Plant receptor-like kinase gene family: diversity, function, and signaling.

Plant receptor-like kinases (RLKs) are transmembrane proteins with putative amino-terminal extracellular domains and carboxyl-terminal intracellular kinase domains, with striking resemblance in domain organization to the animal receptor tyrosine kinases such as epidermal growth factor receptor. The recently sequenced Arabidopsis genome contains more than 600 RLK homologs, representing nearly 2.5% of the annotated protein-coding genes in Arabidopsis. Although only a handful of these genes have known functions and fewer still have identified ligands or downstream targets, the studies of several RLKs such as CLAVATA1, Brassinosteroid Insensitive 1, Flagellin Insensitive 2, and S-locus receptor kinase provide much-needed information on the functions mediated by members of this large gene family. RLKs control a wide range of processes, including development, disease resistance, hormone perception, and self-incompatibility. Combined with the expression studies and biochemical analysis of other RLKs, more details of RLK function and signaling are emerging.

Receptor-like kinases from Arabidopsis form a monophyletic gene family related to animal receptor kinases.

Plant receptor-like kinases (RLKs) are proteins with a predicted signal sequence, single transmembrane region, and cytoplasmic kinase domain. Receptor-like kinases belong to a large gene family with at least 610 members that represent nearly 2.5% of Arabidopsis protein coding genes. We have categorized members of this family into subfamilies based on both the identity of the extracellular domains and the phylogenetic relationships between the kinase domains of subfamily members. Surprisingly, this structurally defined group of genes is monophyletic with respect to kinase domains when compared with the other eukaryotic kinase families. In an extended analysis, animal receptor kinases, Raf kinases, plant RLKs, and animal receptor tyrosine kinases form a well supported group sharing a common origin within the superfamily of serine/threonine/tyrosine kinases. Among animal kinase sequences, Drosophila Pelle and related cytoplasmic kinases fall within the plant RLK clade, which we now define as the RLK/Pelle family. A survey of expressed sequence tag records for land plants reveals that mosses, ferns, conifers, and flowering plants have similar percentages of expressed sequence tags representing RLK/Pelle homologs, suggesting that the size of this gene family may have been close to the present-day level before the diversification of land plant lineages. The distribution pattern of four RLK subfamilies on Arabidopsis chromosomes indicates that the expansion of this gene family is partly a consequence of duplication and reshuffling of the Arabidopsis genome and of the generation of tandem repeats.

Ethylene: a gaseous signal molecule in plants.

Ethylene regulates a multitude of plant processes, ranging from seed germination to organ senescence. Of particular economic importance is the role of ethylene as an inducer of fruit ripening. Ethylene is synthesized from S-adenosyl-L-methionine via 1-aminocyclopropane-1-carboxylic acid (ACC). The enzymes catalyzing the two reactions in this pathway are ACC synthase and ACC oxidase. Environmental and endogenous signals regulate ethylene biosynthesis primarily through differential expression of ACC synthase genes. Components of the ethylene signal transduction pathway have been identified by characterization of ethylene-response mutants in Arabidopsis thaliana. One class of mutations, exemplified by etr1, led to the identification of the ethylene receptors, which turned out to be related to bacterial two-component signaling systems. Mutations that eliminate ethylene binding to the receptor yield a dominant, ethylene-insensitive phenotype. CTR1 encodes a Raf-like Ser/Thr protein kinase that acts downstream from the ethylene receptor and may be part of a MAP kinase cascade. Mutants in CTR1 exhibit a constitutive ethylene-response phenotype. Both the ethylene receptors and CTR1 are negative regulators of ethylene responses. EIN2 and EIN3 are epistatic to CTR1, and mutations in either gene lead to ethylene insensitivity. Whereas the function of EIN2 in ethylene transduction is not known, EIN3 is a putative transcription factor involved in regulating expression of ethylene-responsive genes. Biotechnological modifications of ethylene synthesis and of sensitivity to ethylene are promising methods to prevent spoilage of agricultural products such as fruits, whose ripening is induced by ethylene.

Ethylene perception by the ERS1 protein in Arabidopsis.

Ethylene perception in Arabidopsis is controlled by a family of five genes, including ETR1, ERS1 (ethylene response sensor 1), ERS2, ETR2, and EIN4. ERS1, the most highly conserved gene with ETR1, encodes a protein with 67% identity to ETR1. To clarify the role of ERS1 in ethylene sensing, we biochemically characterized the ERS1 protein by heterologous expression in yeast. ERS1, like ETR1, forms a membrane-associated, disulfide-linked dimer. In addition, yeast expressing the ERS1 protein contains ethylene-binding sites, indicating ERS1 is also an ethylene-binding protein. This finding supports previous genetic evidence that isoforms of ETR1 also function in plants as ethylene receptors. Further, we used the ethylene antagonist 1-methylcyclopropene (1-MCP) to characterize the ethylene-binding sites of ERS1 and ETR1. We found 1-MCP to be both a potent inhibitor of the ethylene-induced seedling triple response, as well as ethylene binding by yeast expressing ETR1 and ERS1. Yeast expressing ETR1 and ERS1 showed nearly identical sensitivity to 1-MCP, suggesting that the ethylene-binding sites of ETR1 and ERS1 have similar affinities for ethylene.

Axillary meristem development in Arabidopsis thaliana.

Axillary shoot apical meristems initiate post-embryonically in the axils of leaves. Their developmental fate is a main determinant of the final plant body plan. In Arabidopsis, usually a single axillary meristem initiates in the leaf axil even though there is developmental potential for formation of multiple branches. While the wild-type plants rarely form multiple branches in the leaf axil, tfl1-2 plants regularly develop two or more branches in the axils of the rosette leaves. Axillary meristem formation in Arabidopsis occurs in two waves: an acropetal wave forms during plant vegetative development, and a basipetal wave forms during plant reproductive development. We report here the morphological and anatomical changes, and the STM expression pattern associated with the formation of axillary and accessory meristems during Arabidopsis vegetative development.

The embryo MADS domain factor AGL15 acts postembryonically. Inhibition of perianth senescence and abscission via constitutive expression.

AGL15 (AGAMOUS-like 15), a member of the MADS domain family of regulatory factors, accumulates preferentially throughout the early stages of the plant life cycle. In this study, we investigated the expression pattern and possible roles of postembryonic accumulation of AGL15. Using a combination of reporter genes, RNA gel blot analysis, and immunochemistry, we found that the AGL15 protein accumulates transiently in the shoot apex in young Arabidopsis and Brassica seedlings and that promoter activity is associated with the shoot apex and the base of leaf petioles throughout the vegetative phase. During the reproductive phase, AGL15 accumulates transiently in floral buds. When AGL15 was expressed in Arabidopsis under the control of a strong constitutive promoter, we noted a striking increase in the longevity of the sepals and petals as well as delays in a selected set of age-dependent developmental processes, including the transition to flowering and fruit maturation. Although ethylene has been implicated in many of these same processes, the effects of AGL15 could be clearly distinguished from the effects of the ethylene resistant1-1 mutation, which confers dominant insensitivity to ethylene. By comparing the petal breakstrength (the force needed to remove petals) for flowers of different ages, we determined that ectopic AGL15 had a novel effect: the breakstrength of petals initially declined, as occurs in the wild type, but was then maintained at an intermediate value over a prolonged period. Abscission-associated gene expression and structural changes were also altered in the presence of ectopic AGL15.

A defect in beta-oxidation causes abnormal inflorescence development in Arabidopsis.

The abnormal inflorescence meristem1 (aim1) mutation affects inflorescence and floral development in Arabidopsis. After the transition to reproductive growth, the aim1 inflorescence meristem becomes disorganized, producing abnormal floral meristems and resulting in plants with severely reduced fertility. The derived amino acid sequence of AIM1 shows extensive similarity to the cucumber multifunctional protein involved in beta-oxidation of fatty acids, which possesses l-3-hydroxyacyl-CoA hydrolyase, l-3-hydroxyacyl-dehydrogenase, d-3-hydroxyacyl-CoA epimerase, and Delta(3), Delta(2)-enoyl-CoA isomerase activities. A defect in beta-oxidation has been confirmed by demonstrating the resistance of the aim1 mutant to 2,4-diphenoxybutyric acid, which is converted to the herbicide 2,4-D by the beta-oxidation pathway. In addition, the loss of AIM1 alters the fatty acid composition of the mature adult plant.

The relationship between ethylene binding and dominant insensitivity conferred by mutant forms of the ETR1 ethylene receptor.

Ethylene responses in Arabidopsis are mediated by a small family of receptors, including the ETR1 gene product. Specific mutations in the N-terminal ethylene-binding domain of any family member lead to dominant ethylene insensitivity. To investigate the mechanism of ethylene insensitivity, we examined the effects of mutations on the ethylene-binding activity of the ETR1 protein expressed in yeast. The etr1-1 and etr1-4 mutations completely eliminated ethylene binding, while the etr1-3 mutation severely reduced binding. Additional site-directed mutations that disrupted ethylene binding in yeast also conferred dominant ethylene insensitivity when the mutated genes were transferred into wild-type Arabidopsis plants. By contrast, the etr1-2 mutation did not disrupt ethylene binding in yeast. These results indicate that dominant ethylene insensitivity may be conferred by mutations that disrupt ethylene binding or that uncouple ethylene binding from signal output by the receptor. Increased dosage of wild-type alleles in triploid lines led to the partial recovery of ethylene sensitivity, indicating that dominant ethylene insensitivity may involve either interactions between wild-type and mutant receptors or competition between mutant and wild-type receptors for downstream effectors.

A copper cofactor for the ethylene receptor ETR1 from Arabidopsis.

The ETR1 receptor from Arabidopsis binds the gaseous hormone ethylene. A copper ion associated with the ethylene-binding domain is required for high-affinity ethylene-binding activity. A missense mutation in the domain that renders the plant insensitive to ethylene eliminates both ethylene binding and the interaction of copper with the receptor. A sequence from the genome of the cyanobacterium Synechocystis sp. strain 6803 that shows homology to the ethylene-binding domain of ETR1 encodes a functional ethylene-binding protein. On the basis of sequence conservation between the Arabidopsis and the cyanobacterial ethylene-binding domains and on in vitro mutagenesis of ETR1, a structural model for this copper-based ethylene sensor domain is presented.

The ethylene-receptor family from Arabidopsis: structure and function.

The gaseous hormone ethylene regulates many aspects of plant growth and development. Ethylene is perceived by a family of high-affinity receptors typified by the ETR1 protein from Arabidopsis. The ETR1 gene codes for a protein which contains a hydrophobic N-terminal domain that binds ethylene and a C-terminal domain that is related in sequence to histidine kinase-response regulator two-component signal transducers found in bacteria. A structural model for the ethylene-binding domain is presented in which a Cu(I) ion is coordinated within membrane-spanning alpha-helices of the hydrophobic domain. It is proposed that binding of ethylene to the transition metal would induce a conformational change in the sensor domain that would be propagated to the cytoplasmic transmitter domain of the protein. A total of four additional genes that are related in sequence to ETR1 have been identified in Arabidopsis. Specific missense mutations in any one of the five genes leads to ethylene insensitivity in planta. Models for signal transduction that can account for the genetic dominance of these mutations are discussed.

EIN4 and ERS2 are members of the putative ethylene receptor gene family in Arabidopsis.

The Arabidopsis ethylene receptor gene ETR1 and two related genes, ERS1 and ETR2, were identified previously. These three genes encode proteins homologous to the two-component regulators that are widely used for environment sensing in bacteria. Mutations in these genes confer ethylene insensitivity to wild-type plants. Here, we identified two Arabidopsis genes, EIN4 and ERS2, by cross-hybridizing them with ETR2. Sequence analysis showed that they are more closely related to ETR2 than they are to ETR1 or ERS1. EIN4 previously was isolated as a dominant ethylene-insensitive mutant. ERS2 also conferred dominant ethylene insensitivity when certain mutations were introduced into it. Double mutant analysis indicated that ERS2, similar to ETR1, ETR2, ERS1, and EIN4, acts upstream of CTR1. Therefore, EIN4 and ERS2, along with ETR1, ETR2, and ERS1, are members of the ethylene receptor-related gene family of Arabidopsis. RNA expression patterns of members of this gene family suggest that they might have distinct as well as redundant functions in ethylene perception.

ETR2 is an ETR1-like gene involved in ethylene signaling in Arabidopsis.

The plant hormone ethylene regulates a variety of processes of growth and development. To identify components in the ethylene signal transduction pathway, we screened for ethylene-insensitive mutants in Arabidopsis thaliana and isolated a dominant etr2-1 mutant. The etr2-1 mutation confers ethylene insensitivity in several processes, including etiolated seedling elongation, leaf expansion, and leaf senescence. Double mutant analysis indicates that ETR2 acts upstream of CTR1, which codes for a Raf-related protein kinase. We cloned the ETR2 gene on the basis of its map position, and we found that it exhibits sequence homology to the ethylene receptor gene ETR1 and the ETR1-like ERS gene. ETR2 may thus encode a third ethylene receptor in Arabidopsis, transducing the hormonal signal through its "two-component" structure. Expression studies show that ETR2 is ubiquitously expressed and has a higher expression in some tissues, including inflorescence and floral meristems, petals, and ovules.

The evolutionary basis of leaf senescence: method to the madness?

Recent studies on the differential expression of genes associated with leaf senescence support the long-standing interpretation of plant senescence as an organized, genetically controlled process. Sequence identities of genes that are differentially expressed in senescing leaves indicate roles in the salvage of nutrients. By considering this salvage function as the selected trait and the degeneration and death of the tissue a pleiotropic consequence of nutrient redistribution, the process of leaf senescence can be reconciled with evolutionary theories on the origins of senescence in animals.

A dominant mutant receptor from Arabidopsis confers ethylene insensitivity in heterologous plants.

Ethylene (C2H4) is a gaseous hormone that affects many aspects of plant growth and development. Ethylene perception requires specific receptors and a signal transduction pathway to coordinate downstream responses. The etr1-1 gene of Arabidopsis encodes a mutated receptor that confers dominant ethylene insensitivity. Evidence is presented here that etr1-1 also causes significant delays in fruit ripening, flower sensecence; and flower abscission when expressed in tomato and petunia plants. The ability of etr1-1 to function in heterologous plants suggests that this pathway of hormone recognition and response is highly conserved and can be manipulated.

An altered body plan is conferred on Arabidopsis plants carrying dominant alleles of two genes.

In this paper, we describe a late-flowering ecotype of Arabidopsis, Sy-0, in which the axillary meristems maintain a prolonged vegetative phase, even though the primary shoot apical meristem has already converted to reproductive development. This novel heterochronic shift in the development of axillary meristems results in the formation of aerial rosettes of leaves at the nodes of the primary shoot axis. We present evidence that the aerial-rosette phenotype arises due to the interaction between dominant alleles of two genes: ART, aerial rosette gene (on chromosome 5) and EAR, enhancer of aerial rosette (on chromosome 4): EAR has been tentatively identified as a new allele of the FRI locus. The possible role of these two genes in the conversion of shoot apical meristems to reproductive development is discussed.

A Mutation Altering Auxin Homeostasis and Plant Morphology in Arabidopsis.

Many aspects of plant development are associated with changing concentrations of the phytohormone auxin. Several stages of root formation exhibit extreme sensitivities to exogenous auxin and are correlated with shifts in endogenous auxin concentration. In an effort to elucidate mechanisms regulating development of adventitious roots, an ethyl methanesulfonate-mutagenized M2 population of Arabidopsis was screened for mutants altered in this process. A recessive nuclear mutant, rooty (rty), displayed extreme proliferation of roots, inhibition of shoot growth, and other alterations suggesting elevated responses to auxin or ethylene. Wild-type Arabidopsis seedlings grown on auxin-containing media phenocopied rty, whereas rty seedlings were partially rescued on cytokinin-containing media. Analysis by gas chromatography-selected ion monitoring-mass spectrometry showed endogenous indole-3-acetic acid concentrations to be two to 17 times higher in rty than in the wild type. Dose-response assays with exogenous indole-3-acetic acid indicated equal sensitivities to auxin in tissues of the wild type and rty. Combining rty with mutations conferring resistance to auxin (axr1-3) or ethylene (etr1-1) suggested that root proliferation and restricted shoot growth are auxin effects, whereas other phenotypic alterations are due to ethylene. Four mutant alleles from independently mutagenized populations were identified, and the locus was mapped using morphological and restriction fragment length polymorphism markers to 3.9 centimorgans distal to marker m605 on chromosome 2. The wild-type RTY gene product may serve a critical role in regulating auxin concentrations and thereby facilitating normal plant growth and development.

Ethylene-binding sites generated in yeast expressing the Arabidopsis ETR1 gene.

Mutations in the ETR1 gene of Arabidopsis thaliana confer insensitivity to ethylene, which indicates a role for the gene product in ethylene signal transduction. Saturable binding sites for [14C]ethylene were detected in transgenic yeast expressing the ETR1 protein, whereas control yeast lacking ETR1 showed no detectable ethylene binding. Yeast expressing a mutant form of ETR1 (etr1-1) also showed no detectable ethylene binding, which provides an explanation for the ethylene-insensitive phenotype observed in plants carrying this mutation. Expression of truncated forms of ETR1 in yeast provided evidence that the amino-terminal hydrophobic domain of the protein is the site of ethylene binding. It was concluded from these results that ETR1 acts as an ethylene receptor in Arabidopsis.

Analysis of ethylene signal-transduction kinetics associated with seedling-growth response and chitinase induction in wild-type and mutant arabidopsis.

Kinetic aspects of ethylene-mediated signal transduction leading to seedling-growth inhibition and chitinase induction in Arabidopsis were investigated by the introduction of defined mutations in components of these pathways. Dose-response analysis of wild-type responses indicated that the rate-limiting steps for seedling responses and Arabidopsis basic-chitinase induction displayed Michaelis-Menten kinetics with apparent dissociation constants of the response (Kr) of 0.1 and 1.4 microL L-1 ethylene, respectively. In the ethylene-insensitive etr1-1 and ein2-32 mutant lines, both Arabidopsis basic-chitinase induction and seedling-growth responses were completely disrupted, whereas the weaker etr1-2 allele eliminated the chitinase-induction response but only partially disrupted the seedling responses. A heterologous reporter gene containing the chitinase promoter from bean (bean basic-chitinase-beta-glucuronidase) displayed subsensitive kinetics (Kr 120 microL L-1 ethylene) compared to the response of the endogenous basic-chitinase response (Kr 1.4 microL L-1 ethylene). A model for ethylene signal transduction that accounts for the observed variation in ethylene dose-response relationships is presented. The relationship between the model and the biochemical mechanisms of well-characterized signal-transduction systems in animals is discussed.