Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Tian, Lining

2nd Supervisor

Karagiannis, Jim

Co-Supervisor

Abstract

Despite evidence that certain HD2 family histone deacetylases (HDACs) play an important role in plant growth and stress response, the coordination of HD2-type HDACs in these processes remains largely unknown. Arabidopsis contains four HD2s called HD2A, HD2B, HD2C, and HD2D. I found that HD2A and HD2C coordinate to positively regulate drought stress response. The hd2a.hd2c double mutant (Mac16) exhibits decreased survival and increased water loss under drought due to modified stomatal closure as compared to the single mutants hd2a and hd2c. Gene expression analysis showed that the expression of stomatal closure-related genes ABI1, ABI2, and SLAC1 was significantly affected in the Mac16 as compared to the hd2a and hd2c. Conversely, plants overexpressing HD2A or HD2C showed enhanced survival under drought stress and decreased water loss from leaves. Both HD2A and HD2C also play role in controlling root growth under stress. Furthermore, I demonstrated that HD2A and HD2C positively regulate root growth. Mac16 showed decreased root growth, compared to hd2a and hd2c. Importantly, the GA2ox1 and GA2ox2, which catabolise bioactive gibberellic acids, were significantly upregulated in the Mac16 as compared to the single mutants, thus caused decreased root growth in the Mac16. Moreover, increased genome-wide H3K9 acetylation was observed in the Mac16 as compared to the single mutants. Additionally, Y2H and BiFC analysis showed that both HD2s can physically interact with each other. Overall, my investigation revealed that HD2A and HD2C coordinate to play a cumulative role in drought stress response and root growth in Arabidopsis.

Summary for Lay Audience

Drought is an important environmental stress affecting the plant growth and production. Drought stress induces several morphological changes in plants at different stages including roots, shoots, leaves, and flowering to limit the plant growth. All growth changes in plants in response to different environmental stresses are controlled by specific DNA sequences which are generally called as genes. Genes are activated only when they are required by the plants to perform their function. Activation or repression of genes at chromatin level is usually controlled by several enzymes. Histone deacetylases (HDACs) are considered gene repressors and play a role in plant growth and stress response. There are three different families of HDACs in plants and I focused on histone deacetylase 2 (HD2) family in the model plant Arabidopsis.

I conducted research to investigate the role of HD2-type HD2A and HD2C in controlling drought stress response and root growth. For this, I obtained HD2 single gene mutant plants and crossed them with each other to develop hd2a.hd2c double gene mutant plants. I compared these mutant plants with wild-type (WT) plants which were expressing normal levels of these HD2 genes. I also developed transgenic plants expressing higher levels of these HD2 genes and compared them with WT. These studies revealed that HD2A and HD2C coordinate to positively regulate plant drought stress response. These HDACs control the expression of abscisic acid related genes which play role in the opening and closing of stomata, thus help the plant to minimize water loss by enabling the stomatal closure under drought conditions.

I also demonstrated that HD2A and HD2C work together to control root growth under drought stress by regulating the expression of GA2ox genes. GA2ox genes play a role in the degradation of specific bioactive gibberellic acids, which are considered to promote root growth. Overall, my research revealed that HD2A and HD2C coordinate to positively regulate the drought stress response and root growth in Arabidopsis. Knowledge gained on the role of these HD2 genes in drought stress response and root growth can be used as a potential molecular strategy to improve drought tolerance in related crops.

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