Mapping QTLs for nitrogen uptake in relation to the early growth of wheat (Triticum aestivum L.)

The objective of this study was to map QTLs for N uptake (NUP) in wheat, and to investigate factors influencing NUP. Two independent field trials with low N (LN) and high N (HN) treatments were conducted in the growing seasons of 2002–2003 (trial 1) and 2003–2004 (trial 2) to measure NUP per plant (N accumulated in the aerial part at maturity stage) of a doubled haploid (DH) population consisting of 120 DH lines derived from winter wheat varieties Hanxuan 10 and Lumai 14. A hydroponic culture with all nutrients supplied sufficiently was conducted to investigate shoot dry weight (SDW), root dry weight (RDW), tiller number (TN) and NUP (total plant N uptake) per plant of this mapping population at seedling stage. SDW, RDW, TN and NUP investigated in the hydroponic culture were significantly and positively correlated with each other, and with NUP under both LN and HN conditions in the field trials. Nine and eight QTLs for NUP were detected under LN and HN conditions in the field trials, respectively. Four to five QTLs for SDW, RDW, TN and NUP were detected in the hydroponic culture. One SDW QTL, three RDW QTLs, two TN QTLs detected in the hydroponic culture were linked with QTLs for NUP under LN or HN condition in the field trials. The positive correlation and genetic linkage for the traits between the field trials and the hydroponic culture demonstrated that greater seedling vigor of root and shoot is an important factor influencing N uptake in wheat. Diaoguo An and Junying Su: These authors contributed equally to this work. Section Editor: H.J. Kronzucker     

Nitrogen redistribution during grain growth in wheat (Triticum aestivum L.)

The flag leaf of wheat was examined for changes in quantity and activity of ribulose-bisphosphate carboxylase (RuBPCase; EC 4.1.1.39), in the proteolytic degradation of RuBPCase and other native proteins, and in the ultrastructure of the leaf cells during grain development. Proteolytic degradation of RuBPCase at pH 4.8 increased until 8–10 d after anthesis, then declined, and increased again 16–18 d after anthesis. The second peak coincided with the onset of a preferential loss of immunologically recognizable RuBPCase. The specific activity and number of active sites per molecule of RuBPCase did not change during senescence. Examination of ultrastructure with the electron microscope showed little change in the appearance of the mitochondria as the flag leaf aged. Prominent cristae were still evident 35 d after anthesis. In contrast, the chloroplasts showed a progressive disruption of the thylakoid structure and an increasing number of osmiophilic glubules. The double membrane envelope surrounding the chloroplast appeared intact until at least 20 d after anthesis. The tonoplast also appeared intact up to 20 d. At later stages of senescence of the leaf the outer membrane of the chloroplast adjacent to the tonoplast appeared to break but the inner membrane of the envelope appeared intact until at least 35 d after anthesis.Abbreviation RuBPCase ribulose-1,5-bisphosphate carboxylase (EC. 4.1.1.39) I=Waters et al. 1980     

Nitrogen redistribution during grain growth in wheat (Triticum aestivum L.)

The technique of EDTA-enhanced phloem exudation (King and Zeevaart, 1974: Plant Physiol. 53, 96–103) was evaluated with respect to the collection and identification of amino acids exported from senescing wheat leaves. Whilst the characteristics of the exudate collected conform with many of the accepted properties of phloem exudate, unexpectedly high molar proportions of phenylalanine and tyrosine were observed. By comparing exudation into a range chelator solutions with exudation into water, the increased exudation of phenylalanine and tyrosine relative to the other amino acids occurring when ethylene-diaminetetracetic acid was used, was considered to an artefact.In plants thought to be relying heavily on mobilisation of protein reserves to satisfy the nitrogen requirements of the grain, the major amino acids present in flag-leaf phloem exudate were glutamate, aspartate, serine, alanine and glycine. Only small proportions of amides were present until late in senescence when glutamine became the major amino acid in phloem exudate (25 molar-%). Glutamine was always the major amino acid in xylem sap (50 molar-%).The activities of glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 1.4.7.1), glutamate dehydrogenase (EC 1.4.1.3) and asparagine synthetase (EC 5.3.5.4) were measured in the flag leaf throughout the grain-filling period. Glutamine synthetase and glutamate-synthase activities declined during this period. Glutamate-dehydrogenase activity was markedly unchanged despite variation in the number of multiple forms visualised after gel electrophoresis. The activity of the enzyme reached a peak only very late in the course of senescence of the flag leaf. No asparagine-synthetase activity could be detected in the flag leaf during the grain-filling period.II. Peoples et al. (1980)     

Nitrogen redistribution during grain growth in wheat (Triticum aestivum L.)

The activity of a range of endo- and exopeptidase enzymes have been measured in the glumes, flag leaf and stem during the period of grain development in wheat. The enzymes show a sequential pattern of appearance with activity peaks occurring at a number of intervals from anthesis until just prior to the cessation of grain growth. Of the enzymes studied only the haemoglobin- and casein-degrading activity and alanylglycine-dipeptidase activity increased during the period of rapid protein loss, while aminopeptidase, carboxypeptidase and leucyltyrosine dipeptidase reached maximum activity prior to this period.     

Mapping loci associated with flour colour in wheat (Triticum aestivum L.)

 An RFLP map constructed using 150 single seed descent (SSD) lines from a cross between two hexaploid wheat varieties (‘Schomburgk’בYarralinka’) was used to identify loci controlling flour colour. Flour colour data were obtained from field trials conducted over two seasons at different sites. The estimated heritability of this trait was calculated as 0.67. Two regions identified in the preliminary analysis on chromosomes 3A and 7A, accounted for 13% and 60% of the genetic variation respectively. A detailed analysis of the major locus on 7A was conducted through fine mapping of AFLP markers identified using bulked segregant analysis (BSA). Seven additional markers were identified by the BSA and mapped to the region of the 7A locus. The applicability of these markers to identify wheat lines with enhanced flour colour is discussed. Received: 30 September 1997 / Accepted: 4 February 1998     

Mapping loci associated with milling yield in wheat (Triticum aestivum L.)

A partial genetic linkage map constructed using 150 single seed descent (SSD) lines generated from a cross between the hexaploid wheat varieties Schomburgk and Yarralinka was used to identify loci controlling milling yield. Milling yield data were obtained using seed collected from field trials conducted at different sites over two seasons. The estimated broad-sense heritability of milling yield in this population was calculated as 0.48. In the preliminary analysis, two regions were identified on chromosomes 3A and 7D, which were significantly associated with milling yield and accounted for 22% and 19% of the genetic variation, respectively. Bulked segregant analysis in combination with AFLP identified other markers linked to these loci, as well as an additional region on chromosome 5A, which accounted for 19% of the genetic variation. The applicability of these markers as selection tools for breeding purposes is discussed.     

Enzyme activities associated with developing wheat(Triticum aestivum L.) grain amyloplasts

Intact amyloplasts from endosperm of developing wheat grains have been isolated by first preparing the protoplasts and then fractionating the lysate of the protoplasts on percoll and ficoll gradients, respectively. Amyloplasts isolated as above were functional and not contaminated by cytosol or by organelles likely to be involved in carbohydrate metabolism. The enzyme distribution studies indicated that ADP-glucose pyrophosphorylase and starch synthase were confined to amyloplasts, whereas invertase, sucrose synthase, UDP-glucose pyrophosphorylase, hexokinase, phosphofructokinase-2 and fructose-2,6-P₂ase were absent fro the amyloplast and mainly confined to the cytosol. Triose-P isomerase, glyceraldehyde-3-P dehydrogenase, phosphohexose isomerase, phosphoglucomutase, phosphofructokinase, aldolase, PPi-fructose-6-P-1 phosphotransferase, and fructose-l,6-P₂ase, though predominantly cytosolic, were also present in the amyloplast. Based on distribution of enzymes, a probable pathway for starch biosynthesis in amyloplasts of developing wheat grains has been proposed.     

Sucrose metabolism in developing endosperm of immature wheat (Triticum aestivum L.) grain

With a view to investigating the role of the enzyme pyrophosphate-fructose-6-phosphate-1-phosphotransferase (PFP) in sucrose breakdown in developing endosperm of wheat grain, the activity of PFP and related enzymes such as phosphofructokinase (PFK), fructose-6-bisphosphatase (FBPase), fructose-6-phosphate-2-kinase (PFK-2) and fructose-2,6-bisphosphatase (F2, 6-P2ase) and the contents of the various intermediates of the pathway serving either the substrate or the effectors of these enzymes such as glu-6-P,glu-1-P,fru-6-P,fru-1,6-P2,DHAP,G3P, UDP-glucose, ADP-glucose, Pi,PPi and fru-2,6-P2 have been determined at 5 days intervals starting from day-5 after anthesis until day-40 after anthesis. These enzymes except PFK-2 had their peak activity at day-25 after anthesis. The activity of PFP was several fold higher than that of PFK at each stage of grain development. PFK-2 exhibited the lowest activity. The various intermediates again had their maximum concentration either at day-20 or day-25 after anthesis. Among hexose phosphates studied, glu-6-P was present in highest concentration at each stage of grain development. The level of Pi was much higher than those of PPi and fru-2,6-P2. Similarly, concentration of UDP-glucose was higher than that of ADP-glucose. Based on these results, it is proposed that the major role of the enzyme PFP in developing wheat grain is to provide PPi for sucrose breakdown via sucrose synthase.     

Effects of foliar applied benzyladenine on grain yield and grain protein in wheat (Triticum aestivum L.)

The effects of foliar applications of nitrogen and benzyladenine (BA) on grain yield and grain protein of wheat grown under field conditions were studied over 2 years with 5 cultivars at 2 locations. Nitrogen (N) at 20 kg.ha^–1, and BA at 100 or 800 mg.l^–1 were applied alone or combined at pre and post-anthesis; applications of BA at 8 mg.l^–1 were also made on individual ears in order to study the effect on cell number. Weekly determinations of the chlorophyll content of the flag leaf were conducted after anthesis to study leaf senescence. At harvest, yield, yield components and grain protein percentage were determined. N and BA applications delayed chlorophyll loss in the flag leaf, but modified neither yield nor yield components. Foliarly applied BA increased grain protein in four of the five cultivars tested. It is concluded that delay of the senescence induced by BA might allow more energy to be available for N uptake by the crop leading to an increase in grain protein.Research supported by a CAFPTA grant 1656/86 and by CONICET, PID 30017700/85.CONICETComisión de Investigaciones Cientificas de la Provincia de Buenos AiresInstituto de Fisiologia Vegetal     

The wheat (Triticum aestivum L.) leaf proteome

The wheat leaf proteome was mapped and partially characterized to function as a comparative template for future wheat research. In total, 404 proteins were visualized, and 277 of these were selected for analysis based on reproducibility and relative quantity. Using a combination of protein and expressed sequence tag database searching, 142 proteins were putatively identified with an identification success rate of 51%. The identified proteins were grouped according to their functional annotations with the majority (40%) being involved in energy production, primary, or secondary metabolism. Only 8% of the protein identifications lacked ascertainable functional annotation. The 51% ratio of successful identification and the 8% unclear functional annotation rate are major improvements over most previous plant proteomic studies. This clearly indicates the advancement of the plant protein and nucleic acid sequence and annotation data available in the databases, and shows the enhanced feasibility of future wheat leaf proteome research.     

Silicon absorption by wheat (Triticum aestivum L.)

Although silicon (Si) is a quantitatively major inorganic constituent of higher plants the element is not considered generally essential for them. Therefore it is not included in the formulation of any of the solution cultures widely used in plant physiological research. One consequence of this state of affairs is that the absorption and transport of Si have not been investigated nearly as much as those of the elements accorded 'essential' status. In this paper we report experiments showing that Si is rapidly absorbed by wheat (Triticum aestivum L.) plants from solution cultures initially containing Si at 0.5 mM, a concentration realistic in terms of the concentrations of the element in soil solutions. Nearly mature plants (headed out) 'preloaded' with Si absorbed it at virtually the same rate as did plants grown previously in solutions to which Si had not been added. The rate of Si absorption increased by more than an order of magnitude between the 2-leaf and the 7-8 leaf stage, with little change thereafter. This revised version was published online in June 2006 with corrections to the Cover Date.     

Identifying loci influencing grain number by microsatellite screening in bread wheat (Triticum aestivum L.)

Grain number (GN) is one of three major yield-related components in wheat. We used the Chinese wheat mini core collection to undertake a genome-wide association analysis of grain number using 531 SSR markers randomly located on all 21 chromosomes. Grain numbers of all accessions were measured in four trials, i.e. two environments in four growing seasons. Association analysis based on a mixed linear model (MLM) revealed that 27 SSR loci were significantly associated with mean GN (MGN) estimated by the best linear unbiased predictor (BLUP) method. These included numerous breeder favorable alleles with strong positive effects at 23 loci. Significant or extremely significant differences were detected on MGN between varieties conveying favored allele and varieties with other alleles. Moreover, statistical simulation showed that the favored alleles have additive genetic effects. Although modern varieties combined larger numbers of favored alleles, the numbers of favored alleles were not significantly different from those in landraces, especially those alleles contributing mostly to the phenotypic variation. These results indicate that there is still considerable genetic potential for use of markers for genome selection of GN for high yield in wheat.     

Marker-assisted pyramiding of eight QTLs/genes for seven different traits in common wheat (Triticum aestivum L.)

Common wheat (Triticum aestivum L.) contributes substantially to global food and nutritional security. Thus, an important goal of wheat breeding is to develop high-yielding varieties with better nutritional quality and resistance to all major diseases. During the present study, in the background of a popular elite wheat cultivar PBW343, we pyramided eight quantitative trait loci (QTLs)/genes for four grain quality traits (high grain weight, high grain protein content, pre-harvest sprouting tolerance, and desirable high-molecular-weight glutenin subunits) and resistance against the three rusts. For pyramiding eight QTLs/genes, four improved PBW343 lines, each carrying different combinations of the desired QTLs/genes (developed by us earlier), were crossed in pairs to produce two single-cross F₁ hybrids. The single-cross F₁ hybrids were intercrossed to produce a double-cross hybrid (DCH). Using marker-assisted selection in five consecutive generations (DCHF₁–DCHF₅), four pyramided lines (PYLs) were selected, each with all the eight desired QTLs/genes in homozygous state. The phenotypic characterization of the progenies of these PYLs suggested that the genetic background of PBW343 was retained in all these four PYLs. Therefore, these PYLs should prove useful in future wheat breeding programs for improving not only the grain quality, but also the durability of resistance against all three rusts. Multi-year/multi-location trials are planned for these pyramided lines to evaluate their potential for release as a next-generation improved version of wheat cv. PBW343 for commercial cultivation.     

The distal portion of the short arm of wheat (Triticum aestivum L.) chromosome 5D controls endosperm vitreosity and grain hardness

Kernel vitreosity is an important trait of wheat grain, but its developmental control is not completely known. We developed back-cross seven (BC(7)) near-isogenic lines in the soft white spring wheat cultivar Alpowa that lack the distal portion of chromosome 5D short arm. From the final back-cross, 46 BC(7)F(2) plants were isolated. These plants exhibited a complete and perfect association between kernel vitreosity (i.e. vitreous, non-vitreous or mixed) and Single Kernel Characterization System (SKCS) hardness. Observed segregation of 10:28:7 fit a 1:2:1 Chi-square. BC(7)F(2) plants classified as heterozygous for both SKCS hardness and kernel vitreosity (n = 29) were selected and a single vitreous and non-vitreous kernel were selected, and grown to maturity and subjected to SKCS analysis. The resultant phenotypic ratios were, from non-vitreous kernels, 23:6:0, and from vitreous kernels, 0:1:28, soft:heterozygous:hard, respectively. Three of these BC(7)F(2) heterozygous plants were selected and 40 kernels each drawn at random, grown to maturity and subjected to SKCS analysis. Phenotypic segregation ratios were 7:27:6, 11:20:9, and 3:28:9, soft:heterozygous:hard. Chi-square analysis supported a 1:2:1 segregation for one plant but not the other two, in which cases the two homozygous classes were under-represented. Twenty-two paired BC(7)F(2):F(3) full sibs were compared for kernel hardness, weight, size, density and protein content. SKCS hardness index differed markedly, 29.4 for the lines with a complete 5DS, and 88.6 for the lines possessing the deletion. The soft non-vitreous kernels were on average significantly heavier, by nearly 20%, and were slightly larger. Density and protein contents were similar, however. The results provide strong genetic evidence that gene(s) on distal 5DS control not only kernel hardness but also the manner in which the endosperm develops, viz. whether it is vitreous or non-vitreous.     

Genetic dissection of grain yield and physical grain quality in bread wheat (Triticum aestivum L.) under water-limited environments

In the water-limited bread wheat production environment of southern Australia, large advances in grain yield have previously been achieved through the introduction and improved understanding of agronomic traits controlled by major genes, such as the semi-dwarf plant stature and photoperiod insensitivity. However, more recent yield increases have been achieved through incremental genetic advances, of which, breeders and researchers do not fully understand the underlying mechanism(s). A doubled haploid population was utilised, derived from a cross between RAC875, a relatively drought-tolerant breeders' line and Kukri, a locally adapted variety more intolerant of drought. Experiments were performed in 16 environments over four seasons in southern Australia, to physiologically dissect grain yield and to detect quantitative trait loci (QTL) for these traits. Two stage multi-environment trial analysis identified three main clusters of experiments (forming distinctive environments, ENVs), each with a distinctive growing season rainfall patterns. Kernels per square metre were positively correlated with grain yield and influenced by kernels per spikelet, a measure of fertility. QTL analysis detected nine loci for grain yield across these ENVs, individually accounting for between 3 and 18% of genetic variance within their respective ENVs, with the RAC875 allele conferring increased grain yield at seven of these loci. These loci were partially dissected by the detection of co-located QTL for other traits, namely kernels per square metre. While most loci for grain yield have previously been reported, their deployment and effect within local germplasm are now better understood. A number of novel loci can be further exploited to aid breeders' efforts in improving grain yield in the southern Australian environment.     

Peroxidase activities in relation to plant height and grain weight in bread wheat (Triticum aestivum L.)

Summary The relationship of peroxidase activity with plant height and grain weight has been studied in seven different varieties of bread wheat belonging to diverse genotypes, and their F₁ crosses. The association between plant height and peroxidase activity was highly significant and negative. Based on the similarity index values of peroxidase isoenzymes, the seven wheat genotypes could be classified into two groups: the first group consisting of triple and quadruple dwarf varieties and the other of tall, single and double dwarf. A negative correlation between peroxidase activity and grain weight was also observed. However, the results of this study indicate a possibility of developing a dwarf plant type with low peroxidase activity and well filled grains.     

Genetic analysis and gene detection of fructan content using DArT molecular markers in spring bread wheat (Triticum aestivum L.) grain

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