Differential activation of two ACC oxidase gene promoters from melon during plant development and in response to pathogen attack

Characterization and Identification of PpEIN3 during the Modulation of Fruit Ripening Process by Ectopic Expressions in Tomato

First study of PpEIN3 by transgenic experiments to verify its function in the maturity process PpEIN3 is a positive regulator of ethylene signal transduction pathway to promote fruits ripening Ethylene is one of the most important phytohormone in plants and plays a critical role during growth, development, maturity, and aging. The framework of the ethylene signaling pathway is well reported. Nevertheless, studies on Ethylene Insensitive 3 (EIN3), the downstream regulator of the ethylene signaling pathway, need to be investigated, especially in peach [Prunus persica (L.) Batsch]. In this study, we cloned PpEIN3 from peach and characterized it in tomato (Solanum lycopersicum L.). Our results depicted that the open-reading frame of PpEIN3 was 1875 bp, encoding a protein with 624 amino acid residues that contained a conserved EIN3 domain, a highly conserved N-terminal region, and seven DNA-binding sites. PpEIN3 showed very close association with homologous EIN genes from apple (Malus domestica Borkh.) and grapevine (Vitis vinifera L.). All investigated EIN proteins shared similar domains and structures. The PpEIN3 promoter possessed several motifs related to hormones that affect fruit development and ripening. Spatial-temporal expression analysis revealed that PpEIN3 was expressed at high levels in the late stage of fruit development vs. the early stage. In transgenic tomato, PpEIIN3 showed overexpression and the key ethylene biosynthesis genes SlACO1, SlACS1, and SlSAMS1 were upregulated and promoted early maturation in fruit. By contrast, PpEIIN3 silencing delayed ripening and reduced SlEIN3 expression in tomato. The results confirmed that PpEIN3 is a positive regulator of the ethylene signal transduction pathway, which promoted fruit ripening. Our findings provide valuable insight to the roles in ethylene signal components in the modulation of peach fruit ripening.

Functional characterization of 1-aminocyclopropane-1-carboxylic acid oxidase gene in Arabidopsis thaliana and its potential in providing flood tolerance

Ethylene is a phytohormone that has gained importance through its role in stress tolerance and fruit ripening. In our study we evaluated the functional potential of the enzyme involved in ethylene biosynthesis of plants called ACC (aminocyclopropane-1-carboxylic acid) oxidase which converts precursor ACC to ethylene. Studies on ethylene have proven that it is effective in improving the flood tolerance in plants. Thus our goal was to understand the potential of ACC oxidase gene overexpression in providing flood tolerance in transgenic plants. ACC oxidase gene was PCR amplified and inserted into the pBINmgfp5-er vector, under the control of a constitutive Cauliflower Mosaic Virus promoter. GV101 strain of Agrobacterium tumefaciens containing recombinant pBINmgfp5-er vector (referred herein as pBIN-ACC) was used for plant transformation by the 'floral dip' method. The transformants were identified through kanamycin selection and grown till T3 (third transgenic) generation. The flood tolerance was assessed by placing both control and transgenic plants on deep plastic trays filled with tap water that covered the soil surface. Our result shows that wild-type Arabidopsis could not survive more than 20 days under flooding while the transgenic lines survived 35 days, suggesting development of flood tolerance with overexpression of ACC oxidase. Further molecular studies should be done to elucidate the role and pathways of ACC oxidase and other phytohormones involved in the development of flood adaptation.

CaHDZ27, a Homeodomain-Leucine Zipper I Protein, Positively Regulates the Resistance to Ralstonia solanacearum Infection in Pepper

Homeodomain-leucine zipper class I (HD-Zip I) transcription factors have been functionally characterized in plant responses to abiotic stresses, but their roles in plant immunity are poorly understood. Here, a HD-Zip I gene, CaHZ27, was isolated from pepper (Capsicum annum) and characterized for its role in pepper immunity. Quantitative real-time polymerase chain reaction showed that CaHDZ27 was transcriptionally induced by Ralstonia solanacearum inoculation and exogenous application of methyl jasmonate, salicylic acid, or ethephon. The CaHDZ27-green fluorescent protein fused protein was targeted exclusively to the nucleus. Chromatin immunoprecipitation demonstrated that CaHDZ27 bound to the 9-bp pseudopalindromic element (CAATAATTG) and triggered β-glucuronidase expression in a CAATAATTG-dependent manner. Virus-induced gene silencing of CaHDZ27 significantly attenuated the resistance of pepper plants against R. solanacearum and downregulated defense-related marker genes, including CaHIR1, CaACO1, CaPR1, CaPR4, CaPO2, and CaBPR1. By contrast, transient overexpression of CaHDZ27 triggered strong cell death mediated by the hypersensitive response and upregulated the tested immunity-associated marker genes. Ectopic CaHDZ27 expression in tobacco enhances its resistance against R. solanacearum. These results collectively suggest that CaHDZ27 functions as a positive regulator in pepper resistance against R. solanacearum. Bimolecular fluorescence complementation and coimmunoprecipitation assays indicate that CaHDZ27 monomers bind with each other, and this binding is enhanced significantly by R. solanacearum inoculation. We speculate that homodimerization of CaHZ27 might play a role in pepper response to R. solanacearum, further direct evidence is required to confirm it.

Effect of jasmonates on ethylene biosynthesis and aroma volatile emission in Japanese apricot infected by a pathogen (Colletotrichum gloeosporioides)

The effects of the application of the jasmonic acid derivative n-propyl dihydrojasmonate (PDJ) on ethylene biosynthesis, volatile compounds, and endogenous jasmonic acid (JA) and methyl jasmonate (MeJA) were examined in Japanese apricot (Prunus mume Sieb.) infected by a pathogen (Colletotrichum gloeosporioides). The fruit were dipped into 0.4 mM PDJ solution before inoculation with the pathogen and stored at 25 °C for 6 days. The inoculation induced an increase in 1-aminocyclopropane-1-carboxylic acid (ACC), ethylene, JA, and MeJA. In contrast, PDJ application reduced the endogenous JA, MeJA, and ethylene production and expression of the ACC oxidase gene (PmACO1) caused by the pathogen infection. The lesion diameter with C. gloeosporioides decreased upon PDJ application. The alcohol, ester, ketone, and lactone concentrations and alcohol acyltransferase (AAT) activity increased in the pathogen-infected fruit, but were decreased by PDJ application. These results suggest that PDJ application might influence ethylene production through PmACO1 and that aroma volatile emissions affected by pathogen infection can be correlated with the ethylene production, which is mediated by the levels of jasmonates.

Distinct colonization patterns and cDNA-AFLP transcriptome profiles in compatible and incompatible interactions between melon and different races of Fusarium oxysporum f. sp. melonis

BACKGROUND

Fusarium oxysporum f. sp. melonis Snyd. & Hans. (FOM) causes Fusarium wilt, the most important infectious disease of melon (Cucumis melo L.). The four known races of this pathogen can be distinguished only by infection on appropriate cultivars. No molecular tools are available that can discriminate among the races, and the molecular basis of compatibility and disease progression are poorly understood. Resistance to races 1 and 2 is controlled by a single dominant gene, whereas only partial polygenic resistance to race 1,2 has been described. We carried out a large-scale cDNA-AFLP analysis to identify host genes potentially related to resistance and susceptibility as well as fungal genes associated with the infection process. At the same time, a systematic reisolation procedure on infected stems allowed us to monitor fungal colonization in compatible and incompatible host-pathogen combinations.

RESULTS

Melon plants (cv. Charentais Fom-2), which are susceptible to race 1,2 and resistant to race 1, were artificially infected with a race 1 strain of FOM or one of two race 1,2 w strains. Host colonization of stems was assessed at 1, 2, 4, 8, 14, 16, 18 and 21 days post inoculation (dpi), and the fungus was reisolated from infected plants. Markedly different colonization patterns were observed in compatible and incompatible host-pathogen combinations. Five time points from the symptomless early stage (2 dpi) to obvious wilting symptoms (21 dpi) were considered for cDNA-AFLP analysis. After successful sequencing of 627 transcript-derived fragments (TDFs) differentially expressed in infected plants, homology searching retrieved 305 melon transcripts, 195 FOM transcripts expressed in planta and 127 orphan TDFs. RNA samples from FOM colonies of the three strains grown in vitro were also included in the analysis to facilitate the detection of in planta-specific transcripts and to identify TDFs differentially expressed among races/strains.

CONCLUSION

Our data suggest that resistance against FOM in melon involves only limited transcriptional changes, and that wilting symptoms could derive, at least partially, from an active plant response.We discuss the pathogen-derived transcripts expressed in planta during the infection process and potentially related to virulence functions, as well as transcripts that are differentially expressed between the two FOM races grown in vitro. These transcripts provide candidate sequences that can be further tested for their ability to distinguish between races.Sequence data from this article have been deposited in GenBank, Accession Numbers: HO867279-HO867981.

Differential responses of the promoters from nearly identical paralogs of loblolly pine (Pinus taeda L.) ACC oxidase to biotic and abiotic stresses in transgenic Arabidopsis thaliana

Promoters from an ACC oxidase gene (PtACO1) and its nearly identical paralog (NIP) (PtACO2) of loblolly pine (Pinus taeda L.) were recovered from genomic DNA using PCR amplification. Transgenic Arabidopsis plants harboring genetic constructs from which beta-glucuronidase (GUS) expression was driven by the full-length (pACO1:GUS, pACO2:GUS) or truncated (pACO1-1.2:GUS, pACO2-1.2:GUS) loblolly pine ACC oxidase gene promoters displayed distinctive patterns of expression for the different promoter constructs. Both full-length promoter constructs, but not those using truncated promoters, responded to indole-3-acetic acid (IAA) and wounding. Both pACO1:GUS and pACO1-1.2:GUS responded to pathogen attack, while neither version of the pACO2 promoter responded to infection. In the inflorescence stalks, the full-length pACO1 promoter construct, but not the truncated pACO1-1.2:GUS or either pACO2 construct, responded to bending stress. When flowering transgenic Arabidopsis plants were placed in a horizontal position for 48 h, expression from pACO2:GUS, but not the other constructs, was induced on the underside of shoots undergoing gravitropic reorientation. The expression pattern for the pACO2:GUS construct in transgenic Arabidopsis was consistent with what might be expected for a gene promoter involved in the compression wood response in loblolly pine. Although near complete sequence identity between PtACO1 and PtACO2 transcripts prevented quantitation of specific gene products, the promoter expression analyses presented in this study provide strong evidence that the two ACC oxidase genes are likely differentially expressed and responded to different external stimuli in pine. These results are discussed with respect to the potential functional differences between these two genes in loblolly pine.

ACC oxidase genes expressed in the wood-forming tissues of loblolly pine (Pinus taeda L.) include a pair of nearly identical paralogs (NIPs)

1-Aminocyclopropane-1-carboxylate (ACC) oxidase catalyzes the final reaction of the ethylene biosynthetic pathway, converting the unusual cyclic amino acid, ACC, into ethylene. Past studies have shown a possible link between ethylene and compression wood formation in conifers, but the relationship has received no more than modest study at the gene expression level. In this study, a cDNA clone encoding a putative ACC oxidase, PtACO1, was isolated from a cDNA library produced using mRNA from lignifying xylem of loblolly pine (Pinus taeda) trunk wood. The cDNA clone comprised an open reading frame of 1461 bp encoding a protein of 333 amino acids. Using PCR amplification techniques, a genomic clone corresponding to PtACO1 was isolated and shown to contain three introns with typical GT/AG boundaries defining the splice junctions. The PtACO1 gene product shared 70% identity with an ACC oxidase from European white birch (Betula pendula), and phylogenetic analyses clearly placed the gene product in the ACC oxidase cluster of the Arabidopsis thaliana 2-oxoglutarate-dependent dioxygenase superfamily tree. The PtACO1 sequence was used to identify additional ACC oxidase clones from loblolly pine root cDNA libraries characterized as part of an expressed sequence tag (EST) discovery project. The PtACO1 sequence was also used to recover additional paralogous sequences from genomic DNA, one of which (PtACO2) turned out to be >98% identical to PtACO1 in the nucleotide coding sequence, leading to its classification as a "nearly identical paralog" (NIP). Quantitative PCR analyses showed that the expression level of PtACO1-like transcripts varied in different tissues, as well as in response to hormonal treatments and bending. Possible roles for PtACO1 in compression wood formation in loblolly pine and the discovery of its NIP are discussed in light of these results.

Biphasic ethylene production during the hypersensitive response in Arabidopsis: a window into defense priming mechanisms?

The hypersensitive response (HR) is a cell death phenomenon associated with localized resistance to pathogens. Biphasic patterns in the generation of H(2)O(2), salicylic acid and ethylene have been observed in tobacco during the early stages of the HR. These biphasic models reflect an initial elicitation by pathogen-associated molecular patterns followed by a second phase, induced by pathogen-encoded avirulence gene products. The first phase has been proposed to potentiate the second, to increase the efficacy of plant resistance to disease. This potentiation is comparable to the "priming" of plant defenses which is seen when plants display systemic resistance to disease. The events regulating the generation of the biphasic wave, or priming, remains obscure, however recently we demonstrated a key role for nitric oxide in this process in a HR occurring in tobacco. Here we use laser photoacoustic detection to demonstrate that biphasic ethylene production also occurs during a HR occurring in Arabidopsis. We suggest that ethylene emanation during the HR represents a ready means of visualising biphasic events during the HR and that exploiting the genomic resources offered by this model species will facilitate the development of a mechanistic understanding of potentiating/priming processes.

Characterization of the 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase multigene family of Malus domestica Borkh

Two 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO) genes have been cloned from RNA isolated from leaf tissue of apple (Malus domestica cv. Royal Gala). The genes, designated MD-ACO2 (with an ORF of 990bp) and MD-ACO3 (966bp) have been compared with a previously cloned gene of apple, MD-ACO1 (with an ORF of 942bp). MD-ACO1 and MD-ACO2 share a close nucleotide sequence identity of 93.9% in the ORF but diverge in the 3' untranslated regions (3'-UTR) (69.5%). In contrast, MD-ACO3 shares a lower sequence identity with both MD-ACO1 (78.5%) and MD-ACO2 (77.8%) in the ORF, and 68.4% (MD-ACO1) and 71% (MD-ACO2) in the 3'-UTR. Southern analysis confirmed that MD-ACO3 is encoded by a distinct gene, but the distinction between MD-ACO1 and MD-ACO2 is not as definitive. Gene expression analysis has shown that MD-ACO1 is restricted to fruit tissues, with optimal expression in ripening fruit, MD-ACO2 expression occurs more predominantly in younger fruit tissue, with some expression in young leaf tissue, while MD-ACO3 is expressed predominantly in young and mature leaf tissue, with less expression in young fruit tissue and least expression in ripening fruit. Protein accumulation studies using western analysis with specific antibodies raised to recombinant MD-ACO1 and MD-ACO3 produced in E. coli confirmed the accumulation of MD-ACO1 in mature fruit, and an absence of accumulation in leaf tissue. In contrast, MD-ACO3 accumulation occurred in younger leaf tissue, and in younger fruit tissue. Further, the expression of MD-ACO3 and accumulation of MD-ACO3 in leaf tissue is linked to fruit longevity. Analysis of the kinetic properties of the three apple ACOs using recombinant enzymes produced in E. coli revealed apparent Michaelis constants (K(m)) of 89.39 microM (MD-ACO1), 401.03 microM (MD-ACO2) and 244.5 microM (MD-ACO3) for the substrate ACC, catalytic constants (K(cat)) of 6.6x10(-2) (MD-ACO1), 3.44x10(-2) (Md-ACO2) and 9.14x10(-2) (MD-ACO3) and K(cat)/K(m) (microMs(-1)) values of 7.38x10(-4) microMs(-1) (MD-ACO1), 0.86x10(-4)Ms(-1) (MD-ACO2) and 3.8x10(-4) microMs(-1) (MD-ACO3). These results show that MD-ACO1, MD-ACO2 and MD-ACO3 are differentially expressed in apple fruit and leaf tissue, an expression pattern that is supported by some variation in kinetic properties.

Melon fruits: genetic diversity, physiology, and biotechnology features

Among Cucurbitaceae, Cucumis melo is one of the most important cultivated cucurbits. They are grown primarily for their fruit, which generally have a sweet aromatic flavor, with great diversity and size (50 g to 15 kg), flesh color (orange, green, white, and pink), rind color (green, yellow, white, orange, red, and gray), form (round, flat, and elongated), and dimension (4 to 200 cm). C. melo can be broken down into seven distinct types based on the previously discussed variations in the species. The melon fruits can be either climacteric or nonclimacteric, and as such, fruit can adhere to the stem or have an abscission layer where they will fall from the plant naturally at maturity. Traditional plant breeding of melons has been done for 100 years wherein plants were primarily developed as open-pollinated cultivars. More recently, in the past 30 years, melon improvement has been done by more traditional hybridization techniques. An improvement in germplasm is relatively slow and is limited by a restricted gene pool. Strong sexual incompatibility at the interspecific and intergeneric levels has restricted rapid development of new cultivars with high levels of disease resistance, insect resistance, flavor, and sweetness. In order to increase the rate and diversity of new traits in melon it would be advantageous to introduce new genes needed to enhance both melon productivity and melon fruit quality. This requires plant tissue and plant transformation techniques to introduce new or foreign genes into C. melo germplasm. In order to achieve a successful commercial application from biotechnology, a competent plant regeneration system of in vitro cultures for melon is required. More than 40 in vitro melon regeneration programs have been reported; however, regeneration of the various melon types has been highly variable and in some cases impossible. The reasons for this are still unknown, but this plays a heavy negative role on trying to use plant transformation technology to improve melon germplasm. In vitro manipulation of melon is difficult; genotypic responses to the culture method (i.e., organogenesis, somatic embryogenesis, etc.) as well as conditions for environmental and hormonal requirements for plant growth and regeneration continue to be poorly understood for developing simple in vitro procedures to culture and transform all C. melo genotypes. In many cases, this has to be done on an individual line basis. The present paper describes the various research findings related to successful approaches to plant regeneration and transgenic transformation of C. melo. It also describes potential improvement of melon to improve fruit quality characteristics and postharvest handling. Despite more than 140 transgenic melon field trials in the United States in 1996, there are still no commercial transgenic melon cultivars on the market. This may be a combination of technical or performance factors, intellectual property rights concerns, and, most likely, a lack of public acceptance. Regardless, the future for improvement of melon germplasm is bright when considering the knowledge base for both techniques and gene pools potentially useable for melon improvement.

Leaf scorch symptoms are not correlated with bacterial populations during Pierce's disease

Xylella fastidiosa (Xf) is a xylem-limited bacterium that lives as a harmless endophyte in most plant species but is pathogenic in several agriculturally important crops such as coffee, citrus, and grapevine (Vitis vinifera L.). In susceptible cultivars of grapevine, Xf infection results in leaf scorch, premature leaf senescence, and eventually vine death; a suite of symptoms collectively referred to as Pierce's disease. A qPCR assay was developed to determine bacterial concentrations in planta and these concentrations were related to the development of leaf-scorch symptoms. The concentration of Xf in leaves of experimental grapevines grown in the greenhouse was similar to the concentration of Xf in leaves of naturally infected plants in the field. The distribution of Xf was patchy within and among leaves. Some whole leaves exhibited severe leaf-scorch symptoms in the absence of high concentrations of Xf. Despite a highly sensitive assay and a range of Xf concentrations from 10(2) to 10(9) cells g(-1) fresh weight, no clear relationship between bacterial population and symptom development during Pierce's disease was revealed. Thus, high and localized concentrations of Xf are not necessary for the formation of leaf-scorch symptoms. The results are interpreted as being consistent with an atiology that involves a systemic plant response.

Transformation of a muskmelon 'Galia' hybrid parental line (Cucumis melo L. var. reticulatus Ser.) with an antisense ACC oxidase gene

'Galia' muskmelon (Cucumis melo L. var. reticulatus Ser.) has been recalcitrant to transformation by Agrobacterium tumefaciens. Transformation of the 'Galia' male parental line, 'Krymka', with an ACC oxidase (CMACO-1) gene in antisense orientation is described herein. Explants were transformed using A. tumefaciens strain ABI, which contained a vector pCmACO1-AS plasmid, bearing an antisense gene of CMACO-1 and the CP4 syn gene (glyphosate-tolerance). Both CMACO-1 and CP4 syn genes were assessed by a polymerase chain reaction method. Flow cytometry analysis was performed to determine plant ploidy level of primary transformants. Two completely diploid independent transgenic plants were obtained. Southern blot and segregation analysis in the T1 generation determined that each independent transgenic line had one single insertion of the transgene. These transgenic muskmelon male parental lines have potential for use in the production of 'Galia' F1 hybrids with improved shelf life.

Expression of 1-aminocyclopropane-1-carboxylate (ACC) oxidase genes during the development of vegetative tissues in white clover (Trifolium repens L.) is regulated by ontological cues

Four 5' flanking sequences, comprising the 5'-UTR and upstream promoter region, have been isolated and cloned from the 1-aminocyclopropane-1-carboxylate (ACC) oxidase gene family of white clover (Trifolium repens L.), and designated TR-ACO1p, TR-ACO2p, TR-ACO3p and TR-ACO4p. Southern analysis confirmed that these sequences correspond to four distinct genes. The four corresponding genomic sequences have also been isolated and each shown to be comprised of four exons interspersed by three introns. The expression pattern, in vivo, directed by all four 5' flanking sequences during leaf development has been examined using GUS fusions and transformation into white clover. Here, the TR-ACO1 5' flanking sequence directs highest expression in the apical tissues, axillary buds, and leaf petiolules in younger tissues and then declines in the ageing tissues, while the TR-ACO2 5' flanking sequence directs expression in both younger, mature green and in ontologically ageing tissue. The TR-ACO3 and TR-ACO4 5' flanking sequences direct more expression in the ontological older tissues, including the axillary buds and leaf petiolules. The TR-ACO1 5' flanking sequence directed expression in the ground meristem and newly emerged leaf tissue at the apical bud of the stolon, but all four 5' flanking sequences directed expression in the ground meristem tissue of axillary buds, vascular tissue, pith and cortex of the internode and node, and the cortex and vascular tissue of the leaf petiolule, with the primacy of each promoter determined by the ontological age of the tissues. These data suggest that in vegetative tissue development of white clover, the primary cues for the transcriptional regulation of the ACO gene family are ontological in nature.

Leaf senescence is delayed in maize expressing the Agrobacterium IPT gene under the control of a novel maize senescence-enhanced promoter

We have genetically modified maize plants to delay leaf senescence. A senescence-enhanced promoter from maize (P(SEE1)) was used to drive expression of the Agrobacterium cytokinin biosynthesis gene IPT in senescing leaf tissue. Three maize lines expressing IPT from P(SEE1), Sg1, Sg2 and Sg3, were analysed in detail, representing mild, intermediate and extreme expression, respectively, of the delayed-senescence phenotype. Backcross populations segregating for the presence or absence of the P(SEE1Xba)IPTNOS transgene also simultaneously segregated for the senescence phenotype. At the time of ear leaf emergence, individuals of lines Sg1 and Sg2 segregating for the presence of the transgene carried about three fewer senescing leaves than control (transgene-minus) segregants, and IPT transcript levels were higher in leaves at incipient senescence than in young leaves. Leaves of transgenic Sg3 plants were significantly greener than controls and progressed directly from fully green to bleached and dead without an intervening yellowing phase. IPT transcript abundance in this line was not related to the initiation of senescence. Extended greenness was accompanied by a delay in the loss of photosynthetic capacity with leaf age. The delayed-senescence trait was associated with relatively minor changes in morphology and development. The phenotype was particularly emphasized in plants grown in low soil nitrogen. The reduced ability of the extreme transgenic line Sg3 to recycle internal nitrogen from senescing lower leaves accounted for significant chlorosis in emerging younger leaves when plants were grown in low nutrient conditions. This study demonstrates that the agronomically important delayed-senescence ('stay-green') trait can be engineered into a monocot crop, and is the first example outside Arabidopsis of senescence modification using a homologous senescence-enhanced promoter.

Delayed Symptom Development in ein2-1, an Arabidopsis Ethylene-Insensitive Mutant, in Response to Bacterial Wilt Caused by Ralstonia solanacearum

ABSTRACT Wilt disease caused by the phytopathogenic bacterium Ralstonia solanacearum is poorly understood at the molecular level. The possible roles of salicylic acid, jasmonic acid, and ethylene, compounds commonly associated with the plant response to pathogens, in wilt symptom development were investigated using various Arabidopsis thaliana mutants in a Col-0 background, an ecotype that develops wilt symptoms in response to the virulent GMI1000 strain. Following root inoculation, wilt symptoms were delayed in ein2-1, an ethylene-insensitive mutant, in response to several virulent strains of the pathogen. In ein2-1, bacteria invade the plant and multiply, reaching concentrations slightly lower than those detected in susceptible plants but 1 to 2 logs higher than in Nd-1, an A. thaliana ecotype resistant to strain GMI1000. This delay in disease symptom development of ein2-1 plants suggests that ethylene signaling plays a critical role in wilt disease development. Furthermore, a strong accumulation of transcripts corresponding to PR-3 and PR-4, two ethylene-responsive genes, was observed in susceptible Col-0 plants, but not in ein2-1 and Nd-1 plants, providing additional evidence for a role of ethylene in wilt symptom production. However, this hormone is probably not involved in the establishment of resistance to R. solanacearum, because homozygous ein2-1 plants in a resistant background remain fully resistant to strain GMI1000.

Reduced symptoms of Verticillium wilt in transgenic tomato expressing a bacterial ACC deaminase

Summary Ethylene evolved during compatible or susceptible disease interactions may hasten and/or worsen disease symptom development; if so, the prevention of disease-response ethylene should reduce disease symptoms. We have examined the effects of reduced ethylene synthesis on Verticillium wilt (causal organism, Verticillium dahliae) of tomato by transforming tomato with ACC deaminase, which cleaves ACC, the immediate biosynthetic precursor of ethylene in plants. Three promoters were used to express ACC deaminase in the plant: (i) CaMV 35S (constitutive expression); (ii) rolD (limits expression specifically to the site of Verticillium infection, i.e. the roots); and (iii) prb-1b (limits expression to certain environmental cues, e.g. disease infection). Significant reductions in the symptoms of Verticillium wilt were obtained for rolD- and prb-1b-, but not for 35S-transformants. The pathogen was detected in stem sections of plants with reduced symptoms, suggesting that reduced ethylene synthesis results in increased disease tolerance. The effective control of formerly recalcitrant diseases such as Verticillium wilt may thus be obtained by preventing disease-related ethylene production via the tissue-specific expression of ACC deaminase.

Characterization and expression of two members of the peach 1-aminocyclopropane-1-carboxylate oxidase gene family

The characterization and expression of PP-ACO1 and PP-ACO2, two members of the peach 1-aminocyclopropane-1-carboxylate (ACC) oxidase (ACO) gene family, are reported. PP-ACO1 is organized in 4 exons interrupted by 3 introns, whereas PP-ACO2 has only 2 of the 3 introns present in PP-ACO1. Comparison of the deduced amino acid sequences of PP-ACO1 and PP-ACO2 reveals a 77.7% identity. PP-ACO1 and PP-ACO2 show highest degree of similarity with petunia (PH-ACO3; 84.1%) and apple (85.4%) ACO genes, respectively. PP-ACO1 is expressed in flowers, fruitlet abscission zones, mesocarp and in young fully expanded leaves. PP-ACO1 transcript accumulation strongly increases during fruitlet abscission, in ripe mesocarp and senescing leaves, and is enhanced by propylene. PP-ACO2 mRNA accumulation is detected in fruits only during early development and is unaffected by propylene treatment. Both ACO genes are expressed in epicotyl and roots of growing seedlings, although a stronger accumulation of PP-ACO2 mRNA is observed.

Characterization of ripening-regulated cDNAs and their expression in ethylene-suppressed charentais melon fruit

Charentais melons (Cucumis melo cv Reticulatus) are climacteric and undergo extremely rapid ripening. Sixteen cDNAs corresponding to mRNAs whose abundance is ripening regulated were isolated to characterize the changes in gene expression that accompany this very rapid ripening process. Sequence comparisons indicated that eight of these cDNA clones encoded proteins that have been previously characterized, with one corresponding to ACC (1-aminocyclopropane-1-carboxylic acid) oxidase, three to proteins associated with pathogen responses, two to proteins involved in sulfur amino acid biosynthesis, and two having significant homology to a seed storage protein or a yeast secretory protein. The remaining eight cDNA sequences did not reveal significant sequence similarities to previously characterized proteins. The majority of the 16 ripening-regulated cDNAs corresponded to mRNAs that were fruit specific, although three were expressed at low levels in vegetative tissues. When examined in transgenic antisense ACC oxidase melon fruit, three distinct patterns of mRNA accumulation were observed. One group of cDNAs corresponded to mRNAs whose abundance was reduced in transgenic fruit but inducible by ethylene treatment, indicating that these genes are directly regulated by ethylene. A second group of mRNAs was not significantly altered in the transgenic fruit and was unaffected by treatment with ethylene, indicating that these genes are regulated by ethylene-independent developmental cues. The third and largest group of cDNAs showed an unexpected pattern of expression, with levels of mRNA reduced in transgenic fruit and remaining low after exposure to ethylene. Regulation of this third group of genes thus appears to ethylene independent, but may be regulated by developmental cues that require ethylene at a certain stage in fruit development. The results confirm that both ethylene-dependent and ethylene-independent pathways of gene regulation coexist in climacteric fruit.

Expression of 1-aminocyclopropane-1-carboxylate oxidase during leaf ontogeny in white clover

We examined the expression of three distinct 1-aminocyclopropane-1-carboxylic acid oxidase genes during leaf ontogeny in white clover (Trifolium repens). Significant production of ethylene occurs at the apex, in newly initiated leaves, and in senescent leaf tissue. We used a combination of reverse transcriptase-polymerase chain reaction and 3'-rapid amplification of cDNA ends to identify three distinct DNA sequences designated TRACO1, TRACO2, and TRACO3, each with homology to 1-aminocyclopropane-1-carboxylic acid oxidase. Southern analysis confirmed that these sequences represent three distinct genes. Northern analysis revealed that TRACO1 is expressed specifically in the apex and TRACO2 is expressed in the apex and in developing and mature green leaves, with maximum expression in developing leaf tissue. The third gene, TRACO3, is expressed in senescent leaf tissue. Antibodies were raised to each gene product expressed in Escherichia coli, and western analysis showed that the TRACO1 antibody recognizes a protein of approximately 205 kD (as determined by gradient sodium dodecyl sulfate-polyacylamide gel electrophoresis) that is expressed preferentially in apical tissue. The TRACO2 antibody recognizes a protein of approximately 36.4 kD (as determined by gradient sodium dodecyl sulfate-polyacylamide gel electrophoresis) that is expressed in the apex and in developing and mature green leaves, with maximum expression in mature green tissue. No protein recognition by the TRACO3 antibody could be detected in senescent tissue or at any other stage of leaf development.

The hypersensitive response. A programmed cell death associated with plant resistance

In plants, the hypersensitive response (HR) is defined as a rapid cell death occurring in response to pathogen attack, and is closely related to active resistance. Initiation of the HR process begins with the recognition of the pathogen by the plant, which is mediated mainly by the pathogen avirulence genes and the plant resistance genes. Then, complex signal transduction pathways intervene, involving changes in protein phosphorylation, production of reactive oxygen species and modification of ion fluxes. Components required for the regulation of the HR cell death are now being identified genetically by the isolation of mutants, in contrast to those involved in the execution of the cell death programme, which are still largely unknown. Further genetic and biochemical analyses will undoubtedly answer the question as to whether this form of programmed cell death (PCD) can be compared with other forms of PCD in plants and with apoptosis in animals.