Exploitation of endophytes of centenarian olive trees in the management of olive knot disease

Abstract: Endophytes are microorganisms that spend most of their life cycle within plant tissues and they are recognized to play important roles in plant protection. These features have attracted the interest of many researchers in the exploitation of endophytes to manage plant diseases and pests. Some recent studies have been showed that endophytes in wild and ancient plants include beneficial endophytes that are absent or underrepresented in domesticated/recent crops. We hypothesized that the loss of these beneficial endophytes may be remediated by transferring endophytes from wild/ancient relatives of crops to crop species. Thus, in this work the effect of the application of a consortium of endophytes, retrieved from centennial olive trees, on the development of olive knot (OK) disease, was evaluated. The mechanisms responsible for the control of OK by endophytes was also evaluated by studying the changes in the indigenous stem endophytic bacterial community, through 16S rRNA gene amplicon-based metagenomic analysis. Accordingly, endophytes were extracted from stems of centennial olive trees using a Histodenz gradient and used to inoculated one-year-old olive plantlets. One week later, these plants were inoculated with the causal agent of OK, the bacterium Pseudomonas savastanoi pv. savastanoi (Pss). Results showed that the inoculation with endophytes reduced significantly both the incidence (up to 1.5-fold) and severity (up to 5.8-fold) of OK, when compared to plants inoculated exclusively with the pathogen Pss. The composition of stem associated bacteria was significantly changed by the single inoculation with the pathogen Pss or endophytes, being the effect of the former greater. However, if plants have been previously inoculated with endophytes, the subsequent inoculation of Pss one week later did not affect the composition of indigenous stem endophytic bacterial community. Overall, the results suggest that the inoculation of olive plants with endophytes from ancient trees could be an innovative strategy to manage olive knot disease. Despite these promising results, further studies are needed to better understand their mechanisms of action.