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

constitutively expressed in developing seeds (117), suggesting that GAs may be playing a role in pea seed development. The oxidization of GA 12 -aldehyde to GA 12 which is primary precursor of bioactive GAs in plant is catalyzed by dioxogenase (GA 7-oxidase, (GA7ox) (105). The conversion of GA 53/ GA 12 to GA 1 /GA 4 is executed through two parallel routes: early 13- hydroxylation and non-13-hyroxylation pathways. In rice, the early 13-hyroxylation pathway involves the activity of 13-hydroxylases which are coded by CYP714b1 and CYP714B2 genes (118), and it has been suggested that the early step could be rate limiting feedback mechanism that checks the growth inducing action of GAs as its over expression resulted GA-deactivation consequently to delay in seed germination and reduced growth in shoot. Transformations of GA 1 /GA 4 to GA 20 /GA 9 is catalyzed by two soluble 2-oxoglutarate-dependent dioxygenases (2ODDS) known as GA20-oxidase (GA20ox) and GA 3-oxidase (GA3ox) (119, 120). Expression of these genes have resulted to reduced dormancy duration phenotype and were also observed to be constitutively expressed in the regions of active growth such as shoot, root and flower initiating organs in several crops (105, 121-124). GA signaling is known to be regulated by a group of repressors called DELLA proteins, including repressor of ga1-3 ( RGA ), GA- INSENSITIVE (GA1) and repressor of ga1-3-like1/2/3 ( RGL 1/2/3) (106). Among these, RGL2 seems to be the major DELLA factor involved in repression of GAs activity and seed germination. Studies have shown that RGL 2 stimulates ABA biosynthesis by inducing the expression of XERICO and ABI5 , whereas, ABA enhances RGL 2 expression (125, 126), indicating that RGL 2 mediates the interaction between GA and ABA in dormancy regulation. Fig. 5 (a): GA biosynthetic pathway indicating reactions in the cytosol, plastid and endoplasmic reticulum in dormancy breaking seed. (b) GA biosynthesis regulation indicating the inactivation action of DELLA proteins and the crosstalk between environmental signals and GA signal transduction pathway that maintain GA homeostasis. The arrows show actions that are successfully executed, while arrow bars show actions that are blocked. Source: Sponsel and Hedden, 2010 © 2022 Global Journals 1 Year 2022 60 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