Host-pathogen studies of wheat leaf rust resistance in Triticum turgidum

Abstract

English: Wheat and the wheat leaf rust pathogen Puccinia tritieina co-evolved for several millennia. The frequency by which host resistance genes are overcome by the pathogen has lead to a constant search for new genes, in particular in wild species related to wheat, to enlarge the resistance gene pool. For this reason four subspecies of T. turgidum that was known to have adult-plant resistance to leaf rust were crossed with the bread wheat cultivar SST55 and studied in terms of expression, inheritance and molecular markers. Seedling plants of T. turgidum ssp. dieoceoides (Kom. ex Aschers. and Graebn.) Theil. var. kotehianum, T. turgidum ssp. pyramidales, T. turgidum ssp. durum (Desf.) Husn var. libyeum and T. turgidum ssp. abyssinieum were inoculated with different races of P. tritieina, P. graminis f. sp. tritiei and P. striiformis f. sp. tntlei. A differential interaction was observed between accessions and pathotypes, indicating that the plants had race-specific resistance. All showed adult-plant resistance with leaf rust infection types ranging between ";" (flecking) and ";1eN" (flecks and small pustules associated with chlorosis and/or necrosis). Despite of low pollen viability, sterility and recessiveness of resistance genes in three of the accessions, F1, F2 and F3'Swere produced. T. turgidum ssp. durum v. libyeum was the only accession that produced resistant F1's, indicating a dominant gene or genes. Mendelian ratios in the F2 progeny suggested that resistance in this accession resulted from two major genes. The F2 of the T. turgidum ssp. abyssinicum and T. turgidum ssp. pyramidales crosses did not segregate according to Mendelian ratios and two minor genes conferred resistance of T. turgidum ssp. dicoccoides v. kotchianum. The molecular AFLP technique was used to to follow the introgression of resistance from tetraploid T. turgidum ssp. dicoccoides v. kotchianum and T. turgidum ssp. durum v. libycum to the hexaploid bread wheat cultivar SST55. In total nine fragments were solely introgressed from T. turgidum ssp. dicoccoides v. kotchianum and four from T. turgidum ssp. duturn v. libycum and were present in all resistant accessions. These fragments have potential to be developed into molecular markers. Histology tests done on accession T. turgidum ssp. durum v. libycum confirmed a post-haustoria I expression of resistance. Resistance in this accession was strongly associated with a hypersensitive response. Necrosis started at the aborted sub-stomatal vesicle stage and continued through to colony formation. All colonies were enveloped in necrotic leaf tissue with the parent having a higher hypersensitivity index than the F1. In some cases it seemed as if necrosis was systemic and carried along the veins. Although hypersensitive resistance as observed in this study is not considered durable it should not be overlooked in the search for new resistance genes. The challenge to the breeder is to use these genes in genetic backgrounds where it will be protected against pathogenic adaptation in the leaf rust fungus.

Afrikaans: Koring en die koringblaarroes patogeen Puccinia triticina het gekoëvoleer vir etlike millennia. Die frekwensie waarteen weerstandsgene deur die patogeen oorkom word, het gelei tot 'n konstante soektog na nuwe gene, veral van die wilde families van koring, om die weerstandsgeenpoel aan te vul. Om hierdie rede is vier subspesies van T. turgidum wat oor volwasseplantsweerstand beskik, gekruis met die broodkoring kultivar SST55 en bestudeerop grond van geenuitdrukking, oorerwing en molekulêre merkers. Saailingplante van T. turgidum ssp. dicoccoides (Kërn, ex Aschers. en Graebn.) Theil. var. kotchianum, T. turgidum ssp. pyramidales, T. turgidum ssp. durum (Desf.) Husn var. libycum en T. turgidum ssp. abyssinicum is geïnokuleer met verskillende rassevan P. triticina, P. striiformis f. sp. tritici asook P. graminis f. sp. tritici. Die verskeie interaksies wat waargeneem is tussen koringlyne en roesrasse het daarop gedui dat die plante oor ras-spesifieke weerstand beskik. Alle plante het volwasseplantsweerstand getoon met blaarroes infeksietipes wat gewissel het tussen ";" (vlek) en ";CN" (vlekke en klein roespuisies geassosieer met chlorose en nekrose). Ten spyte van die lae stuifmeel lewensvatbaarheid, steriliteit en resessiwiteit van weerstandsgene in drie van die koringlyne, is F1, F2 en F3 plante geproduseer. T. turgidum ssp. durum v. libycum was die enigste koringlyn wat weerstandbiedende F1 's geproduseer het. Hierdie aanduiding dat weerstand in dié lyn die resultaat van 'n dominante geen of gene is, is bevestig deur Mendeliese toetse op die F2 nageslag wat aangedui het dat twee dominante gene verantwoordelik was vir weerstand. Alhoewel die F2 plante van die kruisings T. turgidum ssp. pyramidales en T. turgidum ssp. abyssinicum nie volgens Mendeliese verhoudings gesegregeer het nie, het dieselfde toetse aangedui dat weerstand deur twee resessiewe gene veroorsaak word in T. turgidum ssp. dicoccoides v. kotchianum. Die molekulêre AFLP tegniek is gebruik om die introgressie van weerstand vanaf die tetraploïede T. turgidum ssp. durum v. libycum en T. turgidum ssp. dicoccoides v. kotchianum na heksaploïede broodkoring kultivar SST55 te volg. In totaal is nege fragmente uitsluitlik verkry vanaf T. turgidum ssp. dicoccoides v. kotchianum en vier vanaf T. turgidum ssp. durum v. libycum. Hierdie fragmente was teenwoordig in alle weerstandbiedende plante en besit die potensiaaI om as molekulêre merkers ontwikkel te word. Die histologie toetse gedoen op T. turgidum ssp. durum v. libycum het post-haustoria Ie weerstand bevestig. Weerstand in hierdie koringlyn is geasrosieer met die hipersensitiewe reaksie. Nekrose het begin by die geaborteerde substomatale fase en was deurlopend tot by die kolonies. Alle kolonies was omring met nekrotiese blaarweefsel en die ouer het 'n hoër hipersensitiewe indeks as die F1 gehael. In sommige gevalle het dit voorgekom asof die nekrose sistemies was en met are vervoer is. Ten spyte daarvan dat hipersensitiewe weerstand, soos waargeneem in hierdie studie, nie as lanklewend geag word nie, moet dit nie geïgnoreer word in die soektog na nuwe weerstandsgene nie. Die uitdaging vir die koringteler is om hierdie gene te gebruik in In genetiese agtergrond waar hulle beskermd sal wees teen patogeniese mutasies van die blaarroespatogeen.