Histological responses of host and non-host plants to Hyaloperonospora parasitica

Differences in Hyaloperonospora parasitica development and plant tissue responses were compared for 10 cruciferous hosts (including both resistant and susceptible genotypes), 3 leguminous and 1 graminaceous non-host species. Cotyledons, or true leaves in the case of Triticum aestivum and Pisum sativum, were studied at 2, 8, 24 h and 3, 5, 7 days post inoculation (dpi). The high levels of zoosporangial germination observed on all species tested, as well as on glass slides, suggested that inhibition of germination did not play a significant role in distinguishing host versus non-host resistance. During the early stages of infection, at spore germination and host penetration, there was no evidence of a clear-cut difference between Brassica host species which displayed a hypersensitive, partially resistant or susceptible reaction compared with non-host species. Haustoria formation was the key infection phase for the establishment of biotrophy. Across all tested species, haustoria were initiated inside the epidermal cells. However, there were significant differences in the frequency and timing of haustorial formation and the final size of haustoria among the tested species at early infection stage. Fully developed haustoria were never observed in Raphanus raphanistrum, Triticum aestivum, Lupinus angustifolius nor Trifolium subterraneum. Instead, the haustorium development appears to abort in the penetrated epidermal cells of these species. Although haustoria were formed in the epidermal and mesophyll cells of Sinapsis alba and Pisum sativum, subsequent hyphal growth and/or continued haustoria formation were rare or few, respectively. Hypersensitive reaction was the key resistance response observed among the host and non-host resistant species tested. It is noteworthy that, in the initial stages of pathogenesis, there was no differentiating point that separated the non-host species from those that were hosts