Invisible pest causes more than $1B in crop losses by hijacking plants

Schematic representation of the secretion pathways of plant CLE peptides and their mimicry injected by nematodes into the plant cell cytoplasm.

A newly published study led by researchers from the University of Georgia and several partner institutions reveals a discovery that could lead to new control strategies for a tiny-but-persistent agricultural pest that causes enormous soybean losses.

Microscopic soybean cyst nematodes (SCN) Iive in soil and are attracted to the root systems of soybeans.

Once a field is infested, it is nearly impossible to root them out, said lead study investigator Melissa Mitchum, professor of plant nematology in UGA’s Department of Plant Pathology and Institute of Plant Breeding, Genetics and Genomics (IPBGG) in the College of Agricultural and Environmental Sciences and Center for Applied Genetic Technologies.

Invisible to the naked eye, SCN have unique hollow, protrudable mouth spears — called stylets — that they use to pierce through the root of a plant, injecting peptide effectors, which mimic the soybean’s naturally present CLAVATA3/embryo surrounding region (CLE) peptides to hijack the plant.

After hatching from an egg in the soil, the juvenile nematode migrates into the root, where it sets up a feeding site and makes its way toward the circulatory system that the plant uses to transport nutrients to the rest of the plant.

Once a root cell is chosen for feeding, the nematodes transform it into a syncytium, a mass comprised of hundreds of metabolically active cells that continue to secrete CLE, taking over the plant’s natural cellular processes to export the peptides back out of the cell to function as external signaling molecules to distant cells.

This process diverts resources away from the plant in support of the nutrient demands of the developing nematode.

At this point, nematodes change their morphology, turning from wormlike forms into sedentary, lemon-shaped bodies that pop out of the roots, becoming visible to the eye.

Study resultsUsing knowledge that has been developed over the past two decades on how nematode CLEs function, the study, published in the journal New Phytologist, uncovered a new pathway that allows the nematode to disrupt the plant’s natural growth processes to divert resources for its own benefit.

The study was also highlighted in commentary by the editors of New Phytologist.

The study shows that the nematode CLEs provide a way for the nematode peptide to get outside of the cell to interact with the plant protein receptor through the plant’s secretion system, a process that is not well-studied in plants.

Normally occurring plant CLE peptides regulate aspects of cell differentiation in the plant, communicating to the plant’s stem cells, which are constantly regenerating, whether to develop into a root cell, a leaf cell or a cell in another part of th plant.

“The peptides in the plant — and there are many types — tell the plant how to regulate its growth and development. These nematodes have found a way to trick the plant by secreting these similar peptides in the root cell to make a feeding site,” Mitchum said. “Now we want to determine how the nematode CLE peptide forms that feeding site. That is why it is such a successful parasite — it has evolved this peptide that looks like and functions like a plant peptide, so they can put it into the root cell where it does not normally occur and trick the plant into supporting the formation of a feeding cell for the nematode.”

Developing new control strategiesThe next step is to find novel methods to block the nematode peptides from getting out of the feeding cells.

Soybean cyst nematodes are the No. 1 yield-reducing pest of soybean producers, robbing seemingly healthy fields of up to 30% of their yields — and costing the industry up to $1 billion every year.

The primary management practice for soybean cyst nematodes is through developing genetic resistance in soybean varieties. Because SCN has a specialized host range, producers can control nematode damage by rotating fields to a non-host crop plant to reduce the population of nematodes in the field. However, the bodies of dead female nematodes form a cyst around them, protecting hundreds of eggs. These cysts can remain in the ground for years, protecting the eggs until a host crop is planted again and the nematodes reemerge.

A troubling trendWhile creating genetically resistant crops is a very effective tool, SCN have adapted to overcome the source of resistance in the commonly used breeding line PI 88788, which is used in more than 90% of resistant varieties currently planted by producers.

As a result, yields in resistant varieties can be reduced by up to 14 bushels per acre, according to The SCN Coalition, a public/private partnership of university researchers, national extension specialists and agriculture company representatives who are concerned about the evolving threat from soybean cyst nematodes.

Because the symptoms of SCN infestation are often not visible above ground, it is difficult to communicate the severity of the problem to producers and the importance of basic science research to combat it.