Date of Award

8-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Genetics and Biochemistry

Committee Member

Dr. Hong Luo, Committee Chair

Committee Member

Dr. James Morris

Committee Member

Dr. Michael Sehorn

Committee Member

Dr. Haiying Liang

Abstract

Abiotic and biotic stresses such as drought, salt, nutrition starvation, and pathogen infection are major factors threatening our agricultural production. With the rapidly increasing population and limited arable land area, genetic engineering of crops for new products with more stable and higher yield than conventional cultivars under adverse environment provides a powerful new tool for use in developing novel GMOs (Genetically Modified Organisms) to feed the large population in the immediate future. To develop novel GMOs with enhanced performance under adverse conditions, we need first to understand molecular mechanisms underlying plant stress response. To better understand how signaling transduction pathway in plants responds to stresses, we focused on a newly identified Arabidopsis protein kinase family SRF (Stress Responsive Factor). This gene family comprises of four family members ( SRF1-4 ), and their expressions are strongly regulated by abiotic or biotic stress. The four SRF proteins are all localized on plasma membrane, suggesting that they may have similar functions in signaling transduction, but their different expression patterns imply that their functions are temporally and spatially distinct. By using genetic methods, we found that SRF1 and 2 are two negative regulators of salt resistance of Arabidopsis, while SRF2 positively regulates PAMPs (Pathogen-Associated Molecular Patterns)-triggered immunity of Arabidopsis. Results of Western analysis and Northern analysis suggest that the MAPK-mediated signaling transmission and expression of defense-related genes were enhanced in SRF2 overexpressing plants. We also found that BAK1 is a co-receptor of SRF2 kinase. These results suggest that SRFs have important functions in abiotic or biotic stress resistance pathways, and the information obtained may be used to engineer crops for enhanced stress resistance.

Besides further deciphering signaling pathway in plant response to osmotic stress and biotic stress, we also investigated the role of microRNAs (miRNAs) in plant response to nutritional deficiency, specifically, the function of rice miR395 genes responding to sulfate starvation. Our results indicated that under sulfate deficiency conditions, rice miR395 is intensively upregulated, whereas the two predicted target genes of miR395 are down-regulated. Overexpression of the rice miR395h in tobacco impairs its sulfate homeostasis. One sulfate transporter gene NtaSULTR2 was identified to be the target of miR395 in tobacco, which belongs to low affinity sulfate transporter group and may mediate the sulfate transportation and distribution. The critical functions of miR395 and NtaSULTR2 in sulfate transportation and assimilation suggest that these two genes could be utilized to improve the growth of GMOs in sulfate-limited condition.

Development of molecular tools is important in agricultural biotechnology. Tissue specific promoters are of particular interest when developing GMOs with modified traits. For example, their use can lead to reduced accumulation of undesirable heterologous proteins or final metabolites in certain organs such as fruits or seeds. We identified a novel Arabidopsis leaf-specific promoter Srf3abc. Srf3abc exhibits stronger activity than CaMV 35S promoter in the leaves of Arabidopsis. Truncation in Srf3abc abolishes its leaf specificity, and some truncated versions of the promoter exhibit strong constitutive activity in Arabidopsis. Most significantly, Srf3abc and its truncated versions also function across different plant species including dicots and monocots, implying their potential wide applications in agriculture biotechnology.

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