Histopathology provides a phenotype by which to characterize stripe rust resistance genes in wheat

The expression of the resistance phenotypes of QPst.jic‐2D and QPst.jic‐4B, two quantitative trait loci (QTL) for stripe rust resistance in wheat cv. Alcedo, were assessed relative to plant growth stage, while a histopathology analysis was undertaken to characterize the cellular interaction between Puccinia striiformis f. sp. tritici (the causal agent of stripe rust) and each QTL. QPst.jic‐2D expressed a partial resistant phenotype at seedling growth stages, with the level of resistance increasing as the wheat plant matured, conferring a disease‐free phenotype at heading. QPst.jic‐4B, however, did not express a resistant phenotype until booting (growth stage 41 on the Zadoks scale), displaying its full resistant phenotype at heading. Microscopic examination in flag leaves showed that infection sites formed in all genotypes tested, with full infection‐site establishment being observed by 36 h post‐inoculation (hpi). In lines carrying both QPst.jic‐2D and QPst.jic‐4B, as well as the parental cv. Alcedo, no microcolony formation, defined by the appearance of runner hyphae, was observed. Microcolony formation was observed in lines carrying only one, or neither QTL. Cell death associated with infection sites was observed for all genotypes, although the timing of first appearance and the extent of the cell death response varied considerably. In lines carrying both QPst.jic‐2D and QPst.jic‐4B cell death did not extend beyond one to three cells. In lines carrying only one QTL more extensive cell death was observed and cell death appeared later in lines with QPst.jic‐4B than QPst.jic‐2D. Cell death was also occasionally observed in lines without either QTL, although only at 264 hpi.