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

shows ABA biosynthesis from carotenoid pathway and dormancy induction mechanism. ii. Roles ABA signaling networks in dormancy regulation ABA signaling networks also play vital roles in dormancy induction, maintenance and releasing. The core ABA signaling involve in dormancy induction is mediated by pyrabactin resistance proteins/PYR-like proteins/regulatory components of ABA receptor ( PYR/PYL/RCAR ), phosphatase 2C ( PP2C ), SNF1-related protein Kinase 2 ( SnRK2 ), and abscisic acid responsive elements-binding factors ( AREB ) and basic leucine zipper( bZIP ) transcription factors (96-98). In Arabidopsis , ABA signaling genes are also implicated in seed dormancy regulation, for instance, ABA sensitive 1 ( ABI1 ) encodes PP2C phosphatase, and negatively regulated ABA signaling (99). It has been reported that ABI1 loss of function mutant ( abi1 ) exhibited reduced dormancy duration and better seed germination in the presence of optimum ABA content level(100), this could be attributed to lack of ABI1 function in the system and thereby confirm that ABI1 is required for ABA-mediated dormancy induction and that ABA signaling regulatory genes also play key roles. Other PP2C phosphatase, HONSU (HON), also represses ABA signaling specifically in seed, HON expression is associated with both, dormancy induction and releasing (101), it seems to act in rate-limiting manner that enables it to induce both dormancy and dormancy release. Among the ABI genes, ABI3 is the most influential in dormancy induction, and it is expressed in the growing seeds, where it regulates the accumulation of chlorophyll, anthocyanins, and storage proteins together with two other seed-related regulators such as; FUSCA 3 (FUS3) and leafy cotyledon 1 (LEC1) (95, 102). Loss of function mutant of ABI3 (abi3) has been reported to show no dormancy at all and immature seeds are able to germinate (103). ABI3 is regulated by WRKY DNA- binding protein 41 ( WRKY41 ), during seed primary dormancy induction WRYK41 binds directly to ABI3 promoter and to induce its expression (104). The ABA biosynthetic pathway offers opportunity to understand an active ABA pool during plant development that is controlled by various homologous genes. Identification of cofactors of the enzymatic reactions in the ABA biosynthetic pathway would be helpful in understanding of the complete networks of ABA synthesis and offer opportunity for effective dormancy duration manipulation in long duration dormant crop like yam tuber through genetic engineering. b) Role of Gibberellic Acid (GA) in dormancy regulation Gibberellins are phytohormones that comprise of a large family of diterpenoids which possess tetracyclic ent- gibberellane carbon skeletal structure arranged in either four or five ring systems, where the variable fifth ring is a lactone (105). GA promotes seed dormancy release and germination, and its biosynthesis and responses are highly coordinated during dormancy releasing process (106). Activation of GA-responsive genes induces cell wall- remodeling enzymes, such as, as endo ‐β‐ mannase, xyloglucan endotransglycolase, expansin, and β‐ 1,3 ‐ mannase. Their activity leads to the weakening of the embryo ‐ surrounding layers, and thereby stimulate growth in the embryo (92). The complex regulatory events in GA signaling pathway include cross talk with other hormones, environmental signals and regulation of genes involved in promoting cell elongation and division (107). Accumulation of GA in the radicle of embryo is accompanied by a reduction in ABA content suggesting GA and ABA antagonistic roles in dormancy regulation (108). i. GA metabolism and dormancy regulation The biosynthetic pathway of GAs starts from geranyl-geranyl diphosphate (GGDP) through pentenyl diphosphate ( IPP ), which is the 5-carbon building block for all terpenoid /isoprenoid compounds (109). Figure 5a below shows the GAs biosynthetic pathway, indicating the stepwise molecular processes, while 5b indicates the perception of environmental signals by the GAs biosynthetic pathway and crosstalk with other protein molecules in dormancy regulation. The basic isoprenoid unit IPP is generated via two pathways: mevalomic acid (MVA) pathway in cytoplasm and methyl erythrito phosphate (MEP) pathway in plastids (105, 110). The full route is divided into three stages according to their subcellular compartment and enzymes involved. The two-step conversion of GGDP to ent -Kaurene is catalyzed by ent -copaly di-phosphate synthase ( CPS ) and ent -Kaurene synthase ( KS) (105). Both enzymes have been reported to be encoded by single locus in Arabidopsis (GA1 and GA2) respectively and in rice ( OsCPS1 and OsKS 1) respectively, while in pumpkin ( cucucurbita maxima L.) only one gene coding for KS has been identified (111-113). Conversion of ent - Kaurene into GA 12 -aldehyde is catalyzed by the KO and KAO enzymes. In Arabidopsis , one single KO gene ( GA3 ) and two KAO genes ( KAO1 and KAO2 ) have been identified and functionally characterized, where their loss of function mutant ( ga3 ) exhibited growth delay in germination and defective growth phenotype (114). In rice, mutations in the OsKO2 resulted in severe GA- deficiency, prolong dormancy and dwarfism (115), whereas, in maize ( zea mays) two putative KO genes have been identified and CYP701A26 was characterized to exhibit ent -Kaurene oxidase activity which led to increase in the accumulation bioactive GAs and consequently resulted in reduction of dormancy duration, while in barley, one single KAO gene which exhibited similar trit phenotype was found (105, 116). Contrary to previous observations, two KAO genes have been identified in Pea, but one exhibits previously observed phenotype, while the second was 1 Year 2022 59 © 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