Genetic Dissection Of The Maize(Zea Mays L.) MAMP Response And Fine Mapping Of A Resistance QTL For Gray Leaf Spot Of Maize Derived From Teosinte (Z.mays Ssp.parviglumis)

Breeding for disease resistance is a major goal for maize breeding.Generally,plants have two kinds of disease resistance mechanisms:qualitative resistance and quantitative resistance.Qualitative resistance is mostly controlled monogenically or oligogenically,which can confer high resistance,but this kind of resistance will loss when races of pathogens change.Quantitative disease resistance(QDR)is a kind of durable resistance,which is usually controlled by several genes with small effect.Microbe-associated molecular patterns(MAMPs)are highly conserved molecules commonly found in microbes which can be recognized by plant pattern recognition receptors(PRRs).Recognition triggers a suite of responses including production of reactive oxygen species(ROS)and nitric oxide(NO)and expression changes of defense related genes.But our knowledge about the relationship between MAMPs responses and QDR is very limited.Gray leaf spot(GLS)has been one of the most severe foliar diseases threatening maize production,especially in western mountains areas of China.Many researchers have been working on the genetic basis of resistance for GLS.Some QTLs for GLS resistance have been identified,while only several of which have been finely mapped.In this study,we performed two experiments:1.we used two well-studied MAMPs(flg22 and chitooctaose)to challenge different maize lines to determine whether there was variation in the level of responses to these MAMPs,and then used one recombinant inbred lines(RIL)population to dissect the genetic basis underlying that variation and to understand the relationship between MAMP response and QDR;2.we developed F2,F2:3,F4 progenies using the GLS resistance near isogenic line(NIL)Z032E0081 and the susceptibility NIL Z033E0056,and then used these populations to fine map one QTL—Ogls8 from teosinte for GLS resistance.The results are as follows:1.Quantitative variation in NO production and defense genes(ECA、PEX3、PR1 and ECPR4)expression levels triggered by MAMPs was observed in NAM population parental lines B73、CML52、CML322、CML333、Ky21、Ki11、Ki3 and IL14H.As for genes expression,flg22 could not significantly induce ECA expression across different lines,but ECA was strongly induced by chitooctaose in B73;ECPR4 was strongly induced by chitooctaose;both MAMPs triggered strong PEX3 responses in line IL14H,but there were no significant responses in other lines;flg22 and chitooctaose triggerd strong responses of PR1 only in line Ky21.As for the NO production,chitooctaose could strongly trigger the NO production of B73;flg22 more strongly triggered NO production in Kill compared with chitooctaese;NO production was strongly induced by both flg22 and chitooctaose in CML52、CML322、Ki3 and IL14H.2.Based on the above results,we detected MAMPs-triggered ROS production of all of the 26 NAM population paretnal lines.Naturally-occurring variation of ROS production triggered by MAMPs was observed across different parental lines.Compared with B73,CML228 had the strongest responses to both flg22 and chitooctaose,so we investigated the ROS responses of one RIL population derived from the cross of B73×CML228,and used QTL mapping to dissect the genetic basis of this variation.A major quantitative traits locus(QTL)associated with variation in the ROS production response to both flg22 and chitooctaose was identified on chromosome 2.Minor QTLs associated with variation in the flg22 ROS response were identified on chromosomes 1 and 4.Comparison of these results with data previously obtained for variation in QDR for northern leaf blight(NLB),southern leaf blight(SLB),gray leaf spot(GLS)and the defense response in the same RIL population did not provide any evidence for a common genetic basis controlling variation in these traits.3.Our team has identified one GLS resistance QTL—Qgls8 on chromosome 8 using one set of NILs poplation in which segments of the teosinte(Zea mays ssp.parviglumis)genome had been introgressed into the background of the maize line B73.Interestingly,there were two alleles from different NILs:compared with B73,the allele from Z032E0081 increased GLS resistance,while another one from Z033E0056 decreased resistance.In this study,we developed segregating populations at the Qgls8 locus using the two NIL parents(Z032E0081 and Z033E0056)which carried contrasting alleles of Qgls8.In the summer of 2015 and 2016,GLS phenotypes of F2:3 and F4families were scored in both Andrews(North Carolina)and Kentland Research Farm,Blacksburg(Virginia)respectively,combined with molecular markers analysis,Qgls8 was located into a～130 kb region(based on B73 reference genome sequence,v3)which encompassed 5 candidate genes,which were predicted to encod receptor-like protein kinase、ABC transporter ATP-binding protein or other kinds of proteins.