CONICET | Buscador de Institutos y Recursos Humanos

Azospirillum brasilense is a plant growth-promoting rhizobacteria (PGPR) with the ability to produce several phytohormones such as indole-3-acetic acid (IAA). The positive interaction of Azospirillum with plants has been simplified and explained through the bacterial capacity to produce IAA. Typical changes on root architecture by promoting the number of lateral roots and hair formation and reducing the primary root length were established in inoculated plants. These changes increase the root surface and organ volume improving the water and nutrients acquisition by inoculated plants. The mechanism by which Azospirillum induces such changes fails to be explained only by the bacterial capacity to produce IAA. The main objective of this work was to evaluate the role of the IAA produced by A. brasilense Az39 as co-responsible for the changes observed in root architecture of Arabidopsis thaliana and to evaluate the nature of the IAA-independent mechanism. In our experiments A. thaliana root architecture was evaluated after inoculation with A. brasilense Az39 or with its IAA deficient mutant named Az39ΔipdC. Disinfected and stratified seeds of A. thaliana ecotype Col-0 and its mutant tir 1.1 deficient for lateral root formation were germinated vertically in Petri dishes containing Murashige and Skoog (MS) medium for 7 days with 16/8 h photoperiod at 22ºC. Then, seedlings were aseptically transferred to MS plates containing 108 cfu/mL of the wild-type strain (Az39); its mutant (Az39 ipdC-); an equivalent number of heat-inactivated cells of Az39 (Az39φ) or equivalent title of E. coli DH5α used as negative control. After 5 days post-transference, the root architecture was evaluated by the use of an image analysis system. Our results demonstrate the ability of A. brasilense Az39 to modify the primary root development of A. thaliana through IAA biosynthesis, while other IAA-independent mechanisms were related to an increase in the development of lateral roots and the number of root hairs. The physical presence of the bacteria, even metabolically inactive (Az39φ) in the culture medium seems to mediate the development of root hairs in inoculated seedlings and this mechanism being common to other bacteria as the non PGPR strain Escherichia coli DH5α. Our results suggest that inoculation with A. brasilense induces morphological changes in root architecture of A. thaliana through IAA-dependent and IAA-independent mechanisms. The biosynthesis of IAA by A. brasilense Az39 reduces the length of the main root of the inoculated seedlings, while the contact of the bacterial cells with the plant at root level (even inactivated cells) increases the root hair production. Finally, the production of some active compounds by the metabolically active cells of Az39 increases the growth and development of lateral roots. This is the first report presenting some preliminary evidence about this novel interaction for the Azospirillum genus.