Delving into defence: identifying the Pseudomonas protegens Pf-5 gene suite involved in defence against secreted products of fungal, oomycete and bacterial rhizosphere competitors

Involvement of a novel Pseudomonas protegens strain associated with entomopathogenic nematode infective juveniles in insect pathogenesis

BACKGROUND

The bioinsecticidal action of entomopathogenic nematodes (EPNs) typically relies on their symbiosis with core bacteria. However, recent studies highlighted the possible involvement of other noncore species. We have recently isolated a novel Pseudomonas protegens strain as a major agent of septicaemia in larvae of the wax moth, Galleria mellonella, infected with a soil-dwelling Steinernema feltiae strain. The actual role of this bacterium in entomopathogenesis was investigated.

RESULTS

The association of P. protegens with nematodes appeared to be robust, as supported by its direct and repeated isolation from both nematodes and insect larvae infected for several consecutive generations. The bacterium appeared to be well-adapted to the insect haemocoel, being able to proliferate rapidly after the injection of even a small amount of living cells [100 colony forming units (CFU)] to a larva, causing its fast death. The bacterium also was able to act by ingestion against G. mellonella larvae [median lethal concentration (LC₅₀ ) = 4.0 × 10⁷  CFU mL^-1 ], albeit with a slower action, which supports the involvement of specific virulence factors (e.g. chitinases, Fit toxin) to overcome the intestinal barrier to the haemocoel. Varying levels of bacterial virulence were observed on diverse target Diptera and Lepidoptera.

CONCLUSION

The soil-dwelling bacterium P. protegens appears to have evolved its own potential as a stand-alone entomopathogen, yet the establishment of an opportunistic association with entomoparasitic nematodes would represent a special competitive advantage. This finding contributes to a deeper understanding of the nematode-bacteria biocontrol agent complex and the deriving paradigm of their use as biological control agents. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Loss of Gramicidin Biosynthesis in Gram-Positive Biocontrol Bacterium Aneurinibacillus migulanus (Takagi et al., 1993) Shida et al. 1996 Emend Heyndrickx et al., 1997 Nagano Impairs Its Biological Control Ability of Phytophthora

The soil-borne species Aneurinibacillus migulanus (A. migulanus) strains Nagano and NCTC 7096 were shown to be potent biocontrol agents active against several plant diseases in agricultural and forest ecosystems. Both strains produce the cyclic peptide gramicidin S (GS) that was described as the main weapon inhibiting some gram-negative and gram-positive bacteria and fungus-like organisms along with the production of biosurfactant and hemolysis activities. However, the contribution of the cyclic peptide gramicidin S (GS) to the biocontrol ability of A. migulanus has never been studied experimentally. In this paper, using a mutant of the A. migulanus Nagano strain (E1 mutant) impaired in GS biosynthesis we evaluated the contribution of GS in the biocontrol potential of A. migulanus against Phytophthora spp. The two strains of A. migulanus, Nagano and NCTC 7096, were tested in a pilot study for the inhibition of the growth of 13 Phytophthora species in dual culture assays. A. migulanus Nagano was significantly more inhibitory than NCTC 7096 to all species. Additionally, using apple infection assays, P. rosacearum MKDF-148 and P. cryptogea E2 were shown to be the most aggressive on apple fruits displaying clear infection halos. Therefore, the three A. migulanus strains, Nagano, NCTC 7096, and E1, were used in apple infection experiments to check their effect on infection ability of these two Phytophthora species. Treatment with A. migulanus Nagano significantly reduced the severity of symptoms in apple fruits compared with NCTC 7096. A. migulanus E1 mutant showed total loss of biocontrol ability suggesting that GS is a major actor in the biocontrol ability of A. migulanus Nagano strain.

Metabolism of Aldoximes and Nitriles in Plant-Associated Bacteria and Its Potential in Plant-Bacteria Interactions

In plants, aldoximes per se act as defense compounds and are precursors of complex defense compounds such as cyanogenic glucosides and glucosinolates. Bacteria rarely produce aldoximes, but some are able to transform them by aldoxime dehydratase (Oxd), followed by nitrilase (NLase) or nitrile hydratase (NHase) catalyzed transformations. Oxds are often encoded together with NLases or NHases in a single operon, forming the aldoxime-nitrile pathway. Previous reviews have largely focused on the use of Oxds and NLases or NHases in organic synthesis. In contrast, the focus of this review is on the contribution of these enzymes to plant-bacteria interactions. Therefore, we summarize the substrate specificities of the enzymes for plant compounds. We also analyze the taxonomic and ecological distribution of the enzymes. In addition, we discuss their importance in selected plant symbionts. The data show that Oxds, NLases, and NHases are abundant in Actinobacteria and Proteobacteria. The enzymes seem to be important for breaking through plant defenses and utilizing oximes or nitriles as nutrients. They may also contribute, e.g., to the synthesis of the phytohormone indole-3-acetic acid. We conclude that the bacterial and plant metabolism of aldoximes and nitriles may interfere in several ways. However, further in vitro and in vivo studies are needed to better understand this underexplored aspect of plant-bacteria interactions.

Beyond Soil-Dwelling Actinobacteria: Fantastic Antibiotics and Where to Find Them

Bacterial secondary metabolites represent an invaluable source of bioactive molecules for the pharmaceutical and agrochemical industries. Although screening campaigns for the discovery of new compounds have traditionally been strongly biased towards the study of soil-dwelling Actinobacteria, the current antibiotic resistance and discovery crisis has brought a considerable amount of attention to the study of previously neglected bacterial sources of secondary metabolites. The development and application of new screening, sequencing, genetic manipulation, cultivation and bioinformatic techniques have revealed several other groups of bacteria as producers of striking chemical novelty. Biosynthetic machineries evolved from independent taxonomic origins and under completely different ecological requirements and selective pressures are responsible for these structural innovations. In this review, we summarize the most important discoveries related to secondary metabolites from alternative bacterial sources, trying to provide the reader with a broad perspective on how technical novelties have facilitated the access to the bacterial metabolic dark matter.