A soybean cyst nematode suppresses microbial plant symbionts using a lipochitooligosaccharide-hydrolysing enzyme

Cyst nematodes are the most damaging species of plant-parasitic nematodes. They antagonize the colonization of beneficial microbial symbionts that are important for plants’ nutrient acquisition. The molecular mechanism of the antagonism, however, remains elusive.

Here, through biochemical combined with structural analysis, we reveal that Heterodera glycines, the most notorious soybean cyst nematode, suppresses symbiosis by secreting an enzyme named HgCht2 to hydrolyze the key symbiotic signaling molecules lipochitooligosaccharides (LCOs). We solved the 3D structures of apo HgCht2, as well as its chitooligosaccharide-bound and LCO-bound forms. These structures elucidated the substrate binding and hydrolyzing mechanism by the enzyme. We designed an HgCht2 inhibitor, 1516b, which successfully suppresses the antagonism of cyst nematodes towards nitrogen-fixing rhizobia and phosphorus-absorbing arbuscular mycorrhizal symbioses.

As HgCht2 is phylogenetically conserved across all cyst nematodes, our work revealed a molecular mechanism by which parasitic cyst nematodes antagonize the establishment of microbial symbiosis and provided a small-molecule solution.

A soybean cyst nematode suppresses microbial plant symbionts using a lipochitooligosaccharide-hydrolysing enzyme

Chitin is the second most abundant polysaccharide in nature after cellulose and is composed of molecules of N-acetylglucosamine, amide derivatives of glucose. Chitin is the primary component of the insect body, especially the integument and trachea. Perhaps to emphasize this fact, Sir Vincent Wigglesworth, the father of insect physiology coined the phrase ‘insect equals chitin”. What exactly did Wigglesworth mean by this? How about a journey back in time to get the answer to this question?

This presentation will take you on a journey into the mysterious world of chitin that begins in France in 1762 when Pierre Lyonnet first described a sheath surrounding the food bolus of the goat moth caterpillar Cossus cossus larvae. More than a century later, this chitinous tissue was called the “peritrophic membrane”, "the membrane surrounding food" by Édouard-Gérard Balbiani in 1890. The story continues in the 20^th century in Cambridge, England, where Wigglesworth made extensive contributions to PM research between 1930 and 1970. The story then moves across the ocean to the US in the 1990s, where the work of Ping Wang and Robert Granados would indeed be another milestone as they discovered the first PM protein from Trichoplusia ni, an insect intestinal mucin that binds into chitin and is targeted by a baculoviral metalloprotease that helps virions pass through the midgut. The last scene of the play takes place in Canada, where you will meet the "young man" and the "old man" and their mysterious experiment that never works, despite their best efforts. As they search for answers in Wigglesworth's old papers and are about to give up, a third person will tell them the hint of the experiment.

Will the experiment finally work? This is a story of challenge and hope. It's a true story of the "young man", the "old man" and Wigglesworth. And it's also the story of chitin. And this whole story raises a new question: Are insects really equal to chitin? Join us to find out.