Diversity in Pathogenicity to Tobacco and in Elicitin Production among Isolates of Phytophthora parasitica

Abstract

The pathogenicity to tobacco of a large set of Phytophthora parasitica isolates has been assessed using several procedures: root inoculation of young plants, leaf inoculation on detached disks and stem inoculation of decapitated plants, with or without healing. Analysing various aspects of the plantpathogen interaction with this array of tests led to the discrimination between three groups of isolates. In the isolates from hosts other than tobacco, none was truly pathogenic to tobacco, and all but one produced parasiticein, a proteinaceous elicitor of the elicitin family which induces a hypersensitive‐like response in tobacco. Isolates producing parasiticein in vitro induced necrotic fleks on the leaves upon inoculation of roots orof, freshly wounded stems. Most tobacco isolates, including all the highly virulent ones, were characterized by a lack of elicitin production. However, those collected in Australia and Zimbabwe differed in that they exhibited reduced virulence, induced leaf, necrotic flecks and produced parasiticein. The incidence of elicitin production on virulence and the significance of two types of tobacco‐pathogenic strains for tobacco pathology are discussed.

Fungal Infection of Plants.

“…On tobacco, the virulence of /? parasitica is inversely correlated with elicitin secretion (Bonnet et al, 1994;Kamoun et al, 1994), implying that elicitins may be genus-specific determinants of resistance in this (and other solanaceous) species. In contrast, elicitins may be cultivar specific in some Brassica species because they have been shown to function as elicitors of necrosis only in a few cultivars of Raphanus sativus and Brassica campestris (Kamoun et al, 19934 1994).…”

Fungal Avirulence Genes: Structure and Possible Functions

“…The level of elicitin production has been analyzed for isolates of P. parasitica obtained from tobacco and other host plants. A strong negative correlation was found between virulence on tobacco and the level of elicitin production (Ricci et al, 1992;Bonnet et al, 1994). Consequently, the para1 gene encoding the elicitin parasiticein of P. parasitica has been proposed to act as a species-specific Avr gene (Kamoun et al, 1993).…”

Signal perception in plant pathogen defense

“…Thus, cells directly stimulated by fungal elicitors appear to secrete secondary signal molecules that activate defense responses in neighboring cells, thereby amplifying the overall response of challenged plants. The virulence of Ppn on tobacco is inversely correlated with elicitin secretion, implying that elicitins are avirulence factors acting as genus-specific determinants in this plant [83,88]. This has recently been demonstrated by an elegant approach to inhibit elicitin production in P. infestans by gene silencing.…”