An Arabidopsis mutant with deregulated ABA gene expression: implications for negative regulator function

Multifaceted activity of cytokinin in leaf development shapes its size and structure in Arabidopsis

The phytohormone cytokinin has been shown to affect many aspects of plant development ranging from the regulation of the shoot apical meristem to leaf senescence. However, some studies have reported contradictory effects of cytokinin on leaf physiology. Therefore cytokinin treatments cause both chlorosis and increased greening and both lead to decrease or increase in cell size. To elucidate this multifaceted role of cytokinin in leaf development, we have employed a system of temporal controls over the cytokinin pool and investigated the consequences of modulated cytokinin levels in the third leaf of Arabidopsis. We show that, at the cell proliferation phase, cytokinin is needed to maintain cell proliferation by blocking the transition to cell expansion and the onset of photosynthesis. Transcriptome profiling revealed regulation by cytokinin of a gene suite previously shown to affect cell proliferation and expansion and thereby a molecular mechanism by which cytokinin modulates a molecular network underlying the cellular responses. During the cell expansion phase, cytokinin stimulates cell expansion and differentiation. Consequently, a cytokinin excess at the cell expansion phase results in an increased leaf and rosette size fueled by higher cell expansion rate, yielding higher shoot biomass. Proteome profiling revealed the stimulation of primary metabolism by cytokinin, in line with an increased sugar content that is expected to increase turgor pressure, representing the driving force of cell expansion. Therefore, the developmental timing of cytokinin content fluctuations, together with a tight control of primary metabolism, is a key factor mediating transitions from cell proliferation to cell expansion in leaves.

Functionalized mesoporous silica nanoparticles with redox-responsive short-chain gatekeepers for agrochemical delivery

The controlled release of salicylic acid (SA), a key phytohormone, was mediated by using a novel decanethiol gatekeeper system grafted onto mesoporous silica nanoparticles (MSNs). The decanethiol was conjugated only to the external surfaces of the MSNs through glutathione (GSH)-cleavable disulfide linkages and the introduction of a process to assemble gatekeepers only on the outer surface so that the mesopore area can be maintained for high cargo loading. Raman and nitrogen sorption isotherm analyses confirmed the successful linkage of decanethiol to the surface of MSNs. The in vitro release of SA from decanethiol gated MSNs indicated that the release rate of SA in an environment with a certain amount of GSH was significantly higher than that without GSH. More importantly, in planta experiments showed the release of SA from decanethiol gated MSNs by GSH induced sustained expression of the plant defense gene PR-1 up to 7 days after introduction, while free SA caused an early peak in PR-1 expression which steadily decreased after 3 days. This study demonstrates the redox-responsive release of a phytohormone in vitro and also indicates the potential use of MSNs in planta as a controlled agrochemical delivery system.

Abscisic Acid synthesis and response

Abscisic acid (ABA) is one of the "classical" plant hormones, i.e. discovered at least 50 years ago, that regulates many aspects of plant growth and development. This chapter reviews our current understanding of ABA synthesis, metabolism, transport, and signal transduction, emphasizing knowledge gained from studies of Arabidopsis. A combination of genetic, molecular and biochemical studies has identified nearly all of the enzymes involved in ABA metabolism, almost 200 loci regulating ABA response, and thousands of genes regulated by ABA in various contexts. Some of these regulators are implicated in cross-talk with other developmental, environmental or hormonal signals. Specific details of the ABA signaling mechanisms vary among tissues or developmental stages; these are discussed in the context of ABA effects on seed maturation, germination, seedling growth, vegetative stress responses, stomatal regulation, pathogen response, flowering, and senescence.

Genetic analysis reveals a complex regulatory network modulating CBF gene expression and Arabidopsis response to abiotic stress

Arabidopsis CBF genes (CBF1-CBF3) encode transcription factors having a major role in cold acclimation, the adaptive process whereby certain plants increase their freezing tolerance in response to low non-freezing temperatures. Under these conditions, the CBF genes are induced and their corresponding proteins stimulate the expression of target genes configuring low-temperature transcriptome and conditioning Arabidopsis freezing tolerance. CBF2 seems to be the most determinant of the CBFs since it also regulates CBF1 and CBF3 expression. Despite the relevance of CBF genes in cold acclimation, little is known about the molecular components that control their expression. To uncover factors acting upstream of CBF2, mutagenized Arabidopsis containing the luciferase reporter gene under the control of the CBF2 promoter were screened for plants with de-regulated CBF2 expression. Here, the identification and characterization of five of these mutants, named acex (altered CBF2 expression), is presented. Three mutants show increased levels of cold-induced CBF2 transcripts compared with wild-type plants, the other two exhibiting reduced levels. Some mutants are also affected in cold induction of CBF1 and CBF3. Furthermore, the mutants characterized display unique phenotypes for tolerance to abiotic stresses, including freezing, dehydration, and high salt. These results demonstrate that cold induction of CBF2 is subjected to both positive and negative regulation through different signal transduction pathways, some of them also mediating the expression of other CBF genes as well as Arabidopsis responses to abiotic stresses.

Perspectives on Systematic Analyses of Gene Function in Arabidopsis thaliana: New Tools, Topics and Trends

Since the sequencing of the nuclear genome of Arabidopsis thaliana ten years ago, various large-scale analyses of gene function have been performed in this model species. In particular, the availability of collections of lines harbouring random T-DNA or transposon insertions, which include mutants for almost all of the ~27,000 A. thaliana genes, has been crucial for the success of forward and reverse genetic approaches. In the foreseeable future, genome-wide phenotypic data from mutant analyses will become available for Arabidopsis, and will stimulate a flood of novel in-depth gene-function analyses. In this review, we consider the present status of resources and concepts for systematic studies of gene function in A. thaliana. Current perspectives on the utility of loss-of-function and gain-of-function mutants will be discussed in light of the genetic and functional redundancy of many A. thaliana genes.

Systemic low temperature signaling in Arabidopsis

When leaves are exposed to low temperature, sugars accumulate and transcription factors in the C-repeat binding factor (CBF) family are expressed, which, together with CBF-independent pathways, are known to contribute to the cold acclimation process and an increase in freezing tolerance. What is not known, however, is whether expression of these cold-regulated genes can be induced systemically in response to a localized cold treatment. To address this, pre-existing, mature leaves of warm-grown Arabidopsis thaliana were exposed to a localized cold treatment (near 10 °C) whilst conjoined newly developing leaves continued only to experience warmer temperatures. In initial experiments on wild-type A. thaliana (Col-0) using real-time reverse transcription--PCR (RT-PCR) we observed that some genes--including CBF genes, certain downstream cold-responsive (COR) targets and CBF-independent transcription factors--respond to a direct 9 °C treatment of whole plants. In subsequent experiments, we found that the treatment of expanded leaves with temperatures near 10 °C can induce cold-associated genes in conjoined warm-maintained tissues. CBF1 showed a particularly strong systemic response, although CBF-independent transcription factors also responded. Moreover, the localized cold treatment of A. thaliana (C24) plants with a luciferase reporter fused to the promoter region of KIN2 indicated that in warm-maintained leaves, KIN2 might respond to a systemic signal from remote, directly cold-treated leaves. Collectively, our study provides strong evidence that the processes involved in cold acclimation are partially mediated by a signal that acts systemically. This has the potential to act as an early-warning system to enable developing leaves to cope better with the cold environment in which they are growing.

Genetic screens to identify plant stress genes

A powerful means to learn about gene functions in a developmental or physiological context in an organism is to isolate the corresponding mutants with altered phenotypes. Diverse mutagenic agents, including chemical and biological, have been widely employed, and each comes with its own advantages and inconveniences. For Arabidopsis thaliana, whose genome sequence is publicly available, the reliance of reverse genetics to understand the relevant roles of genes particularly those coding for proteins in growth and development is now a common practice. Identifying multiple alleles at each locus is important because they can potentially reveal epistatic relationship in a signaling pathway or components belonging to a common signaling complex by their synergistic or even allele-specific enhancement of the phenotypic severity. In this article, we describe mutagenesis by using ethyl methanesulfonate (EMS) and transfer (T)-DNA-mediated insertion or activation tagging as applied to the most widely used genetic plant model A. thaliana. Also, we demonstrate the utility of several genetic screening approaches to dissect adaptive responses to various abiotic stresses.

Transcriptional regulation by an NAC (NAM-ATAF1,2-CUC2) transcription factor attenuates ABA signalling for efficient basal defence towards Blumeria graminis f. sp. hordei in Arabidopsis

ATAF1 is a member of a largely uncharacterized plant-specific gene family encoding NAC transcription factors, and is induced in response to various abiotic and biotic stimuli in Arabidopsis thaliana. Previously, we showed that a mutant allele of ATAF1 compromises penetration resistance in Arabidopsis with respect to the non-host biotrophic pathogen Blumeria graminis f. sp. hordei (Bgh). In this study, we have used genome-wide transcript profiling to characterize signalling perturbations in ataf1 plants following Bgh inoculation. Comparative transcriptomic analyses identified an over-representation of abscisic acid (ABA)-responsive genes, including the ABA biosynthesis gene AAO3, which is significantly induced in ataf1 plants compared to wild-type plants following inoculation with Bgh. Additionally, we show that Bgh inoculation results in decreased endogenous ABA levels in an ATAF1-dependent manner, and that the ABA biosynthetic mutant aao3 showed increased penetration resistance to Bgh compared to wild-type plants. Furthermore, we show that ataf1 plants show ABA-hyposensitive phenotypes during seedling development and germination. Our data support a negative correlation between ABA levels and penetration resistance, and identify ATAF1 as a new stimuli-dependent attenuator of ABA signalling for the mediation of efficient penetration resistance in Arabidopsis upon Bgh attack.

Suppression by ABA of salicylic acid and lignin accumulation and the expression of multiple genes, in Arabidopsis infected with Pseudomonas syringae pv. tomato

Abscisic acid (ABA) has been implicated in determining the outcome of interactions between many plants and their pathogens. We had previously shown that increased concentrations of ABA within leaves of Arabidopsis induced susceptibility towards an avirulent strain of Pseudomonas syringae pathovar (pv.) tomato. We now show that ABA induces susceptibility via suppression of the accumulation of components crucial for a resistance response. Lignin and salicylic acid concentrations in leaves were increased during a resistant interaction but reduced when plants were treated with ABA. The reduction in lignin and salicylic acid production was independent of the development of the hypersensitive response (HR), indicating that, in this host-pathogen system, HR is not required for resistance. Genome-wide gene expression analysis using microarrays showed that treatment with ABA suppressed the expression of many defence-related genes, including those important for phenylpropanoid biosynthesis and those encoding resistance-related proteins. Together, these results show that resistance induction in Arabidopsis to an avirulent strain of P. syringae pv. tomato is regulated by ABA.

Identification of drought tolerance determinants by genetic analysis of root response to drought stress and abscisic Acid

Drought stress is a common adverse environmental condition that seriously affects crop productivity worldwide. Due to the complexity of drought as a stress signal, deciphering drought tolerance mechanisms has remained a major challenge to plant biologists. To develop new approaches to study plant drought tolerance, we searched for phenotypes conferred by drought stress and identified the inhibition of lateral root development by drought stress as an adaptive response to the stress. This drought response is partly mediated by the phytohormone abscisic acid. Genetic screens using Arabidopsis (Arabidopsis thaliana) were devised, and drought inhibition of lateral root growth (dig) mutants with altered responses to drought or abscisic acid in lateral root development were isolated. Characterization of these dig mutants revealed that they also exhibit altered drought stress tolerance, indicating that this root response to drought stress is intimately linked to drought adaptation of the entire plant and can be used as a trait to access the elusive drought tolerance machinery. Our study also revealed that multiple mechanisms coexist and together contribute to whole-plant drought tolerance.

Arabidopsis mutants deregulated in RCI2A expression reveal new signaling pathways in abiotic stress responses

To uncover new pathways involved in low-temperature signal transduction, we screened for mutants altered in cold-induced expression of RCI2A, an Arabidopsis gene that is not a member of the CBF/DREB1 regulon and is induced not only by low temperature but also by abscisic acid (ABA), dehydration (DH) and NaCl. This was accomplished by generating a line of Arabidopsis carrying a transgene consisting of the RCI2A promoter fused to the firefly luciferase coding sequence. A number of mutants showing low or high RCI2A expression in response to low temperature were identified. These mutants also displayed deregulated RCI2A expression in response to ABA, DH or NaCl. Interestingly, however, they were not altered in stress-induced expression of RD29A, a CBF/DREB1-target gene, suggesting that the mutations affect signaling intermediates of CBF/DREB1-independent regulatory pathways. Several mutants showed alterations in their tolerance to freezing, DH or salt stress, as well as in their ABA sensitivity, which indicates that the signaling intermediates defined by the corresponding mutations play an important role in Arabidopsis tolerance to abiotic stresses. Based on the mutants identified, we discuss the involvement of CBF/DREB1-independent pathways in modulating stress signaling.

Gain-of-function and loss-of-function phenotypes of the protein phosphatase 2C HAB1 reveal its role as a negative regulator of abscisic acid signalling

HAB1 was originally cloned on the basis of sequence homology to ABI1 and ABI2, and indeed, a multiple sequence alignment of 32 Arabidopsis protein phosphatases type-2C (PP2Cs) reveals a cluster composed by the four closely related proteins, ABI1, ABI2, HAB1 and At1g17550 (here named HAB2). Characterisation of transgenic plants harbouring a transcriptional fusion ProHAB1: green fluorescent protein (GFP) indicates that HAB1 is broadly expressed within the plant, including key target sites of abscisic acid (ABA) action as guard cells or seeds. The expression of the HAB1 mRNA in vegetative tissues is strongly upregulated in response to exogenous ABA. In this work, we show that constitutive expression of HAB1 in Arabidopsis under a cauliflower mosaic virus (CaMV) 35S promoter led to reduced ABA sensitivity both in seeds and vegetative tissues, compared to wild-type plants. Thus, in the field of ABA signalling, this work represents an example of a stable phenotype in planta after sustained overexpression of a PP2C genes. Additionally, a recessive T-DNA insertion mutant of HAB1 was analysed in this work, whereas previous studies of recessive alleles of PP2C genes were carried out with intragenic revertants of the abi1-1 and abi2-1 mutants that carry missense mutations in conserved regions of the PP2C domain. In the presence of exogenous ABA, hab1-1 mutant shows ABA-hypersensitive inhibition of seed germination; however, its transpiration rate was similar to that of wild-type plants. The ABA-hypersensitive phenotype of hab1-1 seeds together with the reduced ABA sensitivity of 35S:HAB1 plants are consistent with a role of HAB1 as a negative regulator of ABA signalling. Finally, these results provide new genetic evidence on the function of a PP2C in ABA signalling.

Regulation of the ABA-sensitive Arabidopsis potassium channel gene GORK in response to water stress

The phytohormone abscisic acid (ABA) regulates many stress-related processes in plants. In this context ABA mediates the responsiveness of plants to environmental stresses such as drought, cold or salt. In response to water stress, ABA induces stomatal closure by activating Ca2+, K+ and anion channels in guard cells. To understand the signalling pathways that regulate these turgor control elements, we studied the transcriptional control of the K+ release channel gene GORK that is expressed in guard cells, roots and vascular tissue. GORK transcription was up-regulated upon onset of drought, salt stress and cold. The wilting hormone ABA that integrates responses to these stimuli induced GORK expression in seedlings in a time- and concentration-dependent manner and this induction was dependent on extracellular Ca2+. ABA-responsive expression of GORK was impaired in the ABA-insensitive mutants abi1-1 and abi2-1, indicating that these protein phosphatases are regulators of GORK expression. Application of ABA to suspension-cultured cells for 2 min followed by a 4 h chase was sufficient to manifest transcriptional activation of the K+ channel gene. As predicted for a process involved in drought adaptation, only 12-24 h after the release of the stress hormone, GORK mRNA slowly decreased. In contrast to other tissues, GORK expression as well as K+(out) channel activity in guard cells is ABA insensitive, allowing the plant to adjust stomatal movement and water status control separately.

Analysis of the alternative oxidase promoters from soybean

Alternative oxidase (Aox) is a nuclear-encoded mitochondrial protein. In soybean (Glycine max), the three members of the gene family have been shown to be differentially expressed during normal plant development and in response to stresses. To examine the function of the Aox promoters, genomic fragments were obtained for all three soybean genes: Aox1, Aox2a, and Aox2b. The regions of these fragments immediately upstream of the coding regions were used to drive beta-glucuronidase (GUS) expression during transient transformation of soybean suspension culture cells and stable transformation of Arabidopsis. The expression patterns of the GUS reporter genes in soybean cells were in agreement with the presence or absence of the various endogenous Aox proteins, determined by immunoblotting. Deletion of different portions of the upstream regions identified sequences responsible for both positive and negative regulation of Aox gene expression in soybean cells. Reporter gene analysis in Arabidopsis plants showed differential tissue expression patterns driven by the three upstream regions, similar to those reported for the endogenous proteins in soybean. The expression profiles of all five members of the Arabidopsis Aox gene family were examined also, to compare with GUS expression driven by the soybean upstream fragments. Even though the promoter activity of the upstream fragments from soybean Aox2a and Aox2b displayed the same tissue specificity in Arabidopsis as they do in soybean, the most prominently expressed endogenous genes in all tissues of Arabidopsis were of the Aox1 type. Thus although regulation of Aox expression generally appears to involve the same signals in different species, different orthologs of Aox may respond variously to these signals. A comparison of upstream sequences between soybean Aox genes and similarly expressed Arabidopsis Aox genes identified common motifs.

Tomato tos1 mutation identifies a gene essential for osmotic tolerance and abscisic acid sensitivity

Osmotic stress severely limits plant growth and agricultural productivity. We have used mutagenesis to identify plant genes that are required for osmotic stress tolerance in tomato. As a result, we have isolated a novel mutant in tomato (tos1) caused by a single recessive nuclear mutation that is hypersensitive to general osmotic stress. Growth measurements demonstrated that the tos1 mutant is less sensitive to intracellular abscisic acid (ABA) and this decreased ABA sensitivity of tos1 is a basic cellular trait expressed by the mutant at all developmental stages analysed. It is not caused by a deficiency in the synthesis of ABA because the tos1 seedlings accumulated more ABA than the wild type (WT) after osmotic stress. In contrast, the tss2 tomato mutant, which is also hypersensitive to osmotic stress, is hypersensitive to exogenous ABA. Comparative analysis of tos1 and tss2 indicates that appropriate ABA perception and signalling is essential for osmotic tolerance.

Regulation of gene expression in Arabidopsis thaliana by artificial zinc finger chimeras

The artificial regulation of endogenous gene expression in plants is limited to only a few approaches. Here, we describe the use of artificial zinc finger chimeras to regulate the expression of a known reporter construct. The artificial zinc finger chimera TFIIIAZif is a fusion protein consisting of the four zinc fingers of TFIIIA linked through a spacer region to the three zinc fingers of Zif268. This artificial zinc finger chimera is able to bind specifically to a target DNA sequence (ZBS, zinc finger binding site) of 27 base pairs (bp). TFIIIAZif was fused to a transactivation domain from the herpes simplex virus VP16 or its tetramer VP64 to give ZF-VP16 or ZF-VP64, respectively. In transient expression assays, these two transcription activators were able to activate a target reporter gene (luc and GFP) expressed from a minimal -46 35S promoter linked to four copies of ZBS. The activation was confirmed in transgenic plants using an inducible XVE system [Zuo et al. (2000) Plant J. 24: 265] to express ZF-VP16 or ZF-VP64. Furthermore, to test the specificity of ZF-VP64 we have compared reporter gene expression from a wild type (1xZBS) and a mutant (1xZBSmu) binding site in transgenic plants. The 1xZBS was used to express green fluorescent protein (GFP) whereas the 1xZBSmu was used to express red fluorescent protein (RFP). Upon induction of ZF-VP64 we found a much higher expression of GFP (about 33-fold) as compared to RFP expression. These results suggest that artificial zinc finger chimeras can be used to target specific DNA sequences and to regulate gene expression in plants.

Genetic interactions between ABA, ethylene and sugar signaling pathways

The identification of genes through mutant screens is beginning to reveal the structure of a number of signaling pathways in plants. In the past year, genes that determine the plant's response to the hormones ethylene and abscisic acid have also been shown to be involved in sugar sensing in early seedlings. These results suggest that hormone signaling and carbon homeostasis are tightly coupled but that the architecture of these interactions is complex. Part of this complexity may be because some genetic screens on exogenous compounds produce signaling linkages that are not necessarily pertinent under normal growth conditions. Because many of the genes identified in these screens are cloned, the relevance of these interactions can now be unraveled at the molecular level.

An mRNA cap binding protein, ABH1, modulates early abscisic acid signal transduction in Arabidopsis

The plant hormone abscisic acid (ABA) regulates important stress and developmental responses. We have isolated a recessive ABA hypersensitive mutant, abh1, that shows hormone specificity to ABA. ABH1 encodes the Arabidopsis homolog of a nuclear mRNA cap binding protein and functions in a heterodimeric complex to bind the mRNA cap structure. DNA chip analyses show that only a few transcripts are down-regulated in abh1, several of which are implicated in ABA signaling. Consistent with these results, abh1 plants show ABA-hypersensitive stomatal closing and reduced wilting during drought. Interestingly, ABA-hypersensitive cytosolic calcium increases in abh1 guard cells demonstrate amplification of early ABA signaling. Thus, ABH1 represents a modulator of ABA signaling proposed to function by transcript alteration of early ABA signaling elements.

Abiotic stress signal transduction in plants: Molecular and genetic perspectives

Low temperature, drought and salinity are major adverse environmental factors that limit plant productivity. Understanding the mechanisms by which plants perceive and transduce these stress signals to initiate adaptive responses is essential for engineering stress-tolerant crop plants. Molecular and biochemical studies suggest that abiotic stress signaling in plants involves receptor-coupled phosphorelay, phosphoinositol-induced Ca2+ changes, mitogen-activated protein kinase cascades and transcriptional activation of stress-responsive genes. In addition, protein posttranslational modifications and adapter or scaffold-mediated protein-protein interactions are also important in abiotic stress signal transduction. Most of these signaling modules, however, have not been genetically established to function in plant abiotic stress signal transduction. To overcome the scarcity of abiotic stress-specific phenotypes for conventional genetic screens, molecular genetic analysis using stress-responsive promoter-driven reporter is suggested as an alternative approach to genetically dissect abiotic stress signaling networks in plants.

Pleiotropic morphological and abiotic stress resistance phenotypes of the hyper-abscisic acid producing Abo- mutant in the periwinkle Catharanthus roseus

The pleiotropic properties of a abo abo (Abo-) gamma-ray induced mutant of Catharanthus roseus cv. Nirmal, selected among the M2 generation seeds for ability to germinate at 45 degrees C, are described. The mutant produced seeds possessing tricotyledonous embryos, unlike the typically dicotyledonous embryos present in the wild type Abo+ seeds. In comparison to Abo+ adults, the mutant plants had short stature and lanceolate leaves. The vascular bundles in the leaves and stem were poorly developed. Leaf surfaces were highly trichomatous, epidermal, cortex and mesophyll cells were small sized and a large majority of stomata were closed. Besides high temperature, the mutant was salinity and water-stress tolerant. The abscisic acid (ABA) content in the leaves was about 500-fold higher. The genetic lesion abo responsible for the above pleiotropy was recessive and inherited in Mendelian fashion. The seedlings and adult plants of the mutant accumulated higher proline than Abo+ plants. The phenotypes of abo abo mutants permitted the conclusions that (i) the mutant synthesizes ABA constitutively, (ii) both ABA-dependent and ABA independent pathways for proline and betaine accumulation are functional in the mutant, and (iii) cell division, elongation and differentiation processes in embryo and adult plant stages are affected in the mutant

Trivalent ions activate abscisic acid-inducible promoters through an ABI1-dependent pathway in rice protoplasts

The plant hormone abscisic acid (ABA) mediates many vital processes in plant growth and development, including seed dormancy, cell division, water use efficiency, and adaptation to drought, salinity, chilling, pathogen attack, and UV light. Our understanding of ABA signal transduction is fragmentary and would benefit from specific and facile probes of the process. Protoplasts from rice (Oryza sativa L. cv IR54) embryonic suspension cultures cotransformed with effector plasmids encoding the maize (Zea mays) VIVIPAROUS1 cDNA and/or the Arabidopsis dominant negative mutant (abi1-1) ABA-insensitive cDNA demonstrated genetic interactions of VIVIPAROUS1 and abi1-1 in transactivation of the ABA-inducible HVA1 promoter from barley (Hordeum vulgare), suggesting the mechanisms of these effectors are conserved among monocots and dicots. Trivalent ions have been shown to act as an effector of gene expression in plants and animals, although the mechanism of action is unknown. We show in two complementary transient ABA-inducible gene expression assays (beta-glucuronidase and luciferase enzymatic activities and quantitative flow cytometry of green fluorescent protein) that trivalent ions specifically interact with an ABI1-dependent ABA-signaling pathway leading to gene expression. Trivalent ions mimic ABA effects on gene expression and may be a useful tool to study ABA signaling.

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.

Flow cytometry and surface plasmon resonance analyses demonstrate that the monoclonal antibody JIM19 interacts with a rice cell surface component involved in abscisic acid signalling in protoplasts

Abscisic acid (ABA) is a plant hormone involved in many developmental and physiological processes, but as yet, no ABA receptor has been identified. Flow cytometry of rice protoplasts and immunoblotting of purified plasma membranes (PMs) have been used to demonstrate that the monoclonal antibody JIM19 recognizes carbohydrate epitopes of cell surface glycoproteins. Using surface plasmon resonance technology specific binding of PMs to JIM19 was observed. Such interaction was antagonized significantly by ABA, but not by the biologically inactive ABA catabolite phaseic acid. These in vitro interactions were correlated with the biological activities of JIM19, ABA and phaseic acid on activation of the ABA-inducible Em promoter using two different transient reporter gene assays, beta-glucuronidase/luciferase and quantitative flow cytometry of Aequoria green fluorescent protein. Pre-treatment with JIM19 resulted in significant inhibition of ABA-inducible gene expression. Taken together, these data suggest that JIM19 interacts with a functional PM complex involved in ABA signalling.