Establishment And Application Of Maize Heat Resistance Evaluation System

Maize is one of the main food crops in China and plays an important role in the national economy.However,in recent years,with the global warming and the frequent occurrence of extreme high temperature weather,heat stress has become one of the main abiotic stresses affecting maize yield,which poses a great threat to China’s food security.Screening and cultivating heat-resistant maize varieties is one of the most direct and effective ways to reduce the impact of heat damage on maize.Finding genetic variations that lead to differences in maize heat tolerance and discovering heat-resistant genes are the primary tasks for cultivating heat-resistant varieties.The heat-resistant identification and evaluation system is very important for screening and cultivating excellent heat-resistant varieties of maize and reducing the impact of heat damage on maize.In this study,established an accurate,stable and convenient identification and evaluation system of maize heat resistance,and carried out heat resistance identification to maize varieties at the seedling and flowering stages.The results showed that there were significant differences in heat resistance among different inbred lines after heat treatment,and some excellent maize inbred lines with high heat resistance were screened.Using high-throughput genotype data and maize seedling and flowering heat stress phenotype data to conduct genome-wide association study,multiple SNPs and candidate genes were identified that were significantly associated with maize seedling and flowering heat tolerance.The main results of this study are as follows:1.The maize seedlings were treated at high temperature in an incubator,and the wilting ratio of the second leaf was used as a heat resistance evaluation index to identify the heat resistance of maize seedlings.According to this evaluation system,we screened3 extremely heat-resistant inbred lines at the seedling stage,namely CIMBL41,Chang7-2 and Dan340,from the Association Mapping Population composed of 323inbred lines.These inbred lines can provide excellent genetic resources for heat-resistant breeding in maize seedling stage.2.In order to identify the heat resistance of maize at flowering stage,we established a heat resistance evaluation system of maize at flowering stage.We took the tassel in the pollen-scattering stage outdoors,and then used an indoor incubator to treat the tassel at high temperature,took part of the pollen for tissue culture observation,and the other part of the pollen for field pollination.The relative pollen germination rate and fruiting grade were used as indicators,to measure the heat tolerance level of different inbred lines.We used the F₁ hybrids composed of 312 maize materials to identify the heat tolerance at flowering stage according to the heat tolerance evaluation system we established.CIMBL43,CIMBL107,CIMBL84 and M153 were identified as extremely heat resistant inbred lines at flowering stage.These lines can be used for heat tolerance breeding or heat tolerance gene mapping in maize during flowering.3.Based on about 1.24 million SNP markers covering the whole genome,combined with heat tolerance phenotype data,the genome-wide association study of heat tolerance traits in maize seedling stage was performed,and a total of 102 markers were identified that were significantly associated with phenotype.Then,54 candidate genes were screened by the physical location of significant SNPs site markers,of which6 candidate genes could be repeatedly mapped in at least 2 repeats.Significant markers chr9.S＿12719071 and chr3.S＿173376972 were 2 major loci and were detected in 2repeats.They both explained the maximum 11%of the phenotypic variation and to map candidate genes Zm00001d045089 and Zm00001d042661,respectively.Gene annotation revealed that Zm00001d045089 encodes a pentatricopeptide repeat protein involved in mitochondrial RNA editing and Zm00001d042661 encodes serine hydroxymethyltransferase 7.These significant markers and candidate genes provide potential genetic resources for heat tolerance research or molecular breeding in maize seedling stage.4.Genome-wide association study was performed on maize flowering heat tolerance using relative pollen germination rate and seed setting grade as phenotypes.Based on the relative pollen germination rate phenotype,a total of 71 SNP markers significant associated with the phenotype were identified,and these markers mapped to a total of 39 candidate genes.Among them,the significant marker chr5.S＿215300407has an effect value of 13%,and there are 6 significant markers near it,all of which can locate the candidate gene Zm00001d018455.It encodes 3-ketoacyl-Co A synthase 12.The gene Zm00001d002615 could be co-localized by 5 significant markers,among which the marker chr2.S＿16362014 has the most significant association with the phenotype(-log₁₀P=5.43).This candidate gene encodes a PH homeodomain superfamily protein.Taking the seed setting grade as the phenotype,a total of 100 SNPs were found to be significantly associated with the phenotype,and 57 candidate genes could be screened by these significant markers.The markers chr2.S＿16307301 and chr4.S＿171542923 are both strongly associated with phenotype,and there are multiple significant markers nearby.Their candidate genes are Zm00001d002611 and Zm00001d051915,which encode Copper-zinc superoxide dismutase copper chaperone and Protein kinase superfamily protein,respectively.These significant markers and candidate genes related to heat tolerance at flowering stage of maize can provide reference for heat resistance breeding work and molecular mechanism research of heat tolerance at flowering stage of maize.