Endophytes and recovery from grapevine yellows disease

Plants are naturally associated with different kind of microorganisms. Endophytic bacteria or fungi colonize the host plant systemically, without damaging the host or eliciting symptoms of plant disease according to widely used definition (Quispel, 1992). Endophytes have beneficial effects on plants and they may confer plant protection against pathogens by induction of plant defense mechanisms (ISR), pathogens-antagonistic substances and competition for root colonization (Compant et al., 2010). However, the molecular basis of endophytic interactions is not well understood. Even it is not clear the mechanism, when ISR is activated, it is manifested as a reduction in the rate of disease development, resulting in fewer diseased plants or lesser disease severity (van Loon et al 2007). Flavescence dorée (FD) and Bois noir (BN), the two main phytoplasma associated diseases of grapevine yellows complex (GYs), have been seriously damaging the wine production worldwide. In the last decade, in Italian vineyards was observed the spontaneous remission of symptoms in grapevine plants infected by phytoplasmas (Recovery). Although the bases of this phenomenon are still unclear, different hypothesis have been investigated (Musetti et al., 2007; Bulgari et al., 2011; Grisan et al., 2011). One of this assumptions is the role of endophytic bacteria in recovery. With this purpose the endophytic bacterial community associated grapevine plants was characterized by cultivation-dependent and –independent methods. Composition and structure of endophytic bacterial community were examined in healthy, phytoplasma-diseased and recovered grapevine plants. Length heterogeneity-polymerase chain reaction (LH-PCR) of total DNA from grapevine leaves was used to generate amplicon profiles that were analyzed with univariate and multivariate statistical methods. Jaccard analyses highlighted that microbial diversity and structure are different in healthy, diseased and recovered grapevine plants. Multivariate analyses confirmed this trend and showed which LH-PCR peaks determined the variation in microbial composition. Furthermore, LH-PCR electrophoretic peaks, assigned to isolated cultivable single bacterial strains, were used to monitor their distribution in total DNAs from analyzed plants. Bacterial community associated with healthy plants was characterized by a greater richness (higher number of LH-PCR peaks) than that present in diseased and recovered plants. Interestingly, some isolated strains showed beneficial traits related to mineral nutrition (phosphate solubilization, siderophore production), development (indole acitic acid production) and health (chitinase). In conclusion, from the above data it can be speculated that the alterations induced by phytoplasmas in the grapevine endophytic bacterial community by selecting those bacterial strains more resistant to ROS and able to eliciting plant defense responses, including ROS as well, may ultimately lead to recovery. This view is supported by previously reported findings showing that recovered grapevine plants have higher level of ROS in respect to diseased and healthy ones (Musetti et al., 2007). In order to verify this hypothesis, future studies will focus on determining the relative abundance of putative recovery inducers within microbial community living in grapevines. Furthermore, the possibility that endophytic bacteria are involved in the recovery phenomenon opens new perspectives in the control of these detrimental diseases and could be a starting point for developing environmental friendly, biocontrol strategies to be used in open field.