Transfer of tomato immune receptor Ve1 confers Ave1‐dependent Verticillium resistance in tobacco and cotton

Verticillium wilts caused by soilborne fungal species of the Verticillium genus are economically important plant diseases that affect a wide range of host plants and are notoriously difficult to combat. Perception of pathogen(‐induced) ligands by plant immune receptors is a key component of plant innate immunity. In tomato, race‐specific resistance to Verticillium wilt is governed by the cell surface‐localized immune receptor Ve1 through recognition of the effector protein Ave1 that is secreted by race 1 strains of Verticillium spp. It was previously demonstrated that transgenic expression of tomato Ve1 in the model plant Arabidopsis thaliana leads to Verticillium wilt resistance. Here, we investigated whether tomato Ve1 can confer Verticillium resistance when expressed in the crop species tobacco (Nicotiana tabcum) and cotton (Gossypium hirsutum). We show that transgenic tobacco and cotton plants constitutively expressing tomato Ve1 exhibit enhanced resistance against Verticillium wilt in an Ave1‐dependent manner. Thus, we demonstrate that the functionality of tomato Ve1 in Verticillium wilt resistance through recognition of the Verticillium effector Ave1 is retained after transfer to tobacco and cotton, implying that the Ve1‐mediated immune signalling pathway is evolutionary conserved across these plant species. Moreover, our results suggest that transfer of tomato Ve1 across sexually incompatible plant species can be exploited in breeding programmes to engineer Verticillium wilt resistance.     

First report of <Emphasis Type="Italic">Verticillium tricorpus</Emphasis> isolated from potato tubers in Japan

In 1998, Verticillium sp. (CE98Vt1 and CE98Vt2) were isolated from discolored vascular structures of potato tubers sold at a market in Chiba Prefecture. These isolates were identified as Verticillium tricorpus on the basis of cultural and morphological characteristics and PCR diagnosis. This observed vascular discoloration of the potato tuber was demonstrated in three cultivars (Touya, Toyoshiro, and Waseshiro) among eight cultivars by inoculation to seedlings. External and internal symptoms of these isolates were not distinct in potato plants. The virulence of these isolates to potato was very low as compared with Verticillium dahliae. These two isolates were not pathogenic to Chinese cabbage, eggplant, green pepper, larkspur, parsley, snapdragon, soybean, tobacco, and tomato. This is the first report of V. tricorpus from potato in Japan.     

Isolation of an osmotic stress- and abscisic acid-induced gene encoding an acidic endochitinase from Lycopersicon chilense

We have identified one osmotic stress- and abscisic acid-responsive member of the endochitinase (EC 3.2.1.14) gene family from leaves of drought-stressed Lycopersicon chilense plants, a natural inhabitant of extremely arid regions in South America. The 966-bp full-length cDNA (designated pcht28) encodes an acidic chitinase precursor with an amino-terminal signal peptide. The mature protein is predicted to have 229 amino acid residues with a relative molecular mass of 24 943 and pI value of 6.2. Sequence analysis revealed that pcht28 has a high degree of homology with class II chitinases (EC 3.2.1.14) from tomato and tobacco. Expression of the pcht28 protein in Escherichia coli verified that it is indeed a chitinase. Northern blot analysis indicated that this gene has evolved a different pattern of expression from that of other family members reported thus far. It is highly induced by both osmotic stress and the plant hormone abscisic acid. Southern blot analysis of genomic DNA suggested that the pcht28-related genes may form a small multigene family in this species. The efficiency of induction of the gene by drought stress, in leaves and stems, is significantly higher in L. chilense than in the cultivated tomato. It is speculated that, besides its general defensive function, the pcht28-encoded chitinase may play a particular role in plant development or in protecting plants from pathogen attack during water stress.     

Mapping of Ve in tomato: a gene conferring resistance to the broad-spectrum pathogen, Verticillium dahliae race 1

 The soil-borne fungi Verticillium spp. cause vascular wilt disease in a wide range of crop plants. In tomato, resistance to Verticillium dahliae race 1 is conferred by a single dominant gene, Ve. Previous efforts to map Ve in tomato have yielded confusing results, locating it on different chromosomes, which subsequently raised the possibility that Verticillium resistance may be controlled by a number of loci. We used three different mapping populations to obtain an unambiguous map location of Ve: a recombinant inbred (RI) line population; an F₂ population segregating for Verticillium resistance; and a population of 50 introgression lines (IL). In all of the mapping populations Ve was positioned on the short arm of chromosome 9 tightly linked to the RFLP marker GP39. This linkage was confirmed by screening for GP39 in different breeding lines with known resistance or susceptibility to Verticillium. A perfect match was found between GP39 and the Verticillium response of the lines, indicating the potential of GP39 in the rapid detection of Verticillium resistance and as a starting point for map-based cloning of Ve. This approach is particularly relevant for Verticillium dahliae race 1, since in the present work we also show that the isolate that infects tomato is responsible for wilt disease in other important crop plants. Received: 5 July 1998 / Accepted: 28 July 1998     

Comparative genomics reveals the in planta-secreted Verticillium dahliae Av2 effector protein recognized in tomato plants that carry the V2 resistance locus

Plant pathogens secrete effector molecules during host invasion to promote colonization. However, some of these effectors become recognized by host receptors to mount a defence response and establish immunity. Recently, a novel resistance was identified in wild tomato, mediated by the single dominant V2 locus, to control strains of the soil-borne vascular wilt fungus Verticillium dahliae that belong to race 2. With comparative genomics of race 2 strains and resistance-breaking race 3 strains, we identified the avirulence effector that activates V2 resistance, termed Av2. We identified 277 kb of race 2-specific sequence comprising only two genes encoding predicted secreted proteins that are expressed during tomato colonization. Subsequent functional analysis based on genetic complementation into race 3 isolates and targeted deletion from the race 1 isolate JR2 and race 2 isolate TO22 confirmed that one of the two candidates encodes the avirulence effector Av2 that is recognized in V2 tomato plants. Two Av2 allelic variants were identified that encode Av2 variants that differ by a single acid. Thus far, a role in virulence could not be demonstrated for either of the two variants.     

Isolation of an osmotic stress- and abscisic acid-induced gene encoding an acidic endochitinase from Lycopersicon chilense

We have identified one osmotic stress- and abscisic acid-responsive member of the endochitinase (EC 3.2.1.14) gene family from leaves of drought-stressed Lycopersicon chilense plants, a natural inhabitant of extremely arid regions in South America. The 966-bp full-length cDNA (designated pcht28) encodes an acidic chitinase precursor with an amino-terminal signal peptide. The mature protein is predicted to have 229 amino acid residues with a relative molecular mass of 24 943 and pI value of 6.2. Sequence analysis revealed that pcht28 has a high degree of homology with class II chitinases (EC 3.2.1.14) from tomato and tobacco. Expression of the pcht28 protein in Escherichia coli verified that it is indeed a chitinase. Northern blot analysis indicated that this gene has evolved a different pattern of expression from that of other family members reported thus far. It is highly induced by both osmotic stress and the plant hormone abscisic acid. Southern blot analysis of genomic DNA suggested that the pcht28-related genes may form a small multigene family in this species. The efficiency of induction of the gene by drought stress, in leaves and stems, is significantly higher in L. chilense than in the cultivated tomato. It is speculated that, besides its general defensive function, the pcht28-encoded chitinase may play a particular role in plant development or in protecting plants from pathogen attack during water stress.     

Transgenic sweetpotato plants expressing an <Emphasis Type="Italic">LOS</Emphasis>5 gene are tolerant to salt stress

Transgenic sweetpotato (cv. Lizixiang) plants exhibiting enhanced salt tolerance were developed using LOW OSMOTIC STRESS 5 (LOS5) with Agrobacterium tumefaciens-mediated transformation. A. tumefaciens strain EHA105 harbors the pCAMBIA1300 binary vector with the LOS5 and hygromycin phosphotransferase II (hptII) genes. Selection culture was conducted using 25 mg l⁻¹ hygromycin. A total of 26 plants were produced from the inoculated 200 cell aggregates of Lizixiang via somatic embryogenesis. PCR analysis showed that 23 of the 26 regenerated plants were transgenic plants. All of the transgenic plants exhibited higher salt tolerance compared to the untransformed control plants by in vitro assay for salt tolerance with 86 mM NaCl. When plants were exposed to 86 mM NaCl, 16 transgenic plants had significantly higher levels of superoxide dismutase (SOD), proline, and abscisic acid (ABA) and significantly lower malonaldehyde (MDA) contents than those in untransformed control plants. Salt tolerance of these 16 plants was further evaluated with Hoagland solution containing 86 mM NaCl in a greenhouse. Four of the sixteen had significantly better growth and rooting ability than the remaining 12 plants and control plants. Stable integration of the LOS5 gene into the genome of the 4 salt-tolerant transgenic plants was confirmed by Southern blot analysis, and the copy number of integrated LOS5 gene ranged from 1 to 3. High level of LOS5 gene expression in the 4 salt-tolerant transgenic plants was demonstrated by real-time quantitative PCR analysis. This study provides an important approach for improving salt tolerance of sweetpotato.     

Broad taxonomic characterization of Verticillium wilt resistance genes reveals an ancient origin of the tomato Ve1 immune receptor

Plant‐pathogenic microbes secrete effector molecules to establish themselves on their hosts, whereas plants use immune receptors to try and intercept such effectors in order to prevent pathogen colonization. The tomato cell surface‐localized receptor Ve1 confers race‐specific resistance against race 1 strains of the soil‐borne vascular wilt fungus Verticillium dahliae which secrete the Ave1 effector. Here, we describe the cloning and characterization of Ve1 homologues from tobacco (Nicotiana glutinosa), potato (Solanum tuberosum), wild eggplant (Solanum torvum) and hop (Humulus lupulus), and demonstrate that particular Ve1 homologues govern resistance against V. dahliae race 1 strains through the recognition of the Ave1 effector. Phylogenetic analysis shows that Ve1 homologues are widely distributed in land plants. Thus, our study suggests an ancient origin of the Ve1 immune receptor in the plant kingdom.     

Water Stress and Verticillium Wilt Severity on Eggplant (Solanummelongena L.)

Verticillium wilt is one of the most destructive diseases of eggplant (Solanum melongena L.). Some researchers have reported that wilt was encouraged by sufficient soil humidity, while others stated that it was encouraged by drought. This study investigated the water stress effect on the severity of Verticillium wilt on eggplant, as it is reflected on yield, agronomic traits and fruit quality. Thus, eggplant seedlings cv. ‘Tsakoniki’ were transplanted in three rows, each with 20 plants, during the summer of 1995 and 1996 in a plastic greenhouse, at the Agricultural Research Center of Macedonia and Thrace. Ten of the plants in each row were inoculated with the fungus Verticillium dahliae, while the other 10 were used as controls. Rows were irrigated every 2, 4 or 6 days. Soil humidity was calculated before every irrigation in each row. The disease severity was estimated by the disease index (DI) as the combination product of leaf symptom index (LSI) and vascular discoloration index (VDI). In addition, the plant height, early and total commercial yields, fruit numbers of early and total commercial yields, plant weight, the above-ground plant weight, root weight, pH, total soluble solids and fruit brilliance plus colour intensity were measured. The effect of Verticillium wilt on plants irrigated every 2, 4 or 6 days was estimated by the correlation coefficient (r) between LSI and DI and the aforementioned characteristics. Verticillium wilt had a significant but negative effect on all of the measured or calculated characteristics. This effect, however, was independent of the irrigation applied. On average, the early commercial yield was reduced by 40.8% and the final commercial yield by 39.4%. The only quality characteristic that was affected significantly by irrigation was the fruit brilliance and colour intensity (r =?0.640 to ? 0.727, P ≤ 0.01). Finally, the irrigation frequency (every 2, 4 or 6 days) had a significant but negative effect on all of the characteristics measured on the control plants. The only exception was fruit quality. In conclusion, the combined effect of irrigation and Verticillium wilt infection significantly reduced the early and total production of eggplant and spoiled the fruit quality.     

Overexpression of <Emphasis Type="Italic">Citrus junos</Emphasis> mitochondrial citrate synthase gene in <Emphasis Type="Italic">Nicotiana benthamiana</Emphasis> confers aluminum tolerance

Aluminum (Al) toxicity is one of the major factors that limit plant growth in acid soils. Al-induced release of organic acids into rhizosphere from the root apex has been identified as a major Al-tolerance mechanism in many plant species. In this study, Al tolerance of Yuzu (Citrus Junos Sieb. ex Tanaka) was tested on the basis of root elongation and the results demonstrated that Yuzu was Al tolerant compared with other plant species. Exposure to Al triggered the exudation of citrate from the Yuzu root. Thus, the mechanism of Al tolerance in Yuzu involved an Al-inducible increase in citrate release. Aluminum also elicited an increase of citrate content and increased the expression level of mitochondrial citrate synthase (CjCS) gene and enzyme activity in Yuzu. The CjCS gene was cloned from Yuzu and overexpressed in Nicotiana benthamiana using Agrobacterium tumefaciens-mediated methods. Increased expression level of the CjCS gene and enhanced enzyme activity were observed in transgenic plants compared with the wild-type plants. Root growth experiments showed that transgenic plants have enhanced levels of Al tolerance. The transgenic Nicotiana plants showed increased levels of citrate in roots compared to wild-type plants. The exudation of citrate from roots of the transgenic plants significantly increased when exposed to Al. The results with transgenic plants suggest that overexpression of mitochondrial CS can be a useful tool to achieve Al tolerance.     

Constitutive expression of a fungal glucose oxidase gene in transgenic tobacco confers chilling tolerance through the activation of antioxidative defence system

Scientific evidences in the literature have shown that plants treated exogenously with micromole concentration of hydrogen peroxide (H₂O₂) acquire abiotic stress tolerance potential, without substantial disturbances in the endogenous H₂O₂ pool. In this study, we enhanced the endogenous H₂O₂ content of tobacco (Nicotiana tabaccum L. cv. SR1) plants by the constitutive expression of a glucose oxidase (GO; EC 1.1.3.4) gene of Aspergillus niger and studied their cold tolerance level. Stable integration and expression of GO gene in the transgenic (T₀–T₂) tobacco lines were ascertained by molecular and biochemical tests. Production of functionally competent GO in transgenic plants was confirmed by the elevated levels of H₂O₂ in the transformed tissues. When three homozygous transgenic lines were exposed to different chilling temperatures for 12 h, the electrolyte conductivity was significantly lower in GO-expressing tobacco plants than the control plants; in particular, chilling protection was more prominent at −1°C. In addition, most transgenic lines recovered within a week when returned to normal culture conditions after −1°C–12 h cold stress. However, control plants displayed symptoms of chilling injuries such as necrosis of shoot tip, shoots and leaves, consequently plant death. The protective effect realized in the transgenic plants was comparable to cold-acclimatized wild tobacco. The chilling tolerance of transgenic lines was found associated, at least in part, with elevated levels of total antioxidant content, CAT and APX activities. Based on our findings, we predict that the transgenic expression of GO may be deployed to improve cold tolerance potential of higher plants.     

Competence of oat ( Avena sativa L.) shoot apical meristems for integrative transformation,inherited expression,and osmotic tolerance of transgenic lines containing hva1

Three oat ( Avena sativa L.) cultivars have been successfully transformed using an efficient and reproducible in vitro culture system for differentiation of multiple shoots from shoot apical meristems. The transformation was performed using microprojectile bombardment with two plasmids (pBY520 and pAct1-D) containing linked ( hva1-bar) and non-linked ( gus) genes. The hva1 and bar genes cointegrated with a frequency of 100% as expected, and 61.6% of the transgenic plants carried all three genes. Molecular and biochemical analyses in R0, R1 and R2 progenies confirmed stable integration and expression of all transgenes. Localization of the GUS protein in R0 and R1 plants revealed that high-expression of gus occurred in vascular tissues and in the pollen grains of mature flowers. The constitutive expression of HVA1 protein was observed at all developmental stages of transgenic plants, and was particularly stronger during the early seedling stages. R2 progeny of five independent transgenic lines was tested in vitro for tolerance to osmotic (salt and mannitol) stresses. As compared to non-transgenic control plants, transgenic plants maintained a higher growth and showed significantly ( P < 0.05) increased tolerance to stress conditions. Less than 10% of transgenic plants showed symptoms of wilting or death of leaves and, when these symptoms present were delayed in transgenic plants as compared to 80% of non-transgenic plants, either wilted or died. These symptoms confirmed the increased in vitro tolerance in hva1-expressing transgenic plants to non-transgenic plants, providing strong evidence that the HVA1 protein may play an important role in the protection of oats against salinity and possible water-deficiency stress conditions.     

Over-expression of strawberry d-galacturonic acid reductase in potato leads to accumulation of vitamin C with enhanced abiotic stress tolerance

Vitamin C (ascorbic acid) is an essential component for collagen biosynthesis and also for the proper functioning of the cardiovascular system in humans. Unlike most of the animals, humans lack the ability to synthesize ascorbic acid on their own due to a mutation in the gene encoding the last enzyme of ascorbate biosynthesis. As a result, vitamin C must be obtained from dietary sources like plants. In this study, we have developed transgenic potato plants (Solanum tuberosum L. cv. Taedong Valley) over-expressing strawberry GalUR gene under the control of CaMV 35S promoter with increased ascorbic acid levels. Integration of the GalUR gene in the plant genome was confirmed by PCR and Southern blotting. Ascorbic acid (AsA) levels in transgenic tubers were determined by high-performance liquid chromatography (HPLC). The over-expression of GalUR resulted in 1.6–2-fold increase in AsA in transgenic potato and the levels of AsA were positively correlated with increased GalUR activity. The transgenic lines with enhanced vitamin C content showed enhanced tolerance to abiotic stresses induced by methyl viologen (MV), NaCl or mannitol as compared to untransformed control plants. The leaf disc senescence assay showed better tolerance in transgenic lines by retaining higher chlorophyll as compared to the untransformed control plants. Present study demonstrated that the over-expression of GalUR gene enhanced the level of AsA in potato tubers and these transgenics performed better under different abiotic stresses as compared to untransformed control.