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Soil-borne cereal mosaic virus (SBCMV), the causative agent of wheat mosaic, is a Furovirus challenging wheat production all over Europe. Differently from bread wheat, durum wheat shows greater susceptibility and stronger yield penalties, so identification and genetic characterization of resistance sources are major targets for durum genetics and breeding. The Sbm1 locus providing high level of resistance to SBCMV was mapped in bread wheat to the 5DL chromosome arm (Bass in Genome 49:1140–1148, 2006). This excluded the direct use of Sbm1 for durum wheat improvement. Only one major QTL has been mapped in durum wheat, namely QSbm.ubo-2B, on the 2BS chromosome region coincident with Sbm2, already known in bread wheat as reported (Bayles in HGCA Project Report, 2007). Therefore, QSbm.ubo-2B = Sbm2 is considered a pillar for growing durum in SBCMV-affected areas. Herein, we report the fine mapping of Sbm2 based on bi-parental mapping and GWAS, using the Infinium 90 K SNP array and high-throughput KASP®. Fine mapping pointed out a critical haploblock of 3.2 Mb defined by concatenated SNPs successfully converted to high-throughput KASP® markers coded as KUBO. The combination of KUBO-27, wPt-2106-ASO/HRM, KUBO-29, and KUBO-1 allows unequivocal tracing of the Sbm2-resistant haplotype. The interval harbors 52 high- and 41 low-confidence genes, encoding 17 cytochrome p450, three receptor kinases, two defensins, and three NBS-LRR genes. These results pave the way for Sbm2 positional cloning. Importantly, the development of Sbm2 haplotype tagging KASP® provides a valuable case study for improving efficacy of the European variety testing system and, ultimately, the decision-making process related to varietal characterization and choice. [ABSTRACT FROM AUTHOR]

Given the importance of early cold tolerance in maize ( Zea mays L.), we previously conducted a divergent selection for low (L) and high (H) cold tolerance at germination. The source population was the F of B73 × IABO78 single cross. Selection was first conducted as recurrent selection (four cycles) and then in inbreeding (three generations). Ten lines were finally selected for L and ten for H; these lines were then evaluated to investigate: (i) their field cold tolerance in the early autotrophic phase and the relationship with the cold tolerance expressed at emergence and in the heterotrophic phase; (ii) the inheritance of these cold tolerance traits; (iii) their agronomic performance as lines per se and as crosses; (iv) their SNP-based genetic characterization. For all cold tolerance traits, the H group of lines was superior to the L group; selection responses were often symmetric, suggesting the prevalence of additive effects. The relationship among cold tolerance traits was appreciable, suggesting a common genetic basis. Differences among testcrosses were largely due to general combining ability effects and the relationship between line per se and testcross performance was high, confirming the prevalence of additive effects. Correlated responses were also obtained for agronomic traits, the H lines being superior to L lines both per se and as crosses. For SNP-marker loci, the B73 allelic frequency was greater in the H group (61.7 %) than in L group (39.7 %). Across field trials, an H line excelled for all traits, thus proving to deserve further investigations. [ABSTRACT FROM AUTHOR]

Previous studies revealed that defoliation can bring about differential responses in maize ( Zea mays L.) genotypes for cold tolerance. This research was conducted to study the inheritance of the responses to defoliation for germination at low temperature and for seedling traits in early-sown field trials. Six inbred lines were crossed according to the complete diallel scheme, thus producing 36 genotypes. At milk ripening stage, half of the plants of each genotype were completely defoliated (D) whereas the other half were not (ND). Two experiments were conducted for two years, one in the germinator and one in the field. The response to defoliation was evaluated as (D-ND). Across the two years, the D treatment caused an average kernel weight decrease of −56 mg (−23.1 % as referred to ND), an increase of 2.3 % for germination at 9 °C (G9) and a reduction of −0.3 d for the average time of germination (ATG); in contrast, the defoliation effect was negligible for germination at 25 °C. In the field, the defoliation effects were more notable in the first (colder) year and led to an increase of 2.0 % for field emergence (FE) and to a decrease of −2.7 g for seedling fresh weight (SFW). The genotypic variation for the (D-ND) response was significant for additive, dominance and reciprocal effects for G9, ATG, FE and SFW. There was consistency among lines across traits for additive effects, with Lo1016 and Os420 showing the best and the worst effect, respectively. [ABSTRACT FROM AUTHOR]

Improving cold tolerance in maize ( Zea mays L.) is an important breeding objective, allowing early sowings which result in many agronomic advantages. Using as source the F population of B73 × IABO78 single cross, we previously conducted four cycles of divergent recurrent selection for high (H) and low (L) cold tolerance level, evaluated as the difference (DG) between germination at 9.5 °C and at 25 °C in the germinator. Then, we pursued the divergent selection in inbreeding from S to S. This research was conducted to study (1) the direct response to selection (by testing ten S L and ten S H lines), (2) the trait inheritance (in a complete diallel scheme involving four L and four H lines), (3) the associated responses for cold tolerance in the field (at early and delayed sowings) and (4) the responses for other traits, by testing the ten L and the ten H lines at usual sowing. Selection was effective, leading to appreciable and symmetric responses for DG. Variation among crosses was mainly due to additive effects and the ability to predict hybrid DG based on parental lines DG was appreciable. Associated responses for cold tolerance traits in the field were noticeable, though the relationship between DG and these traits was not outstanding. High tolerance was also associated with early flowering, short plants, less leaves, low kernel moisture, red and thin cob, and flint kernels. These divergently selected lines can represent valuable materials for undertaking basic studies and breeding works concerning cold tolerance. [ABSTRACT FROM AUTHOR]

Recent advances in high-throughput sequencing technologies have triggered a shift toward single-nucleotide polymorphism (SNP) markers. A systematic bias can be introduced if SNPs are ascertained in a small panel of genotypes and then used for characterizing a larger population (ascertainment bias). With the objective of evaluating a potential ascertainment bias of the Illumina MaizeSNP50 array with respect to elite European maize dent and flint inbred lines, we compared the genetic diversity among these materials based on 731 amplified fragment length polymorphisms (AFLPs), 186 simple sequence repeats (SSRs), 41,434 SNPs of the MaizeSNP50 array (SNP-A), and two subsets of it, i.e., 30,068 Panzea (SNP-P) and 11,366 Syngenta markers (SNP-S). We evaluated the bias effects on major allele frequency, allele number, gene diversity, modified Roger's distance (MRD), and on molecular variance (AMOVA). We revealed ascertainment bias in SNP-A, compared to AFLPs and SSRs. It affected especially European flint lines analyzed with markers (SNP-S) specifically developed to maximize differences among North American dent germplasm. The bias affected all genetic parameters, but did not substantially alter the relative distances between inbred lines within groups. For these reasons, we conclude that the SNP markers of the MaizeSNP50 array can be employed for breeding purposes in the investigated material. However, attention should be paid in case of comparisons between genotypes belonging to different heterotic groups. In this case, it is advisable to prefer a marker subset with potentially low ascertainment bias, like in our case the SNP-P marker set. [ABSTRACT FROM AUTHOR]

In a previous study on a maize ( Zea mays L.) population of recombinant inbreds derived from B73 × H99, we identified several quantitative trait loci (QTL) for agronomic traits with high dominance-additive ratio. Then, for four of these QTL, we developed families of near-isogenic lines (NILs) homozygous either for the QTL allele from B73 (BB) or from H99 (HH); for two of these QTL, the NILs' families were produced in two different genetic backgrounds. The present study was conducted to: (1) characterize these QTL for agronomic traits and (2) verify whether their effects were influenced by the genetic background, inbreeding level and plant density (PD). The six NILs' families were tested across 3 years and in three experiments at different inbreeding levels as NILs per se and their reciprocal crosses (Experiment 1), NILs crossed to related inbreds B73 and H99 (Experiment 2) and NILs crossed to four unrelated inbreds (Experiment 3). Experiment 2 was conducted at two PDs (4.5 and 9.0 plants m). Results of Experiments 1 and 2 confirmed previous findings as to QTL effects, with dominance-additive ratio superior to 1 for several traits; as a tendency, dominance effects were more pronounced in Experiment 1. The QTL effects were also confirmed in Experiment 3. The interactions involving QTL effects, families and PD were generally negligible, suggesting a certain stability of the QTL. Results emphasize the importance of dominance effects for these QTL, suggesting that they might deserve further studies, using the NILs' families and their crosses as base materials. [ABSTRACT FROM AUTHOR]

The proteins performing the activity of the inner membrane anion channel (IMAC) and its plant counterpart (PIMAC) are still unknown. Lurin et al. (Biochem J 348: 291-295, ) indicated that a chloride channel (CLC) protein corresponds to PIMAC activity in tobacco seedling mitochondria. In this study, we investigated: (i) the presence of a CLC protein in maize seedling mitochondria; (ii) the involvement of this protein in plant cold tolerance; and (iii) its possible role in PIMAC activity. We validated the presence of a CLC protein (ZmCLCc) in maize mitochondria by immunoassay using a polyclonal antibody against its C-terminus. The differential expression of the ZmCLCc protein in mitochondria was measured in seedlings of maize populations divergently selected for cold tolerance and grown at different temperatures. The ZmCLCc protein level was higher in cold stressed than in non-stressed growing conditions. Moreover, the ZmCLCc level showed a direct relationship with the cold sensitivity level of the populations under both growing conditions, suggesting that selection for cold tolerance induced a constitutive change of the ZmCLCc protein amount in mitochondria. The anti-ZmCLCc antibody inhibited (about 60%) the channel-mediated anion translocations by PIMAC, whereas the same antibody did not affect the free diffusion of potassium thiocyanide through the inner mitochondrial membrane. For this reason, we conclude that the mitochondrial ZmCLCc protein can perform the PIMAC activity in maize seedlings. [ABSTRACT FROM AUTHOR]

Although heterosis is widely exploited in agriculture, a clear understanding of its genetic bases is still elusive. This work describes the development of maize recombinant near-isogenic lines (NILs) for the mendelization of six heterotic QTL previously identified based on a maize ( Zea mays L.) RIL population. The efficient and inexpensive strategy adopted to generate sets of NILs starting from QTL-specific residual heterozygous lines (RHLs) is described and validated. In particular, we produced nine pairs of recombinant NILs for all six QTL starting from RHLs F4:5 originally obtained during the production of the RIL population mentioned above. Whenever possible, two different NIL pairs were generated for each QTL. The efficiency of this procedure was tested by analyzing two segregating populations for two of the selected heterotic QTL for plant height, yield per plant and ears per plant. Both additive and dominant effects were observed, consistently with the presence of the QTL within the introgressed regions. Refinement of QTL detection was consistent with previous observations in terms of effects and position of the considered QTL. The genetic material developed in this work represents the starting point for QTL fine mapping aimed at understanding the genetic bases of hybrid vigor in maize. [ABSTRACT FROM AUTHOR]

The evaluation of recombinant inbred lines (RILs) per se can be biased by inbreeding depression in case of allogamous species. To overcome this drawback, RILs can be evaluated in combination with testers; however, testers can carry dominant alleles at the quantitative trait loci (QTL), thus hampering their detection. This study was conducted on the maize ( Zea mays L.) population of 142 RILs derived from the single cross B73 × H99 to evaluate the role of different testers in affecting: (1) QTL detection, (2) the estimates of their effects, and (3) the consistency of such estimates across testers. Testcrosses (TCs) were produced by crossing RILs with inbred testers B73 [TC(B)], H99 [TC(H)], and Mo17 [TC(M)]. TCs were field tested in three environments. TC(B) mean was higher than TC(H) mean for all traits, while TC(M) mean was the highest for plant vigor traits and grain yield. As to the number of detected QTL, tester Mo17 was superior to H99 and B73 for traits with prevailing additive effects. Several overlaps among the QTL were detected in two or all the three TC populations with QTL effects being almost always consistent (same sign). For traits with prevailing dominance–overdominance effects, as grain yield, the poor performing tester H99 was clearly the most effective; fewer overlaps were found and some of them were inconsistent (different sign). Epistatic interactions were of minor importance. In conclusion, the three testers proved to affect QTL detection and estimation of their effects, especially for traits showing high dominance levels. [ABSTRACT FROM AUTHOR]

Previous studies showed that in materials derived from maize ( Zea mays L.) single cross A632 × Mu195 there is association between agronomic traits and cob color (affected by P1 gene). Objectives of this study were to evaluate the responses to divergent selection for cob color in F2-Syn 0 (obtained by selfing A632 × Mu195), estimate the genetic parameters of the involved putative quantitative trait loci (QTL), and evaluate the responses to divergent selection for cob color in A632 and Mu195 backgrounds. The populations selected in F2-Syn 0 for red (R0) or white cob (W0) were tested in four trials. Differences between R0 and W0 were found for grain yield (85.0 vs. 75.0 g/plant) and other traits; most of these differences were related to leaf number/plant. Then, population F2-Syn 1, derived after one random mating generation, was divergently selected for cob color, thus producing R1 and W1. Populations R0, W0, R1, and W1 were tested in two trials, allowing the estimate of genetic effects and recombination frequencies for putative QTL of several traits. Finally, a divergent selection for cob color was conducted in segregating materials of A632 and Mu195 backgrounds. The two pairs of selected populations were compared in two trials; the responses were similar to those detected by comparing R0 and W0. We conclude that divergent selection for cob color in F2-Syn 0 is effective for several traits, that such responses are due to putative QTL linked to P1, and that selection is also effective in different genetic backgrounds. [ABSTRACT FROM AUTHOR]

Defoliation during maize ( Zea mays L.) kernel development has been observed to induce tolerance to cold of germinating seeds in responsive genotypes. The objectives of this study were to evaluate the response to defoliation of immature embryo and mature seed germinability at cold and to verify if the response was influenced by the developmental stage at which the treatment was applied. In three environments, six inbred lines (B73, IABO78, Lo1016, Lo964, Mo17, Os420) were defoliated (D) approximately 20 days after pollination (DAP) or not defoliated (ND). Immature embryos were excised three days after defoliation and germinated in vitro at 9 or 25 ∘C. At maturation, kernel germination was tested at the same temperatures. Defoliation improved cold tolerance and mean time to germination (MTG) at 9 ∘C of both embryos and kernels of Lo1016. To study the effect of kernel developmental stage on response to defoliation, plants of B73, Lo1016 and Lo964 were defoliated at 15, 18, 21, 24, 27, 30, 33, 36, and 39 DAP, or not defoliated. At the same DAP, immature grains were analyzed for dry weight, water and abscisic acid (ABA) content. In Lo1016, low amounts of kernel ABA were detected at all stages, while in Lo964 and B73 ABA increased during development. Lo1016 mature kernels showed an improvement of cold tolerance due to defoliation at all times, while the other genotypes did not. In conclusion inbred lines showed variability for mature seed and immature embryo tolerance to cold at germination and for the ability to acquire tolerance after defoliation. [ABSTRACT FROM AUTHOR]

The objective of this study was to elucidate the genetic relationship between the specific leaf area (SLA) and the photosynthetic performance of maize ( Zea mays L.) as dependent on growth temperature. Three sets of genotypes: (i) 19 S5 inbred lines, divergently selected for high or low operating efficiency of photosystem II (ΦPSII) at low temperature, (ii) a population of 226 F 2:3 families from the cross of ETH-DL3 × ETH-DH7, and (iii) a population of 168 F 2:4 families from the cross of Lo964 × Lo1016 were tested at low (15/13 °C day/night) or at optimal (25/22 °C day/night) temperature. The latter cross was originally developed to study QTLs for root traits. At 15/13 °C the groups of S5 inbred lines selected for high or low ΦPSII differed significantly for all the measured traits, while at optimal temperature the groups differed only with regard to leaf greenness (SPAD). At low temperature, the SLA of these inbred lines was negatively correlated with ΦPSII ( r = − 0.56, p < 0.05) and SPAD (r = − 0.80, p < 0.001). This negative relationship was confirmed by mapping quantitative trait loci (QTL) in the two mapping populations. A co-location of three QTLs for SLA with QTLs for photosynthesis-related traits was detected in both populations at 15/13 °C, while co-location was not detected at 25/22 °C. The co-selection of SLA and ΦPSII in the inbred lines and the co-location of QTL for SLA, SPAD, and ΦPSII at 15/13 °C in the QTL populations strongly supports pleiotropy. There was no evidence that selecting for high ΦPSII at low temperature leads to a constitutively altered SLA. [ABSTRACT FROM AUTHOR]

Divergently selected genotypes can be used for detecting the genomic regions affecting the selected trait (selection signature). Moreover, the genetic distances (GDs) among divergently selected lines can be correlated with the agronomic performances of the crosses among them. Using as source the maize F2 of B73 × IABO78, we previously conducted four cycles of divergent recurrent selection and three cycles of divergent selection in inbreeding for cold tolerance at germination. We finally obtained 10 lines selected for low (L) and 10 lines selected for high (H) cold tolerance, which exhibited a notable divergence for both the selected and associated traits. Herein, we investigated the 20 lines and the 28 single diallel crosses among eight random lines (four L and four H); the main objectives were to identify the putative regions controlling the selected and associated traits and to study the relationships between crosses performances and GDs among their parental lines. Allele frequencies at 932 recombination blocks based on 19,220 polymorphic SNPs were obtained for the two lines’ groups; the FST calculated across sliding windows indicated 18 regions highly divergent between groups. The increasing alleles for cold tolerance were contributed by both parents, consistently with the transgressive segregations previously found. Several regions associated to DG also affected various agronomic traits. The cross performances showed some relationships with the genetic distances among parental lines for traits affected by dominance, provided that all crosses were considered, while these relationships vanished when only L × H crosses were examined. [ABSTRACT FROM AUTHOR]