| Maize（Zea mays L.）is an important crop for food,feed,and industrial raw-material support.Meanwhile,drought and high temperature are the main abiotic factors affecting maize yield.The seedling stage is the primary phase of maize organs development,which directly affects the subsequent maize growth and yield formation.Therefore,it is of great scientific significance to explore the key loci or genes regulating drought and high temperature tolerance in maize.This is essential for breeding new maize varieties with enhanced tolerance at the seedling stage and ensuring high maize yield.In this study,280 maize inbred lines were used to evaluate five important agronomic traits of seedling survival rate（SR）,chlorophyll content（SPAD）,shoot fresh weight（SFW）,root fresh weight（RFW）and fresh weight（FW）under drought and high temperature stress treatment at the seedling stage.Using the 180,058 high quality single nucleotide polymorphism（SNP）markers,the genetic structure of 280 materials was evaluated.Three models（MLM,mrMLM,and FASTmrMLM）were used for genome-wide association（GWAS）analysis,and candidate genes for drought tolerance and high temperature tolerance in maize were identified.The main results are as follows:1.Analysis of phenotypic data based on best linear-unbiased prediction（BLUP）of five agronomic traits:（1）The phenotypic data under normal and drought conditions showed that the heritability of five traits varied from 0.55（RFW）～0.82（SFW）under normal conditions;under drought condition,the heritability range of five traits was 0.32（SR）～0.63（SFW）.The average survival rate was 83.00%.Meanswhile,the average chlorophyll content,shoot fresh weight,root fresh weight and fresh weight decreased by 28.93%,57.38%,61.28%and 57.72%,respectively.（2）Phenotypic data under normal and high temperature conditions showed that the heritability range of five traits was 0.69（SPAD）～0.89（SFW）under normal conditions;under high temperature condition,the heritability range of five traits was 0.77（SFW）～0.91（SPAD）.The average survival rate was 45.00%,and the average chlorophyll content,shoot fresh weight,root fresh weight and fresh weight decreased by 39.92%,71.88%,74.58%and 80.44%,respectively.（3）Compared to drought stress,the average survival rate of maize under high temperature stress decreased by 38.00%,and the average value of chlorophyll content,aboveground fresh weight,root fresh weight and total fresh weight decreased by 10.99%,14.50%,13.10%and 22.72%,respectively,indicating that long-term high temperature stress had a more serious impact on maize seedling stage.2.Genetic analysis of population structure showed that 280 inbred lines could be divided into temperate and tropical subgroups,containing 45 and 235 inbred lines,respectively.The temperate subgroup was mainly composed of the Expired Plant Variety Protection（Ex-PVP）lines,and the tropical subgroup was mainly composed of the Drought Tolerant Maize for Africa（DTMA）lines.3.MLM,mrMLM and FASTmrMLM models were used for GWAS analysis of five important agronomic traits in maize at the seedling stage.（1）Under normal and drought conditions,116 and 384 significant drought-resistant SNPs were detected,among which 89 SNPs were detected by two or more methods.（2）Under normal and high temperature conditions,202 and 153 significant high temperature tolerance SNPs were detected,respectively,of which 31 SNPs were detected by the combination of two or more methods.（3）103 SNPs were significantly correlated with multiple traits under drought tolerance and high temperature tolerance stress.Among them,75 SNPs were correlated with SFW and FW under drought condition,and 17 SNPs were correlated with SFW and FW under high temperature condition.S7155225900 associated with four traits,i.e.,SFW and FW under drought condition and RFW and FW under high temperature condition.4.There were 384 SNPs associated with drought tolerance corresponding to 185 genes,which significantly enriched in 39 GO annotations.There were 153 SNPs associated with high temperature tolerance corresponding to 72 genes,which significantly enriched in 37 GO annotations.There were 15 GO annotation classifications that were significantly enriched in both biological processes,among which response to stimulus was closely related to stress.22 genes were enriched in response to stimulus under drought conditions,and eight genes were enriched in response to stimulus under high temperature conditions.5.Eight candidate genes were screened by combining the SNPs detected by multiple methods and the GO analysis results.Among them,four genes（Zm00001 eb022180,Zm00001eb221970,Zm00001eb045420,Zm00001eb349780）were related to drought tolerance,two genes（Zm00001eb139570,Zm00001eb386160）were related to heat tolerance,two genes（Zm00001eb172900,Zm0000 1 eb266260）were related to drought and heat tolerance.qTeller database was used to analyze the expression levels of candidate genes under drought and high temperature conditions.The results showed that Zm0000 1eb022180 was down-regulated in ovary,and Zm00001eb221970 and Zm0000 1 eb349780 were up-regulated in leaf meristem under drought conditions.Zm0000 1 eb045420 was down-regulated in leaf meristem.Zm00001eb139570 was down-regulated at high temperature.Zm00001eb172900 was down-regulated in both drought and high temperature conditions,and Zm00001eb266260 was up-regulated in meristem of maize leaves under drought conditions and up-regulated in high temperature conditions.These results indicated that they were involved in maize response to drought and high temperature stress.To sum up:In this study,280 maize inbred lines with wide diversity were used to explore the genetic variation associated with drought tolerance and high temperature tolerance at the maize seedling stage through genome-wide association analysis.This research has important application value for breeding new maize varieties with drought tolerance and high temperature tolerance and ensuring maize yield. |
