Symbiotic microorganisms improve nutrient uptake by plants. To initiate mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi, plants perceive Myc factors, including lipochitooligosaccharides (LCOs) and short-chain chitooligosaccharides (CO4/CO5), secreted by AM fungi. However, the molecular mechanism of Myc factor perception remains elusive. In this study, we identified a heteromer of LysM receptor-like kinases consisting of OsMYR1/OsLYK2 and OsCERK1 that mediates the perception of AM fungi in rice. CO4 directly binds to OsMYR1, promoting the dimerization and phosphorylation of this receptor complex. Compared with control plants, Osmyr1 and Oscerk1 mutant rice plants are less sensitive to Myc factors and show decreased AM colonization. We engineered transgenic rice by expressing chimeric receptors that respectively replaced the ectodomains of OsMYR1 and OsCERK1 with those from the homologous Nod factor receptors MtNFP and MtLYK3 of Medicago truncatula. Transgenic plants displayed increased calcium oscillations in response to Nod factors compared with control rice. Our study provides significant mechanistic insights into AM symbiotic signal perception in rice. Expression of chimeric Nod/Myc receptors achieves a potentially important step toward generating cereals that host nitrogen-fixing bacteria.

Plants encounter various microbes in nature and must respond appropriately to symbiotic or pathogenic ones. In rice, the receptor-like kinase OsCERK1 is involved in recognizing both symbiotic and immune signals. However, how these opposing signals are discerned via OsCERK1 remains unknown. Here, we found that receptor competition enables the discrimination of symbiosis and immunity signals in rice. On the one hand, the symbiotic receptor OsMYR1 and its short-length chitooligosaccharide ligand inhibit complex formation between OsCERK1 and OsCEBiP and suppress OsCERK1 phosphorylating the downstream substrate OsGEF1, which reduces the sensitivity of rice to microbe-associated molecular patterns. Indeed, OsMYR1 overexpression lines are more susceptible to the fungal pathogen Magnaporthe oryzae, whereas Osmyr1 mutants show higher resistance. On the other hand, OsCEBiP can bind OsCERK1 and thus block OsMYR1–OsCERK1 heteromer formation. Consistently, the Oscebip mutant displayed a higher rate of mycorrhizal colonization at early stages of infection. Our results indicate that OsMYR1 and OsCEBiP receptors compete for OsCERK1 to determine the outcome of symbiosis and immunity signals.

BackgroundPathogen perception by plants is mediated by plasma membrane-localized immune receptors that have varied extracellular domains. Lectin receptor kinases (LecRKs) are among these receptors and are subdivided into 3 classes, C-type LecRKs (C-LecRKs), L-type LecRKs (L-LecRKs) and G-type LecRKs (G-LecRKs). While C-LecRKs are represented by one or two members in all plant species investigated and have unknown functions, L-LecRKs have been characterized in a few plant species and have been shown to play roles in plant defense against pathogens. Whereas Arabidopsis G-LecRKs have been characterized, this family of LecRKs has not been studied in tomato.ResultsThis investigation updates the current characterization of Arabidopsis G-LecRKs and characterizes the tomato G-LecRKs, using LecRKs from the monocot rice and the basal eudicot columbine to establish a basis for comparisons between the two core eudicots. Additionally, revisiting parameters established for Arabidopsis nomenclature for LecRKs is suggested for both Arabidopsis and tomato. Moreover, using phylogenetic analysis, we show the relationship among and between members of G-LecRKs from all three eudicot plant species. Furthermore, investigating presence of motifs in G-LecRKs we identified conserved motifs among members of G-LecRKs in tomato and Arabidopsis, with five present in at least 30 of the 38 Arabidopsis members and in at least 45 of the 73 tomato members.ConclusionsThis work characterized tomato G-LecRKs and added members to the currently characterized Arabidopsis G-LecRKs. Additionally, protein sequence analysis showed an expansion of this family in tomato as compared to Arabidopsis, and the existence of conserved common motifs in the two plant species as well as conserved species-specific motifs.Electronic supplementary materialThe online version of this article (10.1186/s12864-018-4606-0) contains supplementary material, which is available to authorized users.

Daytime warm temperature elicits thermomorphogenesis in Arabidopsis by stabilizing the central thermoregulator PHYTOCHROME INTERACTING transcription FACTOR 4 (PIF4), whose degradation is otherwise promoted by the photoreceptor and thermosensor phytochrome B. PIF4 stabilization in the light requires a transcriptional activator, HEMERA (HMR), and is abrogated when HMR’s transactivation activity is impaired in hmr-22. Here, we report the identification of a hmr-22 suppressor mutant, rcb-101, which surprisingly carries an A275V mutation in REGULATOR OF CHLOROPLAST BIOGENESIS (RCB). rcb-101/hmr-22 restores thermoresponsive PIF4 accumulation and reverts the defects of hmr-22 in chloroplast biogenesis and photomorphogenesis. Strikingly, similar to hmr, the null rcb-10 mutant impedes PIF4 accumulation and thereby loses the warm-temperature response. rcb-101 rescues hmr-22 in an allele-specific manner. Consistently, RCB interacts directly with HMR. Together, these results unveil RCB as a novel temperature signaling component that functions collaboratively with HMR to initiate thermomorphogenesis by selectively stabilizing PIF4 in the daytime.

SUMMARYMegasporogenesis is essential for female fertility, and requires the accomplishment of meiosis and the formation of functional megaspores. The inaccessibility and low abundance of female meiocytes make it particularly difficult to elucidate the molecular basis underlying megasporogenesis. We used high-throughput tag-sequencing analysis to identify genes expressed in female meiocytes (FMs) by comparing gene expression profiles from wild-type ovules undergoing megasporogenesis with those from the spl mutant ovules, which lack megasporogenesis. A total of 862 genes were identified as FMs, with levels that are consistently reduced in spl ovules in two biological replicates. Fluorescence-assisted cell sorting followed by RNA-seq analysis of DMC1:GFP-labeled female meiocytes confirmed that 90% of the FMs are indeed detected in the female meiocyte protoplast profiling. We performed reverse genetic analysis of 120 candidate genes and identified four FM genes with a function in female meiosis progression in Arabidopsis. We further revealed that KLU, a putative cytochrome P450 monooxygenase, is involved in chromosome pairing during female meiosis, most likely by affecting the normal expression pattern of DMC1 in ovules during female meiosis. Our studies provide valuable information for functional genomic analyses of plant germline development as well as insights into meiosis.

We demonstrated previously that expression of Macrosiphum euphorbiae salivary protein Me10 enhanced aphid reproduction on its host tomato (Solanum lycopersicum). However, the mechanism of action of Me10 remained elusive.To confirm the secretion of Me10 by the aphid into plant tissues, we produced Me10 polyclonal antibodies. To identify the plant targets of Me10, we developed a tomato immune induced complementary DNA yeast two-hybrid library and screened it with Me10 as bait. Immunoprecipitation and bimolecular fluorescence complementation (BiFC) assays were performed to validate one of the interactions in planta, and virus-induced gene silencing was used for functional characterization in tomato.We demonstrated that Me10 is secreted into the plant tissues and interacts with tomato 14-3-3 isoform 7 (TFT7) in yeast. Immunoprecipitation assays confirmed that Me10 and its homologue in Aphis gossypii, Ag10k, interact with TFT7 in planta. Further, BiFC revealed that Me10 interaction with TFT7 occurs in the plant cell cytoplasm. While silencing of TFT7 in tomato leaves did not affect tomato susceptibility to M. euphorbiae, it enhanced longevity and fecundity of A. gossypii, the non-host aphid.Our results suggest the model whereby TFT7 plays a role in aphid resistance in tomato and effectors of the Me10/Ag10k family interfere with TFT7 function during aphid infestation.

Photoactivated phytochrome B (PHYB) binds to antagonistically acting PHYTOCHROME-INTERACTING transcription FACTORs (PIFs) to regulate hundreds of light responsive genes in Arabidopsis by promoting PIF degradation. However, whether PHYB directly controls the transactivation activity of PIFs remains ambiguous. Here we show that the prototypic PIF, PIF3, possesses a p53-like transcription activation domain (AD) consisting of a hydrophobic activator motif flanked by acidic residues. A PIF3mAD mutant, in which the activator motif is replaced with alanines, fails to activate PIF3 target genes in Arabidopsis, validating the functions of the PIF3 AD in vivo. Intriguingly, the N-terminal photosensory module of PHYB binds immediately adjacent to the PIF3 AD to repress PIF3’s transactivation activity, demonstrating a novel PHYB signaling mechanism through direct interference of the transactivation activity of PIF3. Our findings indicate that PHYB, likely also PHYA, controls the stability and activity of PIFs via structurally separable dual signaling mechanisms.

The potato aphid, Macrosiphum euphorbiae, is an important agricultural pest that causes economic losses to potato and tomato production. To establish the transcriptome for this aphid, RNA-Seq libraries constructed from aphids maintained on tomato plants were used in Illumina sequencing generating 52.6 million 75–105 bp paired-end reads. The reads were assembled using Velvet/Oases software with SEED preprocessing resulting in 22,137 contigs with an N50 value of 2,003bp. After removal of contigs from tomato host origin, 20,254 contigs were annotated using BLASTx searches against the non-redundant protein database from the National Center for Biotechnology Information (NCBI) as well as IntereProScan. This identified matches for 74% of the potato aphid contigs. The highest ranking hits for over 12,700 contigs were against the related pea aphid, Acyrthosiphon pisum. Gene Ontology (GO) was used to classify the identified M. euphorbiae contigs into biological process, cellular component and molecular function. Among the contigs, sequences of microbial origin were identified. Sixty five contigs were from the aphid bacterial obligate endosymbiont Buchnera aphidicola origin and two contigs had amino acid similarities to viruses. The latter two were named Macrosiphum euphorbiae virus 2 (MeV-2) and Macrosiphum euphorbiae virus 3 (MeV-3). The highest sequence identity to MeV-2 had the Dysaphis plantaginea densovirus, while to MeV-3 is the Hubei sobemo-like virus 49. Characterization of MeV-2 and MeV-3 indicated that both are transmitted vertically from adult aphids to nymphs. MeV-2 peptides were detected in the aphid saliva and only MeV-2 and not MeV-3 nucleic acids were detected inside tomato leaves exposed to virus-infected aphids. However, MeV-2 nucleic acids did not persist in tomato leaf tissues, after clearing the plants from aphids, indicating that MeV-2 is likely an aphid virus.