A host-pathogen study of stripe rust resistance in Triticum aestivum

Abstract

English: The successful introgression of the stripe rust seedling resistance gene YrSp into Kariega/ Avs/YrSp is towards the continued pursuit of durable resistance in this cultivar. Following the development of Kariega/ Avs/YrSp hybrid, an F2 and F3 Kariega/YrSp population was obtained and screened for virulence to pathotype 6E22A- of P. striiformis f. sp. tritici. The disease screening of the F2 population deviated from the expected single dominant gene ratio and did not fit a 3:1 gene segregation ratio determined by Chi-square analysis. Disease screening of the resistant F3 families exhibited segregation while susceptible F3 families was not considered to segregate with the observation of singular resistant plants in two susceptible F3 families. Histological studies firstly confirmed the infection pathway of P. striiformis f. sp. tritici as is described for other cereal rusts with the absence of the formation of appressoria and the observation of pseudo-SSVIs the most marked differences. The pseudo-SSVIs are thought to be due to the unsuccessful stomatal penetration by germ tubes as successful penetration results in a single SSVI that is generally larger in size than the vesicles observed but this requires further study. Secondly, quantitative analysis indicated that urediospores of P. striiformis f. sp. tritici have good germination efficiency but germ tubes were rarely observed to penetrate stomata. After penetration, the infection efficiency of the pathogen is high in susceptible cultivars due to systemic colonisation. The resistance conferred by the YrSp gene is considered posthaustorial as no significant difference in infection efficiency could be determined before haustorium formation. An increase in the number of internal infection structures after 7 dpi in the susceptible cultivar was observed while the resistant line reflected a decrease. Thirdly, fluorochrome assessment indicated uvitex 2B and the orange G probe hold the best potential of the fluorochromes tested for use with the confocal laser scanning microscopy (CLSM). Furthermore, the CLSM holds great potential to elucidate host-pathogen interactions if more research into fluorochromes selection and development occurs. Using molecular markers obtained from AFLPs and SSRs the YrSp gene was mapped to the short arm of chromosome 2B. The YrSp gene was located on the short arm of chromosome 2B in the Kariega/ Avs/YrSp hybrid. Two AFLP markers were found to flank the gene namely L15 and L68 at 19.5 and 21.4 cM respectively. In addition to this, 2 QTL markers, QYrSgi-2B.1and QYrSgi-7D for adult plant resistance was present in the Kariega /YrSp population. The previous report of an introgression on chromosome 6A into Avocet/YrSp was not found to contribute to the introgressed trait. The integration of various disciplines provided insight into the hostpathogen interaction of P. striiformis f. sp. tritici on wheat at different levels towards a holistic understanding of such interactions and the mechanisms involved in conditioning resistance.

Afrikaans: Die YrSp geen is suksesvol van Avocet/YrSp na Kariega, ‘n kommersiële Suid-Afrikaanse kultivar oorgedra. Na inkorporering van die geen is die F2 en F3 Kariega/YrSp populasies vir weerstand teen die patotipe 6E22A- van Puccinia striiformis f. sp. tritici geëvalueer. Resultate van die siekteweerstands-evalueering van die F2 populasie het op die teenwoordigheid van ‘n enkel dominante geen gedui en het nie voldoen aan ‘n 3:1 segregasie verhouding. Verdere evaluasie van weerstandbiedende F3 families het segregasie getoon in teenstelling met die vatbare F3 families. Die ontwikkeling van Kariega/YrSp is die volgende stap in die voortdurende strewe na volhoubare weerstand in hierdie kultivar. Histologiese studies, met behulp van transmissie-elektronmikroskopie (TEM) en skandeer-elektronmikroskopie (SEM) het bevestig dat die infeksieproses by P. striiformis f. sp. tritici dieselfde is as wat beskryf is vir ander graanroese, behalwe dat geen appressoria gevorm het nie. Verder is die inisiële fase van die pseudo-substomatale vesikel (SSVIs) van streeproes gedurende hierdie studie waargeneem en beskryf. Die vesikels vorm waarskynlik as gevolg van onsuksesvolle penetrasie van die stomata deur die kiembuis aangesien suksesvolle penetrasie ‘n enkele vesikel vorm en oor die algemeen groter is. Hierdie aspek benodig verdere navorsing. ‘n Kwantitatiewe analise van streeproesinfeksies, het aangetoon dat ten spyte van swak kiembuispenetrasie, die infeksietempo van P. striiformis f. sp. tritici hoog was in vatbare kultivars waarskynlik as gevolg van sistemiese kolonisasie. Daar is gevind dat die weerstand, oorgedra deur die YrSp-geen, na-haustoriaal is, omdat geen verskil in infeksievlak of struktuur tussen Avs/YrSp en Kariega voor haustoriumontwikkeling opgemerk is nie. Die aantal interne infeksiestrukture in die vatbare kultivars het 7 dae na infeksie toegeneem, terwyl die aantal strukture in die weerstandbiedende kultivars afgeneem het. ‘n Vergelyking van verskillende fluorochrome, het bevind dat vir die bestudering van swaminfeksiestrukture, uvitex 2B en oranje G die doeltreffendste fluorochrome was vir gebruik met konfokale-laserskandeermikroskopie (KLSM). Die KLSM kan potensieel help om gasheerpatogeen interaksies op te klaar, maar verdere navorsing in fluorochroom seleksie en ontwikkeling is nodig. Die YrSp geen is suksesvol op die kort arm van chromosoom 2B gekarteer deur gebruik te maak van ‘n AFLP merker, geanker deur ‘n chromosoom spesifieke SSR merker. Die studie het die posisie van YrSp op chromosoom 2B met behulp van molekulêre merkers, asook die kombineering van die geen in Kariega/ Avs/YrSp, bevestig. Twee aangrensende AFLP merkers (L15 en L19, 19.5 en 21.4 cM onderskeidelik vanaf die geen) is gevind. Die teenwoordigheid van twee QTL merkers, QYRSgi-2B.1 en QYRSgi-7D, vir volwasse weerstand in Kariega/YrSp, is bevestig. In teenstelling met ‘n vorige studie, is gevind dat die inkorporering van ‘n gedeelte op chromosoom 6A in Kariega, nie bydra tot die oorgedraagde kenmerk nie. Die integrasie van verskeie dissiplines het tot beter insig in die gasheerpatogeen interaksie van P. striiformis f. sp. tritici in koring gelei, en hoop om by te dra tot ‘n meer holistiese begrip van die interaksies en meganismes betrokke by streeproesweerstand.