Study On Virulence Variation Of Puccinia Striiformis F.sp.tritici Through Sexual Hybridization Between P.striiformis F.sp.tritici And P.striiformis F.sp.leymi

Wheat is a staple crop and widely cultivated in all almost parts of the world.It is also one of the three major cereals and an important economic crop.Wheat stripe rust,caused by obligate biotrophic fungus Puccinia striiformis f.sp.tritici(Pst),is one of the most devastating diseases in all of wheat-producing regions worldwide.The rust pathogen caused the disease epidemics and significant yield losses by repeat infections of wind-borne urediospores.The previous studies have reported that Pst infect wheat as major host and other cereals or grasses,such as barley,rye,Leymus and Elymus,as accessory hosts.Degree of pathogenicity of Pst on host vary as different races of the rust pathogen.Even if the same race exhibited various pathogenicity as difference of host species,showing host specifity.So far,P.striiformis was divided into six formae speciales based on host specifity of the rust fungus,namely f.sp.tritici,f.sp.elymi,f.sp.leymi,f.sp.hordei,f.sp.agropyri,and f.sp.poae(newly changed to P.pseudostriiformis)of P.striiformis.Recently,with the discovery of barberry(Berberis)as alternate host for Pst,which makes it possible to study on sexual reproduction and virulence variation of Pst possible.Previous studies demonstrated that new races can produce through sexual genetic recombination between formae speciales of P.graminis f.sp.tritici(the causal agent of wheat stem rust)and P.triticina(the causal agent of wheat leaf rust),respectively.Up to date,however,little is to known about sexual hybridization of formae speciales of P.striiformis.In this study,first of all,barberry were identified as alternate host for P.striiformis f.sp.leymi by artificially inoculation under controlled conditions.A cross between P.striiformis f.sp.leymi and P.striiformis f.sp.tritici was made to establish a progeny population of single-aeciospore.The progeny population and the parent race of Pst were tested on a set of single Yr lines and compared their virulence variation.The results could explain possibility and function of formae speciales of Pst in virulence variation of the rust pathogen and were as follows.1.Confirmation of f.sp.leymi and f.sp.tritici of P.striiformis.single-urediospore isolate was carried out to test host specifity,observe spores morphology,and sequence comparison,to make sure accuracy of the two formae speciales of P.striiformis.Single-urediospore isolates were induced to produce teliospores.2.Determination of barberry serving as alternate host for P.striiformis f.sp.leymi,verifying the rust pathogen was macrocyclic.Under controlled conditions,basidiospores produced from teliospores of P.striiformis f.sp.leymi infected barberry after inoculation,producing pycnial and aecial stages subsequently.After aeciospores were inoculated wheat cultivar Mingxian 169 and Leymus secalinus,uredia produced on the former with high infection but not on the latter with only necrosis.This indicated that P.striiformis f.sp.leymi complete sexual cycle on barberry.3.The selfed progeny of P.striiformis f.sp.leymi represented immune or low infection on wheat cultivar Mingxian 169.Among 30 isolates,27 were immune and only 3 expressed low infections on wheat.4.Progeny derived from sexual hybridization of P.striiformis f.sp.leymi and P.striiformis f.sp.tritici represented obvious virulence variation.Fifty two single-aeciospore progeny produced 36 various pathotypes,distinguishing from the parent isolates and accounting for approximately 69%.Compared to virulence spectrum on Chinese differential hosts composing of 19 wheat genotype lines,two progeny isolates were similar to P.striiformis f.sp.tritici race CYR32.A few were almost close to CYR32 except for one or a couple of Yr locus/loci.Thirteen were different from the parent isolate CYR32 at one Yr locus.Four were distinguished from CYR32 at two Yr loci and 33 were inconsistent at three or more Yr loci.Obvious virulence variation of some or progeny isolates on some Yr lines were found.Virulence frequency of progeny decreased to 13% at Yr8 and Yr26 loci,19% at Yr24 locus,25% at Yr10 and Yr Tr1 loci,32% at Yr32 locus.Progeny isolate were not virulent to Yr5.5.Virulence tests of selfed progeny derived from sexual hybridization.Three progeny isolates were randomly picked to prepare teliospores,inoculating barberry to complete selfing.Only one were failed to produce aecia and the other two successfully produced.After inoculations of aeciospores on wheat cultivar Mingxian 169 and Leymus secalinus,Su BH-7-3 isolate infected wheat and produced uredia with high infection but produced only necrosis without uredia.Su BH-1-3 were failed to produce uredia on wheat and the grass species.