Chitin lipo-chitin recognition by plants
Plants are sedentary and, therefore, need means to measure environmental conditions and respond to threats or stressful change. Our understanding of how plants recognize and respond to stress (e.g., through membrane bound receptors) has advanced considerably in recent years. However, much less is known about how these signaling pathways are integrated within the plant to effect needed changes in physiology, growth and development.
Research over the past decade has shown that chitooligosaccharides (COs) and modified CO (e.g., lipo-chitooligosaccharides, LCOs) induce a variety of responses in plants. For example, COs, released from the cell walls of plant pathogenic fungi represent examples of Pathogen (Microbe)-Associated-Molecular-Patterns [P(M)AMP]; a group of molecules that play roles in the induction of innate immunity responses in both plants and animals.
In contrast, lipo-chitoolilgosaccharide (LCO) Nod signals, produced by rhizobia, are the key morphogens that induce the formation of symbiotic nodules on legume roots. Numerous recent studies provide strong support for our original hypothesis that “lipo-chitooligosaccharide (LCO) Nod signal recognition pathways likely evolved from more ancient chito-oligosaccharide (CO) recognition pathways that appear to be found in all plants” (Zhang et al., 2007).
Indeed, one central question in our work is how plants recognize very similar molecules (i.e., CO and LCO), using structurally similar receptors (i.e. LysM receptor-like kinases (Wan et al., 2008), but then respond in distinctly different ways (i.e., innate immunity vs. nodulation, respectively).
However, a variety of other plant responses have also been reported for CO or LCO treatment. For example, LCO addition is known to increase root growth in both soybean and corn, which is the basis for a product (Optimize) based, in part, on patents from our laboratory. The means by which CO and LCO mediate these broad effects is unknown.
Indeed, recent results from our laboratory reveal new levels of complexity in CO signaling. Our long term goal is to elucidate plant CO/LCO perception and signaling pathways that play roles in both plant development and pathogen defense. For example, it is clear that CO addition to plants induces a broad array of responses, including a direct impact on photosynthesis, growth and development. Our research also seeks to understand the molecular mechanism of plant receptor function. Furthermore, our studies are strengthened by their multidisciplinary approach, which combines experimental and computational efforts with biochemistry, genetics and molecular biology.