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Can bacteriophages combat bacterial diseases of plants?

The bacterial nature of a plant disease was first proven in 1878–1880 by T. J. Burrill in the University of Illinois when studying fire blight. The number of known species of pathogenic bacteria is in constant change due to the clarification of phylogenetic relatedness upon receipt of new data, but as of 2014 the number of phytopathogenic bacteria genera that unite them was 39 1.

Plant bacteriosis pathogens recognised as significant for science and economics are Pseudomonas syringae causing speck, spot and canker diseases, Ralstonia solanacearum – bacterial wilt of potato, Agrobacterium tumefaciens – crown gall, Xanthomonas oryzae – rice bacterial blight, Xanthomonas campestris – black rot, bacterial leaf spot, Xanthomonas axonopodis pv. manihotis – cassava bacterial blight, Erwinia amylovora – fire blight, Xylella fastidiosa – Pierce’s disease of grapevine, citrus variegated chlorosis, almond leaf scorch disease, Dickeya dadantii and solani – soft rot and black leg disease of potato, Pectobacterium carotovorum and P. atrosepticum – soft rot and blackleg 2.

The difficulty of controlling plant bacterial diseases is the postponement of the moment of diagnosis due to the latent internal development of the pathogen in “silent patients”. For example, the bacteria of E. amylovora species are able to migrate through the plant in upward and downward directions over long distances, herewith not secreting enzymes to dissolve the plant tissues. When establishing the need for treatment, the target bacteria-pathogens often reside at hard-to-reach places – in the thickness of plant tissues.

One of the cardinal and effective methods of combating disease epidemics is pruning, uprooting and crop rotation. Crop rotation is a natural agricultural practice for herbaceous plants, for example, corn and sorghum, whereas eradication of woody plants brings significant economic harm, leaving orchards without a harvest for several years until regeneration. Special sprays of chemical agents such as antibiotics and copper-containing compounds are used to prevent the reproduction and spread of the pathogen in orchards and other plantations during the season.

Due to some restrictions on the use of chemical agents associated with legislation and the emergence of bacterial resistance or sensitivity of plant cultures, the possibility of application biological remedies is being considered. For example, siderophore-producing Pseudomonas fluorescens strains can suppress P. carotovorum.

Positive results in controlling bacterial infections were shown by using direct antagonists-predators – bacteriophages. For example, comparable to copper-mancozeb or better efficacy of reducing the incidence of Xanthomonas leaf blight of onion, as well as tomato bacterial spot, was shown in field tests 3. Under what conditions can bacteriophages serve as a means of controlling phytopathogenic bacteria?

The nucleocapsid organisation of particles of plant viruses and their ability to cause disease epidemics are of interest to the joint consideration of two parasitic systems: the bacteriophage-bacterium and the phytopathogenic virus-plant. Some comparative data on bacteriophages and plant viruses are presented in table 1.

The properties of bacteriophages and plant viruses
Properties Bacteriophages Plant Viruses
Particle size, examples [4-6] T4 92 nm
P2 60 nm
λ 63 nm M13
6×800 nm
Tobacco mosaic virus (TMV) 18×300 nm
Tomato spotted wilt virus (TSWV) 70-80 nm
Potato virus Y (PVY) 15×730-790 nm
Brome mosaic virus (BMV) 25 nm
Virion composition DNA or RNA and proteins, sometimes lipids RNA (for the majority) and proteins, rarely glycoproteins, lipids
Course of infection Lytic or lysogenic life cycle Persistent, acute, chronic or endogenous  lifestyle
Infection conditions A sufficiently large number of bacterial cells to allow contact and subsequent lysis of the bacterial population, as well as overcoming physical barriers by bacteriophage particles, for example, in the case of an endophytic bacterial population Contact with cells when penetrating plants through the pores, as a result of mechanical damage to plants or with the help of carriers
Persistence to external factors, examples[1] Escherichia coli ELP1 phage was persistent in soil with high salinity and low moisture content after 28 days of irrigation [7]
Pseudomonas phaseolicola Phi6 was stable at pH 6 [8]
Pectobacterium phages were stable at 16°C-40°C and pH 6-7, with a slight decrease at pH 8-11 [9]
Ralstonia solanacearum phage φ RSL1 was stable in soil at temperatures of 37–50°C and recovered from roots of treated plants and from soil in 4 months after infection [10]
E. coli phage T4, Salmonella enterica PRD1 and Sulfolobus solfataricus SSV-K were reversibly inactivated by silicification under conditions similar to volcanic hot springs [11]  Pseudomonas syringae pv. actinidiae phages were tolerant to extended UV-B doses [3]
Tobacco mosaic virus (TMV) was viable in cigarettes or cigar tobacco [12]
Barley stripe mosaic virus was stable at pH 6-6.5 [13]
Cucumber mosaic virus (CMV) was more active on cowpea and tobacco leaves at pH 7-8, than at pH 6 [14]
Tomato spotted wilt tospovirus (TSWV) was stable after in vitro 10 min incubation at 44 °C, but not at 46 °C [15]

[1] Examples of phages associated with phytopathogenic bacteria and others are presented. Bacteriophages capable of withstanding extreme conditions and isolated, for example, from hot springs or deserts, are not shown here.

Despite the significant difference between two parasitic systems on the objects of infection, the structural organisation similarity of two types of viruses, as well as the flow of life cycle stages in phytocenosis in the case of plant viruses and phytopathogenic bacteria phages, allow to extrapolate some conclusions about the conditions of successful infection of the host in the phytopathogenic virus-plant system on the bacteriophage-phytopathogenic bacterium system.

There are two important factors playing the role of plant viruses epidemic drivers: climatic conditions and modes of transmission. The climatic conditions of successful circulation of viruses in nature, such as light and temperature, must correspond to the parameters of resistance of virus particles. In the case of organising the control of bacteria with the help of phages, environmental conditions are also required to be “tuned”, that can be implemented in three directions: selection of resistant phages to the required range of physicochemical factors; the use of protective compositions in phage preparations; optimisation of time parameters of treatment with phage preparations, for example, treatment of plants in the evening to minimise the dose of UV irradiation.

Essential to the spread of plant viruses are grafting and vector transport by arthropods. Grafting is an unintentional way of contracting the virus, however, is very common. The role of carriers is illustrated by an example of significant reduction in the incidence viral infections of plants in areas with a low number of insects (sea coasts). Currently, there is no evidence of the effective participation of arthropod vectors in the spread of bacteriophages, however, research in this direction is underway.

In studying the effectiveness of bacteriophage application, direct treatment of plants with bacteriophage-based preparations is used, and drug sprays are the most common treatment method. Application of preparation for a soil drench could serve as an alternative method, as well as injection into a tree trunk – a method tested on antibiotic preparations.

Thus, when considering two parasitic systems, the importance of a combination of factors or epidemic drivers in the success of infection becomes obvious, and therefore an integrated approach is needed to solve the problem of controlling pathogens with the help of bacteriophages.

 

Natalya Vasilievna Besarab is a PhD student at the Belarusian State University. Her research interests include bacteriophages and molecular biology, phytopathology and biological remedies.

 

Have you seen the agenda for the Plant Genomics and Gene Editing Congress: Europe? Take a look now to see what new research will be presented.

View full list of references here.

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