Fungal Pathogen Alternaria Alternata Research Articles

Overexpression of mannitol dehydrogenase in zonal geranium confers increased resistance to the mannitol secreting fungal pathogen Botrytis cinerea

The sugar alcohol mannitol is a carbohydrate with well-documented roles in both metabolism and osmoprotection in plants and fungi. In addition, however, mannitol is an antioxidant, and current research suggests that pathogenic fungi can secrete mannitol into the plant’s extracellular spaces during infection to suppress reactive oxygen-mediated host defenses. In response to pathogen attack, plants have been shown to secrete the normally symplastic enzyme, mannitol dehydrogenase (MTD). Given that MTD converts mannitol to the sugar mannose, extracellular MTD may be an important defense against mannitol-secreting fungal pathogens. Previous work demonstrated that overexpression of MTD in tobacco did, in fact, provide increased resistance to the mannitol-secreting fungal pathogen Alternaria alternata. In the present work we demonstrate that the fungal pathogen Botrytis cinerea also can secrete mannitol, and that overexpression of MTD in zonal geranium (Pelargonium × hortorum) in turn provides increased resistance to B. cinerea. These results are not only an important validation of previous work, but support the idea that MTD-overexpression might be used to engineer a broad variety of plants for resistance to mannitol-secreting fungal pathogens like B. cinerea for which specific resistance is lacking.

A Highly-Conserved Single-Stranded DNA-Binding Protein in Xanthomonas Functions as a Harpin-Like Protein to Trigger Plant Immunity

Harpins are produced by Gram-negative phytopathogenic bacteria and typically elicit hypersensitive response (HR) in non-host plants. The characterization of harpins in Xanthomonas species is largely unexplored. Here we demonstrate that Xanthomonas produce a highly conserved single-stranded DNA-binding protein (SSBX) that elicits HR in tobacco as by harpin Hpa1. SSBX, like Hpa1, is an acidic, glycine-rich, heat-stable protein that lacks cysteine residues. SSBX-triggered HR in tobacco, as by Hpa1, is characterized by the oxidative burst, the expression of HR markers (HIN1, HSR203J), pathogenesis-related genes, and callose deposition. Both SSBX- and Hpa1-induced HRs can be inhibited by general metabolism inhibitors actinomycin D, cycloheximide, and lanthanum chloride. Furthermore, those HRs activate the expression of BAK1 and BIK1 genes that are essential for induction of mitogen-activated protein kinase (MAPK) and salicylic acid pathways. Once applied to plants, SSBX induces resistance to the fungal pathogen Alternaria alternata and enhances plant growth. When ssbX was deleted in X. oryzae pv. oryzicola, the causal agent of bacterial leaf streak in rice, the resulting ssbXoc mutant was reduced in virulence and bacterial growth in planta, but retained its ability to trigger HR in tobacco. Interestingly, ssbXoc contains an imperfect PIP-box (plant-inducible promoter) and the expression of ssbXoc is regulated by HrpX, which belongs to the AraC family of transcriptional activators. Immunoblotting evidence showed that SSBx secretion requires a functional type-III secretion system as Hpa1 does. This is the first report demonstrating that Xanthomonas produce a highly-conserved SSBX that functions as a harpin-like protein for plant immunity.

Cyclic AMP-dependent protein kinase A negatively regulates conidia formation by the tangerine pathotype of Alternaria alternata

The necrotrophic fungal pathogen Alternaria alternata causes brown spot diseases in many citrus cultivars. The FUS3 and SLT2 mitogen-activated protein kinases (MAPK)-mediated signaling pathways have been shown to be required for conidiation. Exogenous application of cAMP to this fungal pathogen decreased conidia formation considerably. This study determined whether a cAMP-activated protein kinase A (PKA) is required for conidiation. Using loss-of-function mutations in PKA catalytic and regulatory subunit-coding genes, we demonstrated that PKA negatively regulates conidiation. Fungal mutants lacking PKA catalytic subunit gene (PKA ( cat )) reduced growth, lacked detectable PKA activity, and produced higher amounts of conidia compared to wild-type. Introduction of a functional copy of PKA ( cat ) into a null mutant partially restored PKA activity and produced wild-type level of conidia. In contrast, fungi lacking PKA regulatory subunit gene (PKA ( reg )) produced detectable PKA activity, exhibited severe growth reduction, formed swelling hyphal segments, and produced no mature conidia. Introduction of the PKA ( reg ) gene to a regulatory subunit mutant restored all phenotypes to wild type. PKA ( reg )-null mutants induced fewer necrotic lesions on citrus compared to wild-type, whereas PKA ( cat ) mutant displayed wild-type virulence. Overall, our studies indicate that PKA and FUS3-mediated signaling pathways apparently have very different roles in the regulation of conidia production and A. alternata pathogenesis in citrus.

Combined expression of antimicrobial genes (Bbchit1 and LJAMP2) in transgenic poplar enhances resistance to fungal pathogens

Populus species are susceptible to infection by microbial pathogens that severely affect their growth and substantially decrease their economic value. In this study, two pathogenesis-related protein genes consisting of Beauveria bassiana chitinase (Bbchit1) and motherwort lipid-transfer protein (LJAMP2) were introduced into Chinese white poplar (Populus tomentosa Carr.) via Agrobacterium-mediated transformation using the hygromycin (hyg) and neomycin phosphotransferase (NPTII) genes as selectable markers, respectively. Polymerase chain reaction analysis confirmed the stable integration of transgenes in the genome of transgenic plants. In vitro assays showed that inhibitory activity against the fungal pathogen Alternaria alternata (Fr.) Keissler was evident from the crude leaf extracts from transgenic plants. Importantly, the double-transgenic plants exhibited significantly higher resistance to the pathogen than either of the single-gene transformants and wild-type plants when inoculated with A. alternata. The level of disease reduction in double-transgenic lines was between 82 and 95%, whereas that of single-gene transformants carrying either LJAMP2 or Bbchit1 was between 65 and 89%. These results indicated that the combined expression of the LJAMP2 and Bbchit-1 genes could significantly enhance resistance to necrotrophic fungal pathogens in poplar.

Roles for SKN7 response regulator in stress resistance, conidiation and virulence in the citrus pathogen Alternaria alternata

“Two-component” histidine kinase (HSK1) is the primary regulator of resistance to sugar osmotic stress and sensitivity to dicarboximide or phenylpyrrole fungicides in the citrus fungal pathogen Alternaria alternata. On the other hand, the mitogen-activated protein kinase HOG1 confers resistance solely to salts and oxidative stress. We report here independent and shared functions of the SKN7-mediated signaling pathway with HSK1 and HOG1. SKN7, a putative transcription downstream regulator of HSK1, is primarily required for cellular resistance to oxidative and sugar-induced osmotic stress. SKN7, perhaps acting in parallel with HOG1, is required for resistance to H2O2, tert-butyl hydroperoxide, and cumyl peroxide, but not to the superoxide-generating compounds – menadione, potassium superoxide, and diamide. Because of phenotypic commonalities, SKN7 is likely involved in resistance to sugar-induced osmotic stress via the HSK1 signaling pathway. However, mutants lacking SKN7 displayed wild-type sensitivity to NaCl and KCl salts. SKN7 is constitutively localized in the nucleus regardless of H2O2 treatment. When compared to the wild type, skn7 mutants exhibited lower catalase, peroxidase, and superoxide dismutase activities and induced significantly fewer necrotic lesions on the susceptible citrus cultivar. The skn7 mutant exhibited fungicide resistance at levels between the hsk1 and the hog1 mutant strains. Skn7/hog1 double mutants exhibited fungicide resistance, similar to the strain with a single AaHSK1 gene mutation. Moreover, the A. alternata SKN7 plays a role in conidia formation. Conidia produced by the skn7 mutant are smaller and have fewer transverse septae than those produced by wild type. All altered phenotypes in the mutant were restored by introducing and expressing a wild-type copy of SKN7 under control of the endogenous promoter.

Stress Response and Pathogenicity of the Necrotrophic Fungal Pathogen Alternaria alternata.

The production of host-selective toxins by the necrotrophic fungus Alternaria alternata is essential for the pathogenesis. A. alternata infection in citrus leaves induces rapid lipid peroxidation, accumulation of hydrogen peroxide (H2O2), and cell death. The mechanisms by which A. alternata avoids killing by reactive oxygen species (ROS) after invasion have begun to be elucidated. The ability to coordinate of signaling pathways is essential for the detoxification of cellular stresses induced by ROS and for pathogenicity in A. alternata. A low level of H2O2, produced by the NADPH oxidase (NOX) complex, modulates ROS resistance and triggers conidiation partially via regulating the redox-responsive regulators (YAP1 and SKN7) and the mitogen-activated protein (MAP) kinase (HOG1) mediated pathways, which subsequently regulate the genes required for the biosynthesis of siderophore, an iron-chelating compound. Siderophore-mediated iron acquisition plays a key role in ROS detoxification because of the requirement of iron for the activities of antioxidants (e.g., catalase and SOD). Fungal strains impaired for the ROS-detoxifying system severely reduce the virulence on susceptible citrus cultivars. This paper summarizes the current state of knowledge of signaling pathways associated with cellular responses to multidrugs, oxidative and osmotic stress, and fungicides, as well as the pathogenicity/virulence in the tangerine pathotype of A. alternata.

Some alternative biological ways for control of pathogenic fungus Alternaria alternata on tobacco

Some alternative biological ways for control of pathogenic fungus Alternaria alternata on tobacco

Gene inactivation in the citrus pathogenic fungus Alternaria alternata defect at the Ku70 locus associated with non-homologous end joining

Low frequency of targeted gene disruption in filamentous fungi has been attributed to the predominance of ectopic integration that is controlled by a non-homologous end-joining (NHEJ) DNA repair mechanism. It has been shown in a number of fungi that suppression of NHEJ by inactivating the yeast Ku70 or Ku80 homolog facilitates homologous recombination, thereby enhancing the frequency of gene targeting. A Ku70 gene homolog was cloned from the necrotrophic fungus Alternaria alternata, causing citrus brown spot. The cloned gene, designated AaKu70, was independently inactivated by insertion of an acetolactate synthase gene (SUR) conferring sulfonylurea resistance or of a bacterial phosphotransferase gene (HYG) cassette conferring hygromycin resistance. The AaKu70 deficient strain reduced conidial formation and pigmentation slightly, but unchanged in toxin production and pathogenicity. Unlike many fungal species, mutation of the AaKu70 gene by inserting either SUR or HYG did not facilitate gene disruption, as tested for disruption targeting an AaAP1gene encoding a redox responsive transcription regulator or an AaHSK1 gene encoding a histidine kinase. However, a split marker approach, using truncated DNA fragments overlapping within the HYG, but not SUR, selectable marker, enhanced the frequency of AaAP1 disruption considerably in the progenitor and the ∆AaKu70 strain. Compared to the disruption targeting the AaAP1 gene, the frequency for recovery of the AaHSK1 impaired mutant was much lower, indicating an allele-dependent disruption. The present study indicates that AaKu70 has little or no effect on gene disruption and homologous integration in A. alternata.

The E3 ubiquitin ligase activity of an insect anti-apoptotic gene (SfIAP) is required for plant stress tolerance

Previously, we showed that transgenic plants expressing SfIAP, an insect anti-apoptotic gene, conferred tolerance to several abiotic stresses and resistance to the fungal pathogen Alternaria alternata. A pronounced and consistent delay in tomato fruit ripening was also observed. Preliminary data suggested that SfIAP negatively affected ethylene synthesis, which could account, at least in part, for the observed phenotypes. A key question is how an insect anti-apoptotic protein functions to enhance stress tolerance in plants, particularly since conserved core apoptotic pathway regulators appear to be absent in plants at the primary sequence level. In this study, we show that the RING domain, a characteristic motif of several IAP family members, has an E3 ubiquitin ligase activity suggesting involvement of SfIAP in proteasome-mediated protein degradation. Proteasome inhibitors were applied to transgenic plants undergoing stress. Proteasome inhibitor treatment restored ethylene signaling and wild type phenotypes in SfIAP expressing transgenic plants. Transient assays of RING domain deletion (Sf∆RING) or point mutations (SfIAPC330A) resulted in the inhibition of ubiquitin ligase activity, while also restoring normal ethylene signaling and the wild type phenotypes including pathogen susceptibility and normal fruit ripening in tomato. During salt stress, increases in ubiquitinylated proteins correlated with salt tolerance in SfIAP expressing plants. Baculovirus inverted repeat (BIR) domains, signature motifs of the IAP family, are also required for the full resistant phenotype of SfIAP plants. SfBIR1, but not SFBIR2, is required for salt tolerance. Only when coupled with the RING domain, was protection conferred in a similar manner to full length SfIAP against salt stress. Taken together, the ubiquitin ligase activity of SfIAP is necessary, but not sufficient to confer plant stress tolerance. The mechanism/target by which SfIAP confers tolerance appears to be via ethylene regulation and the ubiquitin/proteasome pathway.

Contribution of Peroxisomes to Secondary Metabolism and Pathogenicity in the Fungal Plant Pathogen Alternaria alternata

The filamentous fungus Alternaria alternata includes seven pathogenic variants (pathotypes) which produce different host-selective toxins and cause diseases on different plants. The Japanese pear pathotype produces the host-selective AK-toxin, an epoxy-decatrienoic acid ester, and causes black spot of Japanese pear. Previously, we identified four genes, AKT1, AKT2, AKT3, and AKTR, involved in AK toxin biosynthesis. AKT1, AKT2, and AKT3 encode enzyme proteins with peroxisomal targeting signal type 1 (PTS1)-like tripeptides, SKI, SKL, and PKL, respectively, at the C-terminal ends. In this study, we verified the peroxisome localization of Akt1, Akt2, and Akt3 by using strains expressing N-terminal green fluorescent protein (GFP)-tagged versions of the proteins. To assess the role of peroxisome function in AK-toxin production, we isolated AaPEX6, which encodes a peroxin protein essential for peroxisome biogenesis, from the Japanese pear pathotype and made AaPEX6 disruption-containing transformants from a GFP-Akt1-expressing strain. The DeltaAaPEX6 mutant strains did not grow on fatty acid media because of a defect in fatty acid beta oxidation. The import of GFP-Akt1 into peroxisomes was impaired in the DeltaAaPEX6 mutant strains. These strains completely lost AK toxin production and pathogenicity on susceptible pear leaves. These data show that peroxisomes are essential for AK-toxin biosynthesis. The DeltaAaPEX6 mutant strains showed a marked reduction in the ability to cause lesions on leaves of a resistant pear cultivar with defense responses compromised by heat shock. This result suggests that peroxisome function is also required for plant invasion and tissue colonization in A. alternata. We also observed that mutation of AaPEX6 caused a marked reduction of conidiation.

Characterization and expression of an nsLTPs-like antimicrobial protein gene from motherwort (Leonurus japonicus)

In screening for potent antimicrobial proteins (AMPs) from plant seeds, we had purified a heat-stable AMP, LJAMP2, from the seeds of a medicine herb, motherwort (Leonurus japonicus Houtt). In an in vitro assay, the protein can inhibit the growth of both fungi and bacteria. Then a cDNA encoding LJAMP2 was cloned by the rapid amplification of cDNA ends based on the N-terminal amino acid sequence determined. The deduced amino acid sequences of this cDNA show similarity to plant non-specific lipid transfer proteins. Northern blotting assay revealed that this nsLTP-like gene, designated LJAMP2, was expressed in seeds. Overexpression of LJAMP2 in tobacco enhanced resistance to the fungal pathogen Alternaria alternata and the bacterial pathogen Ralstonia solanacearum, significantly, while no visible alteration in plant growth and development. Our data confirm the antifungal and antibacterial function of LJAMP2 from motherwort seeds and suggest the potential of LJAMP2 in improving disease resistance in plants.

Selection of leaf blight-resistant Pelargonium graveolens plants regenerated from callus resistant to a culture filtrate of Alternaria alternata

A protocol has been established for the induction of disease resistance in callus cultures of rose-scented geranium, Pelargonium graveolens cv. Hemanti against leaf blight disease caused by the fungal pathogen Alternaria alternata. The callus cultures were raised and maintained on semi-solid Murashige and Skoog's (MS) medium supplemented with 10 mg l −1 Kn and 1.0 mg l −1 NAA. The calli were subjected to various concentrations of culture filtrate (0%, 4%, 8%, 12%, 16% and 20%) obtained from A. alternata. Resistant calli were selected and placed on regeneration medium (MS supplemented with 0.5 mg l −1 BAP and 0.1 mg l −1 NAA). The regenerants were confirmed for A. alternata resistance by exposing their leaves to the same concentrations of culture filtrate as used previously. While the parental wild type demonstrated typical susceptibility, the leaves of putative resistant clones remained green and viable in the presence of toxin and regenerated shoots directly on the toxin-free regeneration medium (MS supplemented with 5 mg l −1 Kn and l mg l −1 NAA). The above experiment demonstrated the induction of disease resistance in rose-scented geranium plants at the cellular level. This approach could be successfully exploited in raising new disease-resistant cultivars in geranium against various fungal pathogens.

Cytological and molecular analyses of non‐host resistance of Arabidopsis thaliana to Alternaria alternata

SUMMARY When challenged with the necrotrophic fungal pathogen Alternaria alternata Japanese pear pathotype, all tested ecotypes of Arabidopsis plants failed to show hypersensitive cell death, accumulation of detectable levels of reactive oxygen species or accumulation of phytoalexin. We operationally define A. alternata as a non-host pathogen for Arabidopsis plants and show that the protection against A. alternata demonstrated in this study is a non-host penetration resistance. To characterize non-host penetration resistance, we examined the expression patterns of c. 7000 genes by cDNA microarray analysis in Arabidopsis Col-0 plants after inoculation with A. alternata. After inoculation with A. alternata, the transcript levels of 48 genes increased in Col-0 plants. The expression of genes associated with hypersensitive reaction was induced in the non-host penetration resistance to A. alternata, despite the fact that A. alternata had no visible effect on the plants. The non-host penetration resistance to A. alternata was clearly associated with activation of the jasmonate- and ethylene-signalling pathways. In addition, analysis using histochemical staining of GUS activity suggests that defence reactions in non-host penetration resistance are activated locally. The characterization of non-host pathosystem involving Arabidopsis and A. alternata offers an overview of non-host penetration resistance.

Dissection of the host range of the fungal plant pathogen Alternaria alternata by modification of secondary metabolism.

The filamentous fungus Alternaria alternata contains seven pathogenic variants (pathotypes), which produce different host-specific toxins and cause diseases on different plants. The strawberry pathotype produces host-specific AF-toxin and causes Alternaria black spot of strawberry. This pathotype is also pathogenic to Japanese pear cultivars susceptible to the Japanese pear pathotype that produces AK-toxin. The strawberry pathotype produces two related molecular species, AF-toxins I and II: toxin I is toxic to both strawberry and pear, and toxin II is toxic only to pear. Previously, we isolated a cosmid clone pcAFT-1 from the strawberry pathotype that contains three genes involved in AF-toxin biosynthesis. Here, we have identified a new gene, designated AFTS1, from pcAFT-1. AFTS1 encodes a protein with similarity to enzymes of the aldo-ketoreductase superfamily. Targeted mutation of AFTS1 diminished the host range of the strawberry pathotype: Delta aftS1 mutants were pathogenic to pear, but not to strawberry, as is the Japanese pear pathotype. These mutants were found to produce AF-toxin II, but not AF-toxin I. These data represent a novel example of how the host range of a plant pathogenic fungus can be restricted by modification of secondary metabolism.

Epoxide hydrolase: a mRNA induced by the fungal pathogen Alternaria alternata on rough lemon (Citrus jambhiri Lush).

An expression profile of genes induced by non-pathogenic Alternaria alternata on rough lemon leaves was obtained by sequencing 500 subtractive PCR clones generated from mRNA of leaves inoculated with the fungus after subtraction with that of non-inoculated leaves. About 6% of the cDNA sequences had homology to known putative defense-related genes including epoxide hydrolase. A full-length cDNA (951 bp) from rough lemon that encoded epoxy hydrolase was isolated by random amplification of cDNA ends (RACEs), based on sequence information from subtractive PCR, and designated as RlemEH. The product of this gene expressed with an in vitro translation system with Escherichia coli also had activity of a soluble type of epoxide hydrolase. The transcript of rough lemon RlemEH was not detected in flowers, fruits, stems or leaves, but was induced after inoculation of leaves with conidia of Alternaria alternata, wounding, or treatment with C6 volatiles, including trans-2-hexenol and cis-3-hexenol, and methyl jasmonate. The response of the epoxide hydrolase gene correlated well with the activation of defense mechanisms induced in plant-fungus interactions.

Bioassay of the potentiality of Alternaria alternata (Fr.) keissler as a bioherbicide to control waterhyacinth and other aquatic weeds

Waterhyacinth ( Eichhornia crassipes (Mart.) Solms) is one of the world's most important aquatic weeds. The fungal pathogen Alternaria alternata (Fr.) Keissler was isolated from diseased waterhyacinth plants collected in India. No crop tested was susceptible to the fungus. On waterhyacinth, the fungus induces disease symptoms (i.e., lesions) mainly on leaves and less severely on petioles. The application of spores in an oil emulsion (10% oil in water) enhanced the disease on waterhyacinth plants compared with spores suspended in a 0.1% aqueous solution of the surfactant agent, Tween 80. The necrotic leaf area of inoculated plants increased as the length of exposure to 100% relative humidity (RH) and the spore density applied increased. Severe disease developed on plants inoculated with 1×10 6 spores/ml in oil emulsion, even in the absence of exposure to 100% RH. A delay of exposure to 100% RH (up to 8 h) did not have a significant effect on disease severity. Waterhyacinth was susceptible to the fungus at all growth stages tested, but older plants were more susceptible than younger ones. The toxin produced by A. alternata (Fr.) Keissler is known to play an important role in the pathogenesis of the blight disease of waterhyacinth. An experimental investigation was carried out to study the phytotoxicity tests of the metabolite solution or crude toxin of A. alternata on other aquatic weeds. Phytotoxic symptoms were produced on most of the plants tested, demonstrating that phytotoxin produced by the fungus is not host specific. It was concluded that A. alternata has potential as a biocontrol agent of waterhyacinth, especially when applied in an oil emulsion. The oil emulsion maintains the aggressiveness of the pathogen during a dew-free period and provides a favourable microenvironment during the infection process. The toxin may be used as a broad-spectrum herbicide to control many floating aquatic weeds.

In vitro Reaction of Sunflower (Helianthus annuus L.) to the Toxin(s) Produced by Alternaria alternata, the Casual Agent of Brown Leaf Spot

AbstractA bioassay system was developed for studying the in vitro reaction of sunflower (Helianthus annuus L. cv. ‘Nanus’) against the toxin produced by the virulent pathotype IMI 366417 (1) of the pathogenic fungus Alternaria alternata. Cotyledons from 2‐week‐old seedlings were cultured on a MS (Murashige and Skoog) medium supplemented with 0.3 μM NAA (α‐napthylacetic acid) and 1.3 μM BA (6‐benzyladenine). Exponentially growing calli were transferred to selective media containing toxin solutions at various concentrations. The fresh weight of the cultured calli was reduced as the toxin concentration increased, although the viability of the cells, expressed as callus dehydrogenase activity, increased. Selection for toxinresistant genotypes was attempted at 30% toxin concentration, which causes a 90% reduction in callus growth. After one month in culture, 18% of the calli demonstrated resistance to the toxin. However, no plants could be regenerated from those calli after transfer onto a MS medium supplemented with 5.4 μM NAA and 4.4 μM BA. The effect of the toxin purification method on toxin yield and biological activity, as well as its possible mode of cellular action are discussed. The results of these experiments may contribute to a better understanding of the disease mechanism and help establish an efficient selection method of resistant sunflower genotypes.

Mutations at the Asc locus of tomato confer resistance to the fungal pathogen Alternaria alternata f. sp. lycopersici

The fungal pathogen Alternaria alternata f. sp. lycopersici produces host-selective AAL-toxins that cause Alternaria stem canker in tomato. Susceptibility to the disease is based on the relative sensitivity of the host to the AAL-toxins and is controlled by the Asc locus on chromosome 3L. Chemical mutagenesis was employed to study the genetic basis of sensitivity to AAL-toxins and susceptibility to fungal infection. Following the treatment of seeds of a susceptible line with ethyl methanesulphonate (EMS), resistant M2 mutants were obtained. Most plants with induced resistances showed toxin-sensitivity responses that were comparable to those of resistant control lines carrying the Asc locus. In addition, genetic analysis of the mutagenised plants indicated that the mutations occurred at the Asc locus. Furthermore, novel mutants were identified that were insensitive to the AAL-toxins at the seedling stage but toxin-sensitive and susceptible to fungal infection at mature stages. No AAL-toxin-insensitive insertion mutants were identified following a transposon mutagenesis procedure. Molecular mechanisms involved in host defence against A a. lycopersici are discussed.

Inheritance and genetic mapping of resistance to Alternaria alternata f. sp. lycopersici in Lycopersicon pennellii.

The fungal pathogen Alternaria alternata f. sp. lycopersici produces AAL-toxins that function as chemical determinants of the Alternaria stem canker disease in the tomato (Lycopersicon esculentum). In resistant cultivars, the disease is controlled by the Asc locus on chromosome 3. Our aim was to characterize novel sources of resistance to the fungus and of insensitivity to the host-selective AAL-toxins. To that end, the degree of sensitivity of wild tomato species to AAL-toxins was analyzed. Of all members of the genus Lycopersicon, only L. cheesmanii was revealed to be sensitive to AAL-toxins and susceptible to fungal infection. Besides moderately insensitive responses from some species, L. pennellii and L. peruvianum were shown to be highly insensitive to AAL-toxins as well as resistant to the pathogen. Genetic analyses showed that high insensitivity to AAL-toxins from L. pennellii is inherited in tomato as a single complete dominant locus. This is in contrast to the incomplete dominance of insensitivity to AAL-toxins of L. esculentum. Subsequent classical genetics, RFLP mapping and allelic testing indicated that high insensitivity to AAL-toxins from L. pennellii is conferred by a new allele of the Asc locus.

Tentoxin and Valinomycin Affect Electrophysiological Properties and K +( 86Rb +)fluxes in the Liverwort Riccia fluitans

Summary Tentoxin, a cyclic tetrapeptide from the pathogenic fungus Alternaria alternata, and the potassium-ionophor depsipeptide valinomycin were compared in their action on the plasma-lemma of the liverwort Riccia fluitans by membrane potential, current-voltage, and K+(86Rb+) efflux measurements. Whereas 10 µM tentoxin significantly, and sometimes transiently, depolarized the plasmalemma, 0.4 µM valinomycin consistently hyperpolarized it. Neither peptide caused significant changes of the membrane conductance; both peptides, however, stimulated the K+(86Rb+) efflux from preloaded thalli. It is concluded that both peptides interact with the plasmalemma through different mechanisms.