21-BBSRC/NSF-BIO Molecular dissection of symbiosis regulation of plant immunity

Food production is currently achieved through unsustainable farming practices including the application of large amounts of inorganic fertilizers to provide essential nutrients for crop growth. This has led to a loss of biodiversity, degradation of land quality, and environmental pollution. Plants naturally associate with a wide variety of microorganisms, including some beneficial microbes that can infect plant roots to establish mutualistic relationships called symbioses. Among the symbiotic interactions are those between nitrogen-fixing rhizobial bacteria and legumes, as well as arbuscular mycorrhizal fungi (AMF) with most land plants. To establish these symbioses, plants must recognize these microbes as friends, not foes, and adjust their development and cellular physiology to accommodate microbial infection. Symbiosis also necessitates the avoidance of immunity processes, either through action by the microbes or intentional suppression by the plant. In this proposal we develop our understanding of how plant recognition of symbionts interferes with immunity processes in the plant cell, to ensure that the invading microorganisms are not rejected by the immunity processes, but instead are welcomed as symbionts.

In this proposal we attempt to understand the molecular mechanisms in plants by which perception of symbionts allows the suppression of immunity processes during the colonization by beneficial symbiotic microorganisms. Our data has demonstrated that lipo-chitooligosaccharides (LCOs) produced by rhizobia and mycorrhizal fungi inhibit plant immunity through the LCO receptor NFP and the transcription factor NSP2, but independent of the common symbiosis signaling pathway in Medicago truncatula. Our central hypothesis is that LCOs can be recognized by plants to activate a novel signaling process through NFP and NSP2 to inhibit immunity and promote symbiosis. The proposed collaborative project combines expertise in plant immunity (Oklahoma State University group) and plant symbiosis (University of Cambridge group) to develop a better understanding of how LCOs inhibit plant immune signaling utilizing the model legume Medicago and the cereal crop barley. To do this, we intend to analyze large datasets produced by transcriptome and proteome analysis of Medicago and barley roots with or without treatment with LCOs. Our objectives are to: (1) characterize the NFP receptor complex and downstream components for LCO suppression of immunity, and (2) identify the target genes and associated proteins of NSP2 in the nucleus for inhibition of immunity and promotion of symbiosis, and therefore to (3) mechanistically understand how LCOs inhibit plant immune signaling. Furthermore, we expect to (4) engineer the identified candidates into barley to enhance disease resistance and mycorrhizal colonization. Our research might bring new insights into engineering nitrogen-fixing symbioses into crops, an important approach to sustaining cereal yields and reducing dependence on inorganic fertilizers.