A putative leucine-rich repeat receptor kinase involved in brassinosteroid signal transduction

Two novel proteins, PopB, which has functional nuclear localization signals, and PopC, which has a large leucine-rich repeat domain, are secreted through the hrp-secretion apparatus of Ralstonia solanacearum

The Ralstonia solanacearum hrp gene cluster codes for components of a type III secretion pathway necessary for the secretion of PopA1, a hypersensitive response-like elicitor protein. In the present study, we show that several other Hrp-secreted proteins can be detected by growing wild-type bacteria in minimal medium in the presence of Congo red. Two of these proteins, PopB and PopC, are encoded by genes located downstream of popA and constitute an operon with popA. popABC mutants retain the wild-type ability to cause disease in hosts and to elicit the hypersensitive response on non-hosts. Expression of the popABC operon is controlled by the hrpB regulatory gene and is induced upon co-culture with Arabidopsis cell suspensions. This plant cell-specific induction depends on PrhA, a putative receptor for plant specific signal(s). The transcription of the popABC operon is not modified by the addition of Congo red to the growth medium and the intracellular pools of PopB and PopC are very similar in the absence or presence of Congo red. Preliminary data suggest that Congo red stabilizes secreted proteins in the extracellular medium. PopB is a 173-amino-acid-basic protein that contains a functional bipartite nuclear localization signal. PopC is a 1024-amino-acid protein that carries 22 tandem leucine-rich repeats (LRR). The LRR domain of this protein forms a consensus that perfectly matches the predicted eukaryotic cytoplasmic LRR consensus. We propose that PopB and PopC may be translocated into plant cells via the Hrp pathway.

Lesions in the sterol delta reductase gene of Arabidopsis cause dwarfism due to a block in brassinosteroid biosynthesis

The brassinosteroid (BR) biosynthetic pathway, and the sterol pathway which is prerequisite to the BR pathway, are rapidly being characterized because of the availability of a large number of characteristic dwarf mutants in Arabidopsis. Here we show that the Arabidopsis dwarf5 mutants are disrupted in a sterol Delta7 reduction step. dwf5 plants display the characteristic dwarf phenotype typical of other BR mutants. This phenotype includes small, round, dark-green leaves, and short stems, pedicels, and petioles. Metabolite tracing with 13C-labeled precursors in dwf5 verified a deficiency in a sterol Delta7 reductase activity. All six independent alleles contain loss-of-function mutations in the sterol Delta7 reductase gene. These include a putative mRNA instability mutation in dwf5-1, 3' and 5' splice-site mutations in dwf5-2 and dwf5-6, respectively, premature stop codons in dwf5-3 (R400Z) and dwf5-5 (R409Z), and a mis-sense mutation in dwf5-4 (D257N). The dwf5 plant could be restored to wild type by ectopic overexpression of the wild-type copy of the gene. Both the Arabidopsis dwf5 phenotype and the human Smith-Lemli-Opitz syndrome are caused by loss-of-function mutations in a sterol Delta7 reductase gene, indicating that it is required for the proper growth and development of these two organisms.

Brassinosteroid signal transduction: still casting the actors

Significant advances in the genetic dissection of brassinosteroid biosynthesis and signaling have been made during the past few years. Genetic and biochemical data have helped to elucidate the pathways of biosynthesis of brassinolide, the most active brassinosteroid. In addition, several models have been put forward for the perception of brassinolide by its putative receptor, BRI1, a ubiquitously expressed plasma membrane localized protein kinase. These studies provide the basic framework for future analysis of brassinosteroid signaling.

Cell signaling within the shoot meristem

Shoot apical meristems are self-renewing stem cell populations that generate all of the above-ground organs (i.e. stems, leaves and flowers) of higher plants. Recent studies have identified new molecular components required for proper shoot meristem activity, and they have revealed that complex, intercellular communication pathways play important roles in coordinating meristem function.

HAESA, an Arabidopsis leucine-rich repeat receptor kinase, controls floral organ abscission

Abcission, the natural shedding of leaves, flowers and fruits, is a fundamental component of plant development. Abscission is a highly regulated process that occurs at distinct zones of cells that undergo enlargement and subsequent separation. Although some components of abscission, including accumulation of the hormone ethylene and cell wall-degrading enzymes, have been described, the regulatory pathways remain largely unknown. In this paper we describe a critical component required for floral organ abscission in Arabidopsis thaliana, the receptor-like protein kinase HAESA. Histochemical analysis of transgenic plants harboring a HAESA promoter:: β-glucuronidase reporter gene and in situ RNA hybridization experiments show HAESA expression in the abscission zones where the sepals, petals, and stamens attach to the receptacle, at the base of pedicels, and at the base of petioles where leaves attach to the stem. Immunodetection, immunoprecipitation, and protein kinase activity assays reveal HAESA is a plasma membrane serine/threonine protein kinase. The reduction of function of HAESA in transgenic plants harboring an antisense construct results in delayed abscission of floral organs, and the severity of the phenotype is directly correlated with the level of HAESA protein. These results demonstrate that HAESA functions in developmentally regulated floral organ abscission.

The evolution of disease resistance genes

Several common themes have shaped the evolution of plant disease resistance genes. These include duplication events of progenitor resistance genes and further expansion to create clustered gene families. Variation can arise from both intragenic and intergenic recombination and gene conversion. Recombination has also been implicated in the generation of novel resistance specificities. Resistance gene clusters appear to evolve more rapidly than other regions of the genome. In addition, domains believed to be involved in recognitional specificity, such as the leucine-rich repeat (LRR), are subject to adaptive selection. Transposable elements have been associated with some resistance gene clusters, and may generate further variation at these complexes.

A putative role for the tomato genes DUMPY and CURL-3 in brassinosteroid biosynthesis and response

The dumpy (dpy) mutant of tomato (Lycopersicon esculentum Mill.) exhibits short stature, reduced axillary branching, and altered leaf morphology. Application of brassinolide and castasterone rescued the dpy phenotype, as did C-23-hydroxylated, 6-deoxo intermediates of brassinolide biosynthesis. The brassinolide precursors campesterol, campestanol, and 6-deoxocathasterone failed to rescue, suggesting that dpy may be affected in the conversion of 6-deoxocathasterone to 6-deoxoteasterone, similar to the Arabidopsis constitutive photomorphogenesis and dwarfism (cpd) mutant. Measurements of endogenous brassinosteroid levels by gas chromatography-mass spectrometry were consistent with this hypothesis. To examine brassinosteroid-regulated gene expression in dpy, we performed cDNA subtractive hybridization and isolated a novel xyloglucan endotransglycosylase that is regulated by brassinosteroid treatment. The curl-3 (cu-3) mutant (Lycopersicon pimpinellifolium ¿Jusl. Mill.) shows extreme dwarfism, altered leaf morphology, de-etiolation, and reduced fertility, all strikingly similar to the Arabidopsis mutant brassinosteroid insensitive 1 (bri1). Primary root elongation of wild-type L. pimpinellifolium seedlings was strongly inhibited by brassinosteroid application, while cu-3 mutant roots were able to elongate at the same brassinosteroid concentration. Moreover, cu-3 mutants retained sensitivity to indole-3-acetic acid, cytokinins, gibberellin, and abscisic acid while showing hypersensitivity to 2, 4-dichlorophenoxyacetic acid in the root elongation assay. The cu-3 root response to hormones, coupled with its bri1-like phenotype, suggests that cu-3 may also be brassinosteroid insensitive.

BAS1: A gene regulating brassinosteroid levels and light responsiveness in Arabidopsis

The Arabidopsis bas1-D mutation suppresses the long hypocotyl phenotype caused by mutations in the photoreceptor phytochrome B (phyB). The adult phenotype of bas1-D phyB-4 double mutants mimics that of brassinosteroid biosynthetic and response mutants. bas1-D phyB-4 has reduced levels of brassinosteroids and accumulates 26-hydroxybrassinolide in feeding experiments. The basis for the mutant phenotype is the enhanced expression of a cytochrome P450 (CYP72B1). bas1-D suppresses a phyB-null allele, but not a phyA-null mutation, and partially suppresses a cryptochrome-null mutation. Seedlings with reduced BAS1 expression are hyperresponsive to brassinosteroids in a light-dependent manner and display reduced sensitivity to light under a variety of conditions. Thus, BAS1 represents one of the control points between multiple photoreceptor systems and brassinosteroid signal transduction.

The Arabidopsis det3 mutant reveals a central role for the vacuolar H(+)-ATPase in plant growth and development

In all multicellular organisms growth and morphogenesis must be coordinated, but for higher plants, this is of particular importance because the timing of organogenesis is not fixed but occurs in response to environmental constraints. One particularly dramatic developmental juncture is the response of dicotyledonous seedlings to light. The det3 mutant of Arabidopsis develops morphologically as a light-grown plant even when it is grown in the dark. In addition, it shows organ-specific defects in cell elongation and has a reduced response to brassinosteroids (BRs). We have isolated the DET3 gene by positional cloning and provide functional and biochemical evidence that it encodes subunit C of the vacuolar H(+)-ATPase (V-ATPase). We show that the hypocotyl elongation defect in the det3 mutant is conditional and provide evidence that this is due to an alternative mechanism of V-ATPase assembly. Together with the expression pattern of the DET3 gene revealed by GFP fluorescence, our data provide in vivo evidence for a role for the V-ATPase in the control of cell elongation and in the regulation of meristem activity.

Brassinosteroid-insensitive dwarf mutants of Arabidopsis accumulate brassinosteroids

Seven dwarf mutants resembling brassinosteroid (BR)-biosynthetic dwarfs were isolated that did not respond significantly to the application of exogenous BRs. Genetic and molecular analyses revealed that these were novel alleles of BRI1 (Brassinosteroid-Insensitive 1), which encodes a receptor kinase that may act as a receptor for BRs or be involved in downstream signaling. The results of morphological and molecular analyses indicated that these represent a range of alleles from weak to null. The endogenous BRs were examined from 5-week-old plants of a null allele (bri1-4) and two weak alleles (bri1-5 and bri1-6). Previous analysis of endogenous BRs in several BR-biosynthetic dwarf mutants revealed that active BRs are deficient in these mutants. However, bri1-4 plants accumulated very high levels of brassinolide, castasterone, and typhasterol (57-, 128-, and 33-fold higher, respectively, than those of wild-type plants). Weaker alleles (bri1-5 and bri1-6) also accumulated considerable levels of brassinolide, castasterone, and typhasterol, but less than the null allele (bri1-4). The levels of 6-deoxoBRs in bri1 mutants were comparable to that of wild type. The accumulation of biologically active BRs may result from the inability to utilize these active BRs, the inability to regulate BR biosynthesis in bri1 mutants, or both. Therefore, BRI1 is required for the homeostasis of endogenous BR levels.

Interactions and intersections of plant signaling pathways

Plant signal transduction is a rapidly expanding field of research, and during the last decade a wealth of insight into how plants perceive and transmit signals as part of normal development and in response to environmental cues has been and is continuing to be unraveled. Although ?signaling cascades are often viewed as linear chains of events it is now becoming increasingly apparent, through the use of cell biological, molecular and genetic approaches, that plant signal transduction involves extensive cross-talk between different pathways. The numerous interactions and intersections which take place are potentially important to modulate and balance the various inputs from different signaling cascades so that plants can integrate all this information to execute the proper developmental responses.

Shuffling the deck: plant signalling plays a club

The transmission of signals across the plasma membrane of cells plays an integral part in cell communication in unicellular and complex organisms. Protein kinases and their activators serve key roles in this process, and a number of paradigms have been established to describe their mode of action. Signalling in plant cells appears to shuffle these paradigms - as evidenced by two recent reports on the development of the Arabidopsis meristem.

Short communication: developmental control of Xa21-mediated disease resistance in rice

The rice resistance gene Xa21 confers resistance against the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo). The molecular genetic mechanism controlling the integration of the Xa21-mediated disease resistance response with the developmental program in rice is under study in this model system. Reproducible means of infecting plants at certain developmental stages were designed based on the timing of full expansion of the leaf. Xa21-resistance progressively increases from the susceptible juvenile leaf 2 stage through later stages, with 100% resistance at the adult leaf 9/10 stage. We found that Xa21 expression is independent of plant developmental stage, infection with Xoo, or wounding. Expression of the Xa21 gene transcript is not correlated with expression of Xa21 disease resistance indicating that the developmental regulation of Xa21-resistance is either controlled post-transcriptionally or by other factors.

Rice gibberellin-insensitive dwarf mutant gene Dwarf 1 encodes the alpha-subunit of GTP-binding protein

A rice Dwarf 1 gene was identified by using a map-based cloning strategy. Its recessive mutant allele confers a dwarf phenotype. Linkage analysis revealed that a cDNA encoding the alpha-subunit of GTP-binding protein cosegregated with d1 in 3,185 d1 segregants. Southern hybridization analysis with this cDNA as a probe showed different band patterns in several d1 mutant lines. In at least four independent d1 mutants, no gene transcript was observed by Northern hybridization analysis. Sequencing analysis revealed that an 833-bp deletion had occurred in one of the mutant alleles, which resulted in an inability to express GTP-binding protein. A transgenic d1 mutant with GTP-binding protein gene restored the normal phenotype. We conclude that the rice Dwarf 1 gene encodes GTP-binding protein and that the protein plays an important role in plant growth and development. Because the d1 mutant is classified as gibberellin-insensitive, we suggest that the GTP-binding protein might be associated with gibberellin signal transduction.

Inactivation of brassinosteroid biological activity by a salicylate-inducible steroid sulfotransferase from Brassica napus

Recent discoveries from brassinosteroid-deficient mutants led to the recognition that plants, like animals, use steroids to regulate their growth and development. We describe the characterization of one member of a Brassica napus sulfotransferase gene family coding for an enzyme that catalyzes the O-sulfonation of brassinosteroids and of mammalian estrogenic steroids. The enzyme is specific for the hydroxyl group at position 22 of brassinosteroids with a preference for 24-epicathasterone, an intermediate in the biosynthesis of 24-epibrassinolide. Enzymatic sulfonation of 24-epibrassinolide abolishes its biological activity in the bean second internode bioassay. This mechanism of hormone inactivation by sulfonation is similar to the modulation of estrogen biological activity observed in mammals. Furthermore, the expression of the B. napus steroid sulfotransferase genes was found to be induced by salicylic acid, a signal molecule in the plant defense response. This pattern of expression suggests that, in addition to an increased synthesis of proteins having antimicrobial properties, plants respond to pathogen infection by modulating steroid-dependent growth and developmental processes.

Identification of an SCF ubiquitin-ligase complex required for auxin response in Arabidopsis thaliana

The plant hormone auxin regulates diverse aspects of plant growth and development. We report that in Arabidopsis, auxin response is dependent on a ubiquitin-ligase (E3) complex called SCFTIR1. The complex consists of proteins related to yeast Skp1p and Cdc53p called ASK and AtCUL1, respectively, as well as the F-box protein TIR1. Mutations in either ASK1 or TIR1 result in decreased auxin response. Further, overexpression of TIR1 promotes auxin response suggesting that SCFTIR1 is limiting for the response. These results provide new support for a model in which auxin action depends on the regulated proteolysis of repressor proteins.

Expression of a gibberellin-induced leucine-rich repeat receptor-like protein kinase in deepwater rice and its interaction with kinase-associated protein phosphatase

We identified in deepwater rice (Oryza sativa L.) a gene encoding a leucine-rich repeat receptor-like transmembrane protein kinase, OsTMK (O. sativa transmembrane kinase). The transcript levels of OsTMK increased in the rice internode in response to gibberellin. Expression of OsTMK was especially high in regions undergoing cell division and elongation. The kinase domain of OsTMK was enzymatically active, autophosphorylating on serine and threonine residues. A cDNA encoding a rice ortholog of a kinase-associated type 2C protein phosphatase (OsKAPP) was cloned. KAPPs are putative downstream components in kinase-mediated signal transduction pathways. The kinase interaction domain of OsKAPP was phosphorylated in vitro by the kinase domain of OsTMK. RNA gel-blot analysis indicated that the expression of OsTMK and OsKAPP was similar in different tissues of the rice plant. In protein-binding assays, OsKAPP interacted with a receptor-like protein kinase, RLK5 of Arabidopsis, but not with the protein kinase domains of the rice and maize receptor-like protein kinases Xa21 and ZmPK1, respectively.

PLANT PROTEIN SERINE/THREONINE KINASES: Classification and Functions

The first plant protein kinase sequences were reported as recently as 1989, but by mid-1998 there were more than 500, including 175 in Arabidopsis thaliana alone. Despite this impressive pace of discovery, progress in understanding the detailed functions of protein kinases in plants has been slower. Protein serine/threonine kinases from A. thaliana can be divided into around a dozen major groups based on their sequence relationships. For each of these groups, studies on animal and fungal homologs are briefly reviewed, and direct studies of their physiological functions in plants are then discussed in more detail. The network of protein-serine/threonine kinases in plant cells appears to act as a "central processor unit" (cpu), accepting input information from receptors that sense environmental conditions, phytohormones, and other external factors, and converting it into appropriate outputs such as changes in metabolism, gene expression, and cell growth and division.

Recent advances on proteins of plant terminal membranes

Since the beginning of the 1990s, our knowledge of the protein equipment of plant membranes progresses at an accelerating pace, owing to the irruption of molecular biology tools and genetics strategies in plant biology. Map-based cloning strategies and exploration of EST databases rapidly enrich the catalog of cDNA or gene sequences expected to code for membrane proteins. The accumulation of 'putative' membrane proteins reinforces the need for structural, functional and physiological information. Indeed, ambiguities often exist concerning the association to a membrane, the membrane identity and the topology of the protein inserted in the membrane. The combination of directed mutagenesis and heterologous expression of plant genes in various systems and plant reverse genetics has opened the possibility to study molecular and physiological functions. This review will emphasize how these tools have been essential for the exciting recent discoveries on plant terminal membrane proteins. These discoveries concern a variety of transport systems for ions, organic solutes including auxin, water channels, a large collection of systems suspected to act as receptors of chemical signals, proteins thought to control vesicle trafficking and enzymatic systems.

GENETIC ANALYSIS OF HORMONE SIGNALING

Phytohormones influence many diverse developmental processes ranging from seed germination to root, shoot, and flower formation. Recently, mutational analysis using the model plant Arabidopsis thaliana has been instrumental in determining the individual components of specific hormone signal transduction pathways. Moreover, epistasis and suppressor studies are beginning to explain how these genes and their products relate to one another. While no hormone transduction pathway is completely understood, the genes identified to date suggest that simple molecular rules can be established to explain how plant hormone signals are transduced. This review describes some of the shared characteristics of plant hormone signal transduction pathways and the properties for informational transfer common to many of the genes that specify the transduction of the signal.

Biochemical and molecular tools for the production of useful terpene products from pepper (Capsicum annuum)

Among other natural products such as colorants and flavorants, natural fungicides like the pepper phytoalexin capsidiol, and the related biochemical pathways, may be used for practical approaches. Key enzymes such as 3-hydroxy-3-methylglutaryl Coenzyme A: reductase, the farnesyl pyrophosphate synthase and and farnesyl pyrophosphate cyclases are known and some related genes have been isolated. However, specific enzymes for important and final modifications as methylation and others, are still to be studied. Construction of chimeric enzymes allowed already the synthesis of different products and the possibilities of designing new enzymes by gene manipulation to produce unknown and useful chemicals are open.

The specificity of polygalacturonase-inhibiting protein (PGIP): a single amino acid substitution in the solvent-exposed beta-strand/beta-turn region of the leucine-rich repeats (LRRs) confers a new recognition capability

Two members of the pgip gene family (pgip-1 and pgip-2) of Phaseolus vulgaris L. were expressed separately in Nicotiana benthamiana and the ligand specificity of their products was analysed by surface plasmon resonance (SPR). Polygalacturonase-inhibiting protein-1 (PGIP-1) was unable to interact with PG from Fusarium moniliforme and interacted with PG from Aspergillus niger; PGIP-2 interacted with both PGs. Only eight amino acid variations distinguish the two proteins: five of them are confined within the beta-sheet/beta-turn structure and two of them are contiguous to this region. By site-directed mutagenesis, each of the variant amino acids of PGIP-2 was replaced with the corresponding amino acid of PGIP-1, in a loss-of-function approach. The mutated PGIP-2s were expressed individually in N.benthamiana, purified and subjected to SPR analysis. Each single mutation caused a decrease in affinity for PG from F.moniliforme; residue Q253 made a major contribution, and its replacement with a lysine led to a dramatic reduction in the binding energy of the complex. Conversely, in a gain-of-function approach, amino acid K253 of PGIP-1 was mutated into the corresponding amino acid of PGIP-2, a glutamine. With this single mutation, PGIP-1 acquired the ability to interact with F.moniliforme PG.

Characterization of an Arabidopsis thaliana receptor-like protein kinase gene activated by oxidative stress and pathogen attack

An Arabidopsis thaliana cDNA clone that encodes a putative receptor-like protein kinase gene (At-RLK3) was characterized. The deduced 667-amino acid protein consists of an amino-terminal signal sequence, an extracellular domain, a single transmembrane domain, and a cytoplasmic domain with characteristics of serine/threonine protein kinase. Because of the original features of its extracellular domain, the At-RLK3 protein is a member of a new class of receptor-like protein kinases. The At-RLK3 gene is present as a single copy within the Arabidopsis genome and its transcripts are detected in root, stem, leaf and flower. In cultured cells, the At-RLK3 gene is activated upon oxidative stress and salicylic acid treatment. In plants, the gene appears to be differentially regulated during various plant-pathogen interactions: upon inoculation with strains of Pseudomonas syringae pv. tomato harboring or not, different avr genes, At-RLK3 transcripts accumulate transiently at similar levels during both compatible and incompatible interactions. This gene is, however, preferentially expressed during the incompatible interaction induced by the soil-borne vascular bacteria, Ralstonia solanacearum. The involvement of At-RLK3 in signal transduction pathways during pathogen attack is discussed.

Brassinosteroid/Sterol synthesis and plant growth as affected by lka and lkb mutations of Pea

The dwarf pea (Pisum sativum) mutants lka and lkb are brassinosteroid (BR) insensitive and deficient, respectively. The dwarf phenotype of the lkb mutant was rescued to wild type by exogenous application of brassinolide and its biosynthetic precursors. Gas chromatography-mass spectrometry analysis of the endogenous sterols in this mutant revealed that it accumulates 24-methylenecholesterol and isofucosterol but is deficient in their hydrogenated products, campesterol and sitosterol. Feeding experiments using 2H-labeled 24-methylenecholesterol indicated that the lkb mutant is unable to isomerize and/or reduce the Delta24(28) double bond. Dwarfism of the lkb mutant is, therefore, due to BR deficiency caused by blocked synthesis of campesterol from 24-methylenecholesterol. The lkb mutation also disrupted sterol composition of the membranes, which, in contrast to those of the wild type, contained isofucosterol as the major sterol and lacked stigmasterol. The lka mutant was not BR deficient, because it accumulated castasterone. Like some gibberellin-insensitive dwarf mutants, overproduction of castasterone in the lka mutant may be ascribed to the lack of a feedback control mechanism due to impaired perception/signal transduction of BRs. The possibility that castasterone is a biologically active BR is discussed.

A leucine-rich repeat protein of carrot that exhibits antifreeze activity

A gene encoding an antifreeze protein (AFP) was isolated from carrot (Daucus carota) using sequence information derived from the purified protein. The carrot AFP is highly similar to the polygalacturonase inhibitor protein (PGIP) family of apoplastic plant leucine-rich repeat (LRR) proteins. Expression of the AFP gene is rapidly induced by low temperatures. Furthermore, expression of the AFP gene in transgenic Arabidopsis thaliana plants leads to an accumulation of antifreeze activity. Our findings suggest that a new type of plant antifreeze protein has recently evolved from PGIPs.

The Arabidopsis dwarf1 mutant is defective in the conversion of 24-methylenecholesterol to campesterol in brassinosteroid biosynthesis

Since the isolation and characterization of dwarf1-1 (dwf1-1) from a T-DNA insertion mutant population, phenotypically similar mutants, including deetiolated2 (det2), constitutive photomorphogenesis and dwarfism (cpd), brassinosteroid insensitive1 (bri1), and dwf4, have been reported to be defective in either the biosynthesis or the perception of brassinosteroids. We present further characterization of dwf1-1 and additional dwf1 alleles. Feeding tests with brassinosteroid-biosynthetic intermediates revealed that dwf1 can be rescued by 22alpha-hydroxycampesterol and downstream intermediates in the brassinosteroid pathway. Analysis of the endogenous levels of brassinosteroid intermediates showed that 24-methylenecholesterol in dwf1 accumulates to 12 times the level of the wild type, whereas the level of campesterol is greatly diminished, indicating that the defective step is in C-24 reduction. Furthermore, the deduced amino acid sequence of DWF1 shows significant similarity to a flavin adenine dinucleotide-binding domain conserved in various oxidoreductases, suggesting an enzymatic role for DWF1. In support of this, 7 of 10 dwf1 mutations directly affected the flavin adenine dinucleotide-binding domain. Our molecular characterization of dwf1 alleles, together with our biochemical data, suggest that the biosynthetic defect in dwf1 results in reduced synthesis of bioactive brassinosteroids, causing dwarfism.

Detection of membrane-bound cytokinin-binding proteins in Arabidopsis thaliana cells

In order to isolate cytokinin-binding proteins (CBPs), we have developed new affinity probes constituted of a cytokinin such as zeatin riboside ([9R]Z) conjugated to a carrier protein. These probes were used for detecting CBPs in an ELISA procedure. The efficiency of the cytokinin conjugate in detecting CBPs was controlled with protein model: proteins having an affinity for cytokinin such as the monoclonal anti-[9R]Z antibodies did bind the cytokinin conjugate whereas proteins unable to bind cytokinin such as bovine serum albumin did not. Using these new affinity probes, we showed that CBPs are present in the membrane fraction of in vitro cultured Arabidopsis thaliana cells. The nature of the protein at the detected binding sites was demonstrated by submitting the microsomal proteins to a proteolytic treatment, which was found to eradicate the binding. Free biologically active cytokinins or monoclonal anti-[9R]Z antibodies inhibited the binding, thus showing the specificity of the interaction. The detected CBPs were partially solubilized from the membranes with potassium chloride, indicating their peripheral membrane location. The separation by anion exchange chromatography of solubilized microsomal proteins revealed the existence of two different CBPs. They were present at higher levels in cells during the exponential growth phase.

Receptors and signalling components of plant hormones

Recent advances in understanding plant hormonal signalling has resulted in the identification of a variety of signalling components including receptor kinases with homology to the bacterial two component system as well as serine/threonine kinases and protein phosphatases. In addition, the existence of MAP kinase pathways in plants indicates a similar role of these signalling cascades in the relay of exogenous signals into the nucleus as has been disclosed in animal cells. The emerging signalling pathways of the plant hormone abscisic acid and ethylene are presented.

The genome of Melanoplus sanguinipes entomopoxvirus

The family Poxviridae contains two subfamilies: the Entomopoxvirinae (poxviruses of insects) and the Chordopoxvirinae (poxviruses of vertebrates). Here we present the first characterization of the genome of an entomopoxvirus (EPV) which infects the North American migratory grasshopper Melanoplus sanguinipes and other important orthopteran pests. The 236-kbp M. sanguinipes EPV (MsEPV) genome consists of a central coding region bounded by 7-kbp inverted terminal repeats and contains 267 open reading frames (ORFs), of which 107 exhibit similarity to previously described genes. The presence of genes not previously described in poxviruses, and in some cases in any other known virus, suggests significant viral adaptation to the arthropod host and the external environment. Genes predicting interactions with host cellular mechanisms include homologues of the inhibitor of apoptosis protein, stress response protein phosphatase 2C, extracellular matrixin metalloproteases, ubiquitin, calcium binding EF-hand protein, glycosyltransferase, and a triacylglyceride lipase. MsEPV genes with putative functions in prevention and repair of DNA damage include a complete base excision repair pathway (uracil DNA glycosylase, AP endonuclease, DNA polymerase beta, and an NAD+-dependent DNA ligase), a photoreactivation repair pathway (cyclobutane pyrimidine dimer photolyase), a LINE-type reverse transcriptase, and a mutT homologue. The presence of these specific repair pathways may represent viral adaptation for repair of environmentally induced DNA damage. The absence of previously described poxvirus enzymes involved in nucleotide metabolism and the presence of a novel thymidylate synthase homologue suggest that MsEPV is heavily reliant on host cell nucleotide pools and the de novo nucleotide biosynthesis pathway. MsEPV and lepidopteran genus B EPVs lack genome colinearity and exhibit a low level of amino acid identity among homologous genes (20 to 59%), perhaps reflecting a significant evolutionary distance between lepidopteran and orthopteran viruses. Divergence between MsEPV and the Chordopoxvirinae is indicated by the presence of only 49 identifiable chordopoxvirus homologues, low-level amino acid identity among these genes (20 to 48%), and the presence in MsEPV of 43 novel ORFs in five gene families. Genes common to both poxvirus subfamilies, which include those encoding enzymes involved in RNA transcription and modification, DNA replication, protein processing, virion assembly, and virion structural proteins, define the genetic core of the Poxviridae.

A tale of dwarfs and drugs: brassinosteroids to the rescue

Brassinosteroids (BRs), a specific class of low abundance plant steroids, are capable of eliciting strong growth responses and a variety of physiological changes through exogenous application to plants. Recently, BRs gained general acceptance as important regulators (hormones) of plant growth and development through genetic and molecular identification and characterization of genes involved in BR biosynthesis or response in Arabidopsis, pea and tomato. This major advance in the molecular genetics of BR biosynthesis and mode of action disclosed another case of amazing functional conservation of signalling molecules utilized in plants and animals, and provided a set of extremely valuable tools for the functional analysis of BRs.

Recent advances in brassinosteroid molecular genetics

The importance of brassinosteroids (BRs, a specific class of ecdysone-like plant steroids) as essential endogenous regulators of growth and development is demonstrated through a growing number of well characterised Arabidopsis, pea, and tomato mutants deficient in BR biosynthesis or BR response. Thus, a rapid advancement in understanding the molecular genetics of BR biosynthesis and mode of action can be witnessed, which will be further enhanced through the availability of a set of extremely valuable molecular tools for the analysis of the biological function of BRs.

Challenges in understanding RLK function

Plants use receptor-like kinases (RLKs) to transduce extracellular signals into the cell. Recent advancements in RLK research include the cloning of the BRASSINOSTEROID INSENSITIVE 1 and CLAVATA 1 genes, revealing RLK roles in development. Our understanding of RLK function has also been broadened by transgenic approaches in the study of the RLKs pollen receptor kinase 1, and wall associated kinase 1. These results extend the observations that RLKs function in developmental processes and plant defense responses. Additionally, expression based studies suggest roles for other newly reported RLKs in development and light responses. Taken together, the studies confirm the importance of RLKs in diverse plant processes, yet major challenges remain. These include identifying ligands that activate RLKs and characterizing downstream pathways. These challenges can be conquered by coordinated efforts from investigators using molecular, genetic, and biochemical approaches.

Plants embrace a stepchild: the discovery of peptide growth regulators

Over the past decade, peptides have been added to the collection of signalling molecules in plants. As the impact of peptide hormones in non-plants is enormous, a comparison of plant and non-plant peptide signal molecules at this stage deserves our attention-not only to reveal common and unique features, but also to point to new avenues of future research on plant hormones.

Overexpression of an Arabidopsis cDNA encoding a sterol-C24(1)-methyltransferase in tobacco modifies the ratio of 24-methyl cholesterol to sitosterol and is associated with growth reduction

Higher plants synthesize 24-methyl sterols and 24-ethyl sterols in defined proportions. As a first step in investigating the physiological function of this balance, an Arabidopsis cDNA encoding an S-adenosyl-L-methionine 24-methylene lophenol-C24(1)-methyltransferase, the typical plant enzyme responsible for the production of 24-ethyl sterols, was expressed in tobacco (Nicotiana tabacum L.) under the control of a constitutive promoter. Transgenic plants displayed a novel 24-alkyl-Delta5-sterol profile: the ratio of 24-methyl cholesterol to sitosterol, which is close to 1 in the wild type, decreased dramatically to values ranging from 0.01 to 0.31. In succeeding generations of transgenic tobacco, a high S-adenosyl-L-methionine 24-methylene lophenol-C24(1)-methyltransferase enzyme activity and, consequently, a low ratio of 24-methyl cholesterol to sitosterol, was associated with reduced growth compared with the wild type. However, this new morphological phenotype appeared only below the threshold ratio of 24-methyl cholesterol to sitosterol of approximately 0.1. Because the size of cells was unchanged in small, transgenic plants, we hypothesize that a radical decrease of 24-methyl cholesterol and/or a concomitant increase of sitosterol would be responsible for a change in cell division through as-yet unknown mechanisms.

Plant hormone perception and action: a role for G-protein signal transduction?

Plants perceive and respond to a profusion of environmental and endogenous signals that influence their growth and development. The G-protein signalling pathway is a mechanism for transducing extracellular signals that is highly conserved in a range of eukaryotes and prokaryotes. Evidence for the existence of G-protein signalling pathways in higher plants is reviewed, and their potential involvement in plant hormone signal transduction evaluated. A range of biochemical and molecular studies have identified potential components of G-protein signalling in plants, most notably a homologue of the G-protein coupled receptor superfamily (GCR1) and the G alpha and G beta subunits of heterotrimeric G-proteins. G-protein agonists and antagonists are known to influence a variety of signalling events in plants and have been used to implicate heterotrimeric G-proteins in gibberellin and possibly auxin signalling. Antisense suppression of GCR1 in Arabidopsis leads to a phenotype which supports a role for this receptor in cytokinin signalling. These observations suggest that higher plants have at least some of the components of G-protein signalling pathways and that these might be involved in the action of certain plant hormones.

Control of cell elongation and stress responses by steroid hormones and carbon catabolic repression in plants

Molecular analysis of Arabidopsis mutants displaying hypocotyl elongation defects in both the dark and light revealed recently that steroids play an essential role as hormones in plants. Deficiencies in brassinosteroid biosynthesis and signalling permit photomorphogenic development and light-regulated gene expression in the dark, and result in severe dwarfism, male sterility and de-repression of stress-induced genes in the light. A cytochrome P450 steroid hydroxylase (CYP90) controls a rate limiting step in brassinosteroid biosynthesis and appears to function as a signalling factor in stress responses. Another key step in steroid biosynthesis is controlled by the Arabidopsis SNF1 kinases that phosphorylate the 3-hydroxy-3methylglutaryl-CoA reductase. The activity of SNF1 kinases is regulated by PRL1, an evolutionarily conserved alpha-importin-binding nuclear WD-protein. The prl1 mutation results in cell elongation defects, de-repression of numerous stress-induced genes, and augments the sensitivity of plants to glucose, cold stress and several hormones, including cytokinin, ethylene, auxin, and abscisic acid.

Control of meristem development by CLAVATA1 receptor kinase and kinase-associated protein phosphatase interactions

The CLAVATA1 (CLV1) gene encodes a putative receptor kinase required for the proper balance between cell proliferation and differentiation in Arabidopsis shoot and flower meristems. Impaired CLV1 signaling results in masses of undifferentiated cells at the shoot and floral meristems. Although many putative receptor kinases have been identified in plants, the mechanism of signal transduction mediated by plant receptor-like kinases is largely unknown. One potential effector of receptor kinase signaling is kinase-associated protein phosphatase (KAPP), a protein that binds to multiple plant receptor-like kinases in a phosphorylation-dependent manner. To examine a possible role for KAPP in CLV1-dependent plant development, the interaction of CLV1 and KAPP was investigated in vitro and in vivo. KAPP binds directly to autophosphorylated CLV1 in vitro and co-immunoprecipitates with CLV1 in plant extracts derived from meristematic tissue. Reduction of KAPP transcript accumulation in an intermediate clv1 mutant suppresses the mutant phenotype, and the degree of suppression is inversely correlated with KAPP mRNA levels. These data suggest that KAPP functions as a negative regulator of CLV1 signaling in plant development. This may represent a general model for the interaction of KAPP with receptor kinases.

Structure and function of proteins controlling strain-specific pathogen resistance in plants

Recently recognised structural and amino acid sequence similarities between plant disease resistance (R) proteins and animal proteins such as Apaf-1 and CED-4 are providing conceptual models for resistance protein function. Data from extensive DNA sequencing of resistance gene families are indicating that the leucine-rich repeat motif is an important determinant of gene-for-gene specificity and that intergenic DNA sequence exchange is a major contributor to R gene diversity.

Resistance gene evolution

Plant resistance genes are highly polymorphic and have diverse recognition specificities. These genes often occur as members of clustered gene families that have evolved through duplication and diversification. Regions of nucleotides conserved between family members and flanking sequences facilitate equal or unequal recombination events. Transposition contributes to allelic diversity. Resistance gene clusters appear to evolve more rapidly than other regions of the genome, and domains responsible for recognitional specificity, such as the leucine-rich repeat domain, are subject to adaptive selection.

Ethylene responses are negatively regulated by a receptor gene family in Arabidopsis thaliana

A family of genes including ETR1, ETR2, EIN4, ERS1, and ERS2 is implicated in ethylene perception in Arabidopsis thaliana. As only dominant mutations were previously available for these genes, it was unclear whether all of them are components in the ethylene signaling pathway and whether they code for positive or negative regulators of ethylene responses. In this study, we have isolated loss-of-function mutations of four of these genes (ETR1, ETR2, EIN4, and ERS2) and identified an ethylene-independent role of ETR1 in promoting cell elongation. Quadruple mutants had constitutive ethylene responses, revealing that these proteins negatively regulate ethylene responses and that the induction of ethylene response in Arabidopsis is through inactivation rather than activation of these proteins.

Transcription of the Arabidopsis CPD gene, encoding a steroidogenic cytochrome P450, is negatively controlled by brassinosteroids

The Arabidopsis CPD gene encodes a cytochrome P450 steroid side-chain hydroxylase (CYP90) that plays an essential role in the biosynthesis of the plant hormone brassinolide. Expression of the CPD gene is confined to cotyledons and leaf primordia in etiolated seedlings and detectable in the adaxial parenchyma of expanding leaves in light-grown plants. Transcription of the CPD gene is not affected by the plant growth factors auxin, ethylene, gibberellin, cytokinin, jasmonic acid and salicylic acid, but is specifically down-regulated by brassinolide in both dark and light. Steady-state mRNA levels of a CPD promoter-driven uidA reporter gene correlate with the expression of resident CPD gene in transgenic plants. Intermediates of the early and late C-6 oxidation pathways of brassinolide, carrying C-22 and C-23 side-chain hydroxyls, efficiently inhibit the activity of the CPD promoter. Repression of CPD transcription by brassinosteroids is sensitive to the protein synthesis inhibitor cycloheximide, indicating a requirement for de novo synthesis of a regulatory factor.

BRASSINOSTEROIDS: Essential Regulators of Plant Growth and Development

Brassinosteroids (BRs) are growth-promoting natural products found at low levels in pollen, seeds, and young vegetative tissues throughout the plant kingdom. Detailed studies of BR biosynthesis and metabolism, coupled with the recent identification of BR-insensitive and BR-deficient mutants, has greatly expanded our view of steroids as signals controlling plant growth and development. This review examines the microchemical and molecular genetic analyses that have provided convincing evidence for an essential role of BRs in diverse developmental programs, including cell expansion, vascular differentiation, etiolation, and reproductive development. Recent advances relevant to the molecular mechanisms of BR-regulated gene expression and BR signal transduction are also discussed.

Genetics of plant-pathogen interactions

Progress has occurred in understanding the function of disease-resistance genes that govern the resistance of plants to pathogens, and pathogen-produced molecules, called elicitors, that resistance genes key on. Data support the elicitor-receptor model wherein resistant plants contain receptors for pathogen elicitors. This recognition may be complex, however, involving delivery of elicitors to plant cells by specialized pathogen secretion systems and their processing prior to perception. Furthermore, elicitor receptors may not be the resistance gene proteins that govern specificity of the system. It is now also recognized that many elicitors function as virulence factors for the pathogen but have been co-opted by plants as triggers for active resistance. Major recent advances in the cloning and sequencing of clustered plant disease-resistance genes are providing information on the basis of their recognitional specificities and offer the opportunity to engineer new genes that recognize refractory pathogens or exhibit increased efficacy and durability. In combination with the transformation of cloned disease-resistance genes into new plant species, these approaches should facilitate disease control strategies in practical agriculture.