Global Journal of Science Frontier Research, G: Bio-Tech & Genetics, Volume 22 Issue 2
Fig. 6: Hypothetical model of genetic interactions between sugar and hormone signaling. HXK1 -mediated glucose signaling that regulates dormancy induction, germination and seedling development by inducing both ABA biosynthesis and ABA signaling gene expression. Glucose and ethylene signaling converge on the ETHYLENE INSENSITIVE3 ( EIN3) TF to differentially regulate its protein stability. HXK -signaling interacts positively and negatively with auxin and cytokinin signaling respectively. Hypothetical connections are shown with dashed lines , while connections that led to biosynthetic or regulatory product and developmental trait are shown with arrows, whereas connections that result in repression of either biosynthetic or regulatory product and developmental trait are shown with arrow bars. Source: Smekeens, et al, 2010 The multi-level interactions between auxins, cytokinins and sugars are highly complex and are yet to be well understood, even in Arabidopsis. However, some studies have tried to link sucrose to the auxin biosynthesis (184-186), a strong candidate for a long- distance signal promoting lateral root growth. It has been suggested that auxin biosynthesis is induced by soluble sugars, this is support by the fact that daily fluctuations in sugar content highly correlated with fluctuations in auxin levels (184), and circadian clock is responsive to auxin treatment (187). Glucose treatment of Arabidopsis seedlings induces expression of multiple genes encoding auxin biosynthetic enzymes, including; YUCCA8 and YUCCA9 (184), corroborating another report that a putative maize YUCCA gene is strongly induced by glucose (186). Surprisingly, sucrose effects on auxin levels are more pronounced in the roots than in shoots, suggesting that sugars may impact auxin transport and pathways as well. The growth promoting effect of sucrose is likely through its effect on auxin, as it can be partially mimicked by directly adding auxin and can be blocked by adding polar auxin inhibitors (185). Auxin signaling has also been linked to sugar metabolism. For instance, down -regulation of tomato auxin response repressor SIAF4 led to a dramatic increase in chloroplast number and an increase in sugar and starch content in the fruit (188). Sugars and cytokinin interact during plant growth and development, and these interactions can be both direct and indirect, and involve cell-specific and long-distance interactions (175). Transcript profiling of Arabidopsis seedlings after glucose and cytokinin treatment showed that many genes involved in stress responses and developmental pathways were affected (189). It has been reported that glucose and cytokinin acted both agonistically and antagonistically on gene expression, and glucose had a strong effect on genes involved in cytokinin metabolism and signaling (190). Cytokinin deficiency caused by constitutive overexpression of cytokinin oxidase ( CKX ) gene, leads to drastic changes in root and shoot growth (191), though molecular mechanisms are only partly known, and involve changes in the cell cycle and in photosynthetic activity, altered carbohydrate distribution and source/sink relations. Gibberellins (GA) daily fluctuations is also responsive to fluctuations in sugar levels and are regulated by Circadian clock (187, 192). Studies have shown evidence that sucrose stabilizes growth repressor protein ( DELLA ) exert its repression effects by blocking GA regulatory networks ( PIFs ) from interacting 1 Year 2022 65 © 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
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