What lies beneath: root-associated bacteria to improve the growth and health of olive (Olea europaea L.)

Abstract

In recent decades we have witnessed growing public concern on the abuse/misuse of agrochemicals to control plant pathogens. Besides, the fact that some phytopathological agents (for instance, the soil-borne fungus Verticillium dahliae Kleb.) are very difficult to control by alternative methods to those based on chemical-based products, has urged researchers to seek effective measures within integrated disease management frameworks. Biological control, alone or in combination with other approaches, emerges as one of the most promising alternatives to confront plant pathogens in a sustainable, environment-friendly strategy. In this study, a collection of indigenous culturable bacteria (>300) from olive roots was generated. First selection was performed based on their antagonistic activity against V. dahliae. As a result, some 190 isolates were selected and further (i) tested against relevant plant pathogens (Alternaria alternata, Colletotrichum godetiae, Colletotrichum nymphaeae, Fusarium oxysporum f. sp. dianthi, Phytophthora cinnamomi, Pseudomonas savastanoi pv. savastanoi, and Rosellinia necatrix), (ii) identified molecularly (16S rDNA and gyrB genes), and (iii) screened for the presence of phenotypes usually associated to antagonism and/or plant growth promotion (PGP). Results showed prevalence of Proteobacteria (55.5 %) and Firmicutes (36.5 %) phyla, Pseudomonas (28.6 %) and Bacillus (33 %) being the most abundant genera. Although none of the strains showed effective against all tested pathogens, twelve of them belonging to genera Paenibacillus, Rhizobium, Pseudomonas or Bacillus presented a broad antagonistic activity, together with interesting phenotypic characteristics such as enzymatic activities associated to biocontrol and PGP (e.g. production of siderophore or chitinase activity). These results indicate that some of these bacteria could be good candidates to be used in future bioformulations aiming to improve health and growth of this tree crop.