Bacteriophage Application in Agriculture

Bacteriophage Application in Agriculture

Importance of Phages in the Agriculture Sector

Our ever-growing world population requires an ever-growing supply of food. Chakraborty et al. predict that global food production must increase by up to 50% by 2050 to meet the demand. As such, the agricultural sector faces increasing pressure to produce greater volumes of food, even more efficiently. However, plant crop farmers face the onslaught of pest species and diseases, limiting supply with substantial economic losses.

Since as early as the 1880s, commercial growers have used chemical bactericides to control pathogenic diseases in crops. After the Second World War there followed an explosion in the use of antibiotics. For example, the antibiotic streptomycin has been used to fight against phytopathogenic bacteria in plants since the 1950s. The mass overuse of copper-based bactericides and antibiotics has caused many pathogenic bacteria to resist the control methods. The optimal growth conditions for the bacteria within crops mean that they replicate rapidly, and their mutations can be easily passed on. 

The pressure of producing a greater food supply, and the growing resistance of pathogenic bacteria to current chemical controls have pushed research to develop biological-based alternatives for the biocontrol of disease in plants. This is where bacteriophages come into play. Bacteriophages (otherwise known as phages) are the most prevalent biological entity in the biosphere. They are viruses that have a narrow ‘host-range’, meaning that they can target and kill specific bacteria, and they are an ideal biological alternative for bacteria control.

Phages are advantageous for disease biocontrol in plants as they self-replicate and self-limit. Phages will only continue to replicate for as long as the host bacteria are still present, and they have been shown to be non-toxic to eukaryotic cells. This means that the consumer can rest assured that no harm will come to them as a result of bacteriophage application to crops. Whereas, the same could not be said for chemical controls such as bactericides, that cause concern about their impact on human health and the environment. In addition, phages are easy and cheap to prepare and easy to store for long-term use.

Therefore, bacteriophages offer a promising alternative to chemical controls for plant diseases and can be incorporated into integrated disease management strategies for effective biological control of plant pathogens.

Common Challenges with Plant Bacteria

Plant pathology has revealed that more than 200 pathogenic bacterial species can attack plants. The major pathogenic bacteria belong to either the Agrobacterium, Ralstonia, Xanthomonas or Pseudomonas genera.

Bacteria infect plants through natural openings, or through inflicted ‘wounds’ created by insects or environmental damages. Different pathogenic bacteria require a variety of conditions in order to survive and spread and impact a large range of crops. Examples of pathogenic bacteria that affect vegetable crops include:

  • Bacterial leaf spot, Pseudomonas syringae. This bacteria spreads in beetroot, spring onions and leeks, rockets and coriander, causing grey spots, lesions and discolouration in each different plant.
  • Black rot, Xanthomonas campestris pv. campestris affects brassicas and causes lesions on leaves and vein blackening.
  • Bacterial wilt, Ralstonia solanacearum, targets potatoes, tomatoes, peppers and aubergines. This bacteria causes wilting, yellowing and death of the plants.

Erwinia amylovora is another example of a pathogenic bacteria that impact plant crops, specifically affecting species that are part of the Rosaceae family. This pathogen causes the disease known as fire blight. Symptoms of fire blight include the wilting and dying of blossoms during flowering, white oozing liquid exuding from the infected site, shoots dying, and cankers on branches. The control of Erwinia amylovora is difficult, as many strains are now resistant to streptomycin, and other strategies such as plant resistance inducers have shown limited efficacy. It has been estimated that Erwinia amylovora causes over $100 million in the US, due to losses from fire blight and funding spent on its control.

Application of Bacteriophages for Bacterial Disease Management in Plants

Bacteriophages were discovered independently by Frederick Twort in 1915 and Felix d’Herelle in 1917. The first studies using phages for the biocontrol of plant disease were conducted in 1924, showing that bacteriophages could be used to prevent cabbage rot by controlling the bacteria, Xanthomonas campestris pv. campestris. This was a pre-antibiotic era, and research into bacteriophage use was pushed aside following the discovery of antibiotics. However, with antibiotic resistance on the rise, bacteriophage research came back with a vengeance in the early 2000s.  

Phage therapy, the use of bacteriophages to target and treat a bacterial infection, is now commonplace in research for disease biocontrol in plant crops. Rombouts et al. conducted field trials with a cocktail of six bacteriophages, to control the spread of P. syringae, one of the top 10 most important plant pathogens according to a 2012 survey. The study showed the potential of phage mixtures for symptom control of plant disease. 

In addition to their potential for therapeutic use, bacteriophages can also be used for plant disease diagnostics. The current common protocols for disease detection in crops rely heavily on culture-based methods. These are slow and time-consuming, lacking specificity and accuracy in their outputs. However, phage-based methods can be a cost-effective alternative for detecting disease, using either reporter phage or phage amplification techniques. Reporter phages transport reporter genes (e.g. luciferase) into the bacteria, leading to expression and subsequent detection. Whereas phage amplification involves the bioproduction of phages within the bacteria, which is then lysed and plaques are formed by a reporter organism for detection.

Nexabiome has sought to overcome plant health challenges by offering natural and sustainable solutions. They have tailored their solutions for farm or retail customer use. For farms, agriPHIX solutions provide a natural approach to improving the storage of a range of crops. For the retail customer, freshPHIX solutions provide a natural and sustainable answer, which significantly increases shelf life. As an example, trials with bagged spinach have demonstrated a two-day extension of shelf life.

Conclusions and future prospects

Many pathogenic bacteria species can impact plants, disrupting the food supply of our increasing world population, and causing huge economic losses. The use of chemical bactericides and antibiotics is becoming ineffective to control the spread of plant disease, with the rise of resistant bacterial strains. Therefore, there is a growing demand for biological alternatives. Bacteriophages offer a solution to these problems, as discriminatory viruses that target harmful bacteria whilst remaining non-toxic to consumers. 

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