Characterization of Yr15 resistance to stripe rust of wheat

Stripe (yellow) rust, caused by Puccinia striiformis Westend f. sp. tritici, is a major disease of bread wheat, Triticum aestivum L., in South Africa. The objective of this study was to evaluate the Yr15 gene as a potential resistance source for breeding purposes. Histological evidence was provided that Yr15 conditions a hypersensitive reaction typical of non-durable resistance. Fluorescence, phase contrast and scanning electron microscopy complemented each other and may be used to quantify various aspects of resistance expression in the wheat stripe rust pathosystem. The frequency of stomatal penetration was extremely low and no meaningful differences in early infection structures were observed between the resistant Yr15 and susceptible Avocet 'S' lines. Papillae were not involved in the resistance response to pathotype 6E16. In studies of defense-related proteins, an induction of β- 1,3-glucanase activity in both resistant and susceptible wheat genotypes was detected within 96 h post-inoculation (hpi). This induction, however, was constitutive and did not depend on the infection process. Likewise, no changes in chitinase activity were observed between resistant and susceptible plants. A pronounced increase of peroxidase activity between 0 and 168 hpi, in both infected and uninfected resistant and susceptible plants was observed, suggesting a constitutive expression of this enzyme. Using AFLP (Amplified Fragment Length Polymorphism) technology to detect possible molecular markers for Yr15, two primer combinations yielded polymorph isms between the bulked susceptible and resistant SST55 x Yr15/6*AvS F2 DNA, as well as between the susceptible and resistant parents. Fragment sizes ranged from 50 to 420 bp. Of these, one 95 bp fragment identified using primers MseCAA + Eco-ACA and one 205 bp fragment identified using primers MseCAA + Eco-AAC were detected in the resistant parent and the resistant F2 group. This indicates that these fragments are closely associated with Yr15 making them ideal candidates for further investigation.