Bacillales Members from the Olive Rhizosphere Are Effective Biological Control Agents against the Defoliating Pathotype of Verticillium dahliae

Abstract

The use of biological control agents (BCAs) is of interest within an integrated management strategy of Verticillium wilt of olive (VWO) caused by the soil-borne fungus Verticillium dahliae Kleb. Previous studies have shown that the root/rhizosphere of healthy olive plants is an important reservoir of microorganisms displaying biocontrol activity against VWO (i.e., Pseudomonas strains PICF7 and PIC141). Moreover, these BCAs are already adapted to the ecological niche where they are deployed. Three novel bacteria (strains PIC28, PIC73 and PIC167) from nursery-produced olive plants were in-depth characterized using a previously implemented approach consisting of in situ isolation, in vitro antagonism tests, in planta bioassays, phenotypic and metabolic characterization, genome analyses and in silico identification of traits involved in plant-bacteria interactions, and multi-locus sequence analyses. All strains displayed in vitro growth inhibition of different olive pathogens and biocontrol effectiveness against Verticillium dahliae, with strain PIC73 being the most effective BCA. Strains PIC73 and PIC167 were identified as Paenibacillus polymyxa (Prazmowski) Ash et al. and Paenibacillus terrae Yoon et al., respectively. Strain PIC28 belongs to the Bacillus genus. Some of these Bacillales members showed in vitro compatibility with previously characterized BCAs (Pseudomonas spp. strains) also originating from the olive rhizosphere, paving the way for the future development of tailored bacterial consortia effective against VWO.

Linking belowground microbial network changes to different tolerance level towards Verticillium wilt of olive

“…As for biocontrol strategies concerns, the fact that the communities associated with the VWO-tolerant cultivar showed a higher abundance of beneficial genera deserves attention as for the identification of novel potential BCA and/or PGPR. Moreover, assessing the effects caused by the introduction of well-characterized BCA against VWO [12][13][14] on the structure, functionality and network interactions of belowground communities must be investigated as well. This would be relevant for communities exhibiting higher abundance of potential deleterious components as here reported for the VWO-susceptible Picual.…”

“…While studies on specific beneficial components of the olive-associated microbiota have been conducted, some of them aiming to isolate and characterize biological control agents (BCA) against VWO [12][13][14], only very few examples are available on whole indigenous olive microbial communities [15,16] and their potential relationship with susceptibility to biotic constraints [17]. Recently, we described the belowground microbial communities of a range of olive cultivars from different geographical origin grown under the same climatic, agronomical and soil conditions, and in the absence of V. dahliae pressure [18].…”

Verticillium Wilt of Olive and Its Control: What Did We Learn during the Last Decade?

“…members. For instance, following the same scheme of actions mentioned above, three Bacillales strains (PIC28, PIC73, and PIC167) from the olive rhizosphere were also identified as effective BCAs against the D pathotype [165]. Likewise, Paenibacillus alvei K165 was earlier demonstrated as an excellent BCA towards VWO, significantly decreasing the symptoms and severity of the disease on the susceptible cv.…”

Defining the root endosphere and rhizosphere microbiomes from the World Olive Germplasm Collection

“…The olive-associated microbiome harbors an important reservoir of beneficial microorganisms that can be used as plant growth promotion and/or biocontrol tools 15,24 . Moreover, bacterial antagonists of olive phytopathogens isolated from the olive root endosphere or the rhizosphere have the advantage to be adapted to the ecological niche where they can potentially exert their beneficial effect 18 . For instance, Proteobacteria and Firmicutes representatives, usually found as natural inhabitants from the olive rhizosphere, are thus good examples of effective antagonists against V. dahliae 16–18,29 .…”

“…Moreover, bacterial antagonists of olive phytopathogens isolated from the olive root endosphere or the rhizosphere have the advantage to be adapted to the ecological niche where they can potentially exert their beneficial effect 18 . For instance, Proteobacteria and Firmicutes representatives, usually found as natural inhabitants from the olive rhizosphere, are thus good examples of effective antagonists against V. dahliae 16–18,29 . Besides, representatives of these phyla such as the genera Pseudomonas and Bacillus are easy to isolate, manipulate, propagate and formulate as BCAs.…”