Cellular characteristics of temporary partial breakdown of mlo-resistance in barley to powdery mildew

When water-stress is relieved, powdery mildew (Erysiphe graminis f.sp. hordei) infection increases on both Mlo-susceptible and mlo-resistant spring barley cultivars. The breakdown is temporary and is determined by the genetic background rather than the specific resistance allele. The relief of water-stress time-point for maximum mildew infection frequency on mlo-resistant barley is approximately 7 h post-inoculation. The degree of breakdown varies with epidermal cell type; increased infection frequency is greatest on short followed by long and adjacent epidermal cell types, rather than on the stomatal subsidiary cells with which occasional colonies are typically associated. Infection frequency on the short cells of mlo-resistant cv. Aem increases from less than 1 %, under a non-stressed Watering regime to more than 10 % after a relief of water-stress at 7 h post-inoculation. Following haustorium formation, colonies develop and reach sporulation within 7 days post-inoculation. In host epidermal cells, the extent of cross-linked protein at interaction sites is reduced under conditions of water-stress or stress-relief Cross-linked protein is reduced in terms of the frequency of occurrence at primary germ tube interaction sites 7 h post-inoculation and the deposition size at appressorial germ tube interaction sites 24 h post-inoculation. This indicates a delayed or reduced defence response during the recovery period. These data demonstrate a differential cellular response to powdery mildew in barley genotypes prone to resistance expression breakdown following relief of water-stress. (C) 2000 Academic Press.