Study On Genetic Basis Of Resistance To Gray Leaf Spot Of Maize In Mountain Area Of Southwest China

Maize is the largest crop in China in regarding to sown area.Gray Leaf Spot（GLS）is becoming a major disease of maize in the southwest mountain area.In view of the adaptation of current varieties,cultivating and planting GLS-resistant maize varieties is the most cost-effective way.In this study,the genes related to the resistance of GLS in southwest area of china were researched.The main contents of this study were as follows:（1）Following NCⅡmating design,the genetic patterns of maize agronomic,quality and physiological traits of 175 hybrids were analyzed in Sichuan Luding and Yunnan Baoshan under GLS stress.（2）The QTL effects of The F₂ population,constructed by using the GLS-susceptible inbred line 08-641.（S8）and the GLS-resistant inbred line 646（R6）,were analyzed with composite interval mapping method（CIM）.（3）The leaves of the GLS-susceptible inbred line 08-641 and the GLS-resistant inbred line 646 were sequenced after inoculation,and the resistance mechanism of GLS was analyzed.The main conclusions of this study were as follows:1.The disease area has significant negative correlation with biological yield,grain yield,hundred-grain weight,ear coarse,kernel number per row,crude fat content,crude protein content,crude starch content,PI,Fv/Fo,SPAD and Fv/Fm;and significant positive correlation with barren ear tip length and Fo/Fm under GLS stress.The narrow heritability（h_N²）was as follows:hundred-grain weight>biological yield>grain yield>disease area>starch content>fat content>SPAD>Fv/Fm>PI,100-grain weight,biological yield,grain yield and disease area were mainly affected by the additive effect of genes,and early selection was more effective.The additive and non-additive effects of starch content were similar,and the fat content and protein content were mainly affected by non-additive effect.The results showed that the SPAD,Fv/Fm and PI of the physiological traits were influenced by the environment,and dominated by the non-additive effect of the genes.A new evaluation criterion for selecting maize varieties with high yield,good quality and GLS-resistance,in which the principal component analysis was used to score 9 indices including disease area,hundred-grain weight,starch content,PI,Fv/Fm and so on,is put forward.2.Five GLS resistance QTLs were detected on chromosomes 1,2,3,4,and 6,which explained6.7-21.3%ofthephenotypicvariation.TheQTLs qRgls.CH-4,qRgls.CH-1,qRgls.CH-2,and qRgls.CH-6 were stably expressed in the four environments,and all loci for GLS resistance derived from the resistant parent 646.The additive effects of qRgls.CH-4,qRgls.CH-1,and qRgls.CH-6 were significantly greater than their single dominant effects,which may be beneficial for GLS resistance breeding.Compared to previous studies,QTL qRgls.CH-6,located at bins 6.02-6.05,did not overlap with any other known resistance QTL,which was identified for the first time.QTL analysis of PI（leaf performance index）detected three leaf function QTLs related to GLS resistance on chromosomes 4,8,and 9,which explained 4.8-6.2%of the phenotypic variation.Among them,qPI.CH-4 was significantly stronger expressed in several environments,the locus associated with increased leaf function came from the resistant parent 646,and its interval overlapped with the qRgls.CH-4.Furthermore,both the qRgls.CH-4 and the qPI.CH-4 were located in a hotspot area for GLS resistanceat bins4.05-4.06,indicating that GLS resistance was significantly related to leaf performance,and GLS significantly reduced leaf photosynthetic performance.3.A total of 351,404,566 raw reads were obtained from the two inbred lines after RNA sequencing.FPKM（per kilobase of exon model per million mapped reads）was used for analyze the expression level of the gene.A total number of 9627 genes were differentially expressed in the two inbred lines,5471 in 08-641 and 8069 in R6,respectively,and 2462 genes were co-upregulated,and 2228 genes were co-downregulated.the biochemical process of the 2541 constitutive differential genes with high expression level in R6 were significantly enriched in respiratory electron transport chain,purine nucleoside triphosphate metabolic process,defense response to fungus,incompatible interaction and so on.After inoculation,the resistant inbred line R6 had more specific up-regulated or down-regulated genes that related to defense response;whereas,the different Expressed genes of up-regulated or down-regulated in the susceptible inbred lines 08-641 were related to growth and development.53 Resistance-related Genes,including 20 PTI related genes,5 ETI related genes,24 other defense related genes,and 4 plant hormone related genes,were screened out based on their biological functions.In contrast to the susceptible inbred lines 08-641,the resistance inbred lines R6 has more PTI-,ETI-,plant hormones and other resistance-related genes,which mainly enriched in Linoleic acid metabolism,Cyanoamino acid metabolism and alpha-Linolenic acid metabolism.4.Compared with the resultsof QTL analysis with the same materials（646,08-641）,one candidate gene（GRMZM2G702599:139,022,453:139,023,167）with GLS resistance on Chr.4 analysed by transcriptome analysis was located in the interval of the qRgls.CH-4（135,229,161-166,915,798）;Threecandidategenes（GRMZM2G171114:103,870,389:103,873,917;GRMZM2G135108:125,194,486:125,196,087;GRMZM2G447795:129,268,963:129,270,180）was located in the interval of the qRgls.CH-6（91,124,061-132,980,851）.The results indicated that transcriptome and QTL analysis were two reliable methods for studing on GLS-resistance.