Functional diversification of AA17 LPMOs reveals multiple polysaccharide-oxidising activities in plant-pathogenic oomycetes
This study reveals that the AA17 lytic polysaccharide monooxygenase family in oomycetes has functionally diversified beyond its known pectin-targeting role to include enzymes capable of oxidatively cleaving cellulose and xylan, with these expanded activities specifically evolving in plant-pathogenic lineages to target host cell wall components.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
In the microscopic world of plant disease, a group of organisms known as oomycetes acts as a formidable enemy to agriculture. Though they look like fungi, these water-mold pathogens are distinct biological entities responsible for devastating crop losses, from the historical Irish potato famine to modern threats against tomatoes and soybeans. To invade a plant, these microbes must breach the tough outer wall that protects plant cells. This wall is not a single material but a complex mesh of sugars, including pectin, cellulose, and xylan, which act as the bricks and mortar of the plant's structure. For decades, scientists understood that oomycetes produce special enzymes to break down these sugars, but the full extent of their chemical toolkit remained a mystery. Specifically, a recently discovered family of enzymes called AA17 was known to target pectin, a sticky sugar found in the middle of plant cells. However, because oomycetes carry hundreds of copies of the genes for these enzymes, researchers suspected that this family might be far more versatile than previously thought, potentially evolving different tools to tackle different parts of the plant's defense system.
A researcher at the Technical University of Denmark set out to map this hidden diversity. They began by gathering over a thousand genetic sequences of the AA17 enzyme family from various oomycete species, ranging from plant pathogens to those that infect animals. By comparing these sequences, they found that the family had split into distinct evolutionary groups, or clusters, much like a family tree branching out into different cousins. While previous studies had only examined a handful of these enzymes and found they all attacked pectin, this new analysis suggested that the other branches of the family might have developed entirely different jobs. To test this, the researcher selected representatives from eight different clusters and produced them in the lab to see what materials they could actually break down.
The results revealed a surprising expansion of the enzyme family's capabilities. While some enzymes behaved as expected, chewing through pectin, others had evolved to attack different types of sugar chains. The researcher discovered that certain AA17 enzymes could oxidatively cleave beta-1,4-glucans, which are the primary structural components of cellulose, the rigid fiber that gives plant cell walls their strength. Even more unexpectedly, they found a specific group of enzymes capable of breaking down xylan, a complex sugar found in the hemicellulose layer of plant walls. This functional radiation means that the AA17 family is not a single-purpose tool but a versatile platform of weapons. The study showed that the ability to attack pectin and xylan is largely restricted to plant-pathogenic lineages, suggesting these enzymes evolved specifically to match the composition of the plants they infect. In contrast, the ability to break down beta-1,4-glucans was found across a wider range of oomycetes, including those that do not infect plants, hinting that this activity might also serve to remodel the microbe's own cell wall during growth.
To understand how these enzymes recognize such different targets, the researcher looked at their physical structures. They determined the atomic-level arrangement of one of the xylan-breaking enzymes, capturing it in a crystal while it held onto a piece of its sugar target. This structural view revealed a unique binding mode that differed significantly from other known enzymes. The active site of this enzyme featured a specific groove and surface shape that seemed perfectly tailored to grip the helical structure of xylan, whereas the pectin-attacking enzymes had a different surface charge and shape suited for their sticky target. This physical evidence confirmed that the genetic differences observed earlier translated into real, mechanical differences in how the enzymes interact with their food.
The findings suggest that oomycetes have fine-tuned their enzymatic arsenal to match the specific chemical landscape of their hosts. For the plant pathogens, possessing a diverse array of AA17 enzymes allows them to dismantle the various layers of a plant cell wall with precision. The study indicates that this diversification is a key part of their strategy for successful infection. By expanding the known functions of the AA17 family from a single pectin-targeting activity to include cellulose and xylan degradation, the research redefines these enzymes as a major virulence platform. This deeper understanding of how these pathogens operate could eventually inform new ways to protect crops, either by blocking these specific enzymes or by finding new ways to break down plant biomass for industrial use. The work highlights that even within a single family of enzymes, nature can engineer a wide spectrum of specialized tools to conquer complex biological barriers.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.