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 Microbe-Associated-Molecular-Patterns (MAMP); 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”. 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), but then respond in distinctly different ways (i.e., innate immunity vs. nodulation, respectively).
Discoveries in our laboratory identified the major chitin receptor in plants (CERK1; Wan et al., 2008), as well as various co-receptors (Wan et al., 2012) and interacting proteins (Le et al., 2014). Most recently, we defined the quaternary structure of the chitin receptor in Arabidopsis as a likely heterotetramer of CERK1 and LYK5, with the later protein play the major role in chitin binding (Cao et al, 2014).
Virtually since the discovery of symbiotic nitrogen fixing in legumes, scientists have dreamed of transferring this capability to other crops (e.g., corn). A major hypothesis was that non-legumes, such as corn, do not nodulate with rhizobia due to an inability to recognize the essential LCO Nod factor signal. However, past collaborative work from our lab showed that some plants nodulate even in the absence of the Nod factor (Giraud et al., 2007). Moreover, recently we showed that most, if not all, non-legumes recognize the LCO Nod factor but couple this to a suppression of innate immunity, not nodule development. We proposed that it is this function of LCO, suppression of innate immunity, that may have evolved first with the symbiotic role of this molecule arising later (Liang et al., 2014).
We continue to investigate the structure and function of the plant chitin receptor complex, as well as other interacting proteins and downstream signaling processes. We are also interested in the ability of LCO to suppress innate immunity and how this may relate to the mechanism of nodulation (Gourion et al., 2015).
Selected recent publications relating to this research topic:
Cao, Yangrong, Yan Liang, Kiwamu Tanaka, Cuong T. Nguyen, Robert P. Jedrzejczak, Andrzej Joachimiak, Gary Stacey (2014) The kinase LYK5 is a major chitin receptor in Arabidopsis and forms a chitin-induced complex with related kinase CERK1. eLife 2014;3:e03766
Giraud, Eric, Lionel Moulin, David Vallenet, ValérieBarbe, Eddie Cytrin, Jean-Christophe Avarre, Marianne Jaubert, Damien Simon, FabienneCartieaux, Yves Prin, Gilles Bena, Laure Hannibal, Joel Fardoux, Mila Kojadinovic, Laurie Vuillet, AurélieLajus, StéphaneCruveiller, Zoe Rouy, Sophie Mangenot, BéatriceSegurens, Carole Dossat, William L. Franck, Woo-Suk Chang, Elizabeth Saunders, David Bruce, Paul Richardson, Philippe Normand, Bernard Dreyfus, David Pignol, Gary Stacey, David Emerich, André Verméglio, Claudine Medigue, and Michael Sadowsky. (2007) Rhizobial Nod factors are not universally required for legume nodulation. Science 316: 1307-1312.
Gourion, Benjamin,FathiBerrabah, Pascal Ratet and Gary Stacey (2015) Rhizobium-legume symbiosis: the role of plant immunity. Trends Plant Sci., (in press)
Le, Mi Ha, Yangrong Cao, Xue-Cheng Zhang, and Gary Stacey (2014) LIK1, A CERK1-interacting kinase, regulates plant immune responses in Arabidopsis. Plos One. July 18 issue; DOI: 10.1371/journal.pone.0102245
Liang, Yan, KatalinTóth, Yangrong Cao, Kiwamu Tanaka, Catherine Espinoza and Gary Stacey (2014) Lipochitooligosaccharide recognition: an ancient story. New Phytol. 204(2):289-96
Wan, Jinrong,, Xuecheng Zhang, Katrina M. Ramonell, Steve Clough, Sung-yong Kim, Minviluz Stacey, and Gary Stacey (2008) A LysM receptor-like kinase mediates chitin perception and fungal resistance in Arabidopsis. Plant Cell 20: 471-481.
Wan, Jinrong, Xue-Cheng Zhang, Kiwamu Tanaka, Geon-hui Son, Laurent Brechenmacher, Tran Hong Nha Nguyen, Gary Stacey. (2012) The LysM receptor-like kinase AtLysM RLK4 is important for chitin signaling and plant innate immunity in Arabidopsis. Plant Physiol. 160: 396-406.