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

phototropism and auxin signaling (137). Studies have revealed that 21 of 29 Aux/IAA genes are the targets of the three PIFs ( PIF3, PIF4, PIF5 ), and 12 Aux/IAA genes are upregulated in response to natural shade and light (155) Fig 4b). These highlight the crucial roles of Aux/IAA genes in auxin-mediated light, photoperiod responses; two environmental signals that greatly influence dormancy induction and duration in crops, especially in tubers. Altogether, it has been demonstrated that auxin is an emerging master key player in dormancy induction, maintenance and seed germination mechanisms in plant, and that its effect is exerted through crosstalk between it, ABA, GA, their biosynthetic pathways and signaling networks, as well as environmental signals (light and photoperiod). This plasticity of means of auxin action will also provide opportunity for effective manipulation of undesirable long dormant phenotype of crop like yam, through genetic engineering by targeting any of the phytohormone biosynthetic pathways or signaling networks regulated by auxin which might not be detrimental to tuber yield and food quality. The table 1 above shows some key genes involve dormancy regulation, the nature of their effect on dormancy and their action pathways that have been reported in many crops. Many of these genes and action pathways have been utilized in genetic engineering the crops of interest to modify their dormancy duration. d) Roles of sugar metabolism in dormancy regulation As autotrophic organisms, plants produce sugars in mature photosynthetic parts (source organs) to support storage and growth in sink tissues. These sugars drive growth by serving both as metabolic substrates and as signals that tightly interact with hormonal, environmental, and other metabolic cues to coordinate cell growth in specific tissues with storage and nutrient remobilization (64). In doing so, sugars have been linked to stress responses and growth control mechanisms, and an increasing number of studies also implicate sugar signals in developmental decisions such as dormancy induction, senescence, germination and flowering (156-158). The primary sugars in plants are sucrose, glucose and fructose, while sucrose is the primary product of photosynthesis, glucose and fructose are products of breakdown of sucrose by trehalose-6-phosphate (T6P) (150, 151). However, glucose and sucrose are the main metabolic sugars that are widely distributed in plants, and have been recognized as pivotal in integrating regulatory molecules that control gene expression related to plant metabolism, stress responses, and growth and development relented processes including seed dormancy, germination, floral transition, fruit ripening, embryogenesis and senescence (43, 151, 159, 160). Over the years appreciable progress has been made towards understanding and identifying the dominant plant growth regulatory systems that are influenced mostly by sugars and sugar derived metabolic signals. The sugar signaling pathways in plants can be divided into two groups; (1) those that promote growth and are responsive to optimum sugar availability, include; the hexokinase (HXK ) glucose sensor, the trehalose-6-phosphate ( T6P ) signal, and target rapamycin (TOR) kinase; (2) those that inhibit growth and are responsive to sugar starvation (deficiency) condition include; sucrose non-fermenting 1 related protein kinase ( SnRK1 ) and C/S1 bZIP transcription factors (48, 49, 150, 151, 161, 162). The induction of the later pathway is a response to energy deficient (sugar starvation) situation which results in growth arrest. It can be speculated that the same sugar (sucrose) starvation condition is responsible for tuber dormancy induction at the onset of vine senescence of yam crop, during which sucrose photosynthate translocation from the source (leaves and stem) to sink (tuber) is stopped as result of senescence, and to maintain life of the tuber without continuous photosynthate sucrose supply, the tuber might resort to activation and adoption of low energy pathways to ensure optimum utilization of the available sugar by maintaining minimum biological activities associated with tuber dormancy. This argument is supported by the fact that two genes ( SnRK1; C/S1 bZIP ) which are implicated to be positive regulators of the low energy (sugar starving) responsive pathway, have also been implicated to be positive regulators of seed dormancy induction through ABA and auxin/IAA regulatory networks respectively (42, 98, 150, 151, 163). e) Crosstalk between sugar signaling and phytohormone signaling networks in dormancy regulation There have been reports about the existing crosstalk between the sugar growth promoting and inhibiting pathways and phytohormone signaling networks which systematically coordinate the molecular; biochemical; physiological and genetical plant growth and developmental processes. For instance, it has been reported that physiological relevant concentration (between 1µM and 10 µM) of T6p (the universal signal of sucrose in plants) inhibits SnRK1 transcription, and therefore any small changes in T6P concentration within the physiological relevant range produces large changes in SnRK1 activity, resulting in metabolic reprogramming of hundreds of genes involved in regulation of growth and defense responses (164-167). In the absence of T6P, SnRK1 regulates the expression of genes that regulate catabolic processes which are important for growth inhibition in a sucrose deficient condition to prevent acute starvation and death. Similarly, glucose-1-phosphate (G1P) and glucose-6- phosphate (G6P) also inhibit SnRK1 transcription at concentration levels (480 µM, and > 1mM) respectively, 1 Year 2022 63 © 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