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Analysis of Hessian Fly Genome Posits Way It May Induce Wheat Galls

NEW YORK (GenomeWeb) – The Hessian fly genome has provided researchers with clues as to how it induces wheat galls to stunt the plant's growth.

As they reported in Current Biology, an international team of researchers led by Baylor College of Medicine's Stephen Richards sequenced the Hessian fly genome to find that it harbors a wide number of effector genes, which control gene expression and cell signaling and which are thought to suppress host defenses and modulate host activities.

"The Hessian fly is basically a plant pathogen in the shape of an insect," co-author Jeffrey Stuart, professor of insect molecular genetics at Purdue University, said in a statement. "If we have a deeper understanding of how the insect is attacking the plant and how it avoids detection, we may be able to develop new ways of making resistant wheat more durable and better advise growers on which varieties to plant."

Richards, Stuart, and their colleagues sequenced the wheat-galling midge, Mayetiola destructor, generating some 26 million reads and 34-fold genome coverage. They annotated the M. destructor genome using the MAKER2 pipeline to generate more than 20,000 protein-coding models.

The researchers noted that M. destructor appeared to lack some gustatory, ionotropic, and odorant receptors, which they noted is in line with its pathogenic life.

The investigators also sifted through the M. destructor genome to look for effector genes.

They ran the approximately one-third of genes in the M. destructor genome lacking homologs in other organisms through a computational tool searching for signal peptide sequences. From this, they uncovered 466 putative effector genes — representing more than 7 percent of the genes in the M. destructor genome.

Most of those putative effector genes belonged to the SSGP-71 gene family, which the researchers said was marked by a cyclin-like F box domain at the N terminus and a series of leucine-rich repeats.

Plants, the researchers added, encode more than 700 similar F-box, leucine-repeat-rich genes. These plant proteins help add ubiquitin to proteins targeted for degradation in the proteasome and have roles in hormone signaling, plant development, and plant immunity.

This, Richards, Stuart, and their colleagues said, has led them to suspect that SSGP-71 proteins are a novel class of F-box, leucine-repeat-rich mimics that allow the infecting insect to hijack the plant proteasome for its own ends, likely to provide a food source for larva.

But as plants are exposed to these effectors, they build up their defenses. Wheat encodes a number of cognate effectors that recognize invading effectors to tip off an immune response, sparking a sort of arms race between the insect and plant, the investigators said.

"We're just starting to understand how the insect and plant are interacting and which proteins the insect uses to avoid or overcome plant defenses," Lucio Navarro-Escalante, a doctoral student at Purdue, added. "In the future, those effector proteins could be the basis from which we generate new ways of controlling the insect."