Abstract

In the last few decades global food security has been an important issue due to continuously growing population. The common bread wheat, Triticum aestivum, belongs to the tribe Triticeae and serves globally as one of the most important staple foods. The minimization of the losses in the crop yield caused by biotic and abiotic factors will be a beneficial approach to increase the crop production. The identification of the genes that change gene expression in response to stresses has been an important avenue to identify candidate genes that may contribute to stress tolerance. Whole genome and transcriptomic analysis has accelerated the discovery and characterization of genes related to stress responses and tolerance. Several gene families in T. aestivum that respond to abiotic stress conditions have been identified and their characterization is in progress. Heterotrimeric G protein gene families have been long known to be involved in the regulation of plant growth and development under control and stress conditions and these gene family members are also found to play regulatory roles through interaction with other proteins. Here we have characterised the heterotrimeric G protein gene families in the T. aestivum. The heterotrimeric G protein α subunit (Gα) has been shown to interact with caleosins, a class of calcium binding proteins, and regulate stress responses through abscisic acid signalling. T. aestivum Gα is known to interact physically with Caleosin 3. However, the facility for genetic studies in T. aestivum is somewhat limited due to the lack of a readily available set of mutants, it’s polyploid nature and difficulty for transformation. Brachypodium distachyon has been developed as a model experimental species for monocotyledons with the particular relevance to the crop species in the Triticeae. It is important to know if the caleosins in Brachypodium also interact with its Gα subunit and the interaction is conserved among the species. In addition, Brachypodium lines with mutations for two caleosin genes are available and in this study have been characterized for their effects on root growth in response to abiotic stresses.
The first study (Chapter 2) of this thesis characterises the heterotrimeric G protein gene families in T. aestivum. Two of the Gγ’ were validated through in vivo protein-protein interaction by bimolecular fluorescence complementation. The differential expression analysis using RNA-Seq and microarray analysis showed that at least one homeologous gene copy of these members responded to abiotic stress conditions such as drought, heat, or cold, whereas only the Gγ1 paralog was found to be induced after inoculation of spore suspension of the fungus Fusarium graminearum in the resistant line NIL38 compared to the F. graminearum susceptible line NIL 51. This study will create a rationale to elucidate the possible role of heterotrimeric G protein gene family members in wheat under these stress conditions, which can be further investigated through mutant analysis.
The second study (Chapter 3) of this thesis report the physical interaction of B. distachyon heterotrimeric G protein subunit Gα with its CALEOSIN 7 (Bd-CLO7) and the role of CLO7 in regulation of root growth. We investigated the effect of Brachypodium CLO7 mutation on the regulation of primary, coleoptile node and lateral root growth under ABA and osmotic stress. Brachypodium CLO7 has found to regulate the lateral root growth under osmotic stress through ABA independent signalling.
The third study (Chapter 4) of this thesis determines the physical interaction of Brachypodium CALEOSIN 3 (Bd-CLO3) and, its N and C terminal truncations with Bd-Gα. We investigated the role of Brachypodium Caleosin 3 in the regulation of primary, coleoptile node and lateral root growth under ABA and osmotic stress by mutant analysis. Bd-CLO3 has been found to affect the primary root growth under ABA and osmotic stress and negatively regulate the coleoptile node root growth under non stress and osmotic stress conditions. In addition, Brachypodium CLO3 negatively regulates the lateral root growth through ABA signalling, whereas under osmotic stress it affects lateral root growth through both ABA dependent and independent pathways.