The aim of this study is to characterize the regulatory and functional proteins that confer the stress tolerances in plants. The dissertation includes three chapters. The first chapter examines the roles of a late embryogenesis abundant protein (HVA1) in stress tolerance. The second chapter discribes the roles of MYBS2 transcription factor in sugar signaling and stress tolerance in rice. The final chapter demonstrates the roles of two MYB transcription factors in Arabidopsis.
Regulation of root architecture is essential for maintaining plant growth under adverse environments. Stress/ABA inducible HVA1 was highly accumulated in root apical meristem (RAM) and lateral root primordia (LRP) after ABA/stress treatments, leading to enhanced root system expansion. HVA1 promotes lateral root (LR) initiation, elongation and emergence and primary root (PR) elongation via an auxin-dependent process, particularly by intensifying asymmetrical accumulation of auxin in LRP founder cells and RAM, even under ABA/stress-suppressive conditions. We demonstrate a successful application of an inducible promoter in regulating the spatial and temporal expression of HVA1 for improving root architecture and multiple stress tolerance without yield penalty.
Autotrophic plants have evolved distinctive mechanisms for maintaining a range of sugars homeostatic states. The on/off switch of reversible gene expression by sugar starvation/provision represents one of the major mechanisms by which sugar levels are maintained, but the details remain unclear. -Amylase (Amy) is the key enzyme for hydrolyzing starch into sugars for plant growth. It is induced by sugar starvation and repressed by sugar provision. In the second chapter, the on/off switch of Amy expression was found to regulate by two MYB transcription factors competing for the same promoter element. MYBS1 promotes Amy expression under sugar starvation, whereas MYBS2 represses it. Sugar starvation promotes nuclear import of MYBS1 and nuclear export of MYBS2, whereas sugar provision has the opposite effects. Phosphorylation of MYBS2 at distinct serine residues plays important roles in regulating its sugar-dependent nucleocytoplasmic shuttling and maintenance in cytoplasm by 14-3-3 proteins. Moreover, dehydration, heat and osmotic stress repress MYBS2 expression, thereby inducing Amy3. Importantly, activation of Amy3 and suppression of MYBS2 enhances plant growth, stress tolerance and total grain weight per plant in rice.
OsMYBS1 is involved in sugar- and hormone-regulated α-amylase gene expression in rice. In the third chapter, the roles of AtMYBS1 and AtMYBS2, which are OsMYBS1 homologs, in sugar signaling in Arabidopsis were investigated. Germination and seedling growth of atmybs1 mutant were hypersensitive, whereas those of atmybs2 were hyposensitive, to glucose and ABA. Furthermore, the expression of glucose-responsive genes, such as HXK1, CAB1, APL3, and CHS, were up-regulated in the atmybs1 mutant but down-regulated in the mybs2 mutant. Moreover, the mRNA levels of three ABA biosynthesis genes, ABA1, NCED9, and AAO3, and three ABA signaling genes, ABI3, ABI4, and ABI5, were increased upon glucose treatment in atmybs1, but decreased in mybs2, seedlings. These results suggest that AtMYBS1 plays a negative, while AtMYBS2 plays a positive, role in the ABA-dependent sugar repression of germination and seeding development.
The knowledge, gene, and findings obtained from these studies will help in developing tolerance crop varieties for sustainable agriculture.
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