BROWN ALGAL GENES IMPART SALINITY AND HIGH TEMPERATURE STRESS TOLERANCE IN ARABIDOPSIS THALIANA

Abstract

Plants are constantly exposed to unfavorable environmental conditions like salinity, temperature, and water stress, that negatively affects growth and development. The ability of plants to survive these environmental stresses depends on the activation of molecular networks that trigger changes in gene expression and the production of metabolites, mitigating the physiological effects induced by stress. Substantial research has been carried out to develop transgenic plants with improved tolerance to abiotic stresses. Brown alga Ectocarpus sp. belongs to Phaeophyceae, a class of macroalgae in the phylum Heterokonta. Ectocarpus sp. is a dominant seaweed in the temperate regions around the world, abundant mostly in the intertidal zones that are known for harsh environmental conditions. The aim of this thesis was to determine if genes that were found to be upregulated in Ectocarpus sp., when subjected to multiple abiotic stresses, can also impart salinity and high temperature stress tolerance in the model plant Arabidopsis thaliana. Mannitol is one the widely studied sugar alcohol, that plays a role in abiotic stress tolerance in some plants and algae. Expression of mannitol biosynthesis genes of Ectocarpus sp. strain Ec32 in transgenic A. thaliana generated 42.3 – 52.7 nmol g-1 fresh weight of mannitol. Mannitol producing transgenic lines exhibited improved tolerance to salinity and high temperature stress. Transcriptomics analysis revealed significant differences in the expression of numerous genes, in both optimum and salinity stress conditions suggesting reprograming of gene expression in the overexpression lines. Besides mannitol biosynthetic genes EsM1PDH1 and EsM1Pase2, two other genes Esi0379_0027 (that codes HSP70) and Esi0017_0056 that were upregulated under abiotic stress conditions in Ectocarpus sp. Ec 32 were also overexpressed in A. thaliana. Bioinformatics analyses indicated that the proteins encoded by Esi0017_0056 and Esi0379_0027 are monomeric and soluble. In A. thaliana both the genes, under constitutive and stress inducible promoters, improved tolerance to salinity and high temperature stresses. Analysis of the expression of several key abiotic stress-related genes revealed that genes involved in ABA-induced abiotic stress tolerance, K+ homeostasis, Na+ sequestration and chaperon activities were significantly up-regulated in the transgenic lines. Transient expression of Esi0017_0056 in Nicotiana benthamiana revealed localization of the protein in cytosol. This is the first time Ectocarpus sp. genes have been successfully expressed in A. thaliana and suggest the possible use of macro-algal genes in improving abiotic stress tolerance in crop plants.