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Abstract Reverse genetics, facilitated by CRISPR technologies and comprehensive sequence-indexed insertion mutant collections, has advanced the identification of plants genes essential for arbuscular mycorrhizal (AM) symbiosis. However, a mutant phenotype alone is generally insufficient to reveal the specific role of the protein in AM symbiosis and in many cases, identifying interacting partner proteins is useful. To enable identification of protein:protein interactions during AM symbiosis, we established aMedicago truncatula -Diversispora epigaeayeast-two-hybrid (Y2H) library which, through Y2H-seq screening, can provide a rank-ordered list of candidate interactors of a protein of interest. We also developed a vector system to facilitate bimolecular fluorescence complementation assays (BIFC) in mycorrhizal roots so that protein interactions can be assessed in their native cell types and sub-cellular locations. We demonstrate the utility of a Y2H-seq screen coupled with BIFC in mycorrhizal roots, with a search for proteins that interact with CYCLIN DEPENDENT LIKE KINASE 2 (CKL2), a kinase essential for AM symbiosis. The Y2H-seq screen identified three 14-3-3 proteins as the highest ranked CKL2 interacting proteins. BIFC assays in mycorrhizal roots provided evidence for a CKL2:14-3-3 interaction at the periarbuscular membrane (PAM) in colonized root cells. Down-regulation of 14-3-3 by RNA interference provides initial evidence for a function in AM symbiosis. Thus, CKL2 may utilize 14-3-3 proteins to direct signaling from the PAM. The Y2H and BIFC resources will accelerate understanding of protein functions during AM symbiosis. 

Two symbiotic processes, nodulation and arbuscular mycorrhiza, are primarily controlled by the plant’s need for nitrogen (N) and phosphorus (P), respectively. Autoregulation of nodulation (AON) and autoregulation of mycorrhizal symbiosis (AOM) both negatively regulate their respective processes and share multiple components—plants that make too many nodules usually have higher arbuscular mycorrhiza (AM) fungal root colonization. The protein TML (TOO MUCH LOVE) was shown to function in roots to maintain susceptibly to rhizobial infection under low N conditions and control nodule number through AON inLotus japonicus.Medicago truncatulahas two sequence homologs:MtTML1 andMtTML2. We report the generation of stable single and double mutants harboring multiple allelic variations inMtTML1andMtTML2using CRISPR–Cas9 targeted mutagenesis and screening of a transposon mutagenesis library. Plants containing single mutations inMtTML1 orMtTML2 produced two to three times the nodules of wild-type plants, whereas plants containing mutations in both genes displayed a synergistic effect, forming 20× more nodules compared to wild-type plants. Examination of expression and heterozygote effects suggests that genetic compensation may play a role in the observed synergy. Plants with mutations in bothTMLs only showed mild increases in AM fungal root colonization at later timepoints in our experiments, suggesting that these genes may also play a minor role in AM symbiosis regulation. The mutants created will be useful tools to dissect the mechanism of synergistic action ofMtTML1 andMtTML2 inM. truncatulasymbiosis with beneficial microbes. 

Abstract BackgroundLegumes utilize a long-distance signaling feedback pathway, termed Autoregulation of Nodulation (AON), to regulate the establishment and maintenance of their symbiosis with rhizobia. Several proteins key to this pathway have been discovered, but the AON pathway is not completely understood. ResultsWe report a new hypernodulating mutant,defective in autoregulation, with disruption of a gene,DAR(Medtr2g450550/MtrunA17_Chr2g0304631), previously unknown to play a role in AON. Thedar-1mutant produces ten-fold more nodules than wild type, similar to AON mutants with disruptedSUNNgene function. As insunnmutants, suppression of nodulation by CLE peptides MtCLE12 and MtCLE13 is abolished indar. Furthermore,dar-1also shows increased root length colonization by an arbuscular mycorrhizal fungus, suggesting a role for DAR in autoregulation of mycorrhizal symbiosis (AOM). However, unlikeSUNNwhich functions in the shoot to control nodulation,DARfunctions in the root. ConclusionsDARencodes a membrane protein that is a member of a small protein family inM. truncatula. Our results suggest that DAR could be involved in the subcellular transport of signals involved in symbiosis regulation, but it is not upregulated during symbiosis.DARgene family members are also present in Arabidopsis, lycophytes, mosses, and microalgae, suggesting the AON and AOM may use pathway components common to other plants, even those that do not undergo either symbiosis. 

The gaseous hormone ethylene is perceived in plants by membrane-bound receptors, the best studied of these being ETR1 from Arabidopsis. Ethylene receptors can mediate a response to ethylene concentrations at less than one part per billion; however, the mechanistic basis for such high-affinity ligand binding has remained elusive. Here we identify an Asp residue within the ETR1 transmembrane domain that plays a critical role in ethylene binding. Site-directed mutation of the Asp to Asn results in a functional receptor that has a reduced affinity for ethylene, but still mediates ethylene responses in planta. The Asp residue is highly conserved among ethylene receptor-like proteins in plants and bacteria, but Asn variants exist, pointing to the physiological relevance of modulating ethylene-binding kinetics. Our results also support a bifunctional role for the Asp residue in forming a polar bridge to a conserved Lys residue in the receptor to mediate changes in signaling output. We propose a new structural model for the mechanism of ethylene binding and signal transduction, one with similarities to that found in a mammalian olfactory receptor.