The role of the abaxial leaf surface waxes of Lolium spp. in resistance to Erysiphe graminis

Abstract

Field and glasshouse observations of Lolium spp. grasses indicated that the lower, abaxial, leaf surface was rarely infected by powdery mildew (Erysiphe graminis) even when the upper, adaxial, surface was densely colonized. Experiments showed that conidia of two strains of E. graminis, one from Lolium and one from Avena, germinated equally well on both surfaces of Lolium and Avena leaves, but that the subsequent growth and development of germlings was impaired on the lower surface of Lolium leaves, so that most formed only multiple short germ tubes or an abnormal long tube, and only c. 25% or fewer formed infection structures. This contributes to the apparent resistance of the lower Lolium leaf surface to powdery mildew and may help to explain why the disease is relatively unimportant in UK ryegrass crops, since infection structures develop at a high frequency on only 50% of the leaf area, i.e. the upper surface. Scanning electron microscopy showed that the epicuticular waxes on the lower Lolium leaf surface form amorphous sheets. This contrasts with the crystalline plate waxes seen on the upper surface of Lolium leaves and on both surfaces of oat leaves. However, when the lower Lolium leaf surface was washed with chloroform to remove epicuticular wax, normal germling and infection structure development was obtained on the wax‐free surface. This suggests that the sheet waxes prevent the pathogen gaining access to features of the cuticular membrane which trigger normal germling development.

Loss of Abaxial Leaf Epicuticular Wax inMedicago truncatula irg1/palm1Mutants Results in Reduced Spore Differentiation of Anthracnose and Nonhost Rust Pathogens

“…Our results showed that irg1/palm1 mutants were completely devoid of wax crystals on the abaxial leaf surface and suggested a possible role for signals at the cuticle interface, especially the components of the leaf wax in promotion or inhibition of rust preinfection structure differentiation on the abaxial leaf surface of the irg1 mutants (Figures 1 and 2). Consistent with our results, two other previous studies have shown normal development of fungal preinfection structures on the adaxial surface and short germ tubes with few appressoria on abaxial leaf surfaces during host-specific pathogenic interactions involving wild-type pea (Pisum sativum)-E. pisi and wild-type ryegrass (Lolium spp)-Erysiphe graminis interactions (Carver et al, 1990;Gniwotta et al, 2005). Variations in wax crystal composition between adaxial and abaxial leaf surfaces were implicated in pea-E. pisi interactions (Gniwotta et al, 2005).…”

Breeding perennial ryegrass for agriculture

“…There is however evidence of non-race-specific resistance to powdery mildew (Erysiphe graminis) . The epicuticular wax on the undersurface of ryegrass leaves, which gives the genus its distinctive shiny appearance, confers partial resistance to powdery mildew since most conidia fail to germinate on this wax (Carver et al ., 1991) .…”

Infection of ryegrass by three rust fungi (Puccinia coronata, P. graminis and P. loliina) and some effects of temperature on the establishment of the disease and sporulation

“…This is not confined to rust fungi since Carver et al (1990) also found that the sheet-like waxes on the abaxial surface of L. temulentum prevented normal germination of conidia of Erysiphe graminis. On the other hand, the ability of leaves of Lolium species to fold in response to changing environmental conditions and the undulating topography of the upper surface might create a favourable micro-climate for urediospore germination on the adaxial surface.…”

“…The cereal hosts of rusts have crystalline waxes on both leaf surfaces (e.g. Carver et al, 1990) and crown rust (P. coronata f.sp. avenae) can infect through stomata on both these surfaces (Kochman & Brown, 1976;Roderick & Thomas, unpublished observations).…”

“…However, compared to cereals, ryegrass leaves are heavily ribbed on the adaxial surface due to raised ridges over the vascular tissue and deep grooves in between. While the adaxial surface is covered by crystalline epicuticular wax, the lower, abaxial, surface has sheet-like amorphous waxes giving it a shiny appearance (Carver et al, 1990). Part of the current study reports histological observations of the development of P. coronata, P. graminis and P. loliina germlings.…”