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

i. Role of ABA biosynthetic genes in inducing dormancy ABA is synthesized from epoxycarotenoids cleavage (zeaxanthin, violaxanthin, and neoxanthin) which is often initiated in the chloroplast and proceed to cytoplasm, its biosynthesis, signaling and degradation genes have been reported to play important roles in dormancy induction, maintenance and release (42). There are three groups of genes that have reported to be involved in the stepwise ABA biosynthesis which include; Zeaxanthin Epoxidation (Zep), Oxidative Cleavage of 9-Cis-Epoxxycarotenoids (Nced) and Abscic Aldehyde Oxidation (Aao )(88). The first step of ABA biosynthesis is the conversion of zeaxanthin to all- trans- violaxanthin , catalyzed by the zeaxanthin epoxidase ( ZEP )(89). Antheraxanthin is formed as an intermediate product of the reaction. Then, the conversion of all- trans-volaxanthin to 9-cis-violaxanthin or 9-cis- neoxanthin mediated by yet to be identified enzymes(90). The oxidative cleavage of 9-cis- violaxanthin and or/ 9-cis-neoxanthin is catalyzed by 9- cis-epoxy carotenoid ( NECD ) which leads to the formation of a C 15 product, xanthoxin and a C 25 metabolite in a reaction which has been described as rate limiting step and NCED is the key enzyme in the biosynthetic pathway(89). The NCED family of genes comprises of NCED 1-9. The next ABA biosynthetic step takes place in cystol in which Xanthoxin formed earlier is converted to ABA through two enzymatic reactions. In the first reaction, xanthoxin is converted to abscisic acid aldehyde by an enzyme belonging to short-chain dehydrogenase/reductase (SDR) family, the gene responsible for these reactions has been identified as ABSCISIC ACID INSENSITIVE (ABI) which comprises of ABI2, AB13, ABI4 and ABI5 . These are the signaling genes that are responsible for ABA dormancy induction and maintenance capability, however, among them, AB13 is outstanding in this function as expression of its transcripts has been reported to be highest in dormant seed and decrease after germination (88). The second Xanthoxin conversion reaction and final step of ABA biosynthesis is the oxidation of abscisic aldehyde to ABA, catalyzed by an abscisic aldehyde oxidase ( AAO ) (91). ABA biosynthetic pathway is important because it reveals different genes involved the ABA biosynthesis and provides various points of the biosynthetic pathway that can be manipulated to effectively reduce the effects of ABA on dormancy maintenance through genetic engineering. For instance, the ZEP /ABA were first to be identified in Arabidopsis thaliana and Nicotiana plimbaginifolia (42, 89, 92). 1 Year 2022 57 © 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