Effects of Azospirillum brasilense Sp245 on grapevine propagation

Azospirillum brasilense Sp245, a well known PGPR (plant growth-promoting rhizobacteria), has been considered in order to evaluate the effects on the nursery propagation, and especially to test the ability of the inoculated bacterium to improve rooting parameters for some rootstocks which do not easily root by means of conventional techniques. Nursery trials, laboratory trials, growth observations of the obtained vines, and experiments on the drought resistance of inoculated plants, were performed along three years, from 2008 to 2010. The nursery trials were executed in a conventional nursery and in an organic one, during the scheduled daily work of the host nurseries; plant material was furnished by the nurseries as well. A significant increase in the callus diameter was observed in inoculated grafted cuttings at the end of the grafted cuttings forcing (a two-weeks period at 25°C and high humidity rate). As to the rooting parameters, while the rooting percentage was not significantly affected, an improved quality of the root system was observed in the second year, especially as to the number of roots and the total biomass of the rooted vines. The chemical analyses of plant material revealed that roots of inoculated plants had greater dry matter, and greater concentrations of total phenols (in this case in leaves as well). Laboratory trials comprised also the isolation and morphological and molecular characterization of bacterial cells found in the grapevine tissues; different bacterial species were identified, some of these (such as Stenotrophomonas sp., and Lysobacter sp.) not yet known to be capable of living as endophytes in grapevine tissues. No cells of Azospirillum brasilense were found inside the examined tissues, suggesting that the observed inoculation effects might not depend on the penetration of the bacterial cells inside the plants. Growth measures on obtained vines, whether in pots or in field, showed that the number of nodes was in general higher in inoculated plants, even if not significantly. The stomatal conductance of the leaves of the treated plants was lower than the control, suggesting an increased resistance of the treated plants to drought stress (for the rootstocks 420A and 775P - second year trials), and, possibly, to plant pathogens. Moreover, the experiments of drought simulation showed that the shoots growth in the treated plants was not affected by the stress, nor was the stomatal conductance, implying that the bacterial treatment could somehow protect plants during drought conditions. This effect of the bacterial inoculation could be ascribed to the induction of systemic resistance in the host plant, possibly via a bacterial MAMP (microbial-associated molecular pattern), which elicited a variety of plant defence responses; some of the typical plant defence responses to MAMPs have been actually found in this work, such as the accumulation of phenolic compounds, and the stomatal closure. The possibility that the Azospirillum brasilense Sp245 treatment could induce systemic resistance to biotic and abiotic stresses in grapevine could be of great importance, whether in nursery propagation or in grape production.