Engineering the future of food security: Can genetically modified microbes be environmentally sustainable drivers of crop production?

• Chemical pesticides previously helped drive the ‘Green Revolution’, which saw food production triple.
• Unfortunately, these chemicals are highly toxic and pollute the environment.
• Microbes that can naturally kill plant pests could provide alternative means of reducing crop damage.
• Associate Professor Ugur Azizoglu’s group at Kayseri University and Erciyes University, Turkey, is seeking to advance sustainable crop pest management strategies by genetically engineering so-called entomopathogenic microorganisms.
• This could provide an environmentally sustainable approach to pest management.

Throughout history, our ability to feed the growing population has been fraught with doubts over whether crop outputs could match the ever-increasing number of mouths to feed. With the world’s population forecast to increase to ~10.4 billion by 2100, there is an urgent need to increase food production to match the demands of a growing population. During a period of change to agricultural practices in the mid-20th century, termed the ‘Green Revolution’, the use of chemical pesticides, alongside higher-yielding crop varieties and fertilisers, saw significant increases in global crop outputs and reductions in crop losses due to insect pests.

An estimated 385 million people per year are unintentionally poisoned by chemical pesticides globally, with 11,000 deaths.

Pesticides, which include insecticides, are chemicals that are toxic to crop pests and crucial to preventing crop losses through insect damage. Without pesticides, we would see significant losses in fruit production (78%), vegetable production (54%), and cereal crops (32%) – their effects and influence cannot be underappreciated. Despite the benefits associated with crop defence against insects, many pesticides have harmful effects on the environment as well as on human and animal health. Using data collected by the World Health Organization (WHO), an estimated 385 million people per year are unintentionally poisoned by chemical pesticides globally, with 11,000 deaths.

Not only do traditional chemical pesticides have major effects on non-target species, including pollinators, but over-application of pesticides onto crops can result in contamination of water supplies, affecting human health and aquatic ecosystems and damaging the microbiota within agricultural soils. Pesticides also exert selective pressure on insects, forcing them to find new ways to evade their toxic effects and killing off those that cannot. Over time, pests that survive develop methods of resistance that render the pesticide ineffective.

In light of these issues, Associate Professor Ugur Azizoglu’s group at Kayseri University and Erciyes University, Turkey, is looking to use more natural and sustainable means to combat crop pests.

Entomopathogenic microbes

Entomopathogenic microorganisms are organisms like bacteria, fungi, protozoa, viruses, and nematodes (roundworms) that cause diseases in insects. These microbes infect and/or produce compounds that are toxic to or disrupt the growth of insects. Their ability to kill insect pests has been used as a natural approach to crop pest management for some time now.

A widely used example of a bioinsecticide is the entomopathogenic bacteria Bacillus thuringiensis (Bt). These ubiquitous soil-dwelling bacteria produce structures called parasporal crystal (Cry) proteins as part of their reproductive cycle. Caterpillars and other susceptible arthropods ingest these crystals, which dissolve in the alkaline conditions of the insect midgut, releasing and activating toxins that bind to cell receptors on the surface of epithelial cells in the insect midgut. Receptor binding initiates cell signalling events leading to perforation of the gut lining, sepsis, and insect death (Figure 1). Cry protein toxicity generally affects a narrow range of insect species due to the particular receptors Cry toxins bind to on the surface of midgut epithelial cells. Additional benefits of using Bt Cry proteins as pesticides are that they are not harmful to humans and are completely biodegradable, unlike the majority of chemical pesticides, thus minimising environmental damage.

Bt maize and cotton crops have been shown to repress insect damage and reduce the need for pesticide application in China and America.

Bt bacteria can either be sprayed directly onto crops, or plants can be genetically engineered to express the Cry proteins in their leaves and, when consumed, kills insects. Bt maize and cotton crops have been shown to repress insect damage and reduce the need for pesticide application in China and America. However, these benefits were soon undermined by the evolution of insect resistance to the Bt toxin, which is also a major problem for chemical pesticides.

Although entomopathogenic organisms produce natural toxins that kill insects very effectively, their effectiveness as a pesticide is reduced by pests developing resistance. So how can these native soil microbes be better utilised or improved? The continual development of recombinant DNA techniques may hold the answer.

Recombinant DNA technology

Recombinant DNA technology involves isolating DNA, typically genes encoding toxins (eg, Cry toxins), and using expression vectors (plasmids) to replicate and express them. DNA is integrated into plasmids at regions called cloning sites. Using restriction enzymes, which cut DNA in specific locations, the closed-loop structure of the plasmid can be opened to allow DNA to be integrated into the plasmid backbone. Upon integration, the now recombinant vector (plasmid + DNA of interest) can be transformed (put into) into bacteria that will express and produce the recombinant protein. A key feature of expression vectors is the region containing a selectable marker, typically a gene encoding resistance to an antibiotic, which only selects bacteria that have been successfully transformed (Figure 2A). Resistant bacteria can then be grown in culture and induced using various chemicals to express the recombinant protein (Figure 2B). These same principles can be applied to making plants express these pesticidal proteins, which in the context of Bt crops is the Cry protein. Pyramiding or stacking multiple cry genes together, each one affecting different midgut receptors, can help to reduce insects developing resistance to these crops as they’d need to simultaneously evolve resistance to all the Cry proteins.

Dr Azizoglu and his team have used these techniques and Bt bacterium to engineer insecticidal genes, hormones, novel toxins, and other substances into Bt and to search for novel cry genes to improve the pathogenicity of Bt to crop pests. The ability to transfer DNA-encoding insecticidal toxins into entomopathogenic microbes to increase their pathogenicity offers a sustainable alternative to chemical pesticides. The potential that recombinant DNA technology offers is vast, but its use has raised concerns over the effects it may have on the environment.

Implications of recombinant DNA technology

With the ability to engineer entomopathogenic microbes with a variety of insecticidal proteins and toxins, there is some evidence that these compounds have negative effects on non-target organisms in the wider ecosystem. Natural predators of crop pests will consume insects that have ingested recombinant biopesticides, which could then have secondary toxic effects on the predator.

This secondary toxicity could have knock-on effects in tritrophic interactions (plant–herbivore–predator) and disturb ecosystem food chains surrounding agricultural land. More studies on the impacts of recombinant proteins and their effects on the predators of crop pests are needed to minimise ecological impacts of this technology.

Despite concerns, genetically modifying entomopathogenic microbes offers a less environmentally polluting alternative to chemical pesticides. Using these techniques, alongside exploring natural microbial strategies to combat pests, will ultimately facilitate a more sustainable approach to managing crop pests.