Resistance of wheat to Puccinia graminis f.sp. tritici: Histological investigation of resistance caused by the Sr5 gene

Abstract

Seedlings of six wheat ( Triticum aestivum L.) lines containing the Sr5 gene for resistance to wheat stem rust were inoculated with urediospores of Puccinia graminis f.sp. tritici, race 32, which carries the P5 gene for avirulence, and primary leaves were examined for the temporal relation between the hypersensitive reaction (HR) and fungal growth. In up to 95% of all infection sites the first host cell that was penetrated by a fungal haustorium was an epidermal cell. Infected epidermal and mesophyll cells containing the Sr5 gene reacted hypersensitively with an accumulation of yellow autofluorescing compounds. Fungal hyphae that were associated with intensely autofluorescing epidermal cells developed not more than one, two, or three haustorial mother cells (HMCs). Significant inhibition of fungal growth in Prelude- Sr5 occurred after formation of the first haustorium, at a time when the number of infection sites with a necrotic epidermal cell rapidly increased from 11 to 36·5 %. The time interval between formation of the first haustorium in epidermal cells and appearance of the HR of the first-invaded epidermal cells varied from 4·6 to 8·1 h, depending on the wheat line used. In wheat cvs. Napayo ( Sr5, 6, 7 a, 11) and Manitou ( Sr5, 6, 7 a, 9 g, 12) more epidermal cells relative to Prelude- Sr5 reacted hypersensitively indicating that the expression of the Sr5 gene is modified by additional resistance genes in these cultivars. The data demonstrate that resistance controlled by the Sr5 gene is closely associated with the HR of infected epidermal cells.