Classification approaches for sorting maize (Zea mays subsp. mays) haploids using single-kernel near-infrared spectroscopy

Doubled haploids (DHs) are an important breeding tool for creating maize inbred lines. One bottleneck in the DH process is the manual separation of haploids from among the much larger pool of hybrid siblings in a haploid induction cross. Here, we demonstrate the ability of single-kernel near-infrared reflectance spectroscopy (skNIR) to identify haploid kernels. The skNIR is a high-throughput device that acquires an NIR spectrum to predict individual kernel traits. We collected skNIR data from haploid and hybrid kernels in 15 haploid induction crosses and found significant differences in multiple traits such as percent oil, seed weight, or volume, within each cross. The two kernel classes were separated by their NIR profile using Partial Least Squares Linear Discriminant Analysis (PLS-LDA). A general classification model, in which all induction crosses were used in the discrimination model, and a specific model, in which only kernels within a specific induction cross, were compared. Specific models outperformed the general model and were able to enrich a haploid selection pool to above 50% haploids. Applications for the instrument are discussed.     

Identification of QTL controlling adventitious root formation during flooding conditions in teosinte (Zea mays ssp. huehuetenangensis) seedlings

Adventitious root formation (ARF) at the soil surface is one of the most important adaptations to soil flooding or waterlogging. Quantitative trait loci (QTL) controlling ARF under flooding condition were identified in a 94 F₂ individual population by crossing maize (Zea mays L., B64) × teosinte (Z. mays ssp. huehuetenangensis). A base-map was constructed using 66 SSR and 42 AFLP markers, covering 1,378 cM throughout all ten maize chromosomes. The ARF capacity for seedlings was determined by evaluating the degree of root formation at the soil surface following flooding for 2 weeks. ARF showed continuous variation in the F₂ population. Interval mapping and composite interval mapping analyses revealed that the QTL for ARF was located on chromosome 8 (bin 8.05). Utilising a selective genotyping strategy with an additional 186 F₂ population derived from the same cross combination and 32 AFLP primer combinations, regions on chromosomes 4 (bin 4.07) and 8 (bin 8.03) were found to be associated with ARF. Z. mays ssp. huehuetenangensis contributed all of the QTL detected in this study. Results of the study suggest a potential for transferring waterlogging tolerance to maize from Z. mays ssp. huehuetenangensis.     

Glyphosate tolerance in maize (Zea mays L.). 1. Differential response among inbred lines

Summary Variation in susceptibility to the safe broad-spectrum herbicide glyphosate was investigated in maize. Eleven inbred lines, grown in a growth chamber, were evaluated for their tolerance to the herbicide at 2.4 mM (0.2 kg a.i. in 400 I ha^-1 of water). Following treatment with glyphosate at the three-leaf stage, significant variation in damage, expressed as visual injury ratings scored 7, 14 and 21 days after the application of the herbicide, was found. Effects on dry weight and shoot height were consistent with visual scores and the carbon-exchange rate was found to be a sensitive index of differential injury.Biochemical characterization of 5-enol-pyruvyl-shikimate-3-phosphate (EPSP) synthase, the main target of the herbicide, ruled out the possibility that this differential susceptibility was due to variations in the sensitivity of the enzyme. On the contrary, a positive correlation was found between in vivo tolerance and EPSP synthase levels, measured at different stages during seedling growth. This result suggests that a naturally occurring difference in EPSP synthase levels in the tissues may contribute to the differential response observed in vivo in maize inbreds.Abbreviations CER carbon-exchange rate - EPSP 5-enol-pyruvyl-shikimate-3-phosphate - ID₅₀ concentration causing 50% inhibition     

Cytogenetic evidence for the origin of teosinte (Zea mays ssp. Mexicana)

Summary Cytogenetic evidence has shown that teosinte (Zea mays ssp. mexicana (Schrad.) Iltes) and maize (Zea mays L. ssp. mays) are conspecific. They hybridize readily and their offspring are generally fertile. Teosinte could not have originated as a byproduct of maize-Tripsacum hybridization. Such introgression gave rise to plants that are phenotypically maize or Tripsacum, depending on which parent was used as a pollen donor. Compartive morphological and genetical studies indicated that it is more probable that maize originated from a teosinte-like ancestor under domestication, than that a maize-like plant gave rise to teosinte through a series of mutations.Reseach supported financially by the Illinois Agricultural Experiment Station, and Grants GB-40136-X and BM573-01034 A02 from the National Science Foundation.     

QTL mapping of maize (Zea mays) stay-green traits and their relationship to yield

A maize genetic linkage map derived from 115 simple sequence repeat (SSR) markers was constructed from an F₂ population. The F₂ was generated from a cross between a stay-green inbred line (Q319) and a normal inbred line (Mo17). The map resolved 10 linkage groups and spanned 1431.0 cM in length with an average genetic distance of 12.44 cM between two neighbouring loci. A total of 14 quantitative trait loci (QTL) were detected for stay-green traits at different postflowering time intervals and identified by composite interval mapping. The respective QTL contribution to phenotypic variance ranged from 5.40% to 11.49%, with trait synergistic action from Q319. Moreover, maize stay-green traits were closely correlated to grain yield. Additional QTL analyses indicated that multiple intervals of stay-green QTL overlapped with yield QTL.     

A QTL atlas for grain yield and its component traits in maize (Zea mays)

Grain yield and its component traits are essential targets in maize breeding. These traits are genetically complex and controlled by a large number of quantitative trait loci (QTL). The aim of this study was to compile reported QTL and major genes for grain yield and its component traits in a QTL atlas, as a valuable resource for the maize community. To this end, 1,177 QTL related to maize yield were collected from 56 studies published between 1992 and 2018. These QTL were projected to genetic map “IBM2 2008 Neighbors”, which led to the identification of 135 meta-QTL. Some genomic regions appear to be hotspots for yield-related meta-QTL, often affecting more than one of the investigated traits. Moreover, we catalogued 20 major maize loci associated with yield and identified 65 maize homologs of 21 rice yield-related genes. Interestingly, we found that a significant proportion of them are located in meta-QTL regions. Collectively, this study provides a reference for QTL fine-mapping and gene cloning, as well as for molecular marker-assisted breeding of yield-related traits in maize.     

Mapping QTLs in breeding for drought tolerance in maize (Zea mays L.)

Summary Grain yield in the maize (Zea mays L) plant is sensitive to drought in the period three weeks either side of flowering. Maize is well-adapted to the use of restriction fragment length polymorphisms (RFLPs) to identify a tight linkage between gene(s) controlling the quantitative trait and a molecular marker. We have determined the chromosomal locations of quantitative trait loci (QTLs) affecting grain yield under drought, anthesis-silking interval, and number of ears per plant. The F₃ families derived from the cross SD34(tolerant) × SD35 (intolerant) were evaluated for these traits in a two replicated experiment. RFLP analysis of the maize genome included non-radioactive DNA-DNA hybridization detection using chemiluminescence. To identify QTLs underlying tolerance to drought, the mean phenotypic performances of F₃ families were compared based on genotypic classification at each of 70 RFLP marker loci. The genetic linkage map assembled from these markers was in good agreement with previously published maps. The phenotypic correlations between yield and other traits were highly significant. In the combined analyses, genomic regions significantly affecting tolerance to drought were found on chromosomes 1,3,5,6, and 8. For yield, a total of 50% of the phenotypic variance could be explained by five putative QTLs. Different types of gene action were found for the putative QTLs for the three traits.     

Glyphosate tolerance in maize (Zea mays L.). 2. Selection and characterization of a tolerant somaclone

Summary The progeny of 104 regenerated maize plants were screened for tolerance to the safe broad-spectrum herbicide glyphosate during seed germination and early growth. Seven somaclones showed varying degrees of resistance to the application of the herbicide at 1.2 mM (0.1 kg a.i. in 400 1 ha^-1 of water). Plants capable of a normal growth following treatment with 2.4 mM (0.2 kg ha^-1) glyphosate at the three leaf stage were selfed, and their progeny analyzed. A family able to tolerate the exposure to glyphosate at 2.4 mM was isolated and shown to maintain a photosynthetic rate comparable with control after the application of the herbicide.The selfed progeny of the tolerant somaclone was characterized as to the properties of two targets of glyphosate, the shikimate pathway enzymes 5-enol-pyruvyl-shikimate-3-phosphate (EPSP) synthase and 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase. In vitro tests ruled out the possibility that the tolerance was due to altered forms of these enzymes. Families showed significant variability with regard to EPSP and DAHP synthase levels, measured at different stages during seedling growth; however, not even these traits were correlated with in vivo response to glyphosate. The possible role of other physiological processes in determining the increased tolerance to the herbicide is discussed.Abbreviations DAHP 3-deoxy-D-arabino-heptulosonate-7-phosphate - EPSP 5-enol-pyruvyl-shikimate-3-phosphate - CER carbon-exchange rate - ID₅₀ concentration causing 50% inhibition     

Genetic basis of variation for salt tolerance in maize ( Zea mays L).

The genetic basis of salt tolerance was examined in selected salt tolerant and sensitive material from a sample of accessions previously assessed for variability in salinity tolerance. The North Carolina Model 2 Design and analysis was followed, tolerance being assessed in 10-day-old seedlings grown in salinized solution culture at control (0 mM), 60 mM and 80 mM NaCl concentrations). Salinity tolerance was shown to be under the control of genes with additive and non-additive effects, with broad and narrow sense heritability estimates being approximately 0.7 and 0.4 over all treatments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation of the contribution of teosinte to the improvement of agronomic,grain quality and yield traits in maize (Zea mays)

Teosinte, an ancestor to modern maize, displays an excellent performance regarding resistance to stress, but its yield potential has rarely been reported. To evaluate the potential contribution of teosinte to maize improvement, two maize–teosinte backcrossed recombination inbred line (RIL) populations and their corresponding test-cross hybrids were planted for trait assessment. In RN and ZP RIL populations, the average coefficients of variation of 31 agronomic traits were 9.14% (Range, 0.38%‒25.21%) and 6.85% (Range, 0.55%‒27.73%), respectively. The correlation coefficients of 13 common shared traits between RIL populations and test-cross hybrid populations ranged from 0.10 to 0.60 and from 0.06 to 0.72, respectively. A total of 39 and 3 recombined inbred lines, and 29 and 47 test-cross hybrids exhibited higher yields than their checks (RP125, Zheng58, CD189 and ZD958) with the BLUP data, respectively. Furthermore, four test-cross hybrids including RN034/SCML1950, ZP068/Chang7-2, ZP079/Chang7-2 and ZP122/Chang7-2 showed a more stable yield performance, with yield gains of +7.07%, +3.64%, +5.83% and +3.82% over checks, respectively. In conclusion, teosinte could serve as an alien germplasm for maize breeding.     

玉米种子休眠性的QTL定位

选用两个种子休眠性差异较大的普通玉米自交系R08与A318组配的F2群体共331个单株,构建了包含137个SSR标记的分子遗传连锁图谱,覆盖玉米基因组2 076.7 cM,平均图距15.2 cM。采用复合区间作图法对F_2:3家系种子休眠性数据进行分析,共检测到7个QTL,分别位于玉米第1、3、5和10染色体上。7个QTL的贡献率在2.45%~26.     

Maturity interaction and black layer occurrence in opaque-2 and normal hybrids in maize (Zea mays L.)

Summary Six opaque-2 lines and their normal counterparts were crossed in diallel crosses. The crosses were grown in 1970 in a split plot randomized complete block design. Harvests were made at 7-day intervals starting at 28 days after pollination and countinuing through 63 days.The average kernel weight of opaque-2 hybrids was inferior to that of the normal. Nevertheless, the opaque-2 gene performed differently in different hybrids. In B14 x B37 single cross the mutant had similar kernel weight as its normal counterpart in the first and second harvests. In contrast a wide difference was found between the opaque-2 and the normal, both at early and late stages of development in W64A x A545 background.The normal hybrids had greater cob weight, ranging from 9'7 to 11.8% more than the opaque-2. The difference in cob weight of the opaque-2 and the normal remained constant over the different harvest dates.At physiological maturity, the opaque-2 hybrids averaged 3.5% higher moisture content than the normal. In general, a slower accumulation of dry matter in the kernels was accompanied by a retention of more moisture.Shelling percentage was higher for the normal hybrids. Black layer, an indicator of physiological maturity, was formed at about the same time in the opaque-2 and normal.Journal paper No. 6094.     

Genetic improvement of cell-wall digestibility in forage maize (Zea mays L.). I. Performance of inbred lines and related hybrids

Summary The current study deals with genetic improvement of the nutritive value of forage maize. In separate field trials, maize inbred lines without the brown midrib trait and derived hybrids were evaluated for stalk quality as well as some other agronomic traits. The aim was to relate the performance of lines and hybrids. Quality traits studied were the contents of ash and cell walls expressed as percentage of dry matter and the digestibilities of organic matter and cell walls (stalk-dv% and stalk-dcw%, respectively). The performance of hybrids was established in a trial at two locations with three replicates per location and the performance of lines at one location in an unreplicated trial.The range for stalk-dcw% was about 10 percentage units between hybrids and 15 percentage units between inbred lines. Stalk-dcw% had of all quality traits of hybrids the highest broad-sense heritability (h ²=0.74), and determined about 80% of the variation in stalk-dv%. The only stalk quality trait where a significant correlation was found between the mean hybrid performance and the corresponding midparent value was stalk-dcw% (r=0.70, P<0.01).In conclusion, stalk-dcw% proved to be the only stalk quality trait worth evaluating at the inbred line level in a breeding programme aimed at producing commercial hybrid varieties of forage maize.     

Assessment of salinity tolerance based upon seedling root growth response functions in maize (Zea mays L.)

Root growth response of 10-days-oldseedlings of 100 maize accessions at, 0 mM, 60 mM, 80 mM and 150 mM NaCl concentration was assessed in solution culture. The non-linear least square method was used to quantify the salt tolerance of maize accessions. The estimated salinity threshold, Ct, the NaCl concentration at which root growth starts to decrease, C0, and C50, the concentrations at which roots stop growing and 50% of its control value revealed considerable differences between the accessions. No general consistency for tolerance was, however, found between the estimates of Ct and C50. Different genetic systems appeared to be involved in controlling the inheritance of Ct and C50.Both Ct and C50 appeared to quantify accession tolerance, and the expression of root growth as a function of NaCl concentrations provides a useful guideline for salt tolerance. Estimates of broad sense heritability for relative root length were moderate in size (0.62 to 0.82), suggesting the scope for enhancing salt tolerance in maize through selection and breeding. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cover crops and interrow tillage for weed control in short season maize (Zea mays)

Weed competition can cause substantial maize (Zea mays L.) yield reductions. Interseeding maize with cover crops or a combination of interrow cultivation and interseeded cover crops are possible alternative methods of weed control. This study was conducted to examine the potential of interrow cultivation plus cover crops to reduce weed density in maize without reducing the grain yield. Field experiments were conducted in 1993 and 1994 at two sites in Québec to determine the effects of planting 12 cover crops with maize on weed control. Fall rye (Secale cereal L.), hairy vetch (Vicia villosa Roth), a mixture of red clover (Trifolium pratense L.) and ryegrass (Lolium multiflorum Lam), a mixture of white clover (Trifolium repens L.) and ryegrass, subterranean clover (Trifolium subterraneum L.), yellow sweet clover (Meliotus officinalis Lam), black medic (Medicago lupulina L.), Persian clover (Trifolium resupinatum L.), strawberry clover (Trifolium fragiferum L.), crimson clover (Trifolium incarnatum L.), alfalfa (Medicago sativa L.), and berseem clover (Trifolium alexandrinum L.) were seeded at two planting dates, 10 and 20 days after maize emergence. Interrow cultivation was carried out weekly until forage seeding, with a final cultivation being conducted just prior to cover crop seeding. Cover crop planting date did not affect maize yields or the ability of interrow tillage plus cover crops to suppress the development of weed populations. Maize yield was less affected by the interseeded cover crops under conditions of adequate rainfall. Corn planted in fields heavily infested with weeds resulted in substantial yield reductions even when rainfall was adequate. Except for 1993 at l'Assomption interrow tillage plus cover crop treatments had consistently lower weed biomass when compared to the weedy control. Most of the weed control was due to the interrow cultivation performed prior to seeding of the cover crops. The lowest weed density occurred in the herbicide treated plots. The ability of interrow tillage plus cover crops to suppress the development of weeds was affected by the level of weed infestation, the growing conditions and location. The cover crops provide additional weed control but the interrrow tillage or some herbicide application may still be necessary.     

Genomic selection and enablers for agronomic traits in maize (Zea mays): A review

Maize is a commodity crop providing millions of people with food, feed, industrial raw material and economic opportunities. However, maize yields in Africa are relatively stagnant and low, at a mean of 1.7 t ha⁻¹ compared with the global average of 6 t ha⁻¹. The yield gap can be narrowed with accelerated and precision breeding strategies that are required to develop and deploy high-yielding and climate-resilient maize varieties. Genomic and phenotypic selections are complementary methods that offer opportunities for the speedy choice of contrasting parents and derived progenies for hybrid breeding and commercialization. Genomic selection (GS) will shorten the crop breeding cycle by identifying and tracking desirable genotypes and aid the timeous commercialization of market-preferred varieties. The technology, however, has not yet been fully embraced by most public and private breeding programmes, notably in Africa. This review aims to present the importance, current status, challenges and opportunities of GS to accelerate genetic gains for economic traits to speed up the breeding of high-yielding maize varieties. The first section summarizes genomic selection and the contemporary phenotypic selection and phenotyping platforms as a foundation for GS and trait integration in maize. This is followed by highlights on the reported genetic gains and progress through phenotypic selection and GS for grain yield and yield components. Training population development, genetic design and statistical models used in GS in maize breeding are discussed. Lastly, the review summarizes the challenges of GS, including prediction accuracy, and integrates GS with speed breeding, doubled haploid breeding and genome editing technologies to increase breeding efficiency and accelerate cultivar release. The paper will guide breeders in selection and trait introgression using GS to develop cultivars preferred by the marketplace.