An introduction to plant pathogens
Plant pathogens cause infectious diseases to plants, and it is a big problem now. So, Biological control of plant pathogens is essential. It has become a challenge to produce food for the increasing population with the decreasing land availability. Phytopathogens that infect crop plants play an important role in food production because plant diseases reduce the growth of the plants and cause decreased crop yield. There are several strategies to control the causative agents responsible for plant diseases. Prevailing control methods include extrusion of diseased plants, cultural practices, usage of pesticides to control insect vectors, application of antibiotics (tetracycline, streptomycin) and antimicrobial compounds such as copper.
Copper has been used over generations as a chemical control measure against plant pathogens. Usage of copper sprays for a longer time may cause toxic effects to environment. In addition, certain phytopathogens may develop copper resistance. The issues regarding the usage of antibiotics are the intensive and repeated use of antibiotics will promote the development of resistant and it can cause the spread of antibiotic resistance genes to the environment. In circumstances, it is appropriate to use biocontrol agents (BCAs) instead of conventional control methods. BCAs can be either bacteriophages, filamentous fungi or bacteria that have the capability to control the particular pathogens.
Bacteriophages as BCAs
Bacteriophages, simply the viruses that attack and kill bacteria can be used as biopesticides. There are well-established protocols to isolate, purify and culture bacteriophages.
Phages that are used as BCAs should not have detrimental effects on beneficial bacteria or host plant. They should be stable and able to survive in field conditions. In bacteriophage mediated biocontrol, exclusively lytic (virulent) phages are used. Lytic phages result the lysis of host bacterium after the infection releasing progeny bacteriophages. They should be lack of transduction of bacterial DNA. The ideal phage should possess a wide host range which leads to a successful infection of all strains of the phytopathogenic bacterial genus. It should be able to lyse the pathogen rapidly releasing a large number of progeny particles that can be able to diffuse throughout the field that they are being applied. These progeny phages will continue to infect neighbouring phytopathogenic bacteria.
How to use them?
Basically, the first step in developing a bacteriophage mediated BCA is the isolation of a phage from soil, infected plant tissues of the bacterial infection etc. Soft agar overlay technique employs in isolation of the phage. It is better to try for a phage cocktail that is capable of infecting all the phytopathogenic bacterial strains responsible for the disease. To design a cocktail, it is essential to determine the host range of each phage.
Once the phages are isolated, characteristic features, genetics and environmental impacts of them should be well studied to ensure that they are suitable BCAs. Then after several laboratory and field trials, they are introduced as BCAs.
Pros and cons
Bacteriophage mediated biocontrol is a kind of environmental friendly approach that does not cause toxic effects. Phages cause no harm to eukaryotic cells and no toxic residues are remained in food that is obtained from BCA- applied plants. They have the ability to increase their populations by multiplying on the target bacterial host and it is rapid and relatively less expensive. Bacteriophage mediated biocontrol strategy can confer effective disease control as a part of an integrated management strategy.
Though usage of bacteriophages as BCAs is an effective approach, it has not been commonly used as an antibacterial therapy against plant pathogens due to several reasons such as narrow spectrum of activity against specific hosts, possibility of the development of phage resistance, laborious criteria for selecting an ideal phage, persistence of phages in rhizosphere and phyllosphere etc.
Darani Nirasha – Microbiologist