Stem cells in shoot and floral meristems of Arabidopsis thaliana secrete the signaling peptide CLAVATA3 (CLV3) that restricts stem cell proliferation and promotes differentiation. The CLV3 signaling pathway is proposed to comprise the receptor kinase CLV1 and the receptor-like protein CLV2. We show here that the novel receptor kinase CORYNE (CRN) and CLV2 act together, and in parallel with CLV1, to perceive the CLV3 signal. Mutations in CRN cause stem cell proliferation, similar to clv1, clv2, and clv3 mutants. CRN has additional functions during plant development, including floral organ development, that are shared with CLV2. The CRN protein lacks a distinct extracellular domain, and we propose that CRN and CLV2 interact via their transmembrane domains to establish a functional receptor.

Stem cell number in shoot and floral meristems of Arabidopsis (Arabidopsis thaliana) is regulated by the CLAVATA3 (CLV3) signaling pathway. Perception of the CLV3 peptide requires the receptor kinase CLV1, the receptor-like protein CLV2, and the kinase CORYNE (CRN). Genetic analysis suggested that CLV2 and CRN act together and in parallel with CLV1. We studied the intracellular localization of receptor fusions with fluorescent protein tags and their capacities for interaction via efficiency of fluorescence resonance energy transfer. We found that CLV2 and CRN require each other for export from the endoplasmic reticulum and localization to the plasma membrane (PM). CRN readily forms homomers and interacts with CLV2 through the transmembrane domain and adjacent juxtamembrane sequences. CLV1 forms homomers independently of CLV2 and CRN at the PM. We propose that the CLV3 signal is perceived by a tetrameric CLV2/CRN complex and a CLV1 homodimer that localize to the PM and can interact via CRN.In Arabidopsis (Arabidopsis thaliana), the stem cell number in the shoot apical meristem is regulated by negative feedback regulation. Stem cell induction and maintenance are controlled by the homeodomain protein WUSCHEL (WUS), and WUS expression is in turn repressed by CLAVATA3 (CLV3; Brand et al., 2000;Schoof et al., 2000), which encodes a 13-amino acid arabinosylated glycopeptide that is secreted from stem cells (Ohyama et al., 2009). Three genes have been identified that encode receptors for CLV3 signaling. Mutations in CLV1 (Clark et al., 1997), encoding a leucine-rich repeat (LRR) receptor kinase, CLV2, encoding a LRR receptor-like protein (Jeong et al., 1999), and CORYNE (CRN), encoding a receptor-like kinase, disrupt CLV3 signaling and allow the stem cell domain to expand (Sablowski, 2007;Mü ller et al., 2008). Binding of CLV3 to the LRR domains of CLV1 was recently shown (Ogawa et al., 2008).A simple readout for CLV3 signaling is carpel number. Stem cells of floral meristems are normally consumed with the production of two central carpels. Any reduction in CLV3 signaling, which results in increased WUS expression and production of more stem cells, causes an increase in carpel number. Mutations in CLV1, CLV2, or CRN showed an intermediate carpel number phenotype and reduced CLV3 signaling (Mü ller et al., 2008). Double mutants of clv2 with crn were epistatic, but double mutants of clv1 with clv2 or crn were synergistic and abolished CLV3 signaling. This indicated that CLV1 acts independently from, and in parallel with, CLV2 and CRN to transmit the CLV3 signal. Furthermore, clv2 and crn mutants showed additional phenotypes, such as elongated pedicels and defects in stamen development, suggesting that CLV2 and CRN act in a common pathway (Mü ller et al., 2008). Both CRN and CLV2 were proposed to be membrane localized and may physically interact via their transmembrane domains or immediately adjacent sequences (Fig. 1A). A loss-of-function mutation of CRN, crn-1, is caused by an amino acid exchange within the predicted tra...

We hypothesize that these homo- and heteromeric complexes may differentially regulate distal root meristem maintenance. We conclude that essential components of the ancestral shoot stemness regulatory system also act in the root and that the specific interaction of CLV1 with ACR4 serves to moderate and control stemness homeostasis in the root meristem. The structural differences between these two meristem types may have necessitated this recruitment of ACR4 for signaling by CLV1.

Genetic studies have identified the membrane protein EIN2 (ethylene insensitive 2) as a central component of ethylene signalling in Arabidopsis. In addition, EIN2 might take part in multiple hormone signalling pathways and in response to pathogens as demonstrated by recent genetic and biochemical studies. Here we show, by an integrated approach using in vivo and in vitro fluorescence techniques, that EIN2 is localized at the ER (endoplasmic reticulum) membrane where it shows specific interaction with the ethylene receptor protein ETR1.

Cell–cell communication and interaction is critical during fertilization and triggers free cytosolic calcium ([Ca2+]cyto) as a key signal for egg activation and a polyspermy block in animal oocytes. Fertilization in flowering plants is more complex, involving interaction of a pollen tube with egg adjoining synergid cells, culminating in release of two sperm cells and their fusion with the egg and central cell, respectively. Here, we report the occurrence and role of [Ca2+]cyto signals during the entire double fertilization process in Arabidopsis. [Ca2+]cyto oscillations are initiated in synergid cells after physical contact with the pollen tube apex. In egg and central cells, a short [Ca2+]cyto transient is associated with pollen tube burst and sperm cell arrival. A second extended [Ca2+]cyto transient solely in the egg cell is correlated with successful fertilization. Thus, each female cell type involved in double fertilization displays a characteristic [Ca2+]cyto signature differing by timing and behaviour from [Ca2+]cyto waves reported in mammals.

Essential amino acids cannot be synthesized by humans and animals. They often are limiting in plant-derived foods and determine the nutritional value of a given diet. Seeds and fruits often represent the harvestable portion of plants. In order to improve the amino acid composition of these tissues, it is indispensable to understand how these substrates are transported within the plant. Amino acids result from nitrogen assimilation, which often occurs in leaves, the source tissue. They are transported via the vasculature, the xylem, and the phloem into the seeds, the so-called sink tissue, where they are stored or consumed. In seeds, several tissues are symplasmically isolated, i.e., not connected by plasmodesmata, channels in the cell walls that enable a cytoplasmic continuum in plants. Consequently, amino acids must be exported from cells into the apoplast and re-imported many times to support seed development. Several amino acid importers are known, but exporters remained elusive. Here, we characterize four members of the plant-specific UmamiT transporter family from Arabidopsis, related to the amino acid facilitator SIAR1 and the vacuolar auxin transporter WAT1. We show that the proteins transport amino acids along their (electro)chemical potential across the plasma membrane. In seeds, they are found in tissues from which amino acids are exported. Loss-of-function mutants accumulate high levels of free amino acids in fruits and produce smaller seeds. Our results strongly suggest a crucial role for the UmamiTs in amino acid export and possibly a means to improve yield quality.

SUMMARYPlant-parasitic cyst nematodes secrete CLAVATA3 (CLV3)/ESR (CLE)-like effector proteins. These proteins have been shown to act as ligand mimics of plant CLE peptides and are required for successful nematode infection; however, the receptors for nematode CLE-like peptides have not been identified. Here we demonstrate that CLV2 and CORYNE (CRN), members of the receptor kinase family, are required for nematode CLE signaling. Exogenous peptide assays and overexpression of nematode CLEs in Arabidopsis demonstrated that CLV2 and CRN are required for perception of nematode CLEs. In addition, promoter-reporter assays showed that both receptors are expressed in nematode-induced syncytia. Lastly, infection assays with receptor mutants revealed a decrease in both nematode infection and syncytium size. Taken together, our results indicate that perception of nematode CLEs by CLV2 and CRN is not only required for successful nematode infection but is also involved in the formation and/or maintenance of nematode-induced syncytia.

The CLAVATA (CLV) and flagellin (flg) signaling pathways act through peptide ligands and closely related plasma membrane-localized receptor-like kinases (RLKs). The plant peptide CLV3 regulates stem cell homeostasis, whereas the bacterial flg22 peptide elicits defense responses. We applied multiparameter fluorescence imaging spectroscopy (MFIS) to characterize the dynamics of RLK complexes in the presence of ligand in living plant cells expressing receptor proteins fused to fluorescent proteins. We found that the CLV and flg pathways represent two different principles of signal transduction: flg22 first triggered RLK heterodimerization and later assembly into larger complexes through homomerization. In contrast, CLV receptor complexes were preformed, and ligand binding stimulated their clustering. This different behavior likely reflects the nature of these signaling pathways. Pathogen-triggered flg signaling impedes plant growth and development; therefore, receptor complexes are formed only in the presence of ligand. In contrast, CLV3-dependent stem cell homeostasis continuously requires active signaling, and preformation of receptor complexes may facilitate this task.