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BackgroundBreeding for cold tolerance in maize promises to allow increasing growth area and production in temperate zones. The objective of this research was to conduct genome-wide association analyses (GWAS) in temperate maize inbred lines and to find strategies for pyramiding genes for cold tolerance. Two panels of 306 dent and 292 European flint maize inbred lines were evaluated per se and in testcrosses under cold and control conditions in a growth chamber. We recorded indirect measures for cold tolerance as the traits number of days from sowing to emergence, relative leaf chlorophyll content or quantum efficiency of photosystem II. Association mapping for identifying genes associated to cold tolerance in both panels was based on genotyping with 49,585 genome-wide single nucleotide polymorphism (SNP) markers.ResultsWe found 275 significant associations, most of them in the inbreds evaluated per se, in the flint panel, and under control conditions. A few candidate genes coincided between the current research and previous reports. A total of 47 flint inbreds harbored the favorable alleles for six significant quantitative trait loci (QTL) detected for inbreds per se evaluated under cold conditions, four of them had also the favorable alleles for the main QTL detected from the testcrosses. Only four dent inbreds (EZ47, F924, NK807 and PHJ40) harbored the favorable alleles for three main QTL detected from the evaluation of the dent inbreds per se under cold conditions. There were more QTL in the flint panel and most of the QTL were associated with days to emergence and ΦPSII.ConclusionsThese results open new possibilities to genetically improve cold tolerance either with genome-wide selection or with marker assisted selection.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0816-2) contains supplementary material, which is available to authorized users.

Selection against pink stem borer (Sesamia nonagrioides Lef) (PSB) attack was performed in the maize (Zea mays L.) synthetic EPS12. The direct response of the EPS12 population to three cycles of selection to reduce tunnel length damage by PSB while maintaining yield was evaluated. S0 (cycles of selection) and S1 (selfed cycles of selection) generations and testcrosses to three testers were evaluated under artificial infestation with two corn borers, PSB and European corn borer (Ostrinia nubilalis Hubner) (ECB). Genotypes and the genotype × environment interaction were significant for tunnel length and yield. Differences among cycles of selection were significant for tunnel length, and the linear decrease for this trait (−1.80 cm cycle−1) achieved during selection accounted for most of these differences. Yield was not significantly reduced with selection. Inbreeding for yield significantly increased due to selection. EPS12 crossed to EP42 showed a significant increase in yield with selection, while crosses to other testers showed a nonsignificant reduction in yield. Although crosses to EP42 were not significantly different for tunnel length, the high heterosis between EPS12 and EP42 and the increased yield of EPS12 × EP42 during selection suggest that inbred lines developed from advanced cycles of EPS12 could be crossed to EP42 to generate promising hybrids. In general, resistance to PSB and ECB was improved, while yield was maintained, inbreeding was increased, and yield of the cross EP42 × EPS12 was improved.

Maize (Zea mays L.) for northern growing areas requires cold tolerance for extending the 2 vegetative period. Our objectives were to evaluate two large panels of maize inbred lines 3 adapted to Europe for cold tolerance and to estimate the effects of cold-related traits on 4 biomass production. Two inbred panels were evaluated for cold tolerance per se and in 5 testcrosses under cold and control conditions in a growth chamber and under field 6 conditions. Comparisons of inbreds and groups of inbreds were made taking into account 7 the SNP-based genetic structure of the panels, and the factors affecting biomass 8 production were studied. Eight flint and one dent inbreds with diverse origins were the 9 most cold tolerant. The most cold tolerant dent and flint groups were the Iodent Ph207 10 and the Northern Flint D171 groups, respectively. The relationships between inbred per se 11 and testcross performance and between controlled and field conditions were low. 12Regressions with dry matter yield in the field as dependent variable identified plant height combining ability, as well as between both groups and the Northern Flint D171 group. 20Key words: maize, cold tolerance, abiotic stress, germplasm. period that can potentially increase yield and stability, and the probability of escaping 8 summer drought stress (Kucharik, 2006). This is particularly true in some temperate areas, 9 where springs are cold and rainy and summers are hot and dry. But early sowing in 10 temperate areas requires cold tolerance and, consequently, the interest of breeders in cold 11 tolerance is increasing (Darkó et al., 2011; Frascaroli and Landi, 2013; Revilla et al., 2005; 12 Strigens et al., 2012 12 Strigens et al., , 2013. In the northwest of Spain, a breeding goal would be to advance 13 maize sowing two weeks, i.e. from May to mid April, because there are no late frosts. 14 However, in northern areas the gain could be just a few days. 15The main handicap for breeding programs intending to improve cold tolerance in 16 maize has been the narrow genetic base for this trait (Greaves, 1996; Revilla et al., 2005). were not more cold tolerant than the checks. Actually, they were similar to the improved 21 populations and checks already known, including commercial checks and the best cold explaining between 5.7 and 52.5% of the phenotypic variance for early growth and 10 chlorophyll fluorescence. They propose the use of whole genome prediction approaches 11 rather than classical marker assisted selection to improve the chilling tolerance of maize. 12Other major obstacles for breeders are that cold tolerance has large experimental 13 errors, a strong genotype by environment interaction, and a complex genetic regulation 14 (Revilla et al., 2000, Strigens et al., 2013. Moreover, evaluations for cold tolerance are not 15 accurate enough for a precise discrimination of cold tolerance. This is due to two facts. , 1981;Verheul et al., 1996). Given that most previous reports have faced 16 the problem by using limited resources, we belie...

Two corn borer species are the principal maize insect pests in Europe, the European corn borer, Ostrinia nubilalis (Hübner), and the pink stem borer, Sesamia nonagrioides (Lefebvre). Hence, it would be advisable to evaluate the European maize germplasm for corn borer resistance to generate European varieties resistant to corn borer attack. The creation of the European Union Maize Landrace Core Collection (EUMLCC) allowed the screening of most of the variability for European corn borer resistance present among European maize local populations from France, Germany, Greece, Italy, Portugal, and Spain, testing a representative sample. The objective of this study was the evaluation of stem and ear resistance of the EUMLCC to European corn borer and pink stem borer attack. Trials were made at two Spanish locations that represent two very different maize-growing areas. Populations that performed relatively well under corn borer infestation for stem and ear damage were 'PRT0010008' and'GRC0010085', among very early landraces; 'PRT00100120' and 'PRT00100186', among early landraces; 'GRC0010174', among midseason landraces; and 'ESP0070441', among late landraces. Either the selection that could have happen under high insect pressure or the singular origin of determined maize populations would be possible explanations for the higher corn borer resistance of some landraces. Landraces 'PRT0010008', 'FRA0410090', 'PRT00100186', and 'ESP0090214' would be selected to constitute a composite population resistant to corn borers and adapted to short season, whereas populations 'ESP0090033', 'PRT00100530', 'GRC0010174', and 'ITA0370005' would be used to make a resistant composite adapted to longer season.