Un consorzio di batteri è coinvolto nello sviluppo della rogna dell'olivo

Olive knot disease, caused by Pseudomonas savastanoi pv. savastanoi (Pss), is characterized by knots or galls formation on different parts of the plant, mainly on twigs and young branches, caused by the phytohormones indole-3-acetic acid (IAA) and cytokinins produced by the bacterium. Apart from Pss, knots provides a special niche for the development of non-pathogenic bacterial species, and their role in the development of the disease is under investigation. Our attention is point out on Pantoea agglomerans (Pag), Erwinia toletana (Et) and Erwinia oleae (Eo). The co-inoculation of Pss with the three bacterial endophytes, causes an increase in the size of knots and the growth of Pss in planta is stimulated by Et and Eo and depressed by Pag. We have also demonstrated that Pss and the bacterial endophytes possess a quorum sensing (QS) intercellular communication system mediated by N-acyl homoserine lactones (AHL). Using AHL biosensors, we found that the four species possess a conserved AHL profile: Pss and Et produce C6-3oxo-AHL and C8-3oxo-AHL; Pag C4-AHL and C6-AHL; Eo C6-AHL and C8-AHL. Using bacterial genomic knockout mutants we found that QS is pivotal for virulence of Pss and that the lack of AHL production by Pss mutants can be complemented by wild type Et in planta. Our finding suggested that in the knot niche a stable consortium between the above mentioned bacteria are established. All Pag strains (DAPP-PG 729, 731 and 734) obtained from olive knots bear the iaaM, iaaH and iaaL genes, involved in the IAA metabolism; DAPP-PG 731 strain has the ptz gene, involved in the cytokinin synthesis; DAPP-PG 734 strain bears the hrcC of the hrp/hrc cluster. Since Pag strains have the same IAA pathway used by Pss and one strain bears a gene of the hrp/hrc cluster, it is possible to hypothesize that Pag have acquired these genes from Pss by horizontal transfer.