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

c) Role of Auxins in dormancy regulation Auxin is an exceptional plant hormone, it is the only that controls its own long-distance transport system, and it affects all aspects of plant life, including embryo development, cell division and differentiation, general plant architecture and orientation in space, stress responses and tuber wound healing (47, 127). Indole-3-acetic acid (IAA) is the common natural occurring form of auxins in plants, but other natural (4- chloroindole-3-acetic acid, 4-Cl--IAA; phenylacetic; PAA) and synthetic (1-naphthaleneacetic acid, NAA; 2,4- dichlorophenoxyacetic acid, 2, 4-D) etc., also exist (128). Auxin alone was not previously considered an important regulator of seed dormancy and germination. Earlier studies have suggested that exogenous auxin can suppress seed germination under saline stress conditions (129), implying that auxin plays regulatory role in seed germination in response to environmental cues. It has been reported that IAA could inhibit Pre- harvest sprouting in wheat through ABA repression of embryonic axis elongation by stimulating auxin signaling (130, 131). Another study suggested that after-ripening treatment-mediated dormancy release is correlated with decreased seed sensitivity to auxin (132), suggesting that the after-ripening might have deactivated auxin biosynthetic pathway or signaling network. The exact mechanism underlying auxin action on seed dormancy is largely unknown until recently. Genetic data has demonstrated that auxin regulates seed dormancy via the ABA signaling pathway. Auxin responsive factors ( ARFs ); specifically, ARF10 and ARF16 have been reported to indirectly activate ABI3 transcription and ABI3 is the key dormancy inducing ABA biosynthetic transcription factor, therefore, activating ABI3 will result in increased ABA accumulation and consequently dormancy induction (122). Furthermore, another study has revealed that seeds of Arabidopsis abi4 and abi5 mutants are insensitive to auxin treatment during germination indicating that ABI4 and ABI5 are important regulators of auxin-mediated dormancy induction and maintenance (133, 134). The synergistic effects of IAA and ABA on seed dormancy was also demonstrated by the loss of function of mutant abi3-1 which exhibited reduced dormancy phenotype in the presence of optimum IAA concentration (135). Similarly, intense seed dormancy and ABA hypersensitivity of the iaaM-OX line were compromised in the iaaM-OX/abi3 double mutant confirming that the synergistic effect of IAA/ ABI3 is required for auxin seed dormancy induction (136). Furthermore, seed dormancy and ABA sensitivity was also compromised in mARF16/abi3 double mutant suggesting that mutual action of auxin response factor 16 and ABA transcription factor ABI3 also play role in dormancy induction (51). Also, it has been reported that auxin induced high accumulation of ABI5 protein during seed germination acted downstream of ABI3 to inhibit the seed germination which indicate that auxin enhancement of seed dormancy and ABI3 -dependent ABA seed germination inhibition (132). Hussain et al (88) reported that auxin signaling repressor; IAA8 promoted seed dormancy release in Arabidopsis by down-regulating of ABI3 transcription, thereby further establishing that auxin signaling regulates ABI3 transcription and that auxin signaling/ ABI3 synergistically inhibit seed germination during dormancy period. The ultimate determinant of dormant status of any part of plant that has potential to germinated is the GA/ABA ration. It has been reported that exogenous auxin treatment repressed soybean seed germination by enhancing ABA biosynthesis, while impairing GA biogenesis, and consequently reduced GA 1 /ABA and GA 4 /ABA ratios ((122). Consistent with this, ABA biosynthesis inhibitor fluridone reversed the dormancy-induction phenotype associated with auxin treatment, while placlobutrazol a GA biosynthesis inhibitor, inhibited seed germination phenotype due to its action on GA biosynthestic pathway(51). Further quantification of GA and ABA under exogenous auxin treatment, showed that auxin significantly increased ABA content, whereas, bioactive GA 1 and GA 4 levels were decreased, resulting in significant reduction in GA 1 /ABA and GA 4 /ABA ratios (153). These studies have shown that auxin is exert its influence on dormancy induction and maintenance in plant by mediating ABA and GA biosynthesis and consequently determining the GA/ABA ratio in plant at any point. 1 Year 2022 6 © 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