Abstract
The rhizobacterium Pseudomonas pseudoalcaligenes AVO110, isolated by the enrichment of competitive avocado root tip colonizers, controls avocado white root rot disease caused by Rosellinia necatrix Here, we applied signature-tagged mutagenesis (STM) during the growth and survival of AVO110 in fungal exudate-containing medium with the goal of identifying the molecular mechanisms linked to the interaction of this bacterium with R. necatrix A total of 26 STM mutants outcompeted by the parental strain in fungal exudate, but not in rich medium, were selected and named growth-attenuated mutants (GAMs). Twenty-one genes were identified as being required for this bacterial-fungal interaction, including membrane transporters, transcriptional regulators, and genes related to the metabolism of hydrocarbons, amino acids, fatty acids, and aromatic compounds. The bacterial traits identified here that are involved in the colonization of fungal hyphae include proteins involved in membrane maintenance (a dynamin-like protein and ColS) or cyclic-di-GMP signaling and chemotaxis. In addition, genes encoding a DNA helicase (recB) and a regulator of alginate production (algQ) were identified as being required for efficient colonization of the avocado rhizosphere.IMPORTANCE Diseases associated with fungal root invasion cause a significant loss of fruit tree production worldwide. The bacterium Pseudomonas pseudoalcaligenes AVO110 controls avocado white root rot disease caused by Rosellinia necatrix by using mechanisms involving competition for nutrients and niches. Here, a functional genomics approach was conducted to identify the bacterial traits involved in the interaction with this fungal pathogen. Our results contribute to a better understanding of the multitrophic interactions established among bacterial biocontrol agents, the plant rhizosphere, and the mycelia of soilborne pathogens.
Highlights
The rhizobacterium Pseudomonas pseudoalcaligenes AVO110, isolated by the enrichment of competitive avocado root tip colonizers, controls avocado white root rot disease caused by Rosellinia necatrix
The biocontrol rhizobacterium Pseudomonas pseudoalcaligenes (Proteobacteria, Gammaproteobacteria, Pseudomonadales) strain AVO110, isolated by the enrichment of competitive avocado (Persea americana) root tip colonizers, is able to control avocado white root rot disease caused by the soilborne pathogen Rosellinia necatrix (Ascomycota, Sordariomycetes, Xylariales) under greenhouse conditions, reducing disease development up to 45% when compared to that in control plants not inoculated with bacteria [12]
Most bacterial strains isolated by using this strategy were shown to produce several exoenzymes, hydrogen cyanide (HCN), or antifungal antibiotics, the potential biocontrol traits of P. pseudoalcaligenes AVO110 only included the biosynthesis of siderophores, weak cellulose activity [12], and its ability to colonize both the avocado rhizosphere and the R. necatrix hyphae [1]
Summary
The rhizobacterium Pseudomonas pseudoalcaligenes AVO110, isolated by the enrichment of competitive avocado root tip colonizers, controls avocado white root rot disease caused by Rosellinia necatrix. The bacterium Pseudomonas pseudoalcaligenes AVO110 controls avocado white root rot disease caused by Rosellinia necatrix by using mechanisms involving competition for nutrients and niches. Growing fungal hyphae exude a complex mixture of low-molecular-weight metabolites that include organic acids, such as oxalic, citric, and acetic acids, peptides, amino acids, sugars, and sugar alcohols such as mannitol [13,14,15,16] Along this line, P. pseudoalcaligenes AVO110 was shown to efficiently grow on minimal BM medium only when supplemented with R. necatrix exudates (BM-RE medium), reaching higher cell densities than other nonbiocontrol rhizobacterial strains [1]. AVO110 might harbor specific traits conferring a competitive advantage to this bacterium during its interaction with fungi
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