Introgression Of Powdery Mildew Resistance Gene Pm56 On Rye 6RS Into Wheat

Powdery mildew is an important wheat disease.The use of resistance genes is a useful strategy for the disease control.Wild relative species are the important resistance gene pool.Rye(Secale cereale L.,2n=2x=14,RR)is the first specie using for wheat resistance breeding in the tertiary gene pool.Four named powdery mildew resistance genes from rye have been transferred to wheat,including Pm8/Pm17 on IRS,Pm7 on 2RL and Pm20 on 6RL.Due to the presence of virulent races of the pathogen,only Pm20 is currently still effective.The Chinese cereal rye cultivar Qinling is immune to powdery mildew.We obtained nine 6R(6A)disomic substitutions immune to powdery mildew by persistently selfing the hybrid between D-2-3-4 and Qinling.In the present study,a 6R(6A)disomic substitution was crossed to susceptible Kaixian-luohanmai and CSph1b to generate double monosomic 6R(6A)plants.The chromosome composition of their selfing progenies were then analyzed using genomic in situ hybridization(GISH),fluorescence in situ hybridization(FISH),and molecular makers to screen new wheat-rye translocation lines.Meanwhile,we tried to map the resistance gene(s)by comparing the reaction to powdery mildew infection of plants with different 6R chromosome fragments in greenhouse.The main results are as followed:The meiotic chromosome behaviors of double monosomic 6R(6A)hybrids were analyzed by GISH using rye genomic DNA as the probe.The monosomic 6R and 6A chromosomes usually behaved as unpaired univalents at metaphase I;In the following stages,we observed precocious separation of sister chromatids and 6R and 6A chromosome fragments during meiosis I and wheat-6R translocations during meiosis II.This indicated that monosomic 6R and 6A chromosomes were prone to break and breakage-fusion.In total of 69 first generation selfed progeny of 6A/6R double monosomic plants were characterized using molecular maker and FISH/GISH;of these,25 were descended from 6R(6A)× CSph1b hybrids,and 44 from 6R(6A)× KL hybrids.14%(10/69)progeny lacked both the 6AS and 6AL markers,and hence were deemed to lack the entire chromosome;similarly,26%(18/69)lacked the markers for both arms of 6R,so were deemed to have lost the entire 6R.Fourteen(20%)harbored either a misdivided or a translocated version of 6R.The 14 misdivided or translocated versions were classified into the following six types:1)A telosome,either 6RS or 6RL;2)An isochromosome for 6RL;3)A near entire chromosome 6RL 6RSd lacking the telomeric end of 6RS;4)A chromosome 6RSpi possibly carrying an paracentric inversion;5)The Robertsonian translocation 6AL·6RS,and the non-Robertsonian translocations·6AS·6RS and 4AL·6RS;6)A form of 6R(6Ra)carrying two additional major pSc119.2 sites towards the end of 6RL.The transmission frequency of different version of 6R structural variations were different:T6RS· 6AL(8/11,73%)>T6RS·6AS(5/13,38%)>6RL·6RSd(2/7,29%)= 6RS(2/7,29%)>6RSpi(1/8,13%).Plant LM47-6 was disomic for the T6RS·6AL translocation.All the DS6R(6A)/CSphlb and DS6R(6A)/KL 6R(6A)double monosomic plants were immune to powdery mildew.All of their first selfing generation plants harboring an entire 6R proved to be immune to powdery mildew,whereas all those without 6R chromatin were highly susceptible,confirming that powdery mildew resistance was conferred by gene(s)mapping to chromosome 6R.When the disease response of plants derived from the two cross harboring incomplete forms of 6R were compared,it was clear that the presence of the short arm was associated with immunity,while the presence of the long arm alone rather only a high level of resistance.The implication was that both arms harbor a resistance gene(s),with the one(s)on the short arm being more effective than the one(s)on the long arm.Since the plants with the truncated form of 6R(LM99,HM99,HM99-2,HM99-8 and HM103-2)lacked the 6RS subtelomeric region between two pSc119.2 hybrid signals and plants with 6RL showed similar resistance to powdery mildew,the assumption was that the gene conferring immunity to powdery mildew lies in the truncated segment,named as Pm56.