The ways in which plant growth promoting rhizobacteria facilitate the growth of plants are considered and discussed. Both indirect and direct mechanisms of plant growth promotion are dealt with. Th...

The enhancement of plant growth by free-living bacteria

Pathogenic microorganisms affecting plant health are a major and chronic threat to food production and ecosystem stability worldwide As agricultural production intensified over the past few decades, producers became more and more dependent on agrochemicals as a relatively reliable method of crop

Use of Plant Growth-Promoting Bacteria for Biocontrol of Plant Diseases: Principles, Mechanisms of Action, and Future Prospects

The loss of organic material from the roots provides the energy for the development of active microbial populations in the rhizosphere around the root. Generally, saproptrophs or biotrophs such as mycorrhizal fungi grow in the rhizosphere in response to this carbon loss, but plant pathogens may also develop and infect a susceptible host, resulting in disease. This review examines the microbial interactions that can take place in the rhizosphere and that are involved in biological disease control. The interactions of bacteria used as biocontrol agents of bacterial and fungal plant pathogens, and fungi used as biocontrol agents of protozoan, bacterial and fungal plant pathogens are considered. Whenever possible, modes of action involved in each type of interaction are assessed with particular emphasis on antibiosis, competition, parasitism, and induced resistance. The significance of plant growth promotion and rhizosphere competence in biocontrol is also considered. Multiple microbial interactions involving bacteria and fungi in the rhizosphere are shown to provide enhanced biocontrol in many cases in comparison with biocontrol agents used singly. The extreme complexity of interactions that can occur in the rhizosphere is highlighted and some potential areas for future research in this area are discussed briefly.

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Microbial interactions and biocontrol in the rhizosphere

Microbial pathogenesis in cystic fibrosis: mucoid Pseudomonas aeruginosa and Burkholderia cepacia.

The colonization of land by plants appears to have coincided with the appearance of mycorrhiza-like fungi. Over evolutionary time, fungi have maintained their prominent role in the formation of mycorrhizal associations. In addition, however, they have been able to occupy other terrestrial niches of which the decomposition of recalcitrant organic matter is perhaps the most remarkable. This implies that, in contrast to that of aquatic organic matter decomposition, bacteria have not been able to monopolize decomposition processes in terrestrial ecosystems. The emergence of fungi in terrestrial ecosystems must have had a strong impact on the evolution of terrestrial bacteria. On the one hand, potential decomposition niches, e.g. lignin degradation, have been lost for bacteria, whereas on the other hand the presence of fungi has itself created new bacterial niches. Confrontation between bacteria and fungi is ongoing, and from studying contemporary interactions, we can learn about the impact that fungi presently have, and have had in the past, on the ecology and evolution of terrestrial bacteria. In the first part of this review, the focus is on niche differentiation between soil bacteria and fungi involved in the decomposition of plant-derived organic matter. Bacteria and fungi are seen to compete for simple plant-derived substrates and have developed antagonistic strategies. For more recalcitrant organic substrates, e.g. cellulose and lignin, both competitive and mutualistic strategies appear to have evolved. In the second part of the review, bacterial niches with respect to the utilization of fungal-derived substrates are considered. Here, several lines of development can be recognized, ranging from mutualistic exudate-consuming bacteria that are associated with fungal surfaces to endosymbiotic and mycophagous bacteria. In some cases, there are indications of fungal specific selection in fungus-associated bacteria, and possible mechanisms for such selection are discussed.

Living in a fungal world: impact of fungi on soil bacterial niche development⋆

Trichoderma harzianum was applied to cucumber and pepper seedlings as a peat-bran preparation incorporated into the propagative mixture in a commercial production nursery. On marketing day (after 18 and 30 days for cucumber and pepper, respectively), significant increases of 23.8% and 17.2% in seedling height, 96.1% and 50% in leaf area, and 24.7% and 28.6% in plant dry weight were observed in cucumber and pepper seedlings, respectively, as compared to their non-treated counterparts.Trichoderma-treated seedlings were much more developed and vigorous and had higher chlorophyll contents. No significant differences were found in N, P or K content between treatments. Cucumber seedlings were then transplanted to a commercial greenhouse and analyzed over two successive growth cycles following soil fumigation with methyl bromide (500 kg/ha). Results revealed theTrichoderma-treated plants to be more resistant to damping-off disease. During the first cycle, immediately after soil fumigation, no damping-off was observed with either treatment, except in border beds where 4% of the non-treated plants died, as compared to no damping-off in theTrichoderma-treated plants. During the second growing cycle however, significant reductions in damping-off of 67% and 52% were obtained in middle and border beds, respectively, as compared to the non-treated controls.

Plant growth enhancement and disease control byTrichoderma harzianum in vegetable seedlings grown under commercial conditions

A chitinolytic isolate of Aeromonas caviae was isolated from roots of healthy bean plants grown in soil artificially infested with Sclerotium rolfsii. Under greenhouse conditions, the bacterium controlled Rhizoctonia solani and Fusarium oxysporum f.sp. vasinfectum in cotton (78 and 57% disease reduction, respectively) and S. rolfsii in beans (60% disease reduction). Seed coating was the most effective application method for controlling R. solani in cotton. There was no evidence of inhibition of the fungal pathogens by A. caviae. A caviae partially lysed live mycelium of R. solani, S. rolfsii and F. oxysporum f.sp. vasinfectum when their mycelium served as a sole carbon source in liquid medium. A high chitinolytic activity was found when colloidal chitin was used as a sole carbon source, with an optimum pH between 6.0–7.0. No β-3-glucanase was produced by the bacterium. After partial purification of the enzyme by affinity adsorption to colloidal chitin, three bands appear in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE). One strong band with a molecular weight of ca. 80 kDa, and two weak bands with molecular weights of 48 and 59 kDa. Using the chromogenic substrate pNp-chitobiose, the partially purified chitinase from A. caviae was shown to act in an exo-splitting manner.

Evidence that chitinase produced by Aeromonas caviae is involved in the biological control of soil-borne plant pathogens by this bacterium.

Summary: The induction of chitinolytic enzymes in the biocontrol agent Trichoderma harzianum during parasitism on Sclerotium rolfsii and the role of fungal-fungal recognition in this process were studied. A change in the chitinolytic enzyme profile was detected during the interaction between the fungi, grown in dual culture on synthetic medium. Before coming into contact with each other, both fungi contained a protein with constitutive 1,4-†bT-N-acetylglucosaminidase activity. As early as 12 h after contact, the chitinolytic activity in S. rolfsii disappeared, while that of T. harzianum (a protein with a molecular mass of 102 kDa, CHIT 102) greatly increased. After 24 h of interaction, the activity of CHIT 102 diminished concomitantly with the appearance of a 73 kDa 1,4-β-N-acetylglucosaminidase, which became clear and strong at 48 h. This phenomenon did not occur if the S. rolfsii mycelium was autoclaved prior to incubation with T. harzianum, suggesting its dependence on vital elements from the host. Cycloheximide inhibited this phenomenon, indicating that de novo synthesis of enzymes is taking place in Trichoderma during these stages of the parasitism. A biomimetic system based on the binding of a purified surface lectin from the host S. rolfsii to nylon fibres was used to dissect the effect of recognition. An increase in CHIT 102 activity was detected, suggesting that the induction of chitinolytic enzymes in Trichoderma is an early event which is elicited by the recognition signal (i.e. lectin-carbohydrate interactions). It is postulated that recognition is the first step in a cascade of antagonistic events which triggers the parasitic response in Trichoderma.

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The role of recognition in the induction of specific chitinases during mycoparasitism by Trichoderma harzianum

Pokeweed antiviral protein (PAP), a 29-kD protein isolated from Phytolacca americana inhibits translation by catalytically removing a specific adenine residue from the large rRNA of the 60S subunit of eukaryotic ribosomes. Transgenic plants expressing PAP are resistant to a broad spectrum of plant viruses. Nontoxic PAP mutants have been isolated by random mutagenesis and selection in yeast. One of these mutants, PAP-X, had a point mutation at the active-site (E176V) that abolished enzymatic activity, and another mutant, delta C25PAP, had a nonsense mutation near the C-terminus (W237stop) that deleted 25 C-terminal amino acids. Unlike the wild-type PAP, expression of neither mutant was toxic to transgenic plants. We show that both class I (basic) and class II (acidic) isoforms of pathogenesis-related (PR) proteins are overexpressed in transgenic plants expressing PAP and the nontoxic PAP mutants. Although PR-proteins are constitutively expressed, no increase in salicylic acid levels was detected. Homozygous progeny of transgenic plants expressing either PAP or the nontoxic PAP mutants displayed resistance to the fungal pathogen Rhizoctonia solani. These results show that expression of PAP or the nontoxic PAP mutants activates multiple plant defense pathways independently of salicylic acid and confers resistance to fungal infection. The C-terminal 25 amino acids of PAP, which are required for toxicity in vivo, are not critical for resistance to viral or fungal infection, indicating that toxicity of PAP can be separated from pathogen resistance.

Plant resistance to fungal infection induced by nontoxic pokeweed antiviral protein mutants

Two-hundred and three different strains of bacteria were isolated from the rhizosphere of bean, peanut and chickpea plants grown in Sclerotium rolfsii infested soil. A bacterium, identified as Serratia marcescens , was found to be the best biocontrol agent of the pathogen, under greenhouse conditions (up to 75% disease reduction). Populations of 10 5 or 10 6 CFU g −1 soil were the most effective in disease control. The drench and drip application of S. marcescens suspension were more effective in controlling S. rolfsii than spraying, mixing in soil or seed coating. This bacterium significantly reduced damping-off incidence of bean, caused by Rhizoctonia solani , by 50%, but was not effective against Pythium aphanidematum in cucumber. A natural mutant of S. marcescens , resistant to the antibiotic rifampicin, was isolated. The mutant, effective as the wild type, was used to study rhizosphere colonization. The highest population density of the bacteria was found on the proximal portion of the root, decreasing significantly until the tips, where they increased again.

Rhizosphere colonization by serratia marcescens for the control of sclerotium rolfsii

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