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SESSION 1 - Discovery - New horizons in plant pathology - page 31 / 65





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Plant pathogens and sex: results of a case study on Mycosphaerella graminicola

Tim Jürgens, B.A. McDonald

Plant Pathology group, Institute of Plant Sciences, ETH Zentrum / LFW, Universitätstrasse 2, CH - 8092 Zürich

Surprisingly little knowledge is available regarding the processes that affect the evolution of virulence in parasite populations. Few experiments have considered the consequences of sexual reproduction for the evolution of virulence in pathogen populations. A novel approach to study the influence of mating systems on the evolution of virulence in plant pathogens is presented. The specific objective of this project is to determine whether pathogen populations undergoing sexual reproduction evolve virulence faster than strictly asexual pathogen populations. From successful crosses of Mycosphaerella graminicola (anamorph Septoria tritici) strains, families of isolates were chosen to conduct studies on the virulence of individual strains. Virulence of the parent strains was compared to the virulence of genetically distinct offspring strains as well as virulence data that was obtained from the first asexual generation.


Inhibition of plant defence responses by low temperatures

Besenyei E., Ott P.G., Bozsó Z, Szatmári Á., Varga G.J., Czelleng A., Klement Z.

Plant Protection Institute, Hungarian Academy of Sciences, P.O. Box 102, 1525 Budapest, Hungary

According to our knowledge, plants have two local defence mechanisms to protect themselves against bacteria actively. One of these, the early induced resistance (EIR) is the earliest form of the non-specific general defence response which develops against both incompatible pathogenic and non-pathogenic bacteria. The EIR is symptomless and is able to prevent the hypersensitive reaction (HR) causing a subsequent challenge inoculation. The other is the HR which causes rapid cell death on the site of infection and is activated only by pathogenic bacteria in resistant or non-host plants. The effect of temperature was investigated on the EIR and HR in tobacco in controlled conditions from 30oC to 5oC. The speed of development of both defence responses, EIR and HR mainly depended on the temperature. We have found that the induction time of HR depended not only on the given temperature but also on pathogens and host plants. The EIR developed in a few hours in tobacco at temperatures between 20-30oC but was greatly delayed at lower temperatures. Below 10oC three days were required for the development of EIR. Multiplication of the opportunistic, cold-tolerant pathogen (Pseudomonas syringae pv. syringae) and a saprophytic bacterium (Pseudomonas fluorescens) was investigated in pepper. At 30oC both bacteria were inhibited by a general, non-specific defence response of pepper but at 5oC their multiplication was continuous for lack of inhibition. In the case of Pseudomonas syringae pv. syringae different disease symptoms appeared at every temperature. Therefore, at low temperatures both bacteria were able to multiply and colonise the infected tissue without host responses. To prove this relationship between the lack of general defence response and the multiplication of opportunistic pathogens like Pseudomonas syringae pv. syringae at different temperatures we compared protein contents of the intercellular washing fluid from inoculated and non-inoculated pepper leaves on 1-D PAGE. We found some new protein bands that may be connected with the general defence response in pepper, which correlated with the effect of temperatures.


Nitric Oxide in the Capsicum annuum-Phytophthora capsici interaction

Requena, M.E.1, Candela M.E.1 and Egea-Gilabert, C.2

1Departamento de Biología Vegetal, Facultad de Biología, Universidad de Murcia, Campus de Espinardo, Espinardo (Murcia), E-30100 SPAIN. E-mail: mcandela@um.es

2Departamento de Ciencia y Tecnología Agraria, E.T.S.I.A., Universidad Politécnica de Cartagena, C/ Paseo Alfonso XIII 52, Cartagena, E- 0203 SPAIN. E-mail: catalina.egea@upct.es

The fungus Phytophthora capsici L. causes blight in peppers (Capsicum annuum L.). This disease has a destructive phase at the crown of the stem that results in wilting and the development of a hypersensitive reaction, which advances through the main stem and finally kills the susceptible plants. Previous studies of the interaction between C. annuum and P. capsici enabled us to establish a system to study and measure factors involved in the resistance of plants to fungal attack. Recent studies have implicated Nitric oxide (NO) as an essential regulatory molecule in several development processes and in the stress response in both animal and plant systems. Therefore, the purpose of the present work was to study the involvement of NO in the defensive reactions in pepper against the fungus by comparing susceptible and resistant varieties. Pepper plants of two varieties, Americano (AM): Susceptible, and Serrano Criollo de Morelos (SCM): Resistant, were decapitated and inoculated with plugs of actively growing mycelium of P. capsici. The stems of control plants were inoculated in the same way, but only with culture medium. 3, 6 and 9 days after inoculation the stems showed a necrotic zone that was inversely proportional to the resistance to the fungus. The shortest necrosis length corresponded to the resistant variety that was able to stop the fungus invasion by 6 days after inoculation. The fungal hyphal growth, evaluated in stem sections of 0.1 mm immediately after the necrosis zone, confirmed the resistance of var. SCM because the fungus grew until the fourth and the second sections 3 and 6 after inoculation

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