Mechanisms associated with wheat leaf rust resistance derived from Triticum monococcum

The infection of wheat (Triticum aestivum) line KS93U9 (Karl*3//PI 266844/PI 355520) with Puccinia recondita f. sp. tritici was studied to determine histological mechanisms of resistance. The development of fungal structures was studied at two host growth stages in KS93U9, its leaf rust-resistant T. monococcum parent lines PI 266844 and PI 355520, and its susceptible recurrent parent 'Karl'. T. monococcum accession Tm2126/5, previously reported to exhibit prehaustorial resistance to P. recondita f. sp. tritici, was included in the study. Using fluorescence microscopy, infection sites on leaf sections infected with pathotype UVPrt9 and stained with Uvitex 2B were examined for, respectively, the percentage of prestomatal exclusion of the fungus (germ tubes not forming appressoria and appressoria not forming over stomata), aborted penetration (nonpenetrating appressoria and aborted substomatal vesicles), early abortion (less than six haustorium mother cells per infection site), and successfully established colonies. In general, the resistant lines responded similarly for prestomatal exclusion and aborted penetration, but differences in early abortion and colony formation were observed. In seedlings, prestomatal exclusion could be attributed to the inability of fungal germ tubes to produce appressoria, whereas the formation of nonstomatal appressoria occurred more commonly in adult plants. At both growth stages, most aborted penetration attempts were accounted for by arrested substomatal vesicles rather than nonpenetrating appressoria. All infection sites displaying early abortion in KS93U9 were associated with host cell necrosis, whereas no hypersensitivity was observed in primary leaves of Tm2126/5. In adult Tm2126/5 plants, however, a large proportion of the infection sites exhibited hypersensitivity. Similarly, infection sites in PI 266844 and PI 355520 were frequently accompanied by necrotic leaf tissue. Staining of the leaves with trypan blue and a saturated solution of picric acid in methyl salicylate, and viewing with phase contrast microscopy, showed that papillae commonly occurred at infection sites in the T. monococcum lines, but not in KS93U9. Expression of T. monococcum-derived resistance in KS93U9 was not different from hypersensitive reactions typically associated with existing sources of major gene resistance to wheat leaf rust. Furthermore, components of resistance indicated, that at a histological level, mechanisms of resistance in the hexaploid KS93U9 background were altered when compared with parent lines.