Researchers (from left) Rajan Sankaranarayanan, Gudlur Aparna and Ramesh V. Sonti. 
Plant bacteria have to overcome the complex plant cell walls to invade and infect their hosts. To do this, these bacteria have evolved arsenals of hydrolysing enzymes that can degrade the host cell walls.
Previous work has shown that LipA is an important cell wall degrading enzyme of the devastating rice pathogen Xanthomonas oryzae pv. Oryzae, that causes major yield losses across the world.
The researchers have now presented X-ray crystal structure of LipA, showing that it is the prototype of a new family of plant cell wall degrading enzymes with a novel mode of substrate binding. The structure reveals a typical catalytic domain (found commonly in hydrolysing enzymes) in conjunction with a unique domain that binds to glycoside ligands.
The binding at the unique domain takes place through a pocket for anchoring glucose and a hydrophobic tunnel for binding to the acyl/aryl group of the substrate. When the tunnel is blocked far away from the enzyme active site, it disrupts the disease causing ability of the pathogen, thus demonstrating that the tunnel is crucial for enzyme function within the rice host.
Studies on LipA also provide compelling evidence on how plant pathogens might acquire novel features towards becoming efficient pathogens. This new family of plant cell wall degrading enzymes has also been identified in several other plant pathogens of commercial crops such as tomato, cabbage, pepper and citrus.
"This work has important implications in the development of biofuels and a new generation of plant protection chemicals that revolve around an ability to inhibit LipA action," says lead researcher Rajan Sankaranarayanan of the Centre for Cellular and Molecular Biology, Hyderabad.
