The effect of the Sr6 gene for host resistance on histological events during the development of stem rust in near-isogenic wheat lines

Abstract

Seedling leaves of resistant (Sr6) and susceptible (sr6) near-isogenic wheat lines inoculated with urediospores of Puccinia graminis f. sp. tritici race C 17 (56) became infected at similar rates. Host cells of the resistant line became necrotic after haustorial penetration (beginning about 20 h after inoculation), whereas necrosis was rarely seen in the susceptible line and colonies grew to form sporulating pustules. Some colonies in the resistant line appeared to have stopped growing by about 60 h after inoculation, while others grew slowly, the area of necrosis increasing as they expanded.Inoculated resistant-line plants became susceptible when incubated at 25 °C rather than 20 °C. Provided that the plants were kept at 25 °C for at least 1 day before inoculation, no host cell became necrotic. The necrotic response was resumed, and a more resistant infection type developed when infected seedlings were transferred from 25 °C to 20 °C. The converse occurred when resistant plants were grown and incubated at 20 °C, then transferred to 25 °C.Effects on fungal growth and the action of the Sr6 gene were considered to be closely associated with host cell necrosis. Temperature sensitivity appeared to be a property of the host plant.