Zhuanfang Hao

Meta-analysis of constitutive and adaptive QTL for drought tolerance in maize

The response of plants to drought stress is very complex and involves expression of a lot of genes and pathways for diverse mechanisms and interactions with environments. Many quantitative trait loci(QTL) mapping experiments have given heterogeneous results due to use of different genotypes and populations tested in various environments. Our purpose was to identify some important constitutive and adaptive QTL using meta-analysis and to find specific genes and their families for speculating on drought tolerance networks. A total of 239 QTL detected under water-stressed conditions and 160 detected under control conditions from 12 populations tested in 22 experiments were compiled and compared, resulting in identification of 39 consensus QTL under water stress, and 36 under control conditions. Of them, 32 consensus QTL were supposed to be adaptive while others were constitutive QTL. The consensus QTL on chromosomes 1, 2, 3, 5, 6 and 9 were highly overlapped with several different traits and could be identified under multiple environments, most of which were related to traits of high phenotypic variance. Moreover, 48 candidate genes related to stress tolerance were located in silico in these consensus QTL regions what should facilitate the construction of QTL networks and help to understand the mechanisms related to drought tolerance.

Trends of grain yield and plant traits in Chinese maize cultivars from the 1950s to the 2000s

Retrospective analyses may provide an understanding of unexploited genetic potential and indicate possible pathways for future yield improvement. The objectives of this study were to present maize(Zea mays L.)yield trends and plant traits changes for maize cultivars from the 1950s to the 2000s in China. Trials were conducted at three locations in 2007 and 2008, and at four locations in 2009. Twenty-seven single hybrids, four double-cross hybrids, and four open-pollinated varieties, were grown at three densities at each location each year. 56% of total yield gain was contributed to breeding from 1950 to 2000. New hybrids had more resistance to compound stress. Levels of response of all hybrids to higher-yielding environments were similar, and greater than that of OPVs. All maize cultivars showed morphological changes for all characteristics tested in a volatile manner from 1950 to 2000, except for relatively stable leaf number. ASI decreased and tolerance to root lodging improved, which were enhanced at higher plant densities. There were no trends for other characteristics at higher densities. Shorter maturity, smaller plant size and more tolerance to root and stalk lodging will be required for further yield improvement. Chinese maize yield improvement can benefit from agronomic strategies at higher plant densities.

Low-nitrogen stress tolerance and nitrogen agronomic efficiency among maize inbreds: comparison of multiple indices and evaluation of genetic variation

Limited information is available on genetic variation in low-nitrogen (low-N) stress tolerance and N-use efficiency (NUE) among maize inbreds. To unveil this information, a panel of 189 diverse maize inbred lines was evaluated under contrasting levels of N availability over 2xa0years. Low-N agronomic efficiency (LNAE), absolute grain yield (GY) at low-N conditions, and the ratio between GY at low-N and optimum-N conditions were taken into account to represent low-N tolerance. Additionally, N-agronomic efficiency (NAE) along with other agronomic traits was also analyzed. Analysis of variance revealed significant effects of genotype on LNAE, NAE, and GY. The estimated broad-sense heritability was 0.38 for LNAE while it was only 0.11 for NAE, implying that selection based on LNAE should be more effective than NAE. LNAE exhibited highly positive genotypic and phenotypic correlations with GY, ear kernel number (EKN), kernel weight, plant height (PH), and chlorophyll content at low-N conditions, while it was negatively correlated with grain-N content and anthesis-silking interval. Path analysis indicated that the EKN at low-N stress had the highest positive effects on LNAE.

Breeding potential of U.S. maize germplasm for utilization in Chinese temperate conditions

U.S. maize germplasm has been considered as a new alternative source of favorable alleles to broaden the narrowing genetic base of temperate germplasm. However, direct utilization of diverse U.S. germplasm in target mega-environments could be hampered by limited adaptation-related information on parental performance or heterotic response in crosses with local germplasm. The objective of this study was to measure the performance of U.S. maize populations for broadening the Chinese germplasm base. Fifteen diverse U.S. populations were testcrossed to three local tester lines, representing Chinese heterotic groups A, B, and D, to evaluate parental adaptation and combining abilities for days to silking (DS), ear height (EH), and grain yield (GY) in target and intermediate mega-environments in northern China. There was genetic variability among U.S. maize populations for additive and non-additive effects for DS and GY, and predominant additive genetic effects for EH. All 15 U.S. populations, except for BS27 and BS31 due to slightly high EH, could be directly used in target mega-environments similar to the U.S. Corn Belt. U.S. populations BS11(HI)C7, BS13(S)C7, BS17(CB)C4, BS31, BSCB1(R)C12, and BSBB(SRCB)C4 had better effects for increasing GY. Favorable effects for DS were also observed in BS11(HI)C5 and BS31, for DS and EH in BS13(S)C7 and BSCB1(R)C12, and for EH in BSBB(SRCB)C4. The best strategies for utilizing these germplasms may be to introgress BS13(S)C7 and BS17(CB)C4 into group A, BS11(HI)C5 into group B, and BSCB1(R)C12 and BSBB(SRCB)C4 into group D to increase genetic variation within Chinese heterotic pools.

Heterosis and combining ability of seven maize germplasm populations

Understanding the combining ability and heterosis of available germplasm is a prerequisite for successful maize improvement and breeding. The objectives of this study were to analyze the combining ability and heterosis of seven representative maize germplasm populations, and further, to evaluate their potential utility in germplasm improvement. A total of 21 crosses were made among these seven populations in a complete diallel without reciprocals. The parental populations and 21 crosses were evaluated for days to silking (DS), ear height (EH), and grain yield (GY) in the Northeast and Yellow and Huai River maize growing areas in China in 2012. Csyn5, Csyn7, Cpop.11, and Cpop.12 had desirable general combining ability (GCA) effects for DS and EH in both the Northeast China mega-environment (NCM) and the Yellow and Huai River Regions of China mega-environment (YHCM). Cpop.11 possessed a favorable GCA effect for GY in the NCM, as did Csyn5, Cpop.17, and Cpop.18 in the YHCM. Csyn6 and Csyn7 exhibited tremendous yield-enhancing potential in both mega-environments. Additionally, six combinations including Csyn7xa0×xa0Csyn6, Csyn5xa0×xa0Csyn6, Cpop.11xa0×xa0Cpop.18, Cpop.12xa0×xa0Cpop.17, Csyn7xa0×xa0Cpop.17, and Csyn5xa0×xa0Csyn7 exhibited better specific combining ability effects for GY, yield performance, and mid-parent heterosis in the appropriate mega-environment. These results indicated that the seven populations would be very useful for the improvement of related agronomic traits, and the six candidate combinations possessed great potential for further improvement and utilization.

Identification of loci contributing to maize drought tolerance in a genome-wide association study

Genome-wide association studies (GWAS) have been used widely to analyze the genetic control of complex traits in crops. In the present study, seven related phenotypic traits were analyzed in combination to study their association with 41,101 SNPs in 201 maize inbred lines that had been evaluated in seven environments (year/location combinations) under water-stressed (WS) or well-watered (WW) regimes. By comparing the association signals with a fixed P value, GWAS showed that the number of association signals identified varied among traits and in different environments. Data that were missing under the severe water stress treatment had a great impact on the results of this GWAS. A total of 206 significant SNPs were associated with 115 candidate genes for drought tolerance and related traits including final grain yield, total number of ears per plot, kernel number per row, plant height, anthesis-silking interval, days to anthesis (DtA), and days to silking (DtS). Among these, four genes were associated with at least two different related traits, and six genes associated with traits were detected in at least two environments under water stress. Nine candidate QTL identified by GWAS were also discovered, three of which co-located to a consensus QTL region meta-analyzed by linkage mapping for drought tolerance. Some regulatory genes related to abiotic stress responses might also make a strong contribution to drought tolerance. The comprehensive information presented here regarding consensus QTL combined with candidate genes derived from GWAS provides an important reference tool for improving maize drought tolerance.

Breeding potential of maize germplasm populations to improve yield and predominant heterotic pattern in Northeast China

Increasing the frequency of favorable alleles in elite germplasm is necessary for germplasm improvement and innovation in maize (Zea mays L.) breeding programs. Jidan27 (Si287xa0×xa0Si144), an elite commercial single cross hybrid that represents the predominant heterotic pattern Chinese Sipingtouxa0×xa0Stiff Stalk Synthetic, has been widely cultivated in the Northeast China Corn Belt. However, the use of the hybrid Jidan27 has been hampered gradually due to its low yield potential, lack of resistance to lodging, and considerable incidence of head smut disease. Previous studies have evaluated yield potential of exotic populations, but few have considered using these germplasms as donors with favorable alleles to overcome these deficiencies in the hybrid Jidan27. In the present study, 12 exotic populations from CIMMYT and the U.S. and three semi-exotic populations were investigated as sources of favorable alleles to increase yield and lodging resistance, and reducing head smut susceptibility of the hybrid Jidan27. The performance of a set of 30 inbredxa0×xa0populations crosses, the hybrid Jidan27, and the two parental inbred lines Si287 and Si144 was evaluated in three locations for 2xa0years. Two populations, BS10(FR)C10 and Pop43, were identified as the best donors of favorable alleles lacking in the hybrid for improving the target traits. Further, BS13(S)C7 and Cpop.16 were also found to be better choices for improving grain yield and head smut resistance of the hybrid. These results suggest that the elite exotic germplasms evaluated here could provide useful genetic variability to improve other Chinese Sipingtouxa0×xa0Stiff Stalk Synthetic crosses related to Si287xa0×xa0Si144 that are widely grown in Northeast China.