RESUMO
Extreme ecosystems are a possible source of new interesting microorganisms, in this study the isolation of psychrophilic and psychrotolerant plant growth promoting microorganisms was pursued in a cold habitat, with the aim of finding novel microbes that can protect crops from cold. Eight yeast and four bacterial strains were isolated from rhizospheric soil collected from the Xinantécatl volcano in Mexico, and characterized for plant growth promoting properties. Most of the yeasts produced indole acetic acid and hydrolytic enzymes (cellulases, xilanases and chitinases), but none of them produced siderophores, in contrast to their bacterial counterparts. Inorganic phosphate solubilization was detected for all the bacterial strains and for two yeast strains. Yeast and bacterial strains may inhibit growth of various pathogenic fungi, propounding a role in biological control. Microorganisms were identified up to genera level, by applying ribotyping techniques and phylogenetic analysis. Bacterial strains belonged to the genus Pseudomonas, whereas yeast strains consisted of Rhodotorula sp. (4), Mrakia sp. (3) and Naganishia sp. (1). New species belonging to the aforementioned genera seem to have been isolated from both bacteria and yeasts. Germination promoting activity on Solanum lycopersicum seeds was detected for all strains compared to a control, whereas tomato plantlets, grown at 15 °C in the presence of some of the strains, performed better than the non-inoculated plantlets. This study offers the possibility of using these strains as an additive to improve culture conditions of S. lycopersicum in a more environmentally compatible way. This is the first study to propose psychrophilic/psychrotolerant yeasts, as plant growth promoting microbes.