| Wheat stripe rust caused by Puccinia striiformis f.sp.tritici(Pst)is one of the most important diseases on wheat.The disease is transmitted through air currents for long-distance transmission,which has the characteristics of epidemic outbreaks.The epidemic often causes severe wheat yield loss or even no harvest.Wheat stripe rust is a major biological disaster that seriously affects safe production of wheat in China.At present,cultivation of disease-resistant varieties is the most effective measure to control stripe rust.But breakdown of wheat resistance due to rapid virulence variation of Pst to produce new races,the resistance of wheat cultivars has rapidly lost in several years since their release and lack of continuous use value.Previous studies have shown that mutations,heterokaryosis(somatic recombination)and sexual reproduction can lead to virulence variation of plant pathogens.Although in recent years,with the determination of barberry(Berbers)as alternate host for Pst,the causal pathogen of wheat stripe rust,it has been found that Pst completes naturally sexual reproduction on wild barley bushes under natural conditions.Today,only Pst selfing resulting in virulence variation has been known.However,other possible approaches of sexual reproduction Puccinia striiform is in virulence variation have not been reported.P.striiformis is a group of rust fungi and classified into several formae speciales.Thus,whether other formae speciales share the same alternate hosts with Pst.Whether it could possible to complete sexual reproduction between formae speciales on alternate host to cause virulence variation for Pst.Therefore,in this study,we focused on this aspect to carry out related works and the results were as follows1.Elymus cylindricus stripe rust samples were isolated from Pingnan,Gansu Province and determined by host specifity,spore morphology and molecular sequencing as well as sequence analysis.2.Determination of barberry(Berberis)serving as alternate host for P.striiformis f.sp.elymi(Pse),the causal pathogen of Elymus cylindricus.Pse is a macrocyclic rust fungus.Under controlled conditions.B.shensiana was inoculated with basidiospores produced from germinated teliospores of Pse.Six to nine days post inoculation,pycnia successfully generated on the upper surface of Berberis shensiana leaves.Aecia produced on the backside of B.shensiana leaves approximately 20 days after inoculation.Aeciospore were inoculatedon E.cylindricus and wheat cultivar Mingxian 169.The former were infected and produced uredia with high infection type.But the latter represented only necrosis and chlorosis without uredium.This result showed that the Berberis serve as alternate host for Pse.Pse and Pst share the same alternate host that was biological basis for studying on sexual hybridization between the two formae speciales.3.Sexual recombination between formae speciales is a new approach for resulting in virulence variation of Pst.Sexual hybridization between Pse and Pst was successfully carried out on the B.shensiana,achieving evidence for possibility of sexual hybridization between formae speciales of P.striiformis.This way of sexual recombination resulted in virulence variation of Pst to produce new races.Basidiospores derived from germinated teliospores of Pse and Pst were inoculated on seedlings of B.shensiana,respectively.While pycnia were observed obviously on the upper surface of B.shensiana leaves,sexual hybridization between Pse(donator)and Pst(receptor)were artificially conducted.After aecia successfully generated on the backside of B.shensiana leaves,Sixty one selfed progeny isolates were obtained from 29 different aecial cups.The 61 progeny isolates,together with race CYR32 as the parent isolate of Pst,were tested virulence/avirulence on 27 Yr near isogenic lines(NILs)of wheat and 92R137(Yr26).Of the 61 progeny isolates,only 2 were similar to the parent isolate CYR32 in virulent spectrum,the other 59(96.7%)were different from the parent isolate CYR32.Thirty-seven pathotypes were identified among the progeny population.It was demonstrated firstly that sexual genetic recombination between formae specia resulted in virulence variation to produce new pathotypes and caused a high level of virulence variation.In this study,sexual genetic recombination between Pse and Pst was determined and revealed that between formae speciales of P.striiformis was practical and lead to remarkable virulence variation.It is a new approach for producing virulence variation by sexual reproduction.This study showed that sexual hybridization could produce new virulence phenotypes and new genotype sandim proves strategies for a comprehensive improvement of wheat stripe rust in China. |
