An ethylene-inducible component of signal transduction encoded by never-ripe

TOMATO AGAMOUS-LIKE 1 is a component of the fruit ripening regulatory network

After fertilization, the expanding carpel of fleshy fruit goes through a phase change to ripening. Although the role of ethylene signalling in mediating climacteric ripening has been established, knowledge regarding the regulation of ethylene biosynthesis and its association with fruit developmental programs is still lacking. A functional screen of tomato transcription factors showed that silencing of the TOMATO AGAMOUS-LIKE 1 (TAGL1) MADS box gene results in altered fruit pigmentation. Over-expressing TAGL1 as a chimeric repressor suggested a role in controlling ripening, as transgenic tomato fruit showed reduced carotenoid and ethylene levels, suppressed chlorophyll breakdown, and down-regulation of ripening-associated genes. Moreover, fruits over-expressing TAGL1 accumulated more lycopene, and their sepals were swollen, accumulated high levels of the yellow flavonoid naringenin chalcone and contained lycopene. Transient promoter-binding assays indicated that part of the TAGL1 activity in ripening is executed through direct activation of ACS2, an ethylene biosynthesis gene that has recently been reported to be a target of the RIN MADS box factor. Examination of the TAGL1 transcript and its over-expression in the rin mutant background suggested that RIN does not regulate TAGL1 or vice versa. The results also indicated RIN-dependent and -independent processes that are regulated by TAGL1. We also noted that fruit of TAGL1 loss-of-function lines had a thin pericarp layer, indicating an additional role for TAGL1 in carpel expansion prior to ripening. The results add a new component to the current model of the regulatory network that controls fleshy fruit ripening and its association with the ethylene biosynthesis pathway.

Hormone- and light-mediated regulation of heat-induced differential petiole growth in Arabidopsis

Plants react quickly and profoundly to changes in their environment. A sudden increase in temperature, for example, induces differential petiole growth-driven upward leaf movement (hyponastic growth) in Arabidopsis (Arabidopsis thaliana). We show that accessions that face the strongest fluctuations in diurnal temperature in their natural habitat are least sensitive for heat-induced hyponastic growth. This indicates that heat-induced hyponastic growth is a trait subject to natural selection. The response is induced with kinetics remarkably similar to ethylene- and low light-induced hyponasty in several accessions. Using pharmacological assays, transcript analysis, and mutant analyses, we demonstrate that ethylene and the photoreceptor protein phytochrome B are negative regulators of heat-induced hyponastic growth and that low light, phytochrome A, auxin, polar auxin transport, and abscisic acid are positive regulators of heat-induced hyponastic growth. Furthermore, auxin, auxin polar transport, phytochrome A, phytochrome B, and cryptochromes are required for a fast induction of heat-induced hyponastic growth.

Identification of Solanum habrochaites loci that quantitatively influence tomato fruit ripening-associated ethylene emissions

The phytohormone ethylene is essential for ripening of climacteric fruits such as tomato. While many of the genes responsible for ethylene synthesis and perception have been identified, the regulatory network controlling autocatalytic climacteric ethylene synthesis is not well understood. In order to better understand the regulation of ripening-associated ethylene, we have exploited the genetic variation within Solanum Sect. Lycopersicon. In particular, we have used a near-isogenic population of S. habrochaites introgression lines to identify chromosome segments affecting ethylene emissions during ripening. S. habrochaites fruits produce much larger quantities of ethylene during ripening than do cultivated S. lycopersicum tomatoes. A total of 17 segments were identified; 3 had emissions more than twice the level of the tomato parent, 11 had less than a twofold increase and 3 had significantly reduced emissions at one or more ripening stages. While several of these segments co-segregate with known ethylene-related genes, many do not correspond to known genes. Thus, they may identify novel modes of regulation. These results illustrate the utility of wild relatives and their introgression lines to understand regulation of fruit ripening-related processes.

Genome analysis and genetic enhancement of tomato

The Solanaceae is an important family of vegetable crops, ornamentals and medicinal plants. Tomato has served as a model member of this family largely because of its enriched cytogenetic, genetic, as well as physical, maps. Mapping has helped in cloning several genes of importance such as Pto, responsible for resistance against bacterial speck disease, Mi-1.2 for resistance against nematodes, and fw2.2 QTL for fruit weight. A high-throughput genome-sequencing program has been initiated by an international consortium of 10 countries. Since heterochromatin has been found to be concentrated near centromeres, the consortium is focusing on sequencing only the gene-rich euchromatic region. Genomes of the members of Solanaceae show a significant degree of synteny, suggesting that the tomato genome sequence would help in the cloning of genes for important traits from other Solanaceae members as well. ESTs from a large number of cDNA libraries have been sequenced, and microarray chips, in conjunction with wide array of ripening mutants, have contributed immensely to the understanding of the fruit-ripening phenomenon. Work on the analysis of the tomato proteome has also been initiated. Transgenic tomato plants with improved abiotic stress tolerance, disease resistance and insect resistance, have been developed. Attempts have also been made to develop tomato as a bioreactor for various pharmaceutical proteins. However, control of fruit quality and ripening remains an active and challenging area of research. Such efforts should pave the way to improve not only tomato, but also other solanaceous crops.

Callus, shoot and hairy root formation in vitro as affected by the sensitivity to auxin and ethylene in tomato mutants

We analyzed the impact of ethylene and auxin disturbances on callus, shoots and Agrobacterium rhizogenes-induced hairy root formation in tomato (Solanum lycopersicum L.). The auxin low-sensitivity dgt mutation showed little hairy root initiation, whereas the ethylene low-sensitivity Nr mutation did not differ from the control Micro-Tom cultivar. Micro-Tom and dgt hairy roots containing auxin sensitivity/biosynthesis rol and aux genes formed prominent callus onto media supplemented with cytokinin. Under the same conditions, Nr hairy roots did not form callus. Double mutants combining Rg1, a mutation conferring elevated shoot formation capacity, with either dgt or Nr produced explants that formed shoots with little callus proliferation. The presence of rol + aux genes in Rg1 hairy roots prevented shoot formation. Taken together, the results suggest that although ethylene does not affect hairy root induction, as auxin does, it may be necessary for auxin-induced callus formation in tomato. Moreover, excess auxin prevents shoot formation in Rg1.

Ripening-regulated susceptibility of tomato fruit to Botrytis cinerea requires NOR but not RIN or ethylene

Fruit ripening is a developmental process that is associated with increased susceptibility to the necrotrophic pathogen Botrytis cinerea. Histochemical observations demonstrate that unripe tomato (Solanum lycopersicum) fruit activate pathogen defense responses, but these responses are attenuated in ripe fruit infected by B. cinerea. Tomato fruit ripening is regulated independently and cooperatively by ethylene and transcription factors, including NON-RIPENING (NOR) and RIPENING-INHIBITOR (RIN). Mutations in NOR or RIN or interference with ethylene perception prevent fruit from ripening and, thereby, would be expected to influence susceptibility. We show, however, that the susceptibility of ripe fruit is dependent on NOR but not on RIN and only partially on ethylene perception, leading to the conclusion that not all of the pathways and events that constitute ripening render fruit susceptible. Additionally, on unripe fruit, B. cinerea induces the expression of genes also expressed as uninfected fruit ripen. Among the ripening-associated genes induced by B. cinerea are LePG (for polygalacturonase) and LeExp1 (for expansin), which encode cell wall-modifying proteins and have been shown to facilitate susceptibility. LePG and LeExp1 are induced only in susceptible rin fruit and not in resistant nor fruit. Thus, to infect fruit, B. cinerea relies on some of the processes and events that occur during ripening, and the fungus induces these pathways in unripe fruit, suggesting that the pathogen itself can initiate the induction of susceptibility by exploiting endogenous developmental programs. These results demonstrate the developmental plasticity of plant responses to the fungus and indicate how known regulators of fruit ripening participate in regulating ripening-associated pathogen susceptibility.

Effect of tomato pleiotropic ripening mutations on flavour volatile biosynthesis

Ripening is a tightly controlled and developmentally regulated process involving networks of genes, and metabolites that result in dramatic changes in fruit colour, texture and flavour. Molecular and genetic analysis in tomato has revealed a series of regulatory genes involved in fruit development and ripening, including MADS box and SPB box transcription factors and genes involved in ethylene synthesis, signalling and response. Volatile metabolites represent a significant part of the plant metabolome, playing an important role in plant signalling, defence strategies and probably in regulatory mechanisms. They also play an important role in fruit quality. In order to acquire a better insight into the biochemical and genetic control of flavour compound generation and links between these metabolites and the central regulators of ripening, five pleiotropic mutant tomato lines were subjected to volatile metabolite profiling in comparison with wild-type Ailsa Craig. One hundred and seventeen volatile compounds were identified and quantified using SPME (Solid Phase Microextraction) headspace extraction followed by Gas Chromatography-Mass Spectrometry (GC-MS) and the data were subjected to multivariate comparative analysis. We find that the different mutants each produce distinct volatile profiles during ripening. Through principal component analysis the volatiles most dramatically affected are those derived from fatty-acids. The results are consistent with the suggestion that specific isoforms of lipoxygenase located in the plastids and the enzymes that provide precursors and downstream metabolites play a key role in determining volatile composition.

Involvement of ethylene in the accumulation of esculeoside A during fruit ripening of tomato (Solanum lycopersicum)

The composition of glycoalkaloids in tomato fruit changes with ripening. However, it has not been clarified whether the accumulation of glycoalkaloids is controlled by the ripening-inducing phytohormone, ethylene. Here, we report the effect of ethylene on the accumulation of tomato fruit glycoalkaloids. We investigated the effect of exogenously applied ethylene. In response to ethylene treatment, the content of alpha-tomatine decreased, whereas the content of esculeoside A increased. Next, we analyzed the fruits of ripening mutants, rin, nor, and Nr. In fruits of these mutant lines, the level of accumulation of esculeoside A decreased, whereas alpha-tomatine accumulated to higher levels than in wild-type fruit. These results demonstrated that the esculeoside A accumulation was associated with production and perception of ethylene. Additionally, the accumulation profiles of the intermediate metabolites of esculeoside A biosynthesis in ripening mutant fruits suggest that a glycosylation step in the putative pathway from alpha-tomatine to esculeoside A depends on ethylene.

F-box proteins regulate ethylene signaling and more

Ethylene regulates several aspects of plant development, such as fruit ripening. In this issue of Genes & Development, Qiao and colleagues (pp. 512-521) report that the stability of the ethylene signaling protein EIN2 is modulated by two F-box proteins ETP1/2, reminiscent of the finding that another regulator of ethylene response, EIN3, is also targeted by the F-box proteins EBF1/2. ETP1/2 and EBF1/2 show distinct phylogenetic patterns, suggesting that they have different evolutionary constraints.

Gene and metabolite regulatory network analysis of early developing fruit tissues highlights new candidate genes for the control of tomato fruit composition and development

Variations in early fruit development and composition may have major impacts on the taste and the overall quality of ripe tomato (Solanum lycopersicum) fruit. To get insights into the networks involved in these coordinated processes and to identify key regulatory genes, we explored the transcriptional and metabolic changes in expanding tomato fruit tissues using multivariate analysis and gene-metabolite correlation networks. To this end, we demonstrated and took advantage of the existence of clear structural and compositional differences between expanding mesocarp and locular tissue during fruit development (12-35 d postanthesis). Transcriptome and metabolome analyses were carried out with tomato microarrays and analytical methods including proton nuclear magnetic resonance and liquid chromatography-mass spectrometry, respectively. Pairwise comparisons of metabolite contents and gene expression profiles detected up to 37 direct gene-metabolite correlations involving regulatory genes (e.g. the correlations between glutamine, bZIP, and MYB transcription factors). Correlation network analyses revealed the existence of major hub genes correlated with 10 or more regulatory transcripts and embedded in a large regulatory network. This approach proved to be a valuable strategy for identifying specific subsets of genes implicated in key processes of fruit development and metabolism, which are therefore potential targets for genetic improvement of tomato fruit quality.

Brassinosteroids interact negatively with jasmonates in the formation of anti-herbivory traits in tomato

Given the susceptibility of tomato plants to pests, the aim of the present study was to understand how hormones are involved in the formation of tomato natural defences against insect herbivory. Tomato hormone mutants, previously introgressed into the same genetic background of reference, were screened for alterations in trichome densities and allelochemical content. Ethylene, gibberellin, and auxin mutants indirectly showed alteration in trichome density, through effects on epidermal cell area. However, brassinosteroids (BRs) and jasmonates (JAs) directly affected trichome density and allelochemical content, and in an opposite fashion. The BR-deficient mutant dpy showed enhanced pubescence, zingiberene biosynthesis, and proteinase inhibitor expression; the opposite was observed for the JA-insensitive jai1-1 mutant. The dpy x jai1-1 double mutant showed that jai1-1 is epistatic to dpy, indicating that BR acts upstream of the JA signalling pathway. Herbivory tests with the poliphagous insect Spodoptera frugiperda and the tomato pest Tuta absoluta clearly confirmed the importance of the JA-BR interaction in defence against herbivory. The study underscores the importance of hormonal interactions on relevant agricultural traits and raises a novel biological mechanism in tomato that may differ from the BR and JA interaction already suggested for Arabidopsis.

Ethylene contributes to potato aphid susceptibility in a compatible tomato host

Resistance to potato aphid (Macrosiphum euphorbiae) in tomato (Solanum lycopersicum) is conferred by Mi-1. Early during both compatible and incompatible interactions, potato aphid feeding induces the expression of ethylene (ET) biosynthetic genes. Here, we used genetic and pharmacologic approaches to investigate the role of ET signaling in basal defense and Mi-1-mediated resistance to potato aphid in tomato. The effect of potato aphid infestation on ET biosynthesis in susceptible and resistant plants was assessed. Aphid bioassays were performed using plants impaired in ET biosynthesis or perception using virus-induced gene silencing, the Never ripe (Nr) mutant, and 1-methylcyclopropene (MCP) treatment. A burst of ET was observed after aphid feeding in both resistant and susceptible plants, correlated with an increase in the expression of ET biosynthetic genes. However, impairing ET signaling or biosynthesis did not compromise Mi-1-mediated resistance but it did decrease susceptibility to potato aphid in a compatible host. ET may not play a significant role in Mi-1-mediated resistance to potato aphids in tomato but modulates the host basal defense, enhancing its susceptibility to the aphid.

Recent advances in ethylene research

Ethylene regulates many aspects of the plant life cycle, including seed germination, root initiation, flower development, fruit ripening, senescence, and responses to biotic and abiotic stresses. It thus plays a key role in responses to the environment that have a direct bearing on a plant's fitness for adaptation and reproduction. In recent years, there have been major advances in our understanding of the molecular mechanisms regulating ethylene synthesis and action. Screening for mutants of the triple response phenotype of etiolated Arabidopsis seedlings, together with map-based cloning and candidate gene characterization of natural mutants from other plant species, has led to the identification of many new genes for ethylene biosynthesis, signal transduction, and response pathways. The simple chemical nature of ethylene contrasts with its regulatory complexity. This is illustrated by the multiplicity of genes encoding the key ethylene biosynthesis enzymes 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase, multiple ethylene receptors and signal transduction components, and the complexity of regulatory steps involving signalling relays and control of mRNA and protein synthesis and turnover. In addition, there are extensive interactions with other hormones. This review integrates knowledge from the model plant Arabidopsis and other plant species and focuses on key aspects of recent research on regulatory networks controlling ethylene synthesis and its role in flower development and fruit ripening.

Differential petiole growth in Arabidopsis thaliana: photocontrol and hormonal regulation

Environmental challenges such as low light intensity induce differential growth-driven upward leaf movement (hyponastic growth) in Arabidopsis thaliana. However, little is known about the physiological regulation of this response. Here, we studied how low light intensity is perceived and translated into a differential growth response in Arabidopsis. We used mutants defective in light, ethylene and auxin signaling, and in polar auxin transport, as well as chemical inhibitors, to analyze the mechanisms of low light intensity-induced differential growth. Our data indicate that photosynthesis-derived signals and blue light wavelengths affect petiole movements and that rapid induction of hyponasty by low light intensity involves functional cryptochromes 1 and 2, phytochrome-A and phytochrome-B photoreceptor proteins. The response is independent of ethylene signaling. Auxin and polar auxin transport, by contrast, play a role in low light intensity-induced differential petiole growth. We conclude that low light intensity-induced differential petiole growth requires blue light, auxin signaling and polar auxin transport and is, at least in part, genetically separate from well-characterized ethylene-induced differential growth.

Ethylene insensitivity impedes a subset of responses to phosphorus deficiency in tomato and petunia

The role of ethylene in growth and developmental responses to low phosphorus stress was evaluated using ethylene-insensitive 'Never-ripe' (Nr) tomato and etr1 petunia plants. Low phosphorus increased adventitious root formation in 'Pearson' (wild-type) tomato plants, but not in Nr, supporting a role for ethylene in adventitious root development and showing that ethylene is important for this aspect of phosphorus response. Low phosphorus reduced ethylene production by adventitious roots of both genotypes, suggesting that ethylene perception--not production--regulates carbon allocation to adventitious roots at the expense of other roots under low phosphorus stress. With the exception of its effect on adventitious rooting, Nr had positive effects on growth and biomass accumulation in tomato whereas etr1 tended to have negative effects on petunia. This was particularly evident during the recovery from transplanting, when the effective quantum yield of photosystem II of etr1 petunia grown with low phosphorus was significantly lower than 'Mitchell Diploid', suggesting that etr1 petunia plants may undergo more severe post-transplant stress at low phosphorus availability. Our results demonstrate that ethylene mediates adventitious root formation in response to phosphorus stress and plays an important role for quick recovery of plants exposed to multiple environmental stresses, i.e. transplanting and low phosphorus.

Comprehensive resources for tomato functional genomics based on the miniature model tomato micro-tom

Tomato (Solanum lycopersicum L., Solanaceae) is an excellent model plant for genomic research of solanaceous plants, as well as for studying the development, ripening, and metabolism of fruit. In 2003, the International Solanaceae Project (SOL, www.sgn.cornell.edu ) was initiated by members from more than 30 countries, and the tomato genome-sequencing project is currently underway. Genome sequence of tomato obtained by this project will provide a firm foundation for forthcoming genomic studies such as the comparative analysis of genes conserved among the Solanaceae species and the elucidation of the functions of unknown tomato genes. To exploit the wealth of the genome sequence information, there is an urgent need for novel resources and analytical tools for tomato functional genomics. Here, we present an overview of the development of genetic and genomic resources of tomato in the last decade, with a special focus on the activities of Japan SOL and the National Bio-Resource Project in the development of functional genomic resources of a model cultivar, Micro-Tom.

CsPLDalpha1 and CsPLDgamma1 are differentially induced during leaf and fruit abscission and diurnally regulated in Citrus sinensis

Understanding leaf and fruit abscission is essential in order to develop strategies for controlling the process in fruit crops. Mechanisms involved in signalling leaf and fruit abscission upon induction by abscission agents were investigated in Citrus sinensis cv. 'Valencia'. Previous studies have suggested a role for phospholipid signalling; hence, two phospholipase D cDNA sequences, CsPLDalpha1 and CsPLDgamma1, were isolated and their role was examined. CsPLDalpha1 expression was reduced in leaves but unaltered in fruit peel tissue treated with an ethylene-releasing compound (ethephon), or a fruit-specific abscission agent, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP). By contrast, CsPLDgamma1 expression was up-regulated within 6 h (leaves) and 24 h (fruit peel) after treatment with ethephon or CMNP, respectively. CsPLDalpha1 expression was diurnally regulated in leaf blade but not fruit peel. CsPLDgamma1 exhibited strong diurnal oscillation in expression in leaves and fruit peel with peak expression around midday. While diurnal fluctuation in CsPLDalpha1 expression appeared to be light-entrained in leaves, CsPLDgamma1 expression was regulated by light and the circadian clock. The diurnal expression of both genes was modulated by ethylene-signalling. The ethephon-induced leaf abscission and the ethephon- and CMNP-induced decrease in fruit detachment force were enhanced by application during rising diurnal expression of CsPLDgamma1. The results indicate differential regulation of CsPLDalpha1 and CsPLDgamma1 in leaves and fruit, and suggest possible roles for PLD-dependent signalling in regulating abscission responses in citrus.

LeCTR2, a CTR1-like protein kinase from tomato, plays a role in ethylene signalling, development and defence

Arabidopsis AtCTR1 is a Raf-like protein kinase that interacts with ETR1 and ERS and negatively regulates ethylene responses. In tomato, several CTR1-like proteins could perform this role. We have characterized LeCTR2, which has similarity to AtCTR1 and also to EDR1, a CTR1-like Arabidopsis protein involved in defence and stress responses. Protein-protein interactions between LeCTR2 and six tomato ethylene receptors indicated that LeCTR2 interacts preferentially with the subfamily I ETR1-type ethylene receptors LeETR1 and LeETR2, but not the NR receptor or the subfamily II receptors LeETR4, LeETR5 and LeETR6. The C-terminus of LeCTR2 possesses serine/threonine kinase activity and is capable of auto-phosphorylation and phosphorylation of myelin basic protein in vitro. Overexpression of the LeCTR2 N-terminus in tomato resulted in altered growth habit, including reduced stature, loss of apical dominance, highly branched inflorescences and fruit trusses, indeterminate shoots in place of determinate flowers, and prolific adventitious shoot development from the rachis or rachillae of the leaves. Expression of the ethylene-responsive genes E4 and chitinase B was upregulated in transgenic plants, but ethylene production and the level of mRNA for the ethylene biosynthetic gene ACO1 was unaffected. The leaves and fruit of transgenic plants also displayed enhanced susceptibility to infection by the fungal pathogen Botrytis cinerea, which was associated with much stronger induction of pathogenesis-related genes such as PR1b1 and chitinase B compared with the wild-type. The results suggest that LeCTR2 plays a role in ethylene signalling, development and defence, probably through its interactions with the ETR1-type ethylene receptors of subfamily I.

Fruit-specific suppression of the ethylene receptor LeETR4 results in early-ripening tomato fruit

Tomato is an economically important crop and a significant dietary source of important phytochemicals, such as carotenoids and flavonoids. Although it has been known for many years that the plant hormone ethylene is essential for the ripening of climacteric fruits, its role in fruit growth and maturation is much less well understood. In this study, data are presented which indicate that fruit-specific suppression of the ethylene receptor LeETR4 causes early ripening, whereas fruit size, yield and flavour-related chemical composition are largely unchanged. Early fruit ripening is a highly desirable and valuable trait, and the approach demonstrated here should be applicable to any fruit species requiring ethylene to ripen. These results demonstrate that ethylene receptors probably act as biological clocks regulating the onset of tomato fruit ripening.

Influence of Fruit Development Stage on the Physiological Response to Ethylene in Cantaloupe Charentais Melon

An experiment has been designed and performed using Cantaloupe Charentais melon as climacteric fruit with the aim of finding out at which point of their development they are able to sense the plant hormone ethylene. Fruit were harvested at 20, 25, and 35 days after pollination (DAP), treated for 5 days at 20 °C with 5 ppm of ethylene, and stored for a further 10 days. Nontreated wild type fruit was used as control. Ethylene-treated genetically modified 1-aminocyclopropane-1-carboxylic acid oxidase antisense fruits with inhibited autocatalytic ethylene production were also used to avoid masking effects because of endogenous ethylene produced by control fruits. Ethylene-treated wild type fruits with 25 DAP were able to produce autocatalytic ethylene. A climacteric respiration increase was observed in treated wild type fruit with 25 and 35 DAP. The degreening of the rind was induced by ethylene in the youngest fruit, showing a total dependence on the hormone for its activation. Loss of pulp firmness induced by exogenous ethylene was found only in 20 DAP-fruit. The existence of ethylene-dependent and developmental factors in the regulation of this ripening process was observed.

Tomato ethylene receptor-CTR interactions: visualization of NEVER-RIPE interactions with multiple CTRs at the endoplasmic reticulum

In the model plant Arabidopsis, members of a family of two-component system His kinase-like ethylene receptors have direct protein-protein interactions with a single downstream Ser/Thr kinase CTR1. These components of the ethylene signalling network found in Arabidopsis are conserved in the climacteric fruit tomato, but both the ethylene receptors and CTR1-like proteins (LeCTRs) in tomato are encoded by multigene families. Here, using a yeast two-hybrid interaction assay, it is shown that the tomato receptors LeETR1, LeETR2, and NEVER-RIPE (NR) can interact with multiple LeCTRs. In vivo protein localization studies with fluorescent tagged proteins revealed that the ethylene receptor NR was targeted to the endoplasmic reticulum (ER) when transiently expressed in onion epidermal cells, whereas the four LeCTR proteins were found in the cytoplasm and nucleus. When co-expressed with NR, three LeCTRs (1, 3, and 4), but not LeCTR2, also adopted the same ER localization pattern in an NR receptor-dependent manner but not in the absence of NR. The receptor-CTR interactions were confirmed by biomolecular fluorescence complementation (BiFC) showing that NR could form a protein complex with LeCTR1, 3, and 4. This suggested that ethylene receptors recruit these LeCTR proteins to the ER membrane through direct protein-protein interaction. The receptor-CTR interactions and localization observed in the study reinforce the idea that ethylene receptors transmit the signal to the downstream CTRs and show that a single receptor can interact with multiple CTR proteins. It remains unclear whether the different LeCTRs are functionally redundant or have unique roles in ethylene signalling.

Involvement of the ethylene response pathway in dormancy induction in chrysanthemum

Temperature plays a significant role in the annual cycling between growth and dormancy of the herbaceous perennial chrysanthemum (Chrysanthemum morifolium Ramat.). After exposure to high summer temperatures, cool temperature triggers dormancy. The cessation of flowering and rosette formation by the cessation of elongation are characteristic of dormant plants, and can be stimulated by exogenous ethylene. Thus, the ethylene response pathway may be involved in temperature-induced dormancy of chrysanthemum. Transgenic chrysanthemums expressing a mutated ethylene receptor gene were used to assess this involvement. The transgenic lines showed reduced ethylene sensitivity: ethylene causes leaf yellowing in wild-type chrysanthemums, but leaves remained green in the transgenic lines. Extension growth and flowering of wild-type and transgenic lines varied between temperatures: at 20 degrees C, the transgenic lines showed the same stem elongation and flowering as the wild type; at cooler temperatures, the wild type formed rosettes with an inability to flower and entered dormancy, but some transgenic lines continued to elongate and flower. This supports the involvement of the ethylene response pathway in the temperature-induced dormancy of chrysanthemum. At the highest dosage of ethephon, an ethylene-releasing agent, wild-type plants formed rosettes with an inability to flower and became dormant, but one transgenic line did not. This confirms that dormancy is induced via the ethylene response pathway.

The role of protein turnover in ethylene biosynthesis and response

Plant growth and development is controlled by a set of hormones whose responses are tightly regulated in order to direct appropriate responses. In several hormone signaling pathways, protein turnover has emerged as a common regulatory element. Ethylene is a phytohormone that controls a variety of processes, including fruit ripening, senescence, and stress response. This review focuses on the regulation of the ethylene response pathway through protein degradation. Protein turnover has been found to regulate both ethylene biosynthesis and ethylene response. Ethylene production is regulated through the turnover of the biosynthetic enzyme ACS. Recently it was found that ethylene receptors are controlled by protein turnover as well. A third process in the control of ethylene signaling is the targeting of the ethylene response transcription factor ETHYLENE INSENSITIVE3 (EIN3) for degradation by the proteins EIN3-BINDING F-BOX 1 and 2 (EBF1 and EBF2).

Identification of woolliness response genes in peach fruit after post-harvest treatments

Woolliness is a physiological disorder of peaches and nectarines that becomes apparent when fruit are ripened after prolonged periods of cold storage. This disorder is of commercial importance since shipping of peaches to distant markets and storage before selling require low temperature. However, knowledge about the molecular basis of peach woolliness is still incomplete. To address this issue, a nylon macroarray containing 847 non-redundant expressed sequence tags (ESTs) from a ripe peach fruit cDNA library was developed and used. Gene expression changes of peach fruit (Prunus persica cv. O'Henry) ripened for 7 d at 21 degrees C (juicy fruit) were compared with those of fruit stored for 15 d at 4 degrees C and then ripened for 7 d at 21 degrees C (woolly fruit). A total of 106 genes were found to be differentially expressed between juicy and woolly fruit. Data analysis indicated that the activity of most of these genes (>90%) was repressed in the woolly fruit. In cold-stored peaches (cv. O'Henry), the expression level of selected genes (cobra, endopolygalacturonase, cinnamoyl-CoA-reductase, and rab11) was lower than in the juicy fruit, and it remained low in woolly peaches after ripening, a pattern that was conserved in woolly fruit from two other commercial cultivars (cv. Flamekist and cv. Elegant Lady). In addition, the results of this study indicate that molecular changes during fruit woolliness involve changes in the expression of genes associated with cell wall metabolism and endomembrane trafficking. Overall, the results reported here provide an initial characterization of the transcriptome activity of peach fruit under different post-harvest treatments.

SlTPR1, a tomato tetratricopeptide repeat protein, interacts with the ethylene receptors NR and LeETR1, modulating ethylene and auxin responses and development

The gaseous hormone ethylene is perceived by a family of ethylene receptors which interact with the Raf-like kinase CTR1. SlTPR1 encodes a novel TPR (tetratricopeptide repeat) protein from tomato that interacts with the ethylene receptors NR and LeETR1 in yeast two-hybrid and in vitro protein interaction assays. SlTPR1 protein with a GFP fluorescent tag was localized in the plasmalemma and nuclear membrane in Arabidopsis, and SlTPR1-CFP and NR-YFP fusion proteins were co-localized in the plasmalemma and nuclear membrane following co-bombardment of onion cells. Overexpression of SlTPR1 in tomato resulted in ethylene-related pleiotropic effects including reduced stature, delayed and reduced production of inflorescences, abnormal and infertile flowers with degenerate styles and pollen, epinasty, reduced apical dominance, inhibition of abscission, altered leaf morphology, and parthenocarpic fruit. Similar phenotypes were seen in Arabidopsis overexpressing SlTPR1. SlTPR1 overexpression did not increase ethylene production but caused enhanced accumulation of mRNA from the ethylene responsive gene ChitB and the auxin-responsive gene SlSAUR1-like, and reduced expression of the auxin early responsive gene LeIAA9, which is known to be inhibited by ethylene and to be associated with parthenocarpy. Cuttings from the SlTPR1-overexpressors produced fewer adventitious roots and were less responsive to indole butyric acid. It is suggested that SlTPR1 overexpression enhances a subset of ethylene and auxin responses by interacting with specific ethylene receptors. SlTPR1 shares features with human TTC1, which interacts with heterotrimeric G-proteins and Ras, and competes with Raf-1 for Ras binding. Models for SlTPR1 action are proposed involving modulation of ethylene signalling or receptor levels.

Ethylene receptor degradation controls the timing of ripening in tomato fruit

Fruit ripening in tomato requires the coordination of both developmental cues and the phytohormone ethylene. The multigene ethylene receptor family has been shown to negatively regulate ethylene signal transduction and suppress ethylene responses. Here we demonstrate that reduction in the levels of either of two family members, LeETR4 or LeETR6, causes an early-ripening phenotype. We provide evidence that the receptors are rapidly degraded in the presence of ethylene, and that degradation probably occurs through the 26S proteasome-dependent pathway. Ethylene exposure of immature fruits causes a reduction in the amount of receptor protein and earlier ripening. The results are consistent with a model in which receptor levels modulate timing of the onset of fruit ripening by measuring cumulative ethylene exposure.

Fruit ripening mutants yield insights into ripening control

Fruit ripening is a developmental process that is exclusive to plants whereby mature seed-bearing organs undergo physiological and metabolic changes that promote seed dispersal. Molecular investigations into ripening control mechanisms have been aided by the recent cloning of tomato ripening genes that were previously known only through mutation. Advances in the genomics of tomato have provided genetic and molecular tools that have facilitated the positional and candidate-gene-based cloning of several key ripening genes. These discoveries have created new inroads into understanding of the primary ripening control mechanisms, including transcription factors such as those encoded by the RIPENING-INHIBITOR (RIN) MADS-box and COLOURLESS NON-RIPENING (CNR) SPB-box genes, which are necessary for the progression of virtually all ripening processes. They have also facilitated the elucidation of downstream signal transduction components that impact the hormonal and environmental stimuli that coordinate and modulate ripening phenotypes.

Chloroplast His-to-Asp signal transduction: a potential mechanism for plastid gene regulation in Heterosigma akashiwo (Raphidophyceae)

Background

Maintenance of homeostasis requires that an organism perceive selected physical and chemical signals within an informationally dense environment. Functionally, an organism uses a variety of signal transduction arrays to amplify and convert these perceived signals into appropriate gene transcriptional responses. These changes in gene expression serve to modify selective metabolic processes and thus optimize reproductive success. Here we analyze a chloroplast-encoded His-to-Asp signal transduction circuit in the stramenopile Heterosigma akashiwo (Hada) Hada ex Y. Hara et Chihara [syn. H. carterae (Hulburt) F.J.R. Taylor]. The presence, structure and putative function of this protein pair are discussed in the context of their evolutionary homologues.

Results

Bioinformatic analysis of the Heterosigma akashiwo chloroplast genome sequence revealed the presence of a single two-component His-to-Asp (designated Tsg1/Trg1) pair in this stramenopile (golden-brown alga). These data represent the first documentation of a His-to-Asp array in stramenopiles and counter previous reports suggesting that such regulatory proteins are lacking in this taxonomic cluster. Comparison of the 43 kDa H. akashiwo Tsg1 with bacterial sensor kinases showed that the algal protein exhibits a moderately maintained PAS motif in the sensor kinase domain as well as highly conserved H, N, G₁ and F motifs within the histidine kinase ATP binding site. Molecular modelling of the 27 kDa H. akashiwo Trg1 regulator protein was consistent with a winged helix-turn-helix identity – a class of proteins that is known to impact gene expression at the level of transcription. The occurrence of Trg1 protein in actively growing H. akashiwo cells was verified by Western analysis. The presence of a PhoB-like RNA polymerase loop in Trg1 and its homologues in the red-algal lineage support the hypothesis that Trg1 and its homologues interact with a sigma 70 (σ⁷⁰) subunit (encoded by rpoD) of a eubacterial type polymerase. Sequence analysis of H. akashiwo rpoD showed this nuclear-encoded gene has a well-defined 4.2 domain, a region that augments RNA polymerase interaction with transcriptional regulatory proteins and also serves in -35 promoter recognition. The presence/loss of the His-to-Asp pairs in primary and secondary chloroplast lineages is assessed.

Conclusion

His-to-Asp signal transduction components are found in most rhodophytic chloroplasts, as well as in their putative cyanobacterial progenitors. The evolutionary conservation of these proteins argues that they are important for the maintenance of chloroplast homeostasis. Our data suggest that chloroplast gene transcription may be impacted by the interaction of the His-to-Asp regulator protein (which is less frequently lost than the sensor protein) with the RNA polymerase σ⁷⁰ subunit.

The tomato ethylene receptor LE-ETR3 (NR) is not involved in mediating ozone sensitivity: causal relationships among ethylene emission, oxidative burst and tissue damage

The causal relationships among ethylene emission, oxidative burst and tissue damage, and the temporal expression patterns of some ethylene biosynthetic and responsive genes, were examined in the Never ripe (Nr) tomato (Lycopersicon esculentum) mutant and its isogenic wild type (cv. Pearson), to investigate the role played by the ethylene receptor LE-ETR3 (NR) in mediating the plant response to ozone (O(3)). Tomato plants were used in a time-course experiment in which they were exposed to acute O(3) fumigation with 200 nl l(-1) O(3) for 4 h. The pattern of leaf lesions indicated similar sensitivities to O(3) for cv. Pearson and Nr. In both genotypes, O(3) activated a hydrogen peroxide (H(2)O(2))-dependent oxidative burst, which was also ethylene-driven in Nr leaves. Ozone induced some ethylene and jasmonate biosynthetic and inducible genes, although with different timings and to different extents in the two genotypes. The overall data indicate that Nr retains partial sensitivity to ethylene, suggesting only a marginal role of the NR receptor in mediating the complex response of tomato plants to O(3).

The involvement of auxin in the ripening of climacteric fruits comes of age: the hormone plays a role of its own and has an intense interplay with ethylene in ripening peaches

Ethylene has long been regarded as the main regulator of ripening in climacteric fruits. The characterization of a few tomato mutants, unable to produce climacteric ethylene and to ripen their fruits even following treatments with exogenous ethylene, has shown that other factors also play an important role in the control of climacteric fruit ripening. In climacteric peach and tomato fruits it has been shown that, concomitant with ethylene production, increases in the amount of auxin can also be measured. In this work a genomic approach has been used in order to understand if such an auxin increase is functional to an independent role played by the hormone during ripening of the climacteric peach fruits. Besides the already known indirect activity on ripening due to its up-regulation of climacteric ethylene synthesis, it has been possible to show that auxin plays a role of its own during ripening of peaches. In fact, the hormone has shown the ability to regulate the expression of a number of different genes. Moreover, many genes involved in biosynthesis and transport and, in particular, the signalling (receptors, Auxin Response Factors and Aux/IAA) of auxin had increased expression in the mesocarp during ripening, thus strengthening the idea that this hormone is actively involved in the ripening of peaches. This study has also demonstrated the existence of an important cross-talk between auxin and ethylene, with genes in the auxin domain regulated by ethylene and genes in the ethylene domain regulated by auxin.

Expression of ethylene receptors Dl-ERS1-3 and Dl-ERS2, and ethylene response during flower senescence in Delphinium

To clarify the relationships of flower senescence, especially sepal abscission, and ethylene receptor gene expression in different flower parts, we isolated two cDNAs encoding ethylene receptors Dl-ERS1-3 and Dl-ERS2 from Delphinium flowers. Deduced polypeptides possessed no response regulator domain, indicating that they belong to a family of ethylene response sensor (ERS) ethylene receptors. Dl-ERS1-3 and Dl-ERS2 exhibited constitutive levels during flower senescence. Exogenous ethylene increased transcript levels in sepals, which are influenced by ethylene but not in gynoecia and receptacles, which produce ethylene. It was suggested that expression of ethylene receptor genes under ethylene exposure was differentially regulated in each organ of the flower.

The ACC synthase TOE sequence is required for interaction with ETO1 family proteins and destabilization of target proteins

The Arabidopsis ETO1 protein is a negative regulator of ethylene biosynthesis. It specifically inhibits the enzyme activity of type 2 1-aminocyclopropane-1-carboxylate synthases (ACC synthases or ACS) and promotes their degradation by a proteasome-dependent pathway. To further understand the function of the ETO1 family in the plant kingdom, we cloned a cDNA of LeEOL1 (Lycopersicon esculentum ETO 1- LIKE 1), an ETO1 homolog from tomato. LeEOL1 encodes a putative protein with domain architecture conserved in the Arabidopsis ETO1/EOL1/EOL2 proteins and in the predicted rice EOL proteins. LeEOL1 is expressed in leaf, stem, root, flower, and the full ripe stage of fruit, suggesting diverse regulatory roles in the development of tomato. Yeast two-hybrid analysis revealed specific interactions between LeEOL1 and type 2 ACC synthases. When the C-terminal 14 amino acids (TOE; target of ETO1) of LE-ACS3 specific to type 2 ACC synthases were fused to a type 1 ACS, LE-ACS2, at the corresponding position, it allowed LE-ACS2 to strongly interact with LeEOL1. A GFP-TOE(LE-ACS3) fusion protein expressed in rice calli and in the roots of wild-type Arabidopsis showed reduced stability compared to native GFP. However, the fluorescence of GFP-TOE(LE-ACS3) was comparable to that of the native GFP in Arabidopsis eto1-4 mutant. Furthermore, MG132 treatment significantly enhanced the fluorescence of GFP-TOE(LE-ACS3) in the roots of wild-type Arabidopsis. These results suggest that the ETO1-family-mediated ACS protein degradation pathway is conserved in both monocots and dicots, and that TOE acts as a protein destabilization signal recognized by the ETO1 protein family.

How ethylene works in the reproductive organs of higher plants: a signaling update from the third millennium

Ethylene (ET) is a notable signaling molecule in higher plants. In the year 1993 the ET receptor gene, ETR1, was identified; this ETR1 receptor protein being the first plant hormone receptor to be isolated. It is striking that there are six ET receptors in tomato instead of five in Arabidopsis, the two best-known signaling-model systems. Even though over the last few years great progress has been made in elucidating the genes and proteins involved in ET signaling, the complete pathway remains to be established. The present review examines the most representative successive advances that have taken place in this millennium in terms of the signaling pathway of ET, as well as the implications of the signaling in the reproductive organs of plants (i.e., flowers, fruits, seeds and pollen grains). A detailed comparative study is made on the advances in knowledge in the last decade, showing how the characterization of ET signaling provides clues for understanding how higher plants regulate their ET sensitivity. Also, it is indicated that ET signaling is at present sparking interest within phytohormonal molecular physiology and biology, and it is explained why several socio-economic aspects (flowering and fruit ripening) are undoubtedly involved in ET physiology.

Characteristics of transgenic tomatoes antisensed for the ethylene receptor genes LeETR1 [corrected] and LeETR2 [corrected]

Two stable transformed lines containing antisense LeETR1 [corrected] or LeETR2 [corrected] sequences and their hybridized line were investigated to determine the effect of LeETR1 [corrected] and LeETR2 [corrected] specificity in the ethylene receptor family in tomato (Lycopersicon esculentum Mill.) on ethylene signaling. The transgenic line ale1 containing antisense LeETR1 [corrected] displayed shorter length of seedling grown in the dark and adult plant in the light, severe epinastic petiole, and accelerated abscission of petiole explant and senescence of flower explant, compared with its wild type B1. The transgenic line ale2 containing antisense LeETR2 [corrected] also exhibited shorter hypocotyls and slightly accelerated abscission. The phenotypes of cross line dale of LeETR1 [corrected] and LeETR2 [corrected] were close to ale1 in many aspects. These results suggested that LeETR1 [corrected] probably plays a relatively important role in ethylene signaling of tomato growth and development.

Ripening in the tomato Green-ripe mutant is inhibited by ectopic expression of a protein that disrupts ethylene signaling

To achieve full ripening, climacteric fruits, such as tomato require synthesis, perception and signal transduction of the plant hormone ethylene. The nonripening phenotype of the dominant Green-ripe (Gr) and Never-ripe 2 (Nr-2) mutants of tomato is the result of reduced ethylene responsiveness in fruit tissues. In addition, a subset of ethylene responses associated with floral senescence, abscission, and root elongation are also impacted in mutant plants, but to a lesser extent. Using positional cloning, we have identified an identical 334-bp deletion in a gene of unknown biochemical function at the Gr/Nr-2 locus. Consistent with a dominant gain of function mutation, this deletion causes ectopic expression of Gr/Nr-2, which in turn leads to ripening inhibition. A CaMV35::GR transgene recreates the Gr/Nr-2 mutant phenotype but does not lead to a global reduction in ethylene responsiveness, suggesting tissue-specific modulation of ethylene responses in tomato. Gr/Nr-2 encodes an evolutionary conserved protein of unknown biochemical function that we associate here with ethylene signaling. Because Gr/Nr-2 has no sequence homology with the previously described Nr (Never-ripe) ethylene receptor of tomato we now refer to this gene only as GR. Identification of GR expands the current repertoire of ethylene signaling components in plants and provides a tool for further elucidation of ethylene response mechanisms and for controlling ethylene signal specificity in crop plants.

The Wound Response Mutant suppressor of prosystemin-mediated responses6 (spr6) is a Weak Allele of the Tomato Homolog of CORONATINE-INSENSITIVE1 (COI1)

The systemic defense response of tomato plant in response to insect attack and wounding is regulated by the 18 amino acid peptide systemin and the phytohormone jasmonic acid (JA). Recent genetic analyses based mainly on spr (suppressors of prosystemin-mediated responses) mutant screens have led to the hypothesis that systemin acts at, or near, the site of wounding to amplify the production of JA, which in turn functions as a mobile signal to promote the systemic defense response. In order to identify more components involved in the systemin/JA-signaled defense response, we carried out a larger scale screen for new spr mutants in tomato. Here we describe the characterization of spr6, a mutant impaired in wound- and systemin-induced defense gene expression. Using a candidate gene approach based on genetic linkage, we demonstrate that spr6 is allelic to jai1-1, which is a loss-of-function allele of the tomato homolog of CORONATINE-INSENSITIVE1 (COI1), an F-box protein that is required for JA-signaled processes in Arabidopsis. We show several aspects of the spr6 mutant phenotype distinct from that of jai1-1. First, the responsiveness of spr6 plants to exogenous JA shows a dosage dependency, i.e. it is more sensitive to JA than jai1-1 while less sensitive to JA than the wild-type. Secondly, unlike the sterile jai1-1, the spr6 plant displays normal fertility and seed set and thus can be maintained as a pure line and does not require selection. Therefore, spr6 provides a valuable tool, which can complement the limitations of jai1-1, to study JA signaling in tomato. The gene identification process of Spr6 we described herein represents an example showing the convenience of a candidate gene approach, based on genetic linkage, to identify gene functions of genetic loci defined by tomato wound response mutants.

How to decide? Different methods of calculating gene expression from short oligonucleotide array data will give different results

Background

Short oligonucleotide arrays for transcript profiling have been available for several years. Generally, raw data from these arrays are analysed with the aid of the Microarray Analysis Suite or GeneChip Operating Software (MAS or GCOS) from Affymetrix. Recently, more methods to analyse the raw data have become available. Ideally all these methods should come up with more or less the same results. We set out to evaluate the different methods and include work on our own data set, in order to test which method gives the most reliable results.

Results

Calculating gene expression with 6 different algorithms (MAS5, dChip PMMM, dChip PM, RMA, GC-RMA and PDNN) using the same (Arabidopsis) data, results in different calculated gene expression levels. Consequently, depending on the method used, different genes will be identified as differentially regulated. Surprisingly, there was only 27 to 36% overlap between the different methods. Furthermore, 47.5% of the genes/probe sets showed good correlation between the mismatch and perfect match intensities.

Conclusion

After comparing six algorithms, RMA gave the most reproducible results and showed the highest correlation coefficients with Real Time RT-PCR data on genes identified as differentially expressed by all methods. However, we were not able to verify, by Real Time RT-PCR, the microarray results for most genes that were solely calculated by RMA. Furthermore, we conclude that subtraction of the mismatch intensity from the perfect match intensity results most likely in a significant underestimation for at least 47.5% of the expression values. Not one algorithm produced significant expression values for genes present in quantities below 1 pmol. If the only purpose of the microarray experiment is to find new candidate genes, and too many genes are found, then mutual exclusion of the genes predicted by contrasting methods can be used to narrow down the list of new candidate genes by 64 to 73%.

Conversion of MapMan to allow the analysis of transcript data from Solanaceous species: effects of genetic and environmental alterations in energy metabolism in the leaf

The tomato microarray TOM1 offers the possibility to monitor the levels of several thousand transcripts in parallel. The microelements represented on this tomato microarray have been putatively assigned to unigenes, and organised in functional classes using the MapMan ontology (Thimm et al., 2004. Plant J. 37: 914-939). This ontology was initially developed for use with the Arabidopsis ATH1 array, has a low level of redundancy, and can be combined with the MapMan software to provide a biologically structured overview of changes of transcripts, metabolites and enzyme activities. Use of this application is illustrated using three case studies with published or novel TOM1 array data sets for Solanaceous species. Comparison of previously reported data on transcript levels in potato leaves in the middle of the day and the middle of the night identified coordinated changes in the levels of transcripts of genes involved in various metabolic pathways and cellular events. Comparison with diurnal changes of gene expression in Arabidopsis revealed common features, illustrating how MapMan can be used to compare responses in different organisms. Comparison of transcript levels in new experiments performed on the leaves of the cultivated tomato S. lycopersicum and the wild relative S. pennellii revealed a general decrease of levels of transcripts of genes involved in terpene and, phenylpropanoid metabolism as well as chorismate biosynthesis in the crop compared to the wild relative. This matches the recently reported decrease of the levels of secondary metabolites in the latter. In the third case study, new expression array data for two genotypes deficient in TCA cycle enzymes is analysed to show that these genotypes have elevated levels of transcripts associated with photosynthesis. This in part explains the previously documented enhanced rates of photosynthesis in these genotypes. Since the Solanaceous MapMan is intended to be a community resource it will be regularly updated on improvements in tomato gene annotation and transcript profiling resources.

Transcriptome and selected metabolite analyses reveal multiple points of ethylene control during tomato fruit development

Transcriptome profiling via cDNA microarray analysis identified 869 genes that are differentially expressed in developing tomato (Solanum lycopersicum) pericarp. Parallel phenotypic and targeted metabolite comparisons were employed to inform the expression analysis. Transcript accumulation in tomato fruit was observed to be extensively coordinated and often completely dependent on ethylene. Mutation of an ethylene receptor (Never-ripe [Nr]), which reduces ethylene sensitivity and inhibits ripening, alters the expression of 37% of these 869 genes. Nr also influences fruit morphology, seed number, ascorbate accumulation, carotenoid biosynthesis, ethylene evolution, and the expression of many genes during fruit maturation, indicating that ethylene governs multiple aspects of development both prior to and during fruit ripening in tomato. Of the 869 genes identified, 628 share homology (E-value < or = 1 x 10(-10)) with known gene products or known protein domains. Of these 628 loci, 72 share homology with previously described signal transduction or transcription factors, suggesting complex regulatory control. These results demonstrate multiple points of ethylene regulatory control during tomato fruit development and provide new insights into the molecular basis of ethylene-mediated ripening.

Increased calcium levels and prolonged shelf life in tomatoes expressing Arabidopsis H+/Ca2+ transporters

Here we demonstrate that fruit from tomato (Lycopersicon esculentum) plants expressing Arabidopsis (Arabidopsis thaliana) H(+)/cation exchangers (CAX) have more calcium (Ca2+) and prolonged shelf life when compared to controls. Previously, using the prototypical CAX1, it has been demonstrated that, in yeast (Saccharomyces cerevisiae) cells, CAX transporters are activated when the N-terminal autoinhibitory region is deleted, to give an N-terminally truncated CAX (sCAX), or altered through specific manipulations. To continue to understand the diversity of CAX function, we used yeast assays to characterize the putative transport properties of CAX4 and N-terminal variants of CAX4. CAX4 variants can suppress the Ca2+ hypersensitive yeast phenotypes and also appear to be more specific Ca2+ transporters than sCAX1. We then compared the phenotypes of sCAX1- and CAX4-expressing tomato lines. The sCAX1-expressing tomato lines demonstrate increased vacuolar H(+)/Ca2+ transport, when measured in root tissue, elevated fruit Ca2+ level, and prolonged shelf life but have severe alterations in plant development and morphology, including increased incidence of blossom-end rot. The CAX4-expressing plants demonstrate more modest increases in Ca2+ levels and shelf life but no deleterious effects on plant growth. These findings suggest that CAX expression may fortify plants with Ca2+ and may serve as an alternative to the application of CaCl2 used to extend the shelf life of numerous agriculturally important commodities. However, judicious regulation of CAX transport is required to assure optimal plant growth.

Ethylene insensitivity conferred by the Green-ripe and Never-ripe 2 ripening mutants of tomato

The ripening of a fleshy fruit represents the summation of an array of biochemical processes that are regulated by interactions between developmental programs and environmental inputs. Analysis of tomato (Solanum lycopersicum) mutants and inhibitor studies indicate that ethylene is necessary for full development of the ripening program of climacteric fruit such as tomato, yet ethylene alone is not sufficient. This suggests that an interaction between ethylene and nonethylene (or developmental) pathways mediates ripening. In this study, we have examined the physiological basis for ripening inhibition of the dominant Green-ripe (Gr) and Never-ripe 2 (Nr-2) mutants of tomato. Our data suggest that this inhibition is due to ethylene insensitivity in mutant fruit. Further investigation of ethylene responses in Gr and Nr-2 plants also revealed weak ethylene insensitivity during floral senescence and abscission and, during inhibition of root elongation, a phenotype associated with the triple response. However, ethylene-induced inhibition of hypocotyl elongation and petiole epinasty are normal in Gr and Nr-2, suggesting that these loci regulate a subset of ethylene responses. We have mapped both dominant mutations to a 2-cM overlapping region of the long arm of chromosome 1 of tomato, a region not previously linked to any known ethylene signaling loci. The phenotypic similarity and overlapping map location of these mutations suggest Gr and Nr-2 may be allelic and may possibly encode a novel component of the ethylene response pathway.

Patterns of Dwarf expression and brassinosteroid accumulation in tomato reveal the importance of brassinosteroid synthesis during fruit development

Brassinosteroids (BRs) are essential for many physiological functions in plants, however little is known concerning where and when they are synthesized. This is especially true during flower and fruit production. To address this we have used a promoter-GUS reporter fusion and RT-PCR to determine the relative expression levels of the tomato Dwarf (D) gene that encodes a BR C-6 oxidase. In young seedlings GUS reporter activity was observed mainly in apical and root tissues undergoing expansion. In flowers GUS activity was observed in the pedicel joints and ovaries, whereas in fruits it was strongest during early seed development and was associated with the locular jelly and seeds. RT-PCR analysis showed that tissue-specific expression of Dwarf mRNA was consistent with that of the Dwarf:GUS fusion. In good correlation with the high local Dwarf activity, quantitative measurements of endogenous BRs indicated intense biosynthesis in developing tomato fruits, which were also found to contain high amounts of brassinolide. Grafting experiments showed the lack of BR transport indicating that BR action occurs at the site of synthesis.

Making sense of it all: bacterial chemotaxis

Bacteria must be able to respond to a changing environment, and one way to respond is to move. The transduction of sensory signals alters the concentration of small phosphorylated response regulators that bind to the rotary flagellar motor and cause switching. This simple pathway has provided a paradigm for sensory systems in general. However, the increasing number of sequenced bacterial genomes shows that although the central sensory mechanism seems to be common to all bacteria, there is added complexity in a wide range of species.