Principal Investigator
Researcher
Email:fxie@cemps.ac.cn
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National Key Laboratory of Plant Molecular Genetics

The nodule is a unique organ in where the bacteria are hosted and will find the proper conditions to reduce atmospheric nitrogen into ammonia. Generating a nodule involves two main processes: the re-initiation of mitosis in cortical cells leading to the nodule primordium and the formation of infection threads that allow bacterial invasion into the developing nodule. The root hair infection is the most common and it requires rhizobia secreted Nod-factor and surface polysaccharides that act as signals, inducing an invagination of the plant cell membrane to create a tube-like structure called infection thread that delivers rhizobia to the inner root tissue. Nodule formation requires mitotic reactivation of differentiated cortical cells, which will give rise to the nodule primordium. Root nodule is a lateral root organ, but the cellular developmental origin, anatomy and patterns of genes expression of lateral roots and nodule are different. Legume nodule formation and nitrogen fixation is an energy-consuming process, the legume plants using a root-shoot-root long-distance pathway to mediate and produce right nodule number. It was showed that high nitrate or ammonia inhibit nodule formation through local and systemic regulation mechanism. 

My laboratory major interests focus on understanding how legume plants make a nodule which including rhizobial infection and nodule organogenesis, and how legume plants response to abiotic stress signaling to mediate nitrogen fixation. From these basic researches, we’re trying to understand why only legumes can symbiotic nitrogen fixation nodules then increase legume crops symbiotic nitrogen fixation efficiency, and provide scientific knowledge for exploring non-legume nitrogen fixation.  

1) Reveal the molecular mechanism of nitrate inhibition of nodulation in M. truncatula:  

Symbiotic nitrogen fixation is an energy-consuming process and is strongly inhibited when sufficient levels of fixed nitrogen are available, but the molecular mechanisms governing this regulation are largely unknown. The transcription factor NIN is strictly required for nodulation and belongs to a family of NIN-like proteins (NLPs), which have been implicated in the regulation of nitrogen homeostasis in Arabidopsis. We found that mutation or downregulation of NLP genes prevents nitrate inhibition of infection, nodule formation and nitrogen fixation. We showed that NIN and NLPs physically interact through their carboxy-terminal PB1 domains. Furthermore, we find that NLP1 is involved in nitrate signaling and that nitrate triggers NLP1 re-localization from the cytosol to the nucleus. Finally, we show that NLP1 can suppress NIN activation of CRE1 expression in N. benthamiana and M. truncatula. Our findings highlight a central role for NLPs in the suppression of nodulation by nitrate. 

2) Identify and characterize an atypical receptor kinase RINRK1 which is required for rhizobial infection but not nodule development in L. japonicus. 

During the legume-rhizobium symbiotic interaction, rhizobial invasion of legumes is primarily mediated by a plant-made tubular invagination called an infection thread. We identify a gene in L. japonicus encoding a leucine-rich repeat receptor-like kinase (LRR-RLK), RINRK1 (Rhizobial Infection Receptor-like Kinase1), that is induced by Nod factors and is involved in infection thread formation but not nodule organogenesis. A paralog, RINRK2, plays a relatively minor role in infection. RINRK1 is required for full induction of early infection genes, including Nodule Inception (NIN), encoding an essential nodulation transcription factor. RINRK1 displayed an infection-specific expression pattern, and NIN bound to the RINRK1 promoter, inducing its expression. RINRK1 was found to be an atypical kinase localized to the plasma membrane and did not require kinase activity for rhizobial infection. We propose RINRK1 is an infection-specific RLK, which may specifically coordinate output from NF signaling or perceive an unknown signal required for rhizobial infection.  

3) SCARN a novel class of SCAR protein that is required for root-hair infection during legume nodulation 

Rhizobial infection of legume root hairs requires a rearrangement of the actin cytoskeleton to enable the establishment of plant-made infection structures called infection threads. In the SCAR/WAVE (Suppressor of cAMP receptor defect/WASP family verpolin homologous protein) actin regulatory complex, the conserved N-terminal domains of SCAR proteins interact with other components of the SCAR/WAVE complex. The conserved C-terminal domains of SCAR proteins bind to and activate the actin-related protein 2/3 (ARP2/3) complex, which can bind to actin filaments catalyzing new actin filament formation by nucleating actin branching. We have identified, SCARN (SCAR-Nodulation), a gene required for root hair infection of L. japonicus by M. loti. Although the SCARN protein is related to Arabidopsis thaliana SCAR2 and SCAR4, it belongs to a distinct legume-sub clade. We identified other SCARN-like proteins in legumes and phylogeny analyses suggested that SCARN may have arisen from a gene duplication and acquired specialized functions in root nodule symbiosis. Mutation of SCARN reduced formation of infection threads and their extension into the root cortex and slightly reduced root-hair length. Surprisingly two of the scarn mutants showed constitutive branching of root hairs in uninoculated plants. However we observed no effect of scarn mutations on trichome development or on the early actin cytoskeletal accumulation that is normally seen in root hair tips shortly after M. loti inoculation, distinguishing them from other symbiosis mutations affecting actin nucleation. The C-terminal domain of SCARN binds to ARPC3 and ectopic expression of the N-terminal SCAR-homology domain (but not the full-length protein) inhibited nodulation. In addition, we found that SCARN expression is enhanced by M. loti in epidermal cells and that this is directly regulated by the NODULE INCEPTION (NIN) transcription factor.