Characterization and management of autochthonous bacterial strains from semiarid soils of Spain and their interactions with fermented agrowastes to improve drought tolerance in native shrub species

Three bacterial autochthonous strains, namely Enterobacter sp.; Bacillus thuringiensis and Bacillus sp. were isolated from the rhizosphere of Mediterranean shrub species growing in a semiarid environment and analyzed alongside with the allochthonous Bacillus megaterium, used as reference drought tolerant strain, for their drought tolerance and plant growth promoting rhizobacteria (PGPR) capacities. The preliminary studies, done in axenic culture under non-stress and stress conditions show that Enterobacter sp. resulted in the most tolerant bacteria to osmotic stress factors. In contrast, Bacillus sp. was the most sensitive bacteria to osmotic stress factors and concomitantly, under these conditions, produced the highest amounts of ACC deaminase, poly-β-hydroxybutyrate and proline, to compensate its lack of stress tolerance. The PGPR activities of the tested bacterial strains under non-osmotic and osmotic stress conditions were determined by evaluating hormone (SA, ABA, JA and IAA) and ACC-deaminase production and phosphate solubilization. To analyze the bacterial efficiency as inoculants four shrubs species (Thymus vulgaris, Santolina chamaecyparissus, Lavandula dentata and Salvia officinalis), adapted to aridity, were selected. All the tested bacteria improved nutrition and physiological variables related to drought tolerance of the test plant. In addition, in S. chamaecyparissus and S. officinalis also increased mycorrhizal colonization. The application of fermented agrowaste resulted in effectively improving nutrient uptake and also interacted positively with most of the bacteria increasing plant nutrients content and drought tolerance but their effectiveness depended on the plant species and bacteria involved. In fact, in B. megaterium and the fermented agrowaste increased P and K uptake in S. chamaecyparissus (by 109% P and by 66% K), in L. dentata (by 75% P and 33% K) and in S. officinalis (by 63% P and 52% K). However, without amendment, the native B. thuringiensis was the most efficient strain in increasing P content in T. vulgaris (by 51%) and in S. chamaecyparissus (by 11%), and K content in L. dentata (by 63%), which decreased the stomatal conductance. Results show that under axenic conditions the stress applied did not suppress the PGPR abilities of assayed bacteria which indicated their potential to be tested as inoculants under detrimental conditions. The applied treatments resulted fundamental for these shrubs to reach their optimal nutritional and physiological traits suggesting their possible applicability under the natural semiarid drought conditions. The multiplicity and complexity of bacterial activities and the intrinsic characteristics of plant reactions to drought could explain the unpredictable results obtained by using these bacteria as plant inoculants. These and other factors are controlling the PGPR effects therefore it made difficult to generalize and to explain the cause/effect of the variable responses to be obtained. The results suggest the potentiality of the target bacteria and fermented agrowaste to be used as a biotechnological tool to help plants and reforestation in semiarid lands.
E. Armada was financed by Ministry of Science and Innovation (Spain). This work was carried out in the framework of the project reference AGL2009-12530-C02-02.