Insect pathogenic bacteria also help fight fungal infestations

Future food shortages are expected to worsen in many parts of the world. With this in mind, sustainable organic techniques are being explored which could increase the yield of cereals and other food crops and which, unlike the use of chemical pesticides, are compatible with the environment. The bacterium Photorhabdus luminescens is already used as a bioinsecticide to protect crops against a wide range of insect pests. Researchers from Johannes Gutenberg University Mainz (JGU) in Germany recently demonstrated that P.luminescens can also protect plants against fungal infections. A secondary cell form of the bacterium is responsible for this additional effect. This variant first colonizes the fungal mycelium and then destroys it by degrading chitin, a major component of the fungal cell wall. The results of this research could be very significant in the future, especially with regard to cereal production. “We see this as a great opportunity to make agriculture more environmentally friendly and sustainable with the help of these bacteria,” said Professor Ralf Heermann from JGU.

Organic methods can result in higher crop yields

Like other plants, crops are susceptible to environmental stresses, disease, and pest infestation. This impacts crop yields and food production and raises concerns about food security given the growing global population. The greatest agricultural losses are attributable to the invasion of weeds, animal pests, as well as plant diseases caused by bacteria, fungi and viruses. In the past, it was the intensive use of chemical plant protection agents that ensured higher yields and thus improved the food supply. However, this came at the cost of environmental damage, risk of lethal toxicity to humans and non-target organisms such as pollinating insects, and most importantly the undesirable change in the composition of the soil microbiome.

An alternative approach is the use of biological agents such as rhizobacteria which promote plant growth and nematodes which attack harmful insects. These are two examples of new and sustainable agricultural techniques to control plant pests.

Primary cells of Photorhabdus luminescens kill insects and make them glow

Among these more sustainable approaches is the use of Photorhabdus luminescens as a beneficial organism that destroys insect larvae. This bacterium lives in symbiosis with small nematodes which penetrate insect larvae and then release the bacterium inside them. This then secretes numerous toxins which lead to the death of the insect larvae, simultaneously producing a bioluminescent enzyme called luciferase which makes the dead larvae glow.

About two years ago, Professor Ralf Heermann’s research group discovered that there is an additional phenotypic cell of P.luminescens which, although unable to undergo symbiosis with nematodes, is able to survive on its own in soil. This type of secondary cell is genetically identical to the primary form, but lacks certain phenotypic properties, such as bioluminescence. However, according to the group’s new findings, these secondary cells are extraordinarily effective against fungal infections. Using bovine tomato plants as an example, Heermann’s team of microbiologists showed that infestation with the phytopathogenic fungus Fusarium graminearum can be prevented by bacteria colonizing the fungal hyphae, breaking down the chitin therein. The scientists also managed to identify the molecular mechanism responsible involving an enzyme called chitinase and a chitin-binding protein. This allows bacteria to dissolve the structure of a fungus, especially its cell wall, and effectively inhibit fungal growth.

Potential new application to promote plant growth and protect against fungal infections

“Furthermore, we were able to show that the secondary cell type of the bacterium colonizes fungal hyphae in particular. This triggers one of the first mechanisms that protects plants against pathogens,” explained Dr. Nazzareno Dominelli, Member of the Heermann team. and the lead author of the recently published article. “Thanks to our results, we can now propose a new use for P.luminescens – as an organism that both promotes plant growth and protects plants against fungal infections.”

The research team plans to continue investigating the promising potential that P.luminescens offers in the field of biological crop protection. Early indications suggest that the secondary, non-luminescent cell type that actively seeks out plant roots may offer additional biotech benefits for agriculture.

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Material provided by Johannes Gutenberg University Mainz. Note: Content may be edited for style and length.