Phd in ENVIRONMENTAL AND EVOLUTIONARY BIOLOGY

Thesis title: Lipids in Xylella fastidiosa - Olea europaea interaction

Xylella fastidiosa is a phytopathogenic bacterium with an extremely wide host range and a strain of X. fastidiosa subsp. pauca called De Donno has been recognized as responsible for the Olive Quick Decline Syndrome (OQDS) affecting olive trees in Salento area. The aim of the thesis is to elucidate the roles of some lipid species in Xylella fastidiosa physiology, especially in regulating adhesion, biofilm formation and motility. A combination of untargeted and targeted mass spectrometry methods allowed to characterize several lipid entities and to shed light on the lipid composition of Xylella fastidiosa subsp. pauca strain De Donno in pure culture; some specific lipids (e.g., ornithine lipids and the oxylipin 7,10-diHOME) were revealed. Nicotiana tabacum was used for testing the ability of this pathogen in producing such lipids in the host and different lipid compounds revealed clear distribution pattern within the infected plant tissues compared to the uninfected ones. Applying a similar workflow to olive trees naturally infected with X. fastidiosa, in addition with in vitro experiments on biofilm formation, it was possible to highlight for the first time the different accumulation of free fatty acids, oxylipins and diacylglycerols in olive trees affected by OQDS, thus identifying lipid hallmarks of the infection, and to suggest their role in the modulation of X. fastidiosa subsp. pauca biofilm formation. A more extensive dataset was then analyzed, including trees belonging to two different cultivars, being infected or non-infected and being treated or non-treated with the zinc- copper-citric acid biocomplex Dentamet®, a treatment able to delay and limit the OQDS, reducing X. fastidiosa subsp. pauca concentration within the leaves of treated plants, even in susceptible vars of olive tree as Ogliarola salentina. The infection apparently can stimulate oxylipin synthesis in olive trees and the variety Ogliarola salentina exceeds in the synthesis of specific oxylipins such as 13-HODE due to the probable action of a 13-lipoxygenases. 13-HODE is depressed by Dentamet®, that also showed an in vitro bactericidal and anti-biofilm activity in representative strains of X. fastidiosa subspecies, including X. fastidiosa subsp. pauca isolated in Apulia from olive trees. A deletion mutant of X. fastidiosa subsp. fastidiosa strain Temecula1 for the XadA2 afimbrial adhesin gene, showed reduced twitching motility and biofilm formation, compared to wild type strain and a decrease in 7,10DiHOME oxylipin production together with an increase as regards its precursor 10 HPOME. This suggest that 7,10DiHOME could be a crucial signalling molecule to promote biofilm formation and twitching motility, working in positive feedback with the afimbrial adhesin XadA2.