Crop yield is determined by the acquisition and allocation of photoassimilates in sink organs. Therefore, genetic modification of sink size is essential for understanding the complex signaling network regulating sink strength and source activities. Sink size in wheat depends on the number of spikelets per spike, floret/grain number per spikelet as well as the grain weight or dry matter accumulation. Hence, increasing spikelet number and improving sink size are targets for wheat breeding. The main objective of the present work was to genetically modify the wheat spike architecture, i.e., the sink size by introgressing the ‘Miracle wheat’ or the bht-A1 allele into an elite durum wheat cv. Floradur. After four generations of backcrossing to the recurrent parent, Floradur (FL), we have successfully developed Near Isogenic Lines (NILs) with a modified spikelet arrangement thereby increasing spikelet and grain number per spike. Genotyping of bht-A1 NILs using the Genotyping-By-Sequencing approach revealed that the size of the introgressed donor segments carrying bht-A1 ranged from 2.3 to 38 cM. The size of the shortest donor segment introgressed into bht-A1 NILs was estimated to be 9.8 mega base pairs (Mbp). Phenotypic analysis showed that FL-bht-A1-NILs (BC3F2 and BC3F3) carry up to seven additional spikelets per spike, leading to up to 29% increase in spike dry weight at harvest (SDWh). The increased SDWh was accompanied by up to 23% more grains per spike. More interestingly, thousand kernel weight (TKW) did not show significant differences between FL-bht-A1-NILs and Floradur, suggesting that besides increasing spikelet number, bht-A1 could also be targeted for increasing grain yield in wheat. Our study suggests that the genetic modification of spikelet number in wheat can be an entry point for improving grain yield, most interestingly and also unexpectedly without the trade-off effects on TKW. Hence, FL-bht-A1-NILs are not only essential for increasing grain number, but also for understanding the molecular and genetic mechanism of the source–sink interaction for a clearer picture of the complex signaling network regulating sink strength and source activities.
Grain yield increase of wheat in the past were mainly based on linear increase in the number of grains per meter square (Serrago et al. 2013; Ferrante et al. 2017), while spikelet number, grain weight and biomass were largely unchanged (Royo et al. 2007; Alvaro et al. 2008; Sanchez-Garcia et al. 2013). Even the tremendous increase in wheat grain yield after the introduction of the semi-dwarfing varieties has been mainly due to an increase in grains per spikelet without change in spikelet number (Youssefian et al. 1992a, b; Thomas 2017). Interestingly, unlike other cereal crops like barley, maize, rice, and sorghum, each wheat spikelet bears more than one grain, making the wheat spikelet the most essential grain yield component. Since the number and arrangement of each spikelet are under strong genetic, hormonal, and environmental control (McSteen 2009; Boden et al. 2015; Poursarebani et al. 2015; Youssef et al. 2017; Dixon et al. 2018), the isolation and characterization of genes that regulate inflorescence architecture is important for a clearer picture of grain yield barriers in wheat. With the fact that grain growth in wheat is also reported to be sink limited (Borras et al. 2004; Miralles and Slafer 2007; Zhang et al. 2010; Serrago et al. 2013), modifying the sink size, i.e., spikelet number is key to further understand the physiological and genetic basis of the source–sink relationship in wheat.
So far, several approaches were suggested for increasing wheat’s yield potential (Fischer 2007; Reynolds et al. 2009; Foulkes et al. 2011; Gonzalez et al. 2011; Guo et al. 2017), including increasing the spike fertility index. The spike fertility index, also termed as fruiting efficiency (Slafer et al. 2015; Alonso et al. 2018), is the number of grains set per unit spike dry weight at anthesis (Ferrante et al. 2012). Because improving harvest index by directly increasing floret fertility has been very tough (Austin et al. 1980; Guo and Schnurbusch 2015; Guo et al. 2016), an alternative approach, such as enhancing spikelet number per spike should be critically considered.
The wheat inflorescence is characterized by an unbranched spike, whereupon sessile spikelets are distichously attached to the inflorescence’s central axis (i.e., the rachis). In wheat, the number of spikelets per spike is fixed after the initiation of the terminal spikelet (Bonnet 1967; McMaster 1997). However, spike length and spikelet number are also known to be influenced by environmental factors, such as temperature and day length (Friend 1965; Rawson 1971; Rawson and Richards 1993). Generally, a longer spike development phase combined with optimum temperature and light promotes the initiation of more spikelets in wheat (Friend 1965; Rawson 1970, 1971; Pinthus and Millet 1978; Fischer 1985; Rawson and Richards 1993; Shaw et al. 2013). In this regard, the reproductive success of the wheat plant is predominantly controlled by three sets of genes namely vernalization, photoperiod, and the earliness per se genes (Yan et al. 2003, 2004, 2006; Beales et al. 2007; Lewis et al. 2008; Gawroński et al. 2014). The earliness per se genes are classes of genes regulating flowering time independently of vernalization and photoperiod and are important for the wide adaptation of wheat to different environments (Cockram et al. 2007; Lewis et al. 2008; Faricelli et al. 2010; Zikhali et al. 2016). By accelerating flowering time, i.e., shortening of the vegetative phases, Photoperiod-1 (Ppd1) is implicated in the reduction of spikelet and tiller number in wheat (Worland et al. 1998; Shaw et al. 2013; Guo et al. 2018). Furthermore, with the rising global temperature, several crops flower earlier in warmer temperature (Craufurd and Wheeler 2009; Ellwood et al. 2013).
Different studies have also reported that late flowering wheat cultivars are vulnerable to post-anthesis stress that lead to a substantial reduction in yield and grain quality (Liu et al. 2014; Myers et al. 2014; Asseng et al. 2015). Therefore, to offset the trade-off between accelerated flowering and spikelet number, other mechanisms of increasing spikelet number need to be considered.
Genes controlling wheat spikelet arrangement have already been discovered (Boden et al. 2015; Dobrovolskaya et al. 2015; Poursarebani et al. 2015; Dixon et al. 2018). One of these genes is the wheat spikelet meristem identity gene, TtBH-A1 (Poursarebani et al. 2015). A non-synonymous mutation that has occurred in the highly conserved AP2/ERF DNA binding domain of TtBH-A1 altered the function of TtBH-A1, resulting in the so-called ‘Miracle wheat’ or spike-branching wheat phenotype. ‘Miracle wheat’ mutants deviate from the canonical spike form by developing mini-spike-like branches and/or secondary or supernumerary spikelets (SS) along the spike, but similarly have often lost terminal spikelet formation with increased spikelet and grain number per spike (Poursarebani et al. 2015).
Aiming to further characterize the ‘Miracle wheat’ allele, especially from the perspectives of increasing spikelet and grain number, we introgressed the bht-A1 allele into an elite durum wheat cv. Floradur. Unlike the ‘Miracle wheat’ (TRI 19165, which is the donor parent), Floradur-bht-A1-NILs are typically characterized by the formation of supernumerary spikelets (SS) instead of developing mini-spike-like structures. Nevertheless, Floradur-bht-A1-NILs showed increased spike dry weight at harvest that was accompanied by significant increase in grain number per spike, suggesting that Floradur-bht-A1-NILs are interesting genetic materials for the genetic manipulation of grain yield in wheat. This is also because, Floradur-bht-A1-NILs are not only carrying the ‘Miracle wheat’ allele, but also incorporated the semi-dwarfing allele, Rht-B1, which make them useful wheat germplasm that have combined two plant architectural genes in a single elite genetic background to further assess new opportunities for increasing grain yield production.
Materials and methods
A German elite durum spring wheat cultivar, Floradur (FL), was used as a recurrent parent. ‘Miracle wheat’ accession TRI 19165 was used as a donor parent for the ‘Miracle wheat’ or the bht-A1 allele. TRI 19165 is a spike-branching mutant landrace from the gene bank collection of the Leibniz Institute for Plant Genetics and Crop Plant Research (IPK, Germany).
Floradur and TRI 19165 were grown side by side in a greenhouse for crossing. TRI 19165 was used as a pollen parent to generate F1 plants. Emasculation of the female parent was initiated when half of the spike emerged from the flag leaf sheath. All young florets were surgically removed from each spikelet except the basal two florets. The terminal and very young spikelets were completely removed. After removing the anthers, the whole spike was covered with bags to avoid any pollen cross-contamination. After pollination, the spike was covered again to protect any pollen cross-contamination. The complete scheme is shown in Fig. 1. Accordingly, four times backcrossing was made to the recurrent parent Floradur.
One genome-specific CAPS marker, which was developed based on the SNP in the TtBH-A1 gene (T287C) which gave rise to the ‘Miracle wheat’ or bht-A1 allele (Poursarebani et al. 2015), was used for the foreground (FG) selection in all generations. Heterozygosity of the F1 plants was checked using two microsatellite markers, xgwm155 (chr.3A) and xgwm95 (chr.2A). PCR conditions and fragment analysis were conducted as described by Röder et al. (1998). All true-bred heterozygous F1 plants were backcrossed with Floradur following the same procedure described earlier (see Fig. 1).
Embryo rescue (ER)
To shorten the generation time, embryos 21 days after pollination were rescued from the first two generations, i.e., BC1F1 and BC2F1 plants (Fig. 1). This saved the time from ripening to harvest. First, the immature grains were removed from all BC1F1 and BC2F1 plants. Then, the grains were rinsed in 70% alcohol for about 1 min. After rinsing the grains with sterilized water, the samples were washed with a washing buffer containing 20% hypochlorite and tween20. After 7 min of shaking, samples were repeatedly washed with sterilized water. The embryos from the grains were surgically removed in a laminar hood and carefully placed on B5 medium in a Petri dish (Duchefa biochemie, Haarlem, The Netherlands) for about 72 h at 24 °C under a dark condition in a growth chamber. The calli were then transferred to fresh new media in a magenta box under long day conditions (16 h light and 8 h dark) for about 10–12 days at 24 °C. Seedlings were then transplanted under greenhouse conditions in a pot size of 2 L filled with substrate 2 (Klasmann-Deilmann GmbH, 49744 Geeste, Germany), compost and sand with a proportion of 2:2:1, respectively. Plants received all the standard treatments for wheat including fertilizers and pesticides. DNA was extracted from these plants for the background (BG) and FG selections. All heterozygous plants were selected and backcrossed with Floradur following the same procedure described earlier (Fig. 1).
Forty-seven polymorphic wheat microsatellite markers were selected for the background selection (Supplementary Table 1). The selection was made in such a way that the polymorphism can be easily detected on a 3% standard agarose gel. PCR conditions for each of these markers were followed as described earlier by Röder et al. (1998). Then, the background selection was applied to 56 heterozygous BC2F1 plants. The recurrent parent genome recovery (RpGR) was calculated as follows
where RpGR is the recurrent parent genome recovery, HO is the number of homozygous markers (Floradur alleles); HE is the number of heterozygous markers (alleles).
After checking the zygosity of 90 BC4F2 plants using the bht-A1 CAPS marker, all bht-A1 homozygous (n = 23) and heterozygous (n = 44) plants were genotyped following the novel two-enzyme Genotyping-By-Sequencing (GBS) approach for the whole genome (Poland et al. 2012). Adapters were trimmed from reads with cutadapt version 1.8.dev0 (Martin 2011). Trimmed reads were mapped to the chromosome-shotgun assemblies of bread wheat cultivar Chinese Spring (The International Wheat Genome Sequencing Consortium (IWGSC) 2014) with BWA mem version 0.7.12 (Li 2013), converted to BAM format with SAMtools (Li et al. 2009) and sorted with Novosort (Novocraft Technologies Sdn Bhd, Malaysia, http://www.novocraft.com/). Multi-sample variant calling was performed with SAMtools version 0.1.19 (Li 2011). The command “mpileup” was used with the parameters “-C50 –DV”. The resultant VCF file was filtered with an AWK script provided as Text S3 by Mascher et al. (2013). Only bi-allelic SNPs were used. Homozygous genotype calls were set to missing if their coverage was below 1 or their genotype quality was below 3. Heterozygous genotype calls were set to missing if their coverage below 4 or their genotype quality was below 10. An SNP was discarded when (1) its quality score was below 40, (2) its heterozygosity was above 20%, (3) its minor allele frequency was below 10%, or (4) had more than 66% missing data. Genotype calls were filtered and converted into genotype matrix with an AWK script available as Text S3 of Mascher et al. (2013). Chromosomal locations and genetic positions were taken from population sequence (POPSEQ) data (Chapman et al. 2015). Raw sequence data are available from the European Nucleotide Archive under accession PRJEB24999.
Phenotyping of homozygous BC3F2 and BC3F3 plants
Grains from selfed BC3F1 plants were randomly picked and grown as BC3F2 plants on a 96-well tray under greenhouse conditions. After 4 weeks of vernalization (at 4 °C) and hardening for 1 week (at 15 °C), seedlings were directly transplanted into the field characterized by a silty loam soil. The distance between rows and plants was 20 and 10 cm, respectively. DNA was extracted from each of these plants for the foreground selection, i.e., homozygous bht-A1 plants, for phenotyping. Following the field evaluation, homozygous progenies from BC3F2 plants, i.e., BC3F3 plants were also phenotyped under greenhouse conditions. In both environments, the recurrent parent Floradur was used for comparison. In total, about 118, i.e., 27 plants at BC3F2 and 91 plants at BC3F3 generation were evaluated for this study. Crop husbandry (fertilization, watering, and pesticides) was applied uniformly to all plants in the field and the greenhouse following the recommended rate for wheat cultivation as required. For phenotypic data analysis, we used unpaired two-tailed Student’s t test to check for trait differences among progenies.
Marker-assisted foreground and background selections
At all generations, the bht-A1 CAPS marker was used as a diagnostic marker for the selection of the bht-A1 allele (Fig. 2a). BG selection was made once at BC2F1 using 47 polymorphic wheat microsatellite markers (Supplementary Table 1). Calculated RpGR based on 56 heterozygous BC2F1 plants is shown in Fig. 2b. Based on RpGR, five plants namely P-69, P-103, P-114, P-126, and P-170 were selected to establish five families of the NILs. These plants were then backcrossed to Floradur to generate BC3F1 plant families. Heterozygous progenies from BC3F1 were further backcrossed to Floradur to generate BC4F1 grains for all the five NILs families (Fig. 1).
Phenotypic analysis of homozygous BC3F2 and BC3F3 plants
Unlike the ‘mini-spike’-like structures known to develop during genuine spike-branching in TRI 19165, spike-branching in bht-A1-NILs was reduced to the formation of secondary spikelets sharing the same rachis node with that of the primary spikelets forming the so-called supernumerary spikelets (Fig. 3b). This clearly revealed the successful introgression of bht-A1 into the Floradur genome. Thus, homozygous bht-A1-NILs were found to carry up to seven SS per spike (Fig. 3c). Such reduced phenotypic penetrance and expressivity is most likely due, partly, to other branch suppressing loci from Floradur located elsewhere in the genome and/or environmental effects. The summary of other spike traits from the five plant families of bht-A1-NILs is shown in Table 1. Compared to Floradur, the bht-A1-NILs had on average more grain-bearing spikelets resulting in up to 29% of increased SDWh, thereby increasing the grain number per spike (Table 1). Unlike the known trade-off between grain number and TKW (Pinthus and Millet 1978; Slafer and Miralles 1993; Gambín and Borrás 2010; Griffiths et al. 2015), especially for the spike-branching wheat accessions (Poursarebani et al. 2015), no significant difference in TKW between Floradur-bht-A1-NILs and Floradur was found, clearly suggesting that the bht-A1 allele is useful for the manipulation of spikelet number per spike and potentially for increasing grain yield in wheat. As sink size is thought to be one of the yield-limiting factors in wheat (Borras et al. 2004; Miralles and Slafer 2007; Zhang et al. 2010), the Floradur-bht-A1-NILs are similarly an ideal resource for further assessing the source–sink interaction. Interestingly, tiller number (TN) analysis between Floradur-bht-A1-NILs and Floradur was non-significant, suggesting that bht-A1 is not involved in shoot branching (Table 1). Thus, the bht-A1 allele can be utilized in durum wheat breeding.
Genotyping and estimation of introgressed donor segment
GBS of 67 BC4F2 plants yielded about 9400 SNP markers spanning the whole genome. After removing all redundant markers, i.e., markers with the same genetic positions, about 799 polymorphic markers were selected to estimate the introgressed donor segment carrying the bht-A1 allele. The genetic position of the markers was adopted from the POPSEQ linkage map (Chapman et al. 2015). Distribution of the markers across the linkage groups is presented in the Supplementary Table 2. Best 20 NILs with higher RpGR are shown in Table 2. Based on the graphical genotypes of the NILs (Table 3), the size of the introgressed donor segment ranged from 2.28 to 38 cM. P-69-4-4-121 and P-69-4-4-122 from Fam1 plants carry the smallest donor fragment (~ 2.28 cM). According to the genome sequence assembly of the Chinese Spring, Refseq V1.0 (Alaux et al. 2018), markers flanking the introgressed fragment, i.e., 2AS_5296611:2363 and 2AS_5272493:9637 were mapped at position 52,425,262 bp and 62,264,948 bp, respectively (Fig. 4). Therefore, the size of the shortest donor segment introgressed into bht-A1 NILs was estimated to be 9,839,686 base pairs. Graphical representation of chromosome group 2 for the best plant from Fam 1, P-69-4-4-122 is shown in Fig. 4. Graphical representation for the remaining chromosomes for P-69-4-4-122 is shown in Supplementary Figure 1. As expected, average whole genome RpGR, i.e., 96.23%, was similar to the expected RpGR of 96.88% (Table 2). On average, RpGR for chromosomes 2A and 2B was 93.41 and 96.27%, respectively. In general, RpGR for chromosomes 2A ranged from 82 to 99%. The lowered RpGR for some of these plants was due to the drag-over-effect attributed to the suppressed recombination and marker-assisted selection. Taking all together, these results demonstrated that the expected RpGR at BC4 generation and one or two more rounds of backcrossing might even remove the drag-over-effects from the donor parent.
Spikelets are the building blocks of the wheat inflorescence, also known as the spike. The number and arrangements of individual spikelets on the spike dictate the architecture or morphology of the spike and critically determines the grain yield in wheat. Thus, understanding the genetic basis of spike development is key to understand the genetic basis of gain yield formation. Due to limited knowledge of genes controlling wheat spikelet arrangement, genetic modification of spikelet arrangement was not possible until recently. In the last few years, genes altering spikelet arrangement have been discovered paving ways for the genetic modification of the spike architecture in wheat (Boden et al. 2015; Dobrovolskaya et al. 2015; Poursarebani et al. 2015; Debernardi et al. 2017; Greenwood et al. 2017).
The current study aimed to introgress the ‘Miracle wheat’ allele bht-A1 into an elite durum wheat variety, Floradur, to better understand the effect of altered spikelet arrangement in connection with the source–sink balance and grain yield formation.
Although spike-branching increases the sink size in wheat (Dobrovolskaya et al. 2015; Poursarebani et al. 2015), the extremely low spikelet fertility hindered the direct utilization of spike-branching wheat accessions for increasing the grain yield (USDA 1916; Rawson and Ruwali 1972; Pennell and Halloran 1984; Hucl and Fowler 1992). Furthermore, the intensity of spike-branching is affected by the environment; especially by the photoperiod and temperature (Percival 1921; Sharman 1944; Pennell and Halloran 1984a, b). Interestingly, unlike the donor parent, TRI 19165, Floradur-bht-A1-NILs often form supernumerary spikelets thereby increasing spikelet and grain number per spike. Such lowered expression of spike-branching in the NILs significantly reduced the trade-off between spike-branching and spikelet fertility/TKW usually seen in the rather extreme ‘Miracle wheat’ phenotype (Poursarebani et al. 2015). This clearly suggests that a more attenuated phenotypic expression of bht-A1 can be used to genetically modify spikelet number and spike dry weight in wheat without affecting TKW.
Carbon assimilation and partitioning to the developing inflorescence critically determine crop yield and productivity (Satoh-Nagasawa et al. 2006; Lawlor and Paul 2014; Sosso et al. 2015; White et al. 2016). The sink capacity of the spike, namely the number of spikelets per spike, the number of grains per spikelet (spikelet fertility) and grain weight (dry matter accumulation in each grain) strongly affect grain yield in wheat. Hence, sink strength was suggested to be a critical yield-limiting factor (Fischer 1985; Slafer and Savin 1994; Richards 1996; Borrás et al. 2004; Reynolds et al. 2007; Foulkes et al. 2011). As sink strength strongly dictates activities in the source and assimilate partitioning to the sink organs (Yu et al. 2015), it is strongly believed that increasing sink strength and activity are of major interest. Previous studies attempted to better understand to which extent source and/or sink limited wheat yields (Blade and Baker 1991; Slafer and Savin 1994; Cruz-Aguado et al. 1999; Serrago et al. 2013); however, results were rather inconclusive which might suggest that both source and sink might co-limit yield formation in wheat. The communication between the source and sink organs is regulated by a complex signaling network involving sugars, hormones, and environmental factors (Yu et al. 2015).
In this regard, a limited number of genes regulating the source–sink communication has been characterized (Aoki et al. 2002; Martinez-Barajas et al. 2011). Thus, modern molecular genetic approaches are required for a deeper understanding of the source–sink relationship in wheat. Because of the position of the secondary spikelets in relation to the primary spikelets, both are arranged laterally sharing the same rachis node, Floradur-bht-A1-NILs are important genetic resources to elucidate the mechanics of vascular tissue architecture and assimilate partitioning towards wheat spikelets.
The tremendous harvest index increase after the introduction of the semi-dwarfing allele during the green revolution was due, partly, to the diversion of more assimilates to the developing spike which otherwise would be invested in the growth and development of the culm (Flintham et al. 1997; Khush 2001). Interestingly, Floradur-bht-A1-NILs are semi-dwarf carrying the reduced plant height gene (Rht-B1), which makes them an excellent wheat ideotype for studying the physiological and genetic basis of source–sink interaction and yield formation in wheat. Hence, Floradur-bht-A1-NILs are important wheat genetic resources not only for increasing grain number per spike, but also, for the systematic discovery of TtBH-A1 downstream target genes for the detailed understanding of the molecular genetics of wheat spike morphogenesis. Our study further suggests genetic modification of spikelet number as an entry point for improving yield, most interestingly and also unexpectedly without the trade-off effects on TKW. Therefore, despite the negative relationship between grain number and grain weight in wheat, this work illustrates the possibilities of increasing grain number per spike without significantly affecting the grain weight. However, whether this genetic material, or its derivatives, will have any yield advantages under real world conditions is still to be shown in multi-year and multi-location field evaluations. Nevertheless, as the sink size in Floradur-bht-A1-NILs is genetically controlled, Floradur-bht-A1-NILs are also beneficial for understanding the physiological and molecular basis of source–sink interaction for a clearer picture of the complex signaling network regulating sink strength and source activities in wheat.
Alaux M, Rogers J, Letellier T, Flores R, Alfama F, Pommier C, Mohellibi N, Durand S, Kimmel E, Michotey C, Guerche C, Loaec M, Lainé M, Steinbach D, Choulet F, Rimbert H, Leroy P, Guilhot N, Salse J, Feuillet C, Paux E, Eversole K, Adam-Blondon A-F, Quesneville H, Consortium IWGS (2018) Linking the International Wheat Genome Sequencing Consortium bread wheat reference genome sequence to wheat genetic and phenomic data. Genome Biol 19:111
Alonso MP, Mirabella NE, Panelo JS, Cendoya MG, Pontaroli AC (2018) Selection for high spike fertility index increases genetic progress in grain yield and stability in bread wheat. Euphytica 214:112
Alvaro F, Isidro J, Villegas D, del Moral LFG, Royo C (2008) Old and modern durum wheat varieties from Italy and Spain differ in main spike components. Field Crops Res 106:86–93
Aoki N, Whitfeld P, Hoeren F, Scofield G, Newell K, Patrick J, Offler C, Clarke B, Rahman S, Furbank RT (2002) Three sucrose transporter genes are expressed in the developing grain of hexaploid wheat. Plant Mol Biol 50:453–462
Asseng S, Ewert F, Martre P, Rotter RP, Lobell DB, Cammarano D, Kimball BA, Ottman MJ, Wall GW, White JW, Reynolds MP, Alderman PD, Prasad PVV, Aggarwal PK, Anothai J, Basso B, Biernath C, Challinor AJ, De Sanctis G, Doltra J, Fereres E, Garcia-Vile M, Gayler S, Hoogenboom G, Hunt LA, Izaurralde RC, Jabloun M, Jones CD, Kersebaum KC, Koehler AK, Muller C, Kumar SN, Nendel C, O’Leary G, Olesen JE, Palosuo T, Priesack E, Rezaei EE, Ruane AC, Semenov MA, Shcherbak I, Stockle C, Stratonovitch P, Streck T, Supit I, Tao F, Thorburn PJ, Waha K, Wang E, Wallach D, Wolf I, Zhao Z, Zhu Y (2015) Rising temperatures reduce global wheat production. Nat Clim Change 5:143–147
Austin RB, Bingham J, Blackwell RD, Evans LT, Ford MA, Morgan CL, Taylor M (1980) Genetic improvements in winter-wheat yields since 1900 and associated physiological-changes. J Agric Sci 94:675–689
Beales J, Turner A, GriYths S, Snape JW, Laurie DA (2007) A pseudo-response regulator is misexpressed in the photoperiod insensitive Ppd-D1a mutant of wheat (Triticum aestivum L.). Theor Appl Genet 115:721–733
Blade SF, Baker RJ (1991) Kernel weight response to source sink changes in spring wheat. Crop Sci 31:1117–1120
Boden SA, Cavanagh C, Cullis BR, Ramm K, Greenwood J, Finnegan EJ, Trevaskis B, Swain SM (2015) Ppd-1 is a key regulator of inflorescence architecture and paired spikelet development in wheat. Nat Plants 1:1–16
Bonnet OT (1967) Inflorescences of maize, wheat, rye, barley, and oats: their initiation and development, vol 721. University of Illinois, College of Agricultural Experiment Station Bulletin, Champaign
Borras L, Slafer GA, Otegui ME (2004) Seed dry weight response to source–sink manipulations in wheat, maize and soybean: a quantitative reappraisal. Field Crops Res 86:131–146
Borrás L, Slafer GA, Otegui MaE (2004) Seed dry weight response to source–sink manipulations in wheat, maize and soybean: a quantitative reappraisal. Field Crops Res 86:131–146
Chapman JA, Mascher M, Buluc A, Barry K, Georganas E, Session A, Strnadova V, Jenkins J, Sehgal S, Oliker L, Schmutz J, Yelick KA, Scholz U, Waugh R, Poland JA, Muehlbauer GJ, Stein N, Rokhsar DS (2015) A whole-genome shotgun approach for assembling and anchoring the hexaploid bread wheat genome. Genome Biol 16:26
Cockram J, Jones H, Leigh FJ, O’Sullivan D, Powell W, Laurie DA, Greenland AJ (2007) Control of flowering time in temperate cereals: genes, domestication, and sustainable productivity. J Exp Bot 58:1231–1244
Craufurd PQ, Wheeler TR (2009) Climate change and the flowering time of annual crops. J Exp Bot 60:2529–2539
Cruz-Aguado JA, Reyes F, Rodes R, Perez I, Dorado M (1999) Effect of source-to-sink ratio on partitioning of dry matter and 14C-photoassimilates in wheat during grain filling. Ann Bot 83:655–665
Debernardi JM, Lin H, Chuck G, Faris JD, Dubcovsky J (2017) microRNA172 plays a crucial role in wheat spike morphogenesis and grain threshability. Development 144:1966–1975
Dixon LE, Greenwood JR, Bencivenga S, Zhang P, Cockram J, Mellers G, Ramm K, Cavanagh C, Swain SM, Boden SA (2018) TEOSINTE BRANCHED1 regulates inflorescence architecture and development in bread wheat (Triticum aestivum). Plant Cell 30:563–581
Dobrovolskaya O, Pont C, Sibout R, Martinek P, Badaeva E, Murat F, Chosson A, Watanabe N, Prat E, Gautier N, Gautier V, Poncet C, Orlov YL, Krasnikov AA, Berges H, Salina E, Laikova L, Salse J (2015) FRIZZY PANICLE drives supernumerary spikelets in bread wheat. Plant Physiol 167:189–199
Ellwood ER, Temple SA, Primack RB, Bradley NL, Davis CC (2013) Record-breaking early flowering in the eastern United States. PLoS ONE 8:e53788
Faricelli ME, Valarik M, Dubcovsky J (2010) Control of flowering time and spike development in cereals: the earliness per se Eps-1 region in wheat, rice, and Brachypodium. Funct Integr Genom 10:293–306
Ferrante A, Savin R, Slafer GA (2012) Differences in yield physiology between modern, well adapted durum wheat cultivars grown under contrasting conditions. Field Crops Res 136:52–64
Ferrante A, Cartelle J, Savin R, Slafer GA (2017) Yield determination, interplay between major components and yield stability in a traditional and a contemporary wheat across a wide range of environments. Field Crops Res 203:114–127
Fischer RA (1985) Number of kernels in wheat crops and the influence of solar-radiation and temperature. J Agric Sci 105:447–461
Fischer RA (2007) Understanding the physiological basis of yield potential in wheat. J Agric Sci 145:99–113
Flintham J, Börner A, Worland A, Gale M (1997) Optimizing wheat grain yield: effects of Rht (gibberellin-insensitive) dwarfing genes. J Agric Sci 128:11–25
Foulkes MJ, Slafer GA, Davies WJ, Berry PM, Sylvester-Bradley R, Martre P, Calderini DF, Griffiths S, Reynolds MP (2011) Raising yield potential of wheat. III. Optimizing partitioning to grain while maintaining lodging resistance. J Exp Bot 62:469–486
Friend DJC (1965) Ear length and spikelet number of wheat grown at different temperatures and light intensities. Can J Bot 43:345–353
Gambín BL, Borrás L (2010) Resource distribution and the trade-off between seed number and seed weight: a comparison across crop species. Ann Appl Biol 156:91–102
Gawroński P, Ariyadasa R, Himmelbach A, Poursarebani N, Kilian B, Stein N, Steuernagel B, Hensel G, Kumlehn J, Sehgal SK (2014) A distorted circadian clock causes early flowering and temperature-dependent variation in spike development in the Eps-3Am mutant of einkorn wheat. Genetics 196:1253–1261
Gonzalez FG, Miralles DJ, Slafer GA (2011) Wheat floret survival as related to pre-anthesis spike growth. J Exp Bot 62:4889–4901
Greenwood JR, Finnegan EJ, Watanabe N, Trevaskis B, Swain SM (2017) New alleles of the wheat domestication gene Q reveal multiple roles in growth and reproductive development. Development 144:1959–1965
Griffiths S, Wingen L, Pietragalla J, Garcia G, Hasan A, Miralles D, Calderini DF, Ankleshwaria JB, Waite ML, Simmonds J, Snape J, Reynolds M (2015) Genetic dissection of grain size and grain number trade-offs in CIMMYT wheat germplasm. PLoS One 10:e0118847
Guo Z, Schnurbusch T (2015) Variation of floret fertility in hexaploid wheat revealed by tiller removal. J Exp Bot 66:5945–5958
Guo Z, Slafer GA, Schnurbusch T (2016) Genotypic variation in spike fertility traits and ovary size as determinants of floret and grain survival rate in wheat. J Exp Bot 67:4221–4230
Guo Z, Chen D, Alqudah AM, Röder MS, Ganal MW, Schnurbusch T (2017) Genome-wide association analyses of 54 traits identified multiple loci for the determination of floret fertility in wheat. New Phytol 214:257–270
Guo Z, Chen D, Röder M, Ganal M, Schnurbusch T (2018) Genetic dissection of pre-anthesis sub-phase durations during the reproductive spike development of wheat. Plant J 95:909–918
Hucl P, Fowler BJ (1992) Comparison of a branched spike wheat with the cultivars Neepawa and Hy320 for grain-yield and yield components. Can J Plant Sci 72:671–677
Khush GS (2001) Green revolution: the way forward. Nat Rev Genet 2:815–822
Lawlor DW, Paul MJ (2014) Source/sink interactions underpin crop yield: the case for trehalose 6-phosphate/SnRK1 in improvement of wheat. Front Plant Sci 5:418
Lewis S, Faricelli ME, Appendino ML, Valarik M, Dubcovsky J (2008) The chromosome region including the earliness per se locus Eps-Am1 affects the duration of early developmental phases and spikelet number in diploid wheat. J Exp Bot 59:3595–3607
Li H (2011) A statistical framework for SNP calling, mutation discovery, association mapping and population genetical parameter estimation from sequencing data. Bioinformatics 27:2987–2993
Li H (2013) Aligning sequence reads, clone sequences and assembly contigs with BWA-MEM. arXiv preprint. arXiv:1303.3997
Li H, Handsaker B, Wysoker A, Fennell T, Ruan J, Homer N, Marth G, Abecasis G, Durbin R, Proc GPD (2009) The sequence alignment/map format and SAMtools. Bioinformatics 25:2078–2079
Liu B, Liu L, Tian L, Cao W, Zhu Y, Asseng S (2014) Post-heading heat stress and yield impact in winter wheat of China. Glob Change Biol 20:372–381
Martin M (2011) Cutadapt removes adapter sequences from high-throughput sequencing reads. EMBnet J 17:10–12
Martinez-Barajas E, Delatte T, Schluepmann H, de Jong GJ, Somsen GW, Nunes C, Primavesi LF, Coello P, Mitchell RA, Paul MJ (2011) Wheat grain development is characterized by remarkable trehalose 6-phosphate accumulation pregrain filling: tissue distribution and relationship to SNF1-related protein kinase1 activity. Plant Physiol 156:373–381
Mascher M, Wu SY, St Amand P, Stein N, Poland J (2013) Application of genotyping-by-sequencing on semiconductor sequencing platforms: a comparison of genetic and reference-based marker ordering in barley. PLoS One 8:e76925
McMaster GS (1997) Phenology, development, and growth of the wheat (Tritimm aestivum L.) shoot apex: a review. In: Sparks DL (ed) Advances in agronomy. Academic Press Inc, United States, pp 63–118. http://www.apnet.com
McSteen P (2009) Hormonal regulation of branching in grasses. Plant Physiol 149:46–55
Miralles DJ, Slafer GA (2007) Sink limitations to yield in wheat: how could it be reduced? J Agric Sci 145:139–149
Myers SS, Zanobetti A, Kloog I, Huybers P, Leakey AD, Bloom AJ, Carlisle E, Dietterich LH, Fitzgerald G, Hasegawa T, Holbrook NM, Nelson RL, Ottman MJ, Raboy V, Sakai H, Sartor KA, Schwartz J, Seneweera S, Tausz M, Usui Y (2014) Increasing CO2 threatens human nutrition. Nature 510:139–142
Pennell A, Halloran GM (1984a) Influence of vernalization and photoperiod on supernumerary spikelet expression in wheat. Ann Bot 53:821–831
Pennell AL, Halloran GM (1984b) Influence of time of sowing, photoperiod, and temperature on supernumerary spikelet expression in wheat (Triticum). Can J Bot Revue Canadienne De Botanique 62:1687–1692
Percival J (1921) The wheat plant: a monograph. Duckworth and Co., London, pp 241–261
Pinthus MJ, Millet E (1978) Interactions among number of spikelets, number of grains and grain weight in spikes of wheat (Triticum-Aestivum L). Ann Bot 42:839–848
Poland JA, Brown PJ, Sorrells ME, Jannink JL (2012) Development of high-density genetic maps for barley and wheat using a novel two-enzyme genotyping-by-sequencing approach. PLoS One 7:e32253
Poursarebani N, Seidensticker T, Koppolu R, Trautewig C, Gawronski P, Bini F, Govind G, Rutten T, Sakuma S, Tagiri A, Wolde GM, Youssef HM, Battal A, Ciannamea S, Fusca T, Nussbaumer T, Pozzi C, Borner A, Lundqvist U, Komatsuda T, Salvi S, Tuberosa R, Uauy C, Sreenivasulu N, Rossini L, Schnurbusch T (2015) The genetic basis of composite spike form in barley and ‘miracle-wheat’. Genetics 201:155–165
Rawson HM (1970) Spikelet number, its control and relation to yield per ear in wheat. Austr J Biol Sci 23:1–16
Rawson H (1971) An upper limit for spikelet number per ear in wheat as controlled by photoperiod. Crop Pasture Sci 22:537–546
Rawson HM, Richards RA (1993) Effects of high temperature and photoperiod on floral development in wheat isolines differing in vernalisation and photoperiod genes. Field Crops Res 32:181–192
Rawson H, Ruwali K (1972) Ear branching as a means of increasing grain uniformity in wheat. Aust J Agric Res 23:551–559
Reynolds M, Calderini D, Condon A, Vargas M (2007) Association of source/sink traits with yield, biomass and radiation use efficiency among random sister lines from three wheat crosses in a high-yield environment. J Agric Sci 145:3–16
Reynolds M, Foulkes MJ, Slafer GA, Berry P, Parry MAJ, Snape JW, Angus WJ (2009) Raising yield potential in wheat. J Exp Bot 60:1899–1918
Richards R (1996) Increasing the yield potential of wheat: manipulating sources and sinks. In: Reynolds MP, Rajaram S, McNab A (eds) Increasing yield potential in wheat: breaking the barriers. International Maize and Wheat Improvement Center, Sonora, pp 134–149
Röder MS, Korzun V, Wendehake K, Plaschke J, Tixier MH, Leroy P, Ganal MW (1998) A microsatellite map of wheat. Genetics 149:2007–2023
Royo C, Alvaro F, Martos V, Ramdani A, Isidro J, Villegas D, del Moral LFG (2007) Genetic changes in durum wheat yield components and associated traits in Italian and Spanish varieties during the 20th century. Euphytica 155:259–270
Sanchez-Garcia M, Royo C, Aparicio N, Martin-Sanchez JA, Alvaro F (2013) Genetic improvement of bread wheat yield and associated traits in Spain during the 20th century. J Agric Sci 151:105–118
Satoh-Nagasawa N, Nagasawa N, Malcomber S, Sakai H, Jackson D (2006) A trehalose metabolic enzyme controls inflorescence architecture in maize. Nature 441:227–230
Serrago RA, Alzueta I, Savin R, Slafer GA (2013) Understanding grain yield responses to source–sink ratios during grain filling in wheat and barley under contrasting environments. Field Crops Res 150:42–51
Sharman B (1944) Branched heads in wheat and wheat hybrids. Nature 153:497–498
Shaw LM, Turner AS, Herry L, Griffiths S, Laurie DA (2013) Mutant alleles of Photoperiod-1 in wheat (Triticum aestivum L.) that confer a late flowering phenotype in long days. PLoS One 8:e79459
Slafer GA, Miralles DJ (1993) Fruiting efficiency in three bread wheat (Tritkum aestivum) cultivars released at different eras. Number of grains per spike and grain weight. J Agron Crop Sci 170:251–260
Slafer GA, Savin R (1994) Source–sink relationships and grain mass at different positions within the spike in wheat. Field Crops Res 37:39–49
Slafer GA, Elia M, Savin R, García GA, Terrile II, Ferrante A, Miralles DJ, González FG (2015) Fruiting efficiency: an alternative trait to further rise wheat yield. Food Energy Secur 4:92–109
Sosso D, Luo D, Li Q-B, Sasse J, Yang J, Gendrot G, Suzuki M, Koch KE, McCarty DR, Chourey PS, Rogowsky PM, Ross-Ibarra J, Yang B, Frommer WB (2015) Seed filling in domesticated maize and rice depends on SWEET-mediated hexose transport. Nat Genet 47:1489
The International Wheat Genome Sequencing Consortium (IWGSC) (2014) A chromosome-based draft sequence of the hexaploid bread wheat (Triticum aestivum) genome. Science 345:1251788
Thomas SG (2017) Novel Rht-1 dwarfing genes: tools for wheat breeding and dissecting the function of DELLA proteins. J Exp Bot 68:354–358
USDA (1916) Alaska and stoner, or “miracle,” wheats: two varieties much misrepresented. Bulletin No. 357, US Department of Agriculture, Washington, DC
White AC, Rogers A, Rees M, Osborne CP (2016) How can we make plants grow faster? A source–sink perspective on growth rate. J Exp Bot 67:31–45
Worland A, Börner A, Korzun V, Li W, Petrovic S, Sayers E (1998) The influence of photoperiod genes on the adaptability of European winter wheats. Euphytica 100:385–394
Yan L, Loukoianov A, Tranquilli G, Helguera M, Fahima T, Dubcovsky J (2003) Positional cloning of the wheat vernalization gene VRN1. Proc Natl Acad Sci 100:6263–6268
Yan L, Loukoianov A, Blechl A, Tranquilli G, Ramakrishna W, SanMiguel P, Bennetzen JL, Echenique V, Dubcovsky J (2004) The wheat VRN2 gene is a flowering repressor down-regulated by vernalization. Science 303:1640–1644
Yan L, Fu D, Li C, Blechl A, Tranquilli G, Bonafede M, Sanchez A, Valarik M, Yasuda S, Dubcovsky J (2006) The wheat and barley vernalization gene VRN3 is an orthologue of FT. Proc Natl Acad Sci 103:19581–19586
Youssef HM, Eggert K, Koppolu R, Alqudah AM, Poursarebani N, Fazeli A, Sakuma S, Tagiri A, Rutten T, Govind G, Lundqvist U, Graner A, Komatsuda T, Sreenivasulu N, Schnurbusch T (2017) VRS2 regulates hormone-mediated inflorescence patterning in barley. Nat Genet 49:157–161
Youssefian S, Kirby EJM, Gale MD (1992a) Pleiotropic effects of the GA-insensitive Rht dwarfing genes in wheat. 1. Effects on development of the ear, stem and leaves. Field Crops Res 28:179–190
Youssefian S, Kirby EJM, Gale MD (1992b) Pleiotropic effects of the GA-insensitive Rht dwarfing genes in wheat. 2. Effects on leaf, stem, ear and floret growth. Field Crops Res 28:191–210
Yu SM, Lo SF, Ho TH (2015) Source–sink communication: regulated by hormone, nutrient, and stress cross-signaling. Trends Plant Sci 20:844–857
Zhang H, Turner NC, Poole ML (2010) Source–sink balance and manipulating sink–source relations of wheat indicate that the yield potential of wheat is sink-limited in high-rainfall zones. Crop Pasture Sci 61:852–861
Zikhali M, Wingen LU, Griffiths S (2016) Delimitation of the Earliness per se D1 (Eps-D1) flowering gene to a subtelomeric chromosomal deletion in bread wheat (Triticum aestivum). J Exp Bot 67:287–299
We are thankful to Corinna Trautewig and Mechthild Pürschel for excellent technical assistance, Dr. Marion Röder for kindly providing all the microsatellite makers and lab facilities, and Anne Fiebig for technically assisting during the submission of the GBS data.
This study was funded by Wissenschaftsgemeinschaft Gottfried Wilhelm Leibniz (WGL) and the Leibniz-Graduate School Gatersleben “Yield formation in cereals - overcoming yield-limiting factors” (Grant FKZ 401410 to TS). TS received financial support from the HEISENBERG Program of the German Research Foundation (DFG), Grant no. SCHN 768/8-1, and IPK core budget.
Conflict of interest
We declare that there was no conflict of interest.
Human participants or animals
This article does not contain any studies with human participants or animals performed by any of the authors.
Communicated by S. Hohmann.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Gizaw M. Wolde and Thorsten Schnurbusch: HEISENBERG-Research Group Plant Architecture. Martin Mascher: Research Group Domestication Genomics.
About this article
Cite this article
Wolde, G.M., Mascher, M. & Schnurbusch, T. Genetic modification of spikelet arrangement in wheat increases grain number without significantly affecting grain weight. Mol Genet Genomics 294, 457–468 (2019). https://doi.org/10.1007/s00438-018-1523-5