Antagonists Of Plant Pathogens Research Articles

Soil fungal communities under slash‐and‐burn system in Mozambique: A metataxonomic approach

AbstractThis study provides a metataxonomic analysis of the fungal communities in soils under slash‐and‐burn agroforestry system and offers new insights into the relationships between fungal populations and soil physicochemical features such as pH, the particle size distribution, easily oxidizable organic carbon, total nitrogen, available phosphorus, and the mineralogical composition. Soils from three locations in central Mozambique—Vanduzi, Sussundenga, and Macate—that are subjected to slash‐and‐burn were considered to assess the effects of the forest fallow length (temporal variation) and of the land use (charcoal kiln, crop field, and forest; meaning horizontal variation) on the fungal community. The fungi of the genetic horizons (vertical variation) were also considered. Most of the detected fungi were decomposers, antagonists of plant pathogens, and plant‐growth promoters; they were differently distributed in relation to the soil's physicochemical properties and the soil use. The variations in the fungi distribution among the locations and between the horizons were considerable, while there were few variations between the different land‐use types. The limited differences between land uses indicate the inability of a forest fallow period shorter than 50 years to improve soil fertility and induce changes in the fungal community. The pedological approach used to identify and sample soil horizons allowed us to clearly distinguish the fungal community of the A horizons, those richest in organics and nutrients, and that of the Bo horizons, which have poor fertility.

Fighting plant pathogens with cold-active microorganisms: biopesticide development and agriculture intensification in cold climates.

Cold-adapted (CA) microorganisms (= psychrophiles or psychrotolerants) are key players of many ecological interactions in natural ecosystems. Some of them can colonize the rhizosphere of plants and cause damage to their hosts; others, on the contrary, protect plants from their pathogens through direct and indirect mechanisms, thus promoting plant growth and development. These "protective" microbes are known as biocontrol agents (BCA). BCA either limit or inhibit the growth of plant pathogens, owing to the excretion of a panoply of secondary metabolites (including soluble and volatile antibiotics, siderophores, quorum sensing interfering agents). BCA can also control plant pathogens through indirect mechanisms, including competence for nutrients and space, or else by interfering with their chemical communication. That explains why some of these BCA have been included in the formulation of commercial biopesticides, which are environmentally friendly products containing live cells used to control plant diseases and pests. At present, the development of biopesticides from mesophilic microorganisms is an established technology. Unfortunately, these biopesticides are not active at low temperatures. On the other hand, the information concerning the potential use of CA-BCA for the same goal is at its infancy. Here, we review the current knowledge concerning the isolation, identification, and characterization of CA microbes which act as antagonists of plant pathogens, including the mechanisms they deploy to antagonize plant pathogens. We also illustrate their biotechnological potential to develop CA biopesticides and discuss their utility in the context of mountainous agriculture. KEY POINTS: • Many naturally occurring cold-active microbes antagonize plant pathogens. • The mechanisms of biocontrol exerted by these microbes are either direct or indirect. • Cold-active biocontrol agents can be used to develop biopesticides. • Cold-active biopesticides are crucial for sustainably intensifying agriculture in cold climates.

Investigating hyperparasites as potential biological control agents of rust pathogens on cereal crops

Rust pathogens cause damage to cereal crops around the world, leading to reduced yield and profit. Current methods of rust control include fungicides, resistant cultivars, and preventative agronomic practices. Some hyperparasites are antagonists of plant pathogens and may provide a potential method of biocontrol against increasingly virulent strains of rust. Very little is known about the mechanism of inhibition of rust growth by hyperparasites, however, isolation of new strains and subsequent characterisation may reveal new treatment strategies for the control of rust. Here we report the isolation of six new fungal hyperparasites and their effects on the development of three Puccinia rust pathogens were examined in vitro, to determine the potential of each as biocontrol agents of rust. Cut-leaf sections of rust-susceptible wheat, barley and oat cultivars were treated with fungal hyperparasite conidia prior to infection with the rust species; Puccinia triticina, P. hordei, and P. coronata f. sp. avenae, respectively. Inhibition of rust spore germination tests were also performed on water agar plates co-inoculated with the isolates. In leaf sections, rust pustule number was significantly (P < 0.01) lower for all six isolates tested: Penicillium brevicompactum, Clonostachys rosea, Simpicillium aogashimaense, Neoascochyta sp., Lecanicillium psalliotae and Epicoccum nigrum. The results of these experiments suggest that the mechanisms underlying the reduction in pustule number could include one or more of the processes of direct parasitism, antagonism by antibiosis, competition, and/or induction of host plant resistance.

INDIGENOUS RHIZOBACTERIA SCREENING FROM TOMATO TO CONTROL Ralstonia syzigii subsp. indonesiensis AND PROMOTE PLANT GROWTH RATE AND YIELD

Indigenous rhizobacteria screening from tomato to control Ralstonia syzigii subsp. indonesiensis and promote plant growth rate and yield. Bacterial wilt is the most damaging vascular pathogen on tomato and many other crops in tropical, subtropical and warm temperate areas of the world which limits the production. Rhizobacteria have been concerned as potential biological control agents due to their ability to promote plant growth and health, and their role as antagonists of plant pathogens. The purpose of this research was to screen the best indigenous rhizobacteria (IRB) that able to control bacterial wilt disease and increase growth rate and yield of tomato plant. This research was conducted in 3 stages: (1) Isolation and selection of indigenous rhizobacteria as PGPR on tomato seedlings, consisted of 27 IRB isolates and a control, with triplications; (2) Selection of IRB isolates that control R. syzigii subsp. indonesiensis on tomato plants, which consisted of 8 treatments including 7 IRB and a control with 5 replications; (3) Characterization of IRB isolates ability to promote plant growth (indicated with IAA production &amp; phosphate solubilizing). The variables observed were disease development, growth enhancement and IRB isolate ability to produce IAA and solubilize phosphate. The results showed that all IRB isolates were able to control bacterial wilt disease and increase the growth rate and yield of tomato. IR2.3.5, IR1.3.4 and IR1.4.2 were the best isolates in controlling R. syzigii subsp. indonesiensis and increasing the growth rate and yield by 81.25% and 68.72% respectively. All isolates showed various abilities to produce IAA, however, only isolates IR2.3.5 and IR1.3.4 that had abilities to solubilize phosphate.

Phylogenomic analysis shows that Bacillus amyloliquefaciens subsp. plantarum is a later heterotypic synonym of Bacillus methylotrophicus.

The rhizosphere-isolated bacteria belonging to the Bacillus amyloliquefaciens subsp. plantarum and Bacillus methylotrophicus clades are an important group of strains that are used as plant growth promoters and antagonists of plant pathogens. These properties have made these strains the focus of commercial interest. Here, we present the draft genome sequence of B. methylotrophicus KACC 13105(T) ( = CBMB205(T)). Comparative genomic analysis showed only minor differences between this strain and the genome of the B. amyloliquefaciens subsp. plantarum type strain, with the genomes sharing approximately 95% of the same genes. The results of morphological, physiological, chemotaxonomic and phylogenetic analyses indicate that the type strains of these two taxa are highly similar. In fact, our results show that the type strain of B. amyloliquefaciens subsp. plantarum FZB42(T) ( = DSM 23117(T) = BGSC 10A6(T)) does not cluster with other members of the B. amyloliquefaciens taxon. Instead, it clusters well within a clade of strains that are assigned to B. methylotrophicus, including the type strain of that species. Therefore, we propose that the subspecies B. amyloliquefaciens subsp. plantarum should be reclassified as a later heterotypic synonym of B. methylotrophicus.

Fungal entomopathogens: new insights on their ecology

An important mechanism for insect pest control should be the use of fungal entomopathogens. Even though these organisms have been studied for more than 100 y, their effective use in the field remains elusive. Recently, however, it has been discovered that many of these entomopathogenic fungi play additional roles in nature. They are endophytes, antagonists of plant pathogens, associates with the rhizosphere, and possibly even plant growth promoting agents. These findings indicate that the ecological role of these fungi in the environment is not fully understood and limits our ability to employ them successfully for pest management. In this paper, we review the recently discovered roles played by many entomopathogenic fungi and propose new research strategies focused on alternate uses for these fungi. It seems likely that these agents can be used in multiple roles in protecting plants from pests and diseases and at the same time promoting plant growth.

Ice nucleation active bacteria and their potential role in precipitation

Certain bacteria that are commonly found on plants have the capacity to catalyze the freezing of supercooled water at temperatures as warm as −1 ◦ C. This is conferred by a protein in the outer membrane of the bacterial cell. Because of the abundance of these bacteria and the warm temperature at which they function as ice nuclei, they are considered to be among the most active of the naturally-occurring ice nuclei. As plant pathogens, antagonists of plant pathogens and as causal agents of frost damage, these bacteria have well- studied interactions with plants. Here we propose that these bacteria also play a role in atmospheric processes leading to rain, given that they are readily disseminated into the atmosphere and have been found in clouds at altitudes of several kilometers. That they participate in a sort of biological cycle of precipitation - whereby they are transported into clouds from plant canopies and incite rain thereby causing favorable conditions for their growth on plant surfaces - was proposed about 20 years ago. Today, sufficient evidence and meteorological tools have emerged to re-ignite interest in bioprecipitation and in the ways in which plants play a role as cloud seeders.

In vitro screening for the identification of potential biocontrol agents of Allium white rot

A multi-test in vitro system was designed to identify antagonistic micro-organisms of the Allium white rot (AWR) pathogen, Sclerotium cepivorum. Micro-organisms (215) comprising 140 fungi, 58 bacteria and 17 actinomycetes were isolated from AWR soils, pathogen sclerotia and diseased host tissue. Fungi were screened and scored for volatile and non-volatile antibiotic activity, inhibition of sclerotium formation, and overgrowth of colonies of S. cepivorum on agar plates. Bacteria and actinomycetes were screened for antibiotic activity only. Comparisons with known antagonists of plant pathogens were also made. The screens enabled selection of antagonists to be made based on either a ‘total score’ from all tests or on a potential mode of action basis in selected tests. Statistical analysis of total score identified inhibition of sclerotia formation and position of sclerotia formation in dual culture as the most discriminating variates in the screen.