Methylobacterium spp., endophytes of olive trees, as potential biocontrol agents of Xylella fastidiosa subsp. pauca

Interactions between endophytes and plants can promote the health of the host and play a significant role in low-input sustainable agriculture. Understanding this plant–microbe interactions and the mechanisms that enable endophytes to enhance the plant defence response is essential, especially for endophytic bacteria that show biocontrol potential against vascular wilt pathogens. Investigations on the bacterial endophytic population occurring in the xylem of healthy and Xylella fastidiosa subsp. pauca ST53 (XfpST53)-infected olive trees showed that under field conditions, the population level of cultivable endophytic bacteria is highly variable, being mainly affected by the host genotype, host age, and wilting severity. Among the different cultivable bacteria occurring in the wood of olive trees, Methylobacterium spp. are one of the most interesting groups. Methylobacterium strains isolated from the xylem of healthy and XfpST53-infected olive trees have been identified as M. mesophilicum and M. radiotolerans. Species of Methylobacterium have also been reported as potential biocontrol agents, plant-growth-promoting bacteria, and resistance inducers, by producing phytohormones, inducing plant systemic resistance, and supplying or mobilising nutritional elements (siderophores production). In order to evaluate the potential of M. mesophilicum GR19, and M. radiotolerans GR18, GR22 e GR23, as nutrient competitors of XfpST53, the production of siderophores was investigated by using the Chrome Azurol S (CAS) agar and ferric perchlorate assay to detect hydroxamates. M. mesophilicum DSM 1708 and M. radiotolerans DSM 1819 were used as reference strains. All the tested strains produced different levels of siderophores, and the most effective were applied by endotherapy in healthy and XfpST53-infected olive trees, in order to evaluate in planta their activity in containing the olive quick decline syndrome. Moreover, the characterisation of plant-growth-promoting traits of several Methylobacterium strains are currently in progress, i.e. by screening the production of indole-3-acetic acid (IAA), and the 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity.