Role of secondary metabolites in the biocontrol activity of <Emphasis Type="Italic">Pseudomonas corrugata</Emphasis> and <Emphasis Type="Italic">Pseudomonas mediterranea</Emphasis>

Colletotrichum gloeosporioides is the causal agent of Camellia oleifera anthracnose, mainly infecting fruits and leaves. The fungus secretes degrading enzymes to destroy the cuticle of aerial plant parts and help infect the host successfully. To validate whether a cutinase gene (CglCUT1) was required for cutinase activity and pathogenicity of C. gloeosporioides, the CglCUT1 gene was cloned and analyzed. The characterization of CglCUT1 predicted protein suggests that the cloned DNA encoded a cutinase in C. gloeosporioides affecting C. oleifera. The CglCUT1 showed a high homology to those from C. gloeosporioides causing papaya anthracnose and C. capsici causing pepper anthracnose, as well as those of other ascomycetes. The whole CglCUT1 gene was knocked-out and the knockout mutant (?CglCUT39) was subsequently complemented using Agrobacterium tumefaciens mediated transformation. The knockout transformants exhibited significant decreases in cutinase activity and virulence compared with the wild-type strain. The complemented transformants of the disrupted transformant ?CglCUT39 showed a significant increase in cutinase activity and virulence compared with the disrupted transformant ?CglCUT39. This study suggests that the CglCUT1 gene has a positive effect on fungal virulence of the hemibiotrophic C. gloeosporioides on C. oleifera.     

Population features of <Emphasis Type="Italic">Xanthomonas arboricola</Emphasis> pv. <Emphasis Type="Italic">pruni</Emphasis> from <Emphasis Type="Italic">Prunus spp.</Emphasis> orchards in northern Italy

Bacterial leaf/fruit spot and canker of stone fruits, caused by Xanthomonas arboricola pv. pruni, is a recurrent disease in Italy. A set of 23 strains has been isolated in peach and plum orchards in an intensively stone fruit cultivated area located in north-eastern Italy. They were all identified as X. arboricola pv. pruni by means of phytopathological and serological features: hypersensitive reaction on bean pods, pathogenicity test on immature peach or plum fruitlets, identification by immunofluorescence assay and conventional PCR. Phylogenetic analysis based on sequencing of the gyrB housekeeping gene of the isolates showed that they formed a unique clade, well characterised and separated from other xanthomonads. An insight into the genetic population features was attempted by rep-PCR analysis, using the ERIC, REP and BOX primers. The combined rep-PCR fingerprints showed a slight intra-pathovar variation within our isolates, which grouped in five close clusters. Copper resistance has been assessed in vitro for our whole X. arboricola pv. pruni collection, highlighting that two isolates show a level of resistance in vitro up to 200 ppm of copper. Nonetheless, the copLAB gene cluster, present in many other species of Xanthomonads, was not detected in any isolate, confirming the presence of a still unknown mechanism of copper detoxification in our Xanthomonads arboricola pv. pruni tolerant/resistant strains.     

Comparative colonisation by virulent versus avirulent <Emphasis Type="Italic">Pyricularia oryzae</Emphasis> on wild <Emphasis Type="Italic">Oryza australiensis</Emphasis>

Pyricularia oryzae (rice blast) conidial development at pre-penetration stage determines success or otherwise of infection inside the rice host plants. Studies on conidial germination and growth on the leaf surface in commercial rice (Oryza sativa) report differently, dependent upon host type and level of blast resistance. Although wild rice (O. australiensis) is known to be an alternative host of blast, the interaction between P. oryzae conidia and wild O. australiensis on its leaf surface has not been previously studied. We found significant (P?<?0.001) differences in conidial development between two blast isolates with different virulence in terms of conidial germination, germ tube growth and appressoria formation on both wild and cultivated rice. Conidial germination at 6 h post-inoculation (hpi) for the virulent isolate was significantly (P?<?0.001) delayed. Germ tubes of the avirulent isolate conidia grew significantly (P?<?0.001) faster and with significantly (P?<?0.001) longer germ tubes than from virulent conidia. Appressoria development for the virulent isolate was significantly (P?<?0.001) faster at its later growth stages of 12 and 18 hpi when approximately 100% of germ tubes formed appressoria. In contrast, formation rate of appressoria for the avirulent isolate was significantly (P?<?0.001) slower and only reached 76% of germ tubes forming appressoria. Appressoria formation on O. australiensis was significantly (P?<?0.001) greater than the formation on O. sativa for both virulent and avirulent P. oryzae at 12 hpi, a clear indication that host type influences the extent of appressoria formation.     

Infection process of <Emphasis Type="Italic">Pseudocercospora musae</Emphasis> on banana leaf

Yellow Sigatoka that is caused by Pseudocercospora musae is an important banana disease. The aim of this study was to elucidate the infection process of P. musae in banana leaves by scanning electron microscopy. Leaf samples were inoculated on the abaxial surface with P. musae and then analysed at 12, 24, 36, 48, 72, 96, 120, 144, and 168 h post inoculation (hpi) and at 36 and 50 days post inoculation (dpi). The conidia were found to be germinated between 24 and 36 hpi and penetrated through the stomata between 96 and 120 hpi, or more generally from 144 hpi. P. musae colonized the spongy parenchyma at 36 dpi and the palisade parenchyma at 50 dpi. Sporulation occurred at 50 dpi on the adaxial surface of leaves through the emergence of conidia on conidiophores through the stomata. Considering the importance of yellow Sigatoka in banana production, our results provide a better understanding of the life cycle of the fungus for treatment processes.     

Genetic structure of <Emphasis Type="Italic">Pyrenophora teres</Emphasis> f. <Emphasis Type="Italic">teres</Emphasis> and <Emphasis Type="Italic">P. teres</Emphasis> f. <Emphasis Type="Italic">maculata</Emphasis> populations from western Canada

Infection by Pyrenophora teres f. teres (Ptt) or P. teres f. maculata (Ptm), the causal agents of the net and spot forms of net blotch of barley, respectively, can result in significant yield losses. The genetic structure of a collection of 128 Ptt and 92 Ptm isolates from the western Canadian provinces of Alberta (55 Ptt, 27 Ptm), Saskatchewan (58 Ptt, 46 Ptm) and Manitoba (15 Ptt, 19 Ptm) were analyzed by simple sequence repeat (SSR) marker analysis. Thirteen SSR loci were examined and found to be polymorphic within both Ptt and Ptm populations. In total, 110 distinct alleles were identified, with 19 of these shared between Ptt and Ptm, 75 specific to Ptt, and 16 specific to Ptm. Genotypic diversity was relatively high, with a clonal fraction of approximately 10 % within Ptt and Ptm populations. Significant genetic differentiation (PhiPT = 0.230, P = 0.001) was found among all populations; 77 % of genetic variation occurred within populations and 23 % between populations. Lower, but still significant genetic differentiation (PhiPT = 0.038, P = 0.001) was detected in Ptt, with 96 % of genetic variation occurring within populations. No significant genetic differentiation (PhiPT = 0.010, P = 0.177) was observed among Ptm populations. Isolates clustered in two distinct groups conforming to Ptt or Ptm, with no intermediate cluster. The high number of haplotypes observed, combined with an equal mating type ratio for both forms of the fungus, suggests that P. teres goes through regular cycles of sexual recombination in western Canada.     

Infection of <Emphasis Type="Italic">Sorghum bicolor</Emphasis>, selected grass species and <Emphasis Type="Italic">Zea mays</Emphasis> by G<Emphasis Type="Italic">loeocercospora sorghi</Emphasis>, causal pathogen of zonate leaf spot

Zonate leaf spot (Gloeocercospora sorghi) is a common disease in Sorghum bicolor producing areas of the U.S., but little is known about its biology, virulence and severity on S. bicolor, Zea mays, and related crop grassweeds. Greenhouse studies were conducted to determine and compare the virulence and severity of G. sorghi on 10 commercially available sorghum hybrids, four Z. mays hybrids and selected grassweed species including Sorghum bicolor (grain sorghum and shattercane biotypes) and Sorghum halepense (Johnsongrass), two of the most problematic arable weeds. Plants from the respective species were inoculated with a local G. sorghi isolate and maintained in a dew-chamber at 24 °C for 24 h and then incubated under greenhouse conditions for 4 weeks. Plants were observed for lesion expression and rated using a modified Horsfall-Barrett scale (0–10). The first symptoms of infection were visible within 24 h following inoculation on shattercane and S. bicolor hybrids. Symptoms consisted of small, non-diagnostic purple lesions on the leaves. Results showed that S. bicolor, S. halepense and shattercane were susceptible to G. sorghi. All other species tested in this study were not infected. More particularly, disease severity, increased from a rating of 3 to 10 on sorghum and from 2 to 7 on S. halepense between 2 and 23 days after inoculation, respectively. However, disease severity on shattercane increased rapidly from 3.5 to 10 between 2 and 8 days after inoculation, respectively. Among the sorghum hybrids tested, FFR-322 appeared to be the most resistant to G. sorghi while Pioneer 83G66 appeared to be the most susceptible. Z. mays hybrids were not infected by the fungus used in this study. G. sorghi could be used effectively to manage shattercane and S. halepense infestations occurring in Z. mays and S. bicolor fields consisting of specific G. sorghi-resistant hybrids.     

The chemotaxis regulator <Emphasis Type="Italic">pilG</Emphasis> of <Emphasis Type="Italic">Xylella fastidiosa</Emphasis> is required for virulence in <Emphasis Type="Italic">Vitis vinifera</Emphasis> grapevines

Type IV pili of X. fastidiosa are regulated by pilG, a response regulator protein putatively involved in chemotaxis-like operon sensing stimuli through signal transduction pathways. To elucidate the roles of pilG in pathogenicity of X. fastidiosa, the pilG-deletion mutant XfΔpilG and complemented strain XfΔpilG-C were generated. While all strains had similar growth curves in vitro, XfΔpliG showed significant reduction in cell-matrix adherence and biofilm production compared with wild-type X. fastidiosa and XfΔpilG-C. The genes pilE, pilU, pilT, and pilS were down-regulated in XfΔpliG when compared with its complemented strain and wild-type X. fastidiosa. Finally, no Pierce’s disease symptoms were observed in grapevines inoculated with XfΔpilG, whereas grapevines inoculated with the wild-type X. fastidiosa and complemented strain of XfΔpilG-C developed typical Pierce’s Disease (PD) symptoms. The results indicate that pilG has a role in X. fastidiosa virulence in grapevines.     

Competence of <Emphasis Type="Italic">Frankliniella occidentalis</Emphasis> and <Emphasis Type="Italic">Frankliniella intonsa</Emphasis> strains as vectors for <Emphasis Type="Italic">Chrysanthemum stem necrosis virus</Emphasis>

The vector competence of Frankliniella occidentalis for Chrysanthemum stem necrosis virus (CSNV) was evaluated. Three vector strains with distinct competences for Tomato spotted wilt virus (TSWV) transmission were investigated, including an artificially selected strain (TsH) that has a particularly high competence (>90 %). Newly hatched larvae of F. occidentalis were given an acquisition access period of 5 days on CSNV-infected D. stramonium leaves, and reared to maturity. Their transmission efficiencies were examined using a leaf disk assay using Petunia x hybrida leaves. Following the leaf disk assay, the virus accumulation in the vectors was examined via a double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) of their bodies. The results showed that the CSNV acquisition and transmission efficiency of the TsH strain did not differ from those of the others, indicating that the competence of F. occidentalis as a vector for CSNV is not related to that for TSWV. The CSNV transmission and acquisition efficiencies of two F. intonsa strains (Hiroshima and Fukuoka) were also evaluated. In Hiroshima strain, 35 % of adults were viruliferous, but only two transmitters (3 %) were observed. In Fukuoka strain, 6 % were viruliferous, and no transmitters were observed. These results indicate that F. intonsa cannot be a major vector for CSNV. The accumulation of CSNV in the adults of F. occidentalis and F. intonsa evaluated using DAS-ELISA showed a significant difference in ELISA values among transmitter, viruliferous non-transmitter, and non-viruliferous individuals. These results clearly demonstrated that only transmitters that accumulated a threshold quantity of virus can transmit CSNV to plants.     

<Emphasis Type="Italic">Fusarium ershadii</Emphasis> sp. nov., a Pathogen on <Emphasis Type="Italic">Asparagus officinalis</Emphasis> and <Emphasis Type="Italic">Musa acuminata</Emphasis>

Two Fusarium strains, isolated from Asparagus in Italy and Musa in Vietnam respectively, proved to be members of an undescribed clade within the Fusarium solani species complex based on phylogenetic species recognition on ITS, partial RPB2 and EF-1α gene fragments. Macro- and micro-morphological investigations followed with physiological studies done on this new species: Fusarium ershadii sp. nov can be distinguished by its conidial morphology. Both isolates of Fusarium ershadii were shown to be pathogenic to the monocot Asparagus officinalis when inoculated on roots and induced hollow root symptoms within two weeks in Asparagus officinalis seedlings. In comparison mild disease symptoms were observed by the same strains on Musa acuminata seedlings.     

Parasitism of <Emphasis Type="Italic">Psytallia concolor</Emphasis> (Hymenoptera: Braconidae) on <Emphasis Type="Italic">Bactrocera oleae</Emphasis> (Diptera: Tephritidae) infesting different olive varieties

Psytallia concolor (Szépligeti) is a koinobiont endoparasitoid of many Tephritidae larvae, including Bactrocera oleae (Rossi), and has been used in Mediterranean areas for biological control of olive fruit fly by inundative release. The present study evaluates the influence of olive fruit variety (Amfissis, Arbequina, Branquita de Elvas, Carolea, Kalamon, Koroneiki, Leccino, Manzanilla, Mastoidis, Moroccan Picholine and Picholine) on P. concolor parasitism efficiency and performance in the field during two successive years. The results showed that the percentage of parasitism was significantly higher (>30%) in Mastoidis and Koroneiki (light-weight varieties <1.5 g) than Leccino which has a medium fruit weight, followed by Amfissis, Moroccan Picholine, Picholine and Branquita de Elvas. Only Manzanilla among large weight varieties, exhibited high percentage of parasitization (42.72%) during 2013. The mean weight of the pupae (>4.21 mg) as well as the length of the developed adult parasitoids (>3.5 cm) in Mastoidis and Manzanilla were significantly higher than these individuals developed from other varieties such as Koroneiki and Kalamon. Finally, the optimal host fruit for P. concolor development seems to be Mastoidis variety with great biological parameters and percentage of parasitism.     

Host status of selected cultivated fruit crops to <Emphasis Type="Italic">Meloidogyne enterolobii</Emphasis>

Meloidogyne enterolobii (syn. M. mayaguensis) has been reported to cause severe damage in commercial guava orchards and other plants in Central and South American countries. Considering the risk of introduction and dissemination of this pest in the European region, M. enterolobii was placed on the EPPO A2 list in 2010. The use of non-host fruit species is a recommended strategy to manage root-knot nematodes in infested guava orchards. This study screened 89 plant genotypes from 25 fruit plants of economic importance, plus two susceptible controls (guava and tomato) for its host status to M. enterolobii. Three to eight months after inoculation, nematode reproduction factor (RF) was used to characterize host suitability of fruit crops to this nematode. Ten banana genotypes, six Barbados cherries, one fig, two grape rootstocks and six melons were rated as good hosts for this nematode. Sixteen fruit plants behaved either as non-hosts or poor hosts to M. enterolobii, including assaí, atemoya, avocado, cashew nut, citrus, coconut, grape, jabuticaba, mango, mulberry, papaya, passion fruit, sapodilla, soursop, starfruit and strawberry. For the future, field experiments in areas infested by this nematode are essential to confirm the greenhouse results. These non-host fruit species can replace in the future eradicated guava trees in fields severely infested by this nematode and become an economic option for growers where M. enterolobii is considered a serious problem.     

Diversity of <Emphasis Type="Italic">Colletotrichum</Emphasis> species in coffee (<Emphasis Type="Italic">Coffea arabica</Emphasis>) plantations in Mexico

The aim of this study was to identify the Colletotrichum species associated with anthracnose symptoms in coffee (Coffea arabica L.) plantations in northern Puebla, Mexico. In 2013, five surveys were conducted in different production areas and at different altitudes. Symptomatic leaves, shoots, and ripe and unripe fruits of the coffee variety Red Caturra were collected. Isolates were obtained and the Colletotrichum species were identified morphologically and characterized by multilocus sequence analyses of the ACT, CAL, GAPDH, and TUB2 genes and the rDNA region. Additionally, pathogenicity tests were conducted using six isolates. We identified C. gigasporum, C. gloeosporioides, C. karstii (two isolates), C. siamense, and C. theobromicola. This is the first report of these five species infecting leaves of coffee. The symptoms caused by these species were characterized, but the species causing Coffee Berry Disease was not found. This is the first report of a complex of species affecting coffee plants in the same geographical area in Mexico, and suggests that other complexes of species may be important pathogens in coffee-producing areas elsewhere.     

Natural enemies of <Emphasis Type="Italic">Diaspis echinocacti</Emphasis> in Greece and first records of <Emphasis Type="Italic">Aphytis debachi</Emphasis> and <Emphasis Type="Italic">Plagiomerus diaspidis</Emphasis>

Two hymenopteran parasitoids of the cactus scale Diaspis echinocacti (Bouché) (Hemiptera: Diaspididae) on Opuntia ficus-indica (L.) Mill. (Cactaceae) are recorded in Greece. Aphytis debachi Azim, 1963 (Aphelinidae) is first recorded for Europe and Plagiomerus diaspidis Crawford, 1910 (Encyrtidae) is first recorded for Greece. Preliminary data on phenology and natural enemies of the scale D. echinocacti on O. ficus-indica are presented. Parasitism of D. echinocacti by P. diaspidis reached 86% in southern Greece (Kalamata) and parasitism by A. debachi reached 9.3% and 12% in Kalamata and Athens, respectively. Two predators, Cybocephalus fodori Endrödy-Youga (Coleoptera: Nitidulidae) and a mite species (Prostigmata: Bdellidae), were found to be associated with D. echinocacti.     

Diversity of the cultivable gut bacterial communities associated with the fruit flies <Emphasis Type="Italic">Bactrocera dorsalis</Emphasis> and <Emphasis Type="Italic">Bactrocera cucurbitae</Emphasis> (Diptera: Tephritidae)

Gut microbes play an important role in insect morphogenesis, nutrition, development of resistance against parasitoids and detoxification of toxic compounds. A culture-based approach is therefore an useful tool for the characterization of cultivable microbial communities associated with the insect gut. In the present study an attempt was made to decipher the gender specificity of gut bacterial communities of two major fruit fly species of India viz., Bactrocera dorsalis (Hendel) and Bactrocera cucurbitae (Conquillett) (Diptera: Tephritidae). Based on molecular identification, B. dorsalis females were found to predominantly harbor the bacterial species Enterobacter cloacae, Enterobacter asburiae and Citrobacter freundii, while B. dorsalis males were found to harbor Providencia rettgerii, Klebsiella oxytoca, Enterococcus faecalis and Pseudomonas aeruginosa The cultivable diversity from females of B. cucurbitae comprised mainly of Morganella morganii and Bacillus pumilis while B.cucurbitae males were predominantly colonized by aerobic endospore formers viz., Bacillus cereus, B. licheniformis and B. subtilis. The above findings have thrown light on a distinct pattern of gender specific gut bacterial colonization in fruit flies, which have to be factored in for the formulation of fruit fly management strategies.     

Temperature and plant age drive downy mildew disease epidemics on oilseed <Emphasis Type="Italic">Brassica napus</Emphasis> and <Emphasis Type="Italic">B. juncea</Emphasis>

Studies were undertaken on the effects of temperature (14/10 °C and 22/17 °C day/night) and plant age (15, 23, 31 and 40 day-old-plants) on the severity of downy mildew (Hyaloperonospora parasitica) on oilseed Brassica cultivars (temperature: Brassica juncea Montara, B. napus Atomic, ATR-Hyden, Hyola 432, Hyola 450 TT, Thunder TT; plant age: B. juncea Dune, B. napus Surpass 402 and Hyola 450 TT). For temperature studies, there were significant (P?<?0.001) effects of temperature, cultivar, and cultivar x temperature interaction. On cotyledons of susceptible cultivars (B. napus Hyola 450 TT and Thunder TT), plants were symptomatic at 22/17 °C by 48 h post inoculation (hpi) and with abundant sporulation evident by 72 hpi, and with all cotyledons of B. napus Thunder TT collapsed by 7 days post inoculation (dpi). However, at 14/10 °C, there were no symptoms on the same cultivars until 5 dpi, and sporulation only observed at 7 dpi. Percent disease index values (DI%) at 22/17 °C of B. juncea Montara and B. napus ATR-Hyden, Hyola 432, Atomic, Hyola 450 TT and Thunder TT were 4.5, 49.0, 51.4, 65.8, 86.3 and 96.0, respectively, with all except B. juncea Montara having significantly lower (P?<?0.001) disease at 14/10 °C with DI% values of 2.8, 30.4, 27.9, 31.1, 44.4 and 76.4, respectively. For plant age studies, there were significant (P?<?0.001) effects of plant age, cultivar, and cultivar x plant age interaction. DI% was significantly higher at 15 compared to 40 day-old-plants (dop) across all cultivars. B. juncea Dune showed greatest resistance, particularly on 40 dop, with DI% values of 25.8, 24.6, 22.9 and 7.5, for 15, 23, 31 and 40 dop, respectively. B. napus Surpass 402 showed high susceptibility on cotyledons of 15 dop but moderate resistance on leaves of other ages, with DI% values of 59.0, 31.2, 27.1 and 26.2 for 15, 23, 31 and 40 dop, respectively. B. napus Hyola 450 TT showed very high susceptibility at the cotyledon stage on 15 dop, but some resistance on 23 dop and more so on 31 and 40 dop, with DI% values of 84.0, 41.2, 35.4 and 32.9 for 15, 23, 31 and 40 dop, respectively. Together, these findings explain for the first time why development of downy mildew epidemics on susceptible cultivars occurs early in the growing season when warmer seasonal temperatures in autumn coincide with presence of seedlings; in contrast to later in the growing season on less susceptible older plants coinciding with cooler and less favourable winter temperatures. Increasing maximum and minimum temperatures associated with climate change have likely fostered the increased severity of downy mildew over the past 15 years.     

Identification and partial characterization of a <Emphasis Type="Italic">Carica papaya</Emphasis>-infecting isolate of <Emphasis Type="Italic">Alfalfa mosaic virus</Emphasis> in Brazil

A Carica papaya plant with severe yellow leaf mosaic, leaf distortion, and systemic necrosis was found in the municipality of Piracicaba, state of São Paulo, Brazil. Transmission electron microscopy (TEM) analysis revealed the presence of potyvirus-like particles and bacilliform particles similar to those of the Alfamovirus genus. The potyvirus was identified as Papaya ringspot virus-type P (PRSV-P). Biological, serological, and molecular studies confirmed the bacilliform virus as an isolate of Alfalfa mosaic virus (AMV). Partial nucleotide and amino acid sequences of the coat protein gene of this AMV isolate shared 97–98% identity with the AMV isolates in the GenBank database. This report is the first of the natural infection of papaya plants by AMV.     

<Emphasis Type="Italic">RB</Emphasis> and <Emphasis Type="Italic">Ph</Emphasis> resistance genes in potato and tomato minimize risk for oospore production in the presence of mating pairs of <Emphasis Type="Italic">Phytophthora infestans</Emphasis>

Composed mostly of fungivorous species, the genus Aphelenchoides also comprises 14 plant-parasitic species. The most common and devastating, A. besseyi, A. fragariae, A. ritzemabosi and A. subtenuis have been reported on more than 900 plant species. The combination of low inter-specific and high intra-specific morphological variability makes morphology-based identification extremely difficult within this genus, and has led to molecular tools being employed to ensure accurate diagnoses. rDNA markers are widely used for the identification of nematodes while the Cytochrome Oxidase I gene (COI) remains relatively unexplored despite its role as the standard barcode for almost all animal groups. To explore its suitability as a diagnostic tool, we studied a fragment of the mtCOI region of the four main plant-parasitic Aphelenchoides within a phylogenetic framework. We generated 69 mtCOI and 123 rDNA sequences of diverse Aphelenchoides taxa; 67 belong to the main plant-parasitic species including the first mtCOI sequence of A. fragariae and the first mtCOI and 28S sequences of A. subtenuis. mtCOI had a similar success rate for PCR amplification. Phylogenetic trees based on the three studied markers are largely in agreement with one another, validating their use for Aphelenchoides diagnosis; additionally, we were able to locate several misidentified sequences of plant-parasitic Aphelenchoides in existing databases. The concatenated analysis from the three markers resulted in a more robust insight into the phylogeny and evolution of Aphelenchoides, revealing that plant-parasitism has evolved independently at least three times within this genus, presumably from fungal-feeding ancestors.     

Overexpression of <Emphasis Type="Italic">SlARG2</Emphasis> or <Emphasis Type="Italic">SlTD2</Emphasis> in Arabidopsis enhances resistance against <Emphasis Type="Italic">Plutella xylostella</Emphasis> L.

Tomato (Solanum lycopersicum L.) ARGINASE2 (ARG2) and THREONINE DEAMINASE2 (TD2) are involved in plant defense. These enzymes act in the midgut of herbivores fed on tomato plants to degrade the essential amino acids Arg and Thr, respectively. Although it has been demonstrated that overexpression of the SlARG2 gene in tomato enhanced its resistance against M. sexta larvae, knock-down the expression of SlTD2 reduced the resistance of tomato to lepidopteran herbivores; it remains unclear whether overexpression of SlTD2 could enhance the resistance of the host plants to herbivores, or whether combined overexpression of SlARG2 and SlTD2 could lead to synergistically enhanced resistance to insects. Here, we generated transgenic Arabidopsis plants overexpressing SlARG2 (SlARG2 OE) and SlTD2 (SlTD2 OE) individually as well as in combination (SlARG2-SlTD2 OE). Overexpression of these genes did not affect Arabidopsis development, seed yield, or Arg and Thr content. Insect-feeding bioassay was performed by feeding diamondback moth (Plutella xylostella L.) larvae on detached leaves of wild-type, SlARG2 OE, SlTD2 OE, and SlARG2-SlTD2 OE plants. Larvae fed on SlARG2 OE leaves showed approximately 31% to 35% reduction in weight and 6% to 10% reduction in survival rate compared to those fed on wild-type leaves. Although larvae fed on SlTD2 OE leaves showed no reduction in survival rate, they gained less weight. Whereas larvae fed on SlARG2-SlTD2 OE leaves showed neither reduction in weight nor reduction in survival rate. We further investigated the arginase enzymatic activity of the SlARG2 OE and SlARG2-SlTD2 OE transgenic plants. The SlARG2 OE line most resistant to diamondback moth larvae displayed the highest arginase activity. Our data indicate that overexpression of SlARG2 or SlTD2 in Arabidopsis can enhance its resistance against diamondback moth, whereas combined overexpression of SlARG2 and SlTD2 did not generate synergistically increased resistance to diamondback moth.     

Nonhost resistance of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> against <Emphasis Type="Italic">Colletotrichum</Emphasis> species

Arabidopsis thaliana exhibits a durable resistance called nonhost resistance against nonadapted fungal pathogens. A. thaliana activates preinvasive resistance and terminates entry attempts by nonadapted fungi belonging to the genus Colletotrichum, which cause anthracnose disease in many plants. In the interaction between A. thaliana and nonadapted C. tropicale, the preinvasive resistance involves the PENETRATION 2-related antifungal secondary metabolite pathway and the ENHANCED DISEASE RESISTANCE 1-dependent antifungal peptide pathway. The development of invasive hyphae by C. tropicale owing to the reduction of preinvasive resistance then triggers the blockage of further hyphal expansion via the activation of the second layer of resistance, i.e., postinvasive resistance, which guarantees the robustness of the nonhost resistance of A. thaliana against Colletotrichum pathogens. Both the tryptophan-derived metabolic pathway and glutathione synthesis play critical roles in the postinvasive resistance against C. tropicale, although the molecular mechanism of postinvasive resistance remains to be elucidated. In this review, we describe the current understanding of the molecular background of the Arabidopsis nonhost resistance against Colletotrichum fungi and discuss perspectives for future research on this durable resistance.