Resistance to Peronospora viciae expressed as differential colony growth in two cultivars of Pisum sativum

Abstract

Peronospora viciae sporulated frequently following localized inoculation of leaflets of Pisum sativum cv. Krupp Pelushka but rarely on cv. Early Onward (60 and < 1% of plants respectively). Similar results were obtained following inoculation of seedling shoot apices. The proportions of sporangia that germinated, formed appressoria, invaded via stomata or cuticle, and established colonies, were similar on leaflets of both cultivars. After 2 days, total hyphal lengths of most colonies were greater than 100 μm (94% in Krupp Pelushka, 64% in Early Onward), but in Krupp Pelushka there were five times as many haustoria per unit length of hypha. Subsequently, only occasional colonies in Early Onward grew to the same extent as in Krupp Pelushka, with 8% and 88% respectively attaining lengths > 800 μm after 4 days. Reduced growth of hyphae in Early Onward was associated with increased frequency of host cells stained by trypan blue.

The Compatible Interaction in Downy Mildew Infections

“…In P. manshurica on soybean, virtually all penetration was via anticlinal cell walls (Riggle, 1977), whist in P. destructor on onion penetration was reported to be entirely via stomata (Kofoet and Zinkernagel, 1991). Approximately 20% of infections by P. viciae on pea leaves were via stomata, in experiments that compared two cultivars with a two-fold difference in the number of stomata per unit area and in the total circumference of aperture available for penetration (Clark and Spencer-Phillips, 1994). This suggested that Figure 2.…”

“…Autofluorescence is probably due to deposition of phenolics in the cell walls (Stumpf and Heath, 1985). Substantial numbers of these cells have been seen in a compatible interaction between P. viciae and the pea cv Krupp Pelushka, in which the cells were only lightly stained by Trypan Blue (although colonies within this interaction had less necrotic cells per length of hyphae than colonies within the resistant cv Early Onward; Clark and Spencer-Phillips, 1994). It is not known whether these cells were undergoing necrosis as part of a hypersensitive response, or whether they were undergoing a mild reaction which resulted in recovery and maintenance of the compatible interaction.…”

“…Certainly oospore densities can be similar in cultivars from different resistance categories (van der Gaag and Frinking, 1996). An investigation of P. viciae infections of cv Early Onward, classed as resistant (Dixon, 1981), showed limited growth ( Figure 1) within plant tissues with infrequent sporulation (Clark and Spencer-Phillips, 1994) and it is clear that the extent of pathogen growth within host tissues is not reflected by conventional keys. It should be noted that occasional colonies in the resistant cv Early Onward grew to the same extent as in the susceptible cv Krupp Pelushka, and presumably it was these colonies that gave rise to sporulating infections.…”

“…At the light microscope level, infected cells can appear to have a faint brown colour and autofluorescence when unstained, and can be stained with Trypan Blue (Clark and Spencer-Phillips, 1994). Enzyme-catalysed browning reactions are a common feature of the response of plant cells to invasion by pathogens (Kuc, 1972).…”

“…Diagrammatic representation of Peronospora viciae colonies at 2 and 4 days postinoculation of Pisum sativum leaves of the resistant cv Early Onward and the susceptible cv Krupp Pelushka. Traced from images of colonies but adjusted to mean hyphal length and haustorial frequency data of Clark and Spencer-Phillips (1994).…”