explanation for the negative effects GA on the sucrose-dependent induction of the anthocyanin biosynthetic pathway (193, 194). Loreti et al (195) showed that GA repress the expression of several sucrose-induced genes involved in the anthocyanin synthesis (195). Conversely, the repressive effect was drastically reduced in gai mutant expressing a stabilized DELLA protein, indicating that DELLA s are involved in the Suc-GA interaction (195). Li et al, (2014) showed that sucrose, not glucose, stabilizes the DELLA protein repressor of GA ( RGA) , given that DELLA proteins are stabilized by sucrose and sucrose content increased in plant during the day due to photosynthesis, it will be tempting to speculate that increased DELLA level during the day is positively correlated with increased sucrose level during the day (192). But contrary to this, a high growth rate during the day was observed in a starchless mutant that displays high sucrose levels during the light period (196). This increase in growth during the day when sucrose content is rather high contradict the growth repressive effect expected from the sucrose-GA interaction and suggests that there could be other pathway(s) than GA pathway which sucrose is not responsive that drive the high growth rate observed. Gene set Enrichment Analysis (GSEA) in Arabidopsis, poplar and grapevine dormant buds revealed a very significant enrichment of genes responsive to AKIN10 , one of the catalytic subunits of SnRK1 , among them, were robust bud dormant markers such as histone HISI-3 and DORMANCY1 (197). Also, the SnRK1 regulatory subunit AKINBETAI , whose mRNA levels correlated directly with dark period duration induced buds dormancy (198). SnRK1 activates autophagy, controls senescence, down regulates anabolism, cell division and protein synthesis (52, 197- 199), which are all parameters that characterize dormant buds and were as expected observed in buds entering dormancy. Theses observation further highlight the potency of sugar-SnRK1 interaction mediated dormancy induction. f) Cell cycle and dormancy regulation Eukaryotic cell cycle consists of mainly five phases (G0, G1, S, G2, M), each phase shares a set of unique activities in the division of labour that cumulate in cell reproduction. Mitogenic signals are required for completion of cell cycle in each phase, but most especially during the transitions from G1 to S (DNA synthesis) phase and G2 to M phase; for proper coordination of activities and precise progression of the cycle (145), otherwise the cell cycle will experience defects which often lead to different biological phenomenon such as; different degree of ploidy. Different plant hormones and sugars act in crosstalk during cell cycle to induce dormancy by causing cell arrest in G1 phase and subsequent release during germination. Earlier studies have shown that in plant meristematic cells, sucrose deficiency induces endogenous principal control points (PCPI and PCP2), which block cell cycle at G1 and G2 respectively (200- 203), this cycle blockade or arrest is what that constitute dormancy induction and is reversible during germination. It has been shown to be reversed by sucrose application which switch on the cell cycle process again though with a delay. The molecular mechanisms regulating the action of PCP1 and PCP2 in this blockade have not yet been elucidated. However, as stated earlier in crosstalk between sugar signaling pathways, it can be speculated that in yam tuber during senescence which is characterized by sucrose deficiency as result of cut in sucrose supply from non- photosynthesizing senescing vine, low energy sugar signaling pathways ( SnRK1 ) and C/S1 bZIP ) which function in crosstalk with auxin biosynthetic pathway to induce growth arrest in response to low energy condition in plants might have elicited the action of PCP1 and PCP2 to effect the cell arrest. This however need to be properly investigated through an organized study. During this period of temporary growth arrest, it has been reported that numerous phosphorylation and dephosphorylation processes occur, both in metabolic pathways and in regulation of the cell cycle. For instance, at the beginning of regeneration, in the presence of sucrose, meristematic cells are strongly sensitive to inhibitors of protein kinases [Cylin- dependent Kinases ( CDK )] and protein phosphatases 1 or 2A ( PP1/PP2A ), which further results in prolonged blockade of cell cycle (dormancy) (200, 201, 204). It has been demonstrated that this sensitivity decreases with time, and consequently allow the cells to resume regenerative activities through the action of [Cylin- dependent Kinases ( CDK )], however, the mechanism that regulate the decrease in sensitivity and reduction in the effects of PCP1, PCP2 and possibly SnRK1 and C/S1 bZIP on the blockade in order to allow the action of Cylin-dependent Kinases ( CDK ) pull through is not yet understood and is vital missing link that will be pivotal in dormancy manipulation through genetic engineering. During G1 phase, auxin was reported to induce expression of cylin D gene; cyD3-1 and cylin-dependent kinase gene CDKA-1, and to play important roles in CDKA/CYCD complex assembling (37). Meanwhile, KRP1 and KRP2 transcripts, encoding two of the CDK inhibitors were reported to be down-regulated after auxin treatment (152, 205-207), thereby sustaining the phosphorylated CDKA/CYCD complex. It is this activation of CDKA/CYCD complex that is believed to stimulate the phosphorylation of the transcriptional repressor retinoblastoma-related ( RBR ) protein, and release its target; Adenovirus E2 promoter-binding factor A/B (E2FA/B) and dimerization partner A ( DPA ) Physiological and Molecular basis of Dormancy in Yam Tuber: A Way Forward towards Genetic Manipulation of Dormancy in Yam Tubers © 2022 Global Journals 1 Year 2022 66 Global Journal of Science Frontier Research Volume XXII Issue ersion I VII ( G )