Microbial tool to improve salt tolerance in plants

Soil salinization, caused by enhanced concentrations of different salts in soils, usually is a consequence of anthropic activity. Saline soils induce stressful conditions to plants, thus leading to growth reduction and physiological dysfunctions, such as overproduction of reactive oxygen species (ROS), reduction photosynthetic activity and osmotic imbalance. Several attempts have been tried to improve salt tolerance in different crops. Among them, seed priming and Plant Growth Promoting Bacteria (PGPB) inoculation are promising. PGPB is a class of bacteria (mostly rhizobacteria) capable of enhancing plant growth and possibly ameliorate plant response to abiotic stress. The aim of my project was the study of the major physiological and biochemical changes that occur in plants when exposed to saline conditions and the determination of the effects of different species of PGPB on salinity stress response in the salt sensitive cv. Sy Saveo of Brassica napus L. Rapeseed inoculation was performed with the following PGPB strains: Azospirillum brasilense CD, Arthrobacter globiformis CD, Burkholderia ambifaria PHP7, Herbaspirillum seropedicae Z67 and Pseudomonas sp. UW4. Surface sterilized seeds were inoculated and sown in saline (EC=8 dS/m) and control (EC= 0.3 dS/m) soil. The effects of salinity on growth and biomass of non-inoculated plants were significant; while beneficial effects of bacteria on growth were observed in plants exposed or not to salt. Antioxidant responses, enzymatic (superoxide dismutase and ascorbate peroxidase activities) and not enzymatic (phenolic compounds) were improved in inoculated plants under saline condition. Photosynthetic performance was evaluated by in vivo chlorophyll fluorescence image analysis and the detection of leaf photosynthetic pigment content. Significant differences were observed between stressed (control) and stressed and inoculated plants, confirming the beneficial effects of bacteria on plant response to salt stress. These positive results suggest the possibility to use PGPB as a tool to ameliorate the fitness of rapeseed salt sensitive cultivar. Considering the possibility to use PGPB inocula under field condition, we tried to establish an efficient protocol for the storage and easy management of the bacteria. For this purpose, we have selected the most promising strains of A. globiformis and H. seropedicae, differing in some of plant growth promoting characteristics, and immobilized the cells in alginate beads. This polymer, extracted from algae, was chosen because it is non-toxic, biodegradable and easy to handle and it is commonly used for biotechnological applications. Bacterial cells were encapsulated in alginate beads and stored either at room temperature or at 4° C. The latter were the best conditions for storing, therefore was chosen for further experiments. The length of the period was examined, and beads were kept under the above conditions up to 4 months. To verify bacterial survival after beads encapsulation and storage, bacterial growth was analysed. We compared the growth of beads in different media in order to determine the possibility to use unexpensive media for the use of PGPB in field. The beads were used to produce 4 different bacterial cultures grown either in TSB medium (control medium: casein 17 g/l + soy peptone 3 g/l + NaCl 5 g/l + dipotassium phosphate 2,5 g/l + dextrose 2,5 g/l) or in minimal medium (experimental medium 5 g/l NaCl + 2.5 g/l raw sugar in tap water). Each medium was inoculated with 1 month or 4 months old beads. The results obtained proved the survival of both A. globiformis and H. seropedicae after alginate encapsulation, even after 4 months storage. To prove the permanence of plant growth promoting activity after the encapsulation, the cultures were used as inoculants in rapeseed. The results obtained showed that the cultures of both PGPB, prepared with alginate beads, induced significant enhancement of plant growth even after 4 months of storage, confirming the maintenance of plant growth promoting activity for both of strains. These results provide new information about the possibility of biotechnological application for preservation procedure of PGPB, thus simplifying their use in agriculture.