Plant PRRs and the Activation of Innate Immune Signaling

Legumes can enter into symbiotic relationships with both nitrogen-fixing bacteria (rhizobia) and mycorrhizal fungi. Nodulation by rhizobia results from a signal transduction pathway induced in legume roots by rhizobial Nod factors. DMI3 ,a Medicago truncatula gene that acts immediately downstream of calcium spiking in this signaling pathway and is required for both nodulation and mycorrhizal infection, has high sequence similarity to genes encoding calcium and calmodulin-dependent protein kinases (CCaMKs). This indicates that calcium spiking is likely an essential component of the signaling cascade leading to nodule development and mycorrhizal infection, and sheds light on the biological role of plant CCaMKs.

Dependent Protein Kinase Required for Bacterial and Fungal Symbioses

Plants encounter a myriad of microorganisms, particularly at the root–soil interface, that can invade with detrimental or beneficial outcomes. Prevalent beneficial associations between plants and microorganisms include those that promote plant growth by facilitating the acquisition of limiting nutrients such as nitrogen and phosphorus. But while promoting such symbiotic relationships, plants must restrict the formation of pathogenic associations. Achieving this balance requires the perception of potential invading microorganisms through the signals that they produce, followed by the activation of either symbiotic responses that promote microbial colonization or immune responses that limit it.

Plant signalling in symbiosis and immunity

Various conceptual models to describe the plant immune system have been presented. The most recent paradigm to gain wide acceptance in the field is often referred to as the zigzag model, which reconciles the previously formulated gene-for-gene hypothesis with the recognition of general elicitors in a single model. This review focuses on the limitations of the current paradigm of molecular plant-microbe interactions and how it too narrowly defines the plant immune system. As such, we discuss an alternative view of plant innate immunity as a system that evolves to detect invasion. This view accommodates the range from mutualistic to parasitic symbioses that plants form with diverse organisms, as well as the spectrum of ligands that the plant immune system perceives. Finally, how this view can contribute to the current practice of resistance breeding is discussed.

Understanding Plant Immunity as a Surveillance System to Detect Invasion

Rhizobia are some of the best-studied plant microbiota These oligotrophic Alphaproteobacteria or Betaproteobacteria form symbioses with their legume hosts Rhizobia must exist in soil and compete with other members of the microbiota before infecting legumes and forming N2-fixing bacteroids These dramatic lifestyle and developmental changes are underpinned by large genomes and even more complex pan-genomes, which encompass the whole population and are subject to rapid genetic exchange The ability to respond to plant signals and chemoattractants and to colonize nutrient-rich roots are crucial for the competitive success of these bacteria The availability of a large body of genomic, physiological, biochemical and ecological studies makes rhizobia unique models for investigating community interactions and plant colonization

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Rhizobia: from saprophytes to endosymbionts.

Plants are constantly exposed to threats from pathogenic microbes and thus developed an innate immune system to protect themselves. On the other hand, many plants also have the ability to establish endosymbiosis with beneficial microbes such as arbuscular mycorrhizal (AM) fungi or rhizobial bacteria, which improves the growth of host plants. How plants evolved these systems managing such opposite plant-microbe interactions is unclear. We show here that knockout (KO) mutants of OsCERK1, a rice receptor kinase essential for chitin signaling, were impaired not only for chitin-triggered defense responses but also for AM symbiosis, indicating the bifunctionality of OsCERK1 in defense and symbiosis. On the other hand, a KO mutant of OsCEBiP, which forms a receptor complex with OsCERK1 and is essential for chitin-triggered immunity, established mycorrhizal symbiosis normally. Therefore, OsCERK1 but not chitin-triggered immunity is required for AM symbiosis. Furthermore, experiments with chimeric receptors showed that the kinase domains of OsCERK1 and homologs from non-leguminous, mycorrhizal plants could trigger nodulation signaling in legume-rhizobium interactions as the kinase domain of Nod factor receptor1 (NFR1), which is essential for triggering the nodulation program in leguminous plants, did. Because leguminous plants are believed to have developed the rhizobial symbiosis on the basis of AM symbiosis, our results suggest that the symbiotic function of ancestral CERK1 in AM symbiosis enabled the molecular evolution to leguminous NFR1 and resulted in the establishment of legume-rhizobia symbiosis. These results also suggest that OsCERK1 and homologs serve as a molecular switch that activates defense or symbiotic responses depending on the infecting microbes.

The Bifunctional Plant Receptor, OsCERK1, Regulates Both Chitin-Triggered Immunity and Arbuscular Mycorrhizal Symbiosis in Rice

OsCERK1 is a rice receptor-like kinase that mediates the signal of a fungal cell wall component, chitin, by coordinating with a lysin motif (LysM)-containing protein CEBiP. To further elucidate the...

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-03-14-0068-R

Targeted Gene Disruption of OsCERK1 Reveals Its Indispensable Role in Chitin Perception and Involvement in the Peptidoglycan Response and Immunity in Rice

CEBiP, a plasma membrane-localized glycoprotein of rice, directly binds with chitin elicitors (CE), and has been identified as a receptor for CE by using CEBiP-RNAi rice cells. To further clarify the function of CEBiP, we produced CEBiP-disrupted rice plants by applying an efficient Agrobacterium-mediated gene-targeting system based on homologous recombination, which has recently been developed for rice. Homologous recombination occurred at the CEBiP locus in ~0.5 % of the positive/negative selected calli. In the self-pollinated next generation, it was confirmed that the first exon of CEBiP was replaced with the hygromycin selection cassette as designed, and that the expression of CEBiP was completely deficient in homozygous cebip lines. Affinity-labeling analysis using biotinylated N-acetylchitooctaose demonstrated that CEBiP is the major CE-binding protein in rice cultured cells and leaves, which was consistent with the result that the response to CE in cebip cells was greatly diminished. Nevertheless, we observed a significant decrease in disease resistance against Magnaporthe oryzae, the causal agent of rice blast disease, only when the cebip leaf sheaths were inoculated with a weakly virulent strain, suggesting that CE perception during the infection process of M. oryzae is limited. The response to peptidoglycan and lipopolysaccharides in cebip cells was not affected, strongly suggesting that CEBiP is a CE-specific receptor.

CEBiP is the major chitin oligomer-binding protein in rice and plays a main role in the perception of chitin oligomers.

The objective is to isolate and identify a gene relating to the regeneration ability of a plant, and to provide the gene and methods of use thereof. Isogenic lines to which culture characteristics of Konansou was introduced were produced by backcrossing. Culture cells were induced from this isogenic line and its parent variety. After confirming the regeneration ability of culture cells cultured for 3 to 6 months, isolation of a gene expressed in culture cells derived from Konansou and an isogenic line having a high regeneration ability, but not expressed in culture cells derived from Sasanishiki and Koshihikari was conducted using the differential display method. As a result, a regeneration-related gene was successfully cloned.

Kenjirou Ozawa

Papers

The Bifunctional Plant Receptor, OsCERK1, Regulates Both Chitin-Triggered Immunity and Arbuscular Mycorrhizal Symbiosis in Rice

Targeted Gene Disruption of OsCERK1 Reveals Its Indispensable Role in Chitin Perception and Involvement in the Peptidoglycan Response and Immunity in Rice

CEBiP is the major chitin oligomer-binding protein in rice and plays a main role in the perception of chitin oligomers.

Gene related to regeneration ability of plants and uses thereof