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

Fig. 4: Dormancy regulation and ABA biosynthesis through the carotenoid pathway starting from β -carotene in the plastid and ending with Abscisic aldehyde conversion to ABA in the cytoplasm, indicating the major genes and enzymes responsible for each conversion reaction. The arrows with question marks indicate the reactions which conversion factor are not yet identified Source: Ali et al, (2021) Their ABA deficient mutants with (aba1/aba2) exhibited impaired oxidation of zeaxanthin into antheraxanthin and violaxanthin which is the initial step of ABA biosynthesis (Fig3). Similarly, n rice, viviparous mutant genotype was identified to exhibit viviparous germination as a result of defect in the oxidation of zeaxanthin during ABA synthesis (93). In maize, different auxotrophic mutants ( vp2, vp5, vp7 and vp9 ) have been identified through genetic screening and they exhibit defects in zeaxanthin epoxidase activity and block the early steps of carotenoid biosynthesis (42). Whereas, overexpression of maize VP1 in wheat induced increased duration of seed dormancy and prevented pre-harvest sprouting (92). All these provided evidence that the oxidation of zeaxanthin is an important, and a conservative phase in the ABA synthesis in plants. Another important gene and stage of ABA biosynthesis is the NCEDs and the conversion of all-trans-violaxanthin to 9-cis-violaxanthin or 9-cis-neoxanthin. NCED9 was first cloned from maize mutant VP14, the VP14 mutant which exhibited defect in the oxidation of all-trans- violaxanthin to 9-cis-violaxanthin or 9-cis-neoxanthin and exhibited reduced ABA content in matured seed and consequently has reduced dormancy duration (42). In Arabidopsis NCED2, NCED3, NCED5, NCED6, and NCED9 have been identified as the homologs of VP14 participating in the rate-limiting step of ABA biosynthesis (94). Also, the PvNCED1. LeNCED1 and BdNCED1 were identified in bean, tomato, and Brachypodium distachyon , respectively and they showed important roles in ABA biosynthesis and seed development and dormancy induction (95). These studies have provided evidence that the oxidative cleavage of xanthophylls is the main step during ABA biosynthesis regulation of seed development and dormancy. Mutants facca and sittens , which are defective in abscisic aldehyde oxidatively conversion into ABA were first identified in tomato, and later abscisic aldehyde oxidase3 ( AAO3 ) was identified in Arabidopsis which functions in the last steps of ABA biosynthesis in seed and its expression was also observed in the embryo vascular tissues during mid and late maturation phases (89, 91). Figure 4 © 2022 Global Journals 1 Year 2022 58 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