Mara Quaglia

Role of pathogen-induced volatiles in the Nicotiana tabacum–Golovinomyces cichoracearum interaction

Plant injuries activate signal transduction cascades mediated by the plant hormones, which lead to enhanced expression of defence related genes and/or to changes in the emission of volatile organic compounds that can act as semiochemicals. In this research we demostrated that infection with the biotrophic pathogen Golovinomyces cichoracearum (DC.) V.P. Heluta (ex Erysiphe cichoracearum DC.), the causal agent of powdery mildew, led in the susceptible host Nicotiana tabacum L. cv Havana 425 to an increased emission of volatile compounds including Methyl-jasmonate (MeJA), (E)-2-hexenal and (E)-β-ocimene. Furthermore we investigated the role of these volatiles in the plant-pathogen interaction. Exogenous application of MeJA induced in tobacco an increase in the transcripts level of the defence related genes lipoxygenase, allene oxide cyclase and defensin and a decrease in the severity of the infection. Qualitative and quantitative differences in volatile compounds emission were showed also in MeJA-treated plants, where the emission of (E)-β-ocimene was significantly increased instead (E)-2-hexenal was not detected. Application of (E)-2-hexenal reduced the severity of powdery mildew while application of (E)-β-ocimene did not. Since (E)-2-hexenal did not activate in tobacco the accumulation of the above reported genes transcripts and the plant cell death, the reduction of the infection severity could be attributable to its inhibitory activity on the fungal germ tube growth. Our data highlight the contributions of natural substances that can act, directly or indirectly, against phytopathogens. In the global context of sustainability, food safety and environmental protection, such semiochemicals represent an alternative and promising approach to integrated pest management.

Arabidopsis flower specific defense gene expression patterns affect resistance to pathogens

We investigated whether the Arabidopsis flower evolved protective measures to increase reproductive success. Firstly, analyses of available transcriptome data show that the most highly expressed transcripts in the closed sepal (stage 12) are enriched in genes with roles in responses to chemical stimuli and cellular metabolic processes. At stage 15, there is enrichment in transcripts with a role in responses to biotic stimuli. Comparative analyses between the sepal and petal in the open flower mark an over-representation of transcripts with a role in responses to stress and catalytic activity. Secondly, the content of the biotic defense-associated phytohormone salicylic acid (SA) in sepals and petals is significantly higher than in leaves. To understand whether the high levels of stress responsive transcripts and the higher SA content affect defense, wild-type plants (Col-0) and transgenic plants defective in SA accumulation (nahG) were challenged with the biotrophic fungus Golovinomyces cichoracearum, the causal agent of powdery mildew, and the necrotrophic fungus Botrytis cinerea. NahG leaves were more sensitive than those of Col-0, suggesting that in leaves SA has a role in the defense against biotrophs. In contrast, sepals and petals of both genotypes were resistant to G. cichoracearum, indicating that in the flower, resistance to the biotrophic pathogen is not critically dependent on SA, but likely dependent on the up-regulation of stress-responsive genes. Since sepals and petals of both genotypes are equally susceptible to B. cinerea, we conclude that neither stress-response genes nor increased SA accumulation offers protection against the necrotrophic pathogen. These results are interpreted in the light of the distinctive role of the flower and we propose that in the early stages, the sepal may act as a chemical defense barrier of the developing reproductive structures against biotrophic pathogens.

Molecular characterization of Diplodia seriata, a new pathogen of Prunus laurocerasus in Italy.

Twig cankers and diebacks were observed on cherry laurel (Prunus laurocerasus L.) hedges located in a private garden of Perugia (Central Italy). Four fungal isolates, forming gray colonies and aerial mycelium, were obtained from cankers. For each isolate, Koch’s postulates were fulfilled by inoculating cherry laurel plants. Based on cultural characteristics, rep-PCR and phylogenetic analyses on ITS and EF1-α nucleotides sequences, the isolates were identified as Diplodia seriata De Not. To the best of our knowledge, this is the first report of D. seriata as a new pathogen of P. laurocerasus. The severe symptoms and the wide distribution of the host plant make the pathogen potentially dangerous for this ornamental species.

First report of Clonostachys rhizophaga as a pathogen of Dendrocalamus giganteus in Mozambique.

Dendrocalamus giganteus Wall. ex Munro, a wild plant belonging to the family Poaceae, is widespread in Mozambique where it is used as a construction material. At the end of 2007, disease symptoms have been observed on D. giganteus plants growing in the neighborhood of Maputo. Diseased plants showed longitudinal dark streaks on the stem surface to which corresponded internal vascular browning and chlorosis in wide leaves that gradually developed into necrosis. At the final stage of the disease plants died. To isolate the pathogen, stem segments collected during September 2008 were surface sterilized with 1% HgCl2 for 30 s, rinsed with sterile deionized water for 30 s, and incubated on potato dextrose agar (PDA) medium at 22°C in the dark for 2 weeks. Monosporic cultures of the isolated fungus formed dimorphic Verticillium-like (primary) or penicillate (secondary) conidiophores and ovoidal to elongate, minutely curved, hyaline conidia, 5 to 9.5 × 2.5 to 4.5 μm, with laterally displaced hilum. These characteristics are typical of Clonostachys rhizophaga Schroers (3). Identification was confirmed by the Centraalbureau voor Schimmelcultures (Utrecht, the Netherlands) on the basis of the β-tubulin (tub2) gene sequence (3). For our isolate CBS 125416, the tub2 sequence was 100% similar to that of the C. rizhophaga strain CBS 124511 (GenBank Accession No. FJ 593883) (1). To verify the pathogenicity of the fungus, a 5-mm-diameter mycelial plug obtained from 2-week-old colonies grown on PDA was affixed to a portion of the stem of D. giganteus from which the superficial tissues had been removed and the inoculation site was covered with wet cotton and wrapped with Parafilm. Control plants were treated by the same method but using PDA plugs without mycelium. Twenty plants were used, ten of which were controls. They were grown in a controlled climatic chamber at 22°C with a photoperiod of 16 h at 40 μE·m-2·s-1. Two months after inoculation, all plants showed a dark area surrounded by an idropic halo on the stem surface and internal browning, whereas control plants remained healthy. C. rhizophaga was recovered from all infected plants. C. rhizophaga is apparently rare. The fungus (as Verticillium rhizophagum Tehon & Jacobs, nom. invalid.) has been previously reported from the United States, Chile, and Ecuador (4) and as a culture contaminant in Switzerland (3). More recently C. rhizophaga has been found to be associated with Pinus canariensis in Argentina (2) and it has been reported as a causal agent of chickpea wilt in Syria (1). To our knowledge, this is the first report of C. rhizophaga for subsaharian Africa. It may be under reported and more common than has been thought because of the difficulty in identifying Clonostachys species, but with the ability to identify species using tub2 (3), there can be no doubt of its identity. References: (1) M. M. Abang et al. Plant Dis. 93:666, 2009. (2) L. Eduardo Piontelli and G. Giusiano. Bol. Micol. 18:89, 2003. (3) H. J. Schroers. Stud. Mycol. 46:85, 2001. (4) L. R. Tehon and H. L. Jacobs. Bull. Davey Tree Expert Company, Kent, OH. 6:3, 1936.