Dissecting water stress-induced molecular responses in the maize primary root
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Drought is an important environmental factor limiting crop growth and productivity. With additional pressures arising from population growth and global warming, drought is an increasingly significant challenge for global agriculture and food security. Despite that numerous studies have been conducted over several decades, a clear strategy to improve the productivity of crop plants like maize under low water availability has not been identified. Roots are essential for the plant to survive under severe water-stressed conditions, because not only can they absorb water from soil but they also can penetrate hard dry soil and keep growing under water-stressed conditions that completely inhibit the growth of other organs. Previous studies showed that the maize (cv. FR697) primary root adapts to water stress by maintaining root growth in the apical 3 mm, whereas in the adjacent 4 mm the growth is progressively inhibited in the water-stressed roots compared to the well-watered roots. Furthermore, a recent proteomic study has identified a subset of plasma membrane proteins that showed different accumulation patterns in the growth zone of the maize primary root, suggesting that they are involved in the responses to water stress. However, the molecular mechanisms leading to such protein accumulation remain unclear. The accumulation of gene products is controlled at different cellular levels from DNA to the final product like protein. Transcription and translation are two major targets for regulating gene expression and for potentially altering the protein abundance. While post-translational regulations like protein trafficking might not change the total protein abundance in the cell, they can have an impact on the protein localization, thus affecting the protein activity, especially the activities of plasma membrane (PM) proteins. In chapter I, I selected several candidates from the subset of proteins identified by a PM proteomic study to investigate whether their changes in protein accumulation were due to transcriptional or post-transcriptional regulation by comparing their transcript accumulation patterns to the corresponding protein accumulation patterns. The results suggest that the regulatory mechanisms may vary from candidate to candidate, and the regulation can be region-specific. For those candidates with a discrepancy between their transcript and protein accumulation, the transcript changes do not indicate the changes in protein levels. Immunoblotting can be used to directly study the protein accumulation in response to water stress; therefore, antibodies against the target proteins are required for immunoblot analyses. In chapter II, the antigen of a candidate, fucosyltransferase-7, was cloned and bacterially expressed for antibody production. Moreover, the technical strategy established in this chapter can be used for producing the antigens of other molecular markers to further study their responses to water stress using immunoblotting. In chapter III, some future directions of my study are discussed. This summary includes the hypothetical development of experimental approaches to understand more about the water stress-induced signaling pathway network in maize primary roots based on the knowledge and tools established in this study. Taken together, this study found that proteins involved in the water stress responses are regulated through various molecular mechanisms in a region-specific manner. My results also suggest that the strategies and tools such as qRT-PCR and antigen production for antibody development established in this study can contribute to the further dissection and characterization of the signaling pathways involved in drought responses, which will give us a better understanding of how plants respond to water stress in order to engineer drought-tolerant crops in the future.