Editor’s Choice Series on the Next Generation of Biotech Crops
Molecular Plant Breeding as the Foundation for21st Century Crop Improvement1
Stephen P.Moo*and Rita H.Mumm
Department of Crop Sciences(S.P.M.,R.H.M.)and Energy Biosciences Institute(S.P.M.),University of Illinois, Urbana-Champaign,Illinois61801;and GeneMax Services,Savoy,Illinois61874(R.H.M.)
The fundamental discoveries of Darwin and Mendel established the scientific basis for plant breeding and genetics at the turn of the20th century.Similarly,the recent integration of advances in biotechnology,ge-nomic rearch,and molecular marker applications with conventional plant breeding practices has created the foundation for molecular plant breeding,an inter-disciplinary science that is revolutionizing21st century crop improvement.Though the methods of molecular plant breeding continue to evolve and are a topic of inten interest among plant breeders and crop scien-tists(for review,e Cooper et al.,2004;Nelson et al., 2004;Lo¨rz and Wenzel,2005;Varshney et al.,2006; Eathington et al.,2007;Mumm,2007),they have re-ceived relatively little attention from the majority of plant biologists engaged in basic scientific rearch. The objective of this article for an Editor’s Choice ries on future advances in crop biotechnology is to briefly review important historic
al developments in molecu-lar plant breeding,key principles influencing the cur-rent practice of molecular plant breeding,and factors that influence the adoption of molecular plant breed-ing in crop improvement programs.Furthermore,we emphasize how the application of molecular plant breeding is now contributing to discoveries of genes and their functions that open new avenues for basic plant biology rearch.
HISTORICAL DEVELOPMENT OF MOLECULAR PLANT BREEDING
Plant breeding describes methods for the creation, lection,andfixation of superior plant phenotypes in the development of improved cultivars suited to needs of farmers and consumers.Primary goals of plant breeding with agricultural and horticultural crops have typically aimed at improved yields,nutritional qualities,and other traits of commercial value.The plant breeding paradigm has been enormously suc-cessful on a global scale,with such examples as the development of hybrid maize(Zea mays;Duvick,2001), the introduction of wheat(Triticum aestivum)and rice (Oryza sativa)varieties that spawned the Green Revo-lution(Everson and Golin,2003),and the recent com-mercialization of transgenic crops(James,2007).The and many other products of plant breeding have con-tributed to the numerous benefits global society has received from greater sustainable supplies of carbon that may be harvested as food,feed,forests,fibe
惊喜英文怎么写surprir,and fuel.
Plant breeding has a long history of integrating the latest innovations in biology and genetics to enhance crop improvement.Prehistoric lection for visible phe-notypes that facilitated harvest and incread produc-tivity led to the domestication of thefirst crop varieties (Harlan,1992)and can be considered the earliest ex-amples of biotechnology.Darwin outlined the scientific principles of hybridization and lection,and Mendel defined the fundamental association between genotype and phenotype,discoveries that enabled a scientific approach to plant breeding at the beginning of the20th Shull,1909).Despite the immediate rec-ognition among some plant breeders of the importance of Mendelian genetics,full integration was delayed for nearly20years until quantitative genetics reconciled Mendelian principles with the continuous variation obrved for most traits considered important by most plant breeders(Paul and Kimmelman,1988).Sub-quent advances in our understanding of plant biology, the analysis and induction of genetic variation,cytoge-netics,quantitative genetics,molecular biology,bio-technology,and,most recently,genomics have been successively applied to further increa the scientific ba and its application to the plant breeding process (e.g.Baenziger et al.,2006;Jauhar,2006;Varshney et al., 2006).
The plant biotechnology era began in the early1980s with the landmark reports of producing transge
nic plants using Agrobacterium(Bevan et al.,1983;Fraley et al.,1983;Herrera-Estrella et al.,1983).Molecular marker systems for crop plants were developed soon thereafter to create high-resolution genetic maps and
1This work was supported by the National Science Foundation (grant no.NSF–PGRP–0501700)and the U.S.Department of Agri-culture(award no.2007–35100–18335).
*Corresponding author;e-mail smoo@uiuc.edu.
The author responsible for distribution of materials integral to the findings prented in this article in accordance with the policy described in the Instructions for Authors(www.plantphysiol)is: Stephen P.Moo(smoo@uiuc.edu).
鲁迅与周作人www.plantphysiol/cgi/doi/10.1104/pp.108.118232
exploit genetic linkage between markers and important crop traits(Edwards et al.,1987;Paterson et al.,1988). By1996,the commercialization of transgenic crops demonstrated the successful integration of biotechnol-ogy into plant breeding and crop improvement pro-grams(Koziel et al.,1993;Delannay et al.,1995).As depicted in Figure1,introgression of one or a few genes into a current elite cultivar via
backcrossing is a com-mon plant breeding practice.Methods for marker-assisted backcrossing were developed rapidly for the introgression of transgenic traits and reduction of link-age drag,where molecular markers were ud in ge-nome scans to lect tho individuals that contained both the transgene and the greatest proportion of favorable alleles from the recurrent parent genome (e.g.Ragot et al.,1995;Johnson and Mumm,1996). During the past25years,the continued development and application of plant biotechnology,molecular markers,and genomics has established new tools for the creation,analysis,and manipulation of genetic variation and the development of improved cultivars (for review,e Sharma et al.,2002;Varshney et al.,2006; Collard and Mackill,2008).Molecular breeding is currently standard practice in many crops,with the following ctions briefly reviewing how molecular information and genetic engineering positively im-pacts the plant breeding paradigm.
PRINCIPLES AND PRACTICES OF MOLECULAR PLANT BREEDING
Breeding Schemes and the Genetic Gain Concept Conceptually,plant breeding is simple:cross the best parents,and identify and recover progeny that outperform the parents.In practice,plant breeding is a three step process,wherein populations or germplasm collections with uful genetic variation are created or asmbled,individuals with superior phenotypes are identified,and improved cultivars
are developed from lected individuals.A wide diversity of approaches, tailored to the crop species and breeding objectives, have been developed for improving cultivars(Fehr, 1987;Stoskopf et al.,1993).The breeding methods feature different types of populations,lection proce-dures,and outcomes.
Figure1summarizes the three breeding methods that are commonly employed in crop improvement programs.As mentioned previously,when the goal is to upgrade an established elite genotype with trait(s) controlled by one or a few loci,backcrossing is ud either to introgress a single gene(Fig.1A)or to pyramid a few genes(Fig.1B).For genetically complex traits, germplasm improvement instead requires reshuffling of the genome to produce new favorable gene combi-nations in the progeny.The pedigree breeding method produces such novelty via crossing and recombina-tion among superior,yet complementary,parents and lection among gregating progeny for
improved Figure1.Common breeding and lection schemes.Each vertical bar is a graphical reprentation of the genome for an individual within a breeding population,with colored gments indicating genes and/or QTLs that influence traits under lection.Genes associated with different traits are shown in different d,blue).‘‘X’’indicates a cross between parents,and arrows depict successive cross of the same type. Asterisk below an individual signifies a desirable genotype.A,Back-crossing.A donor line(blue bar)featuring a specific gene of interest(red) is crosd to an elite line targeted for improvement(white bar),with progeny repeatedly backcrosd to the elite line.Each backcross cycle involves lection for the gene of interest and recovery of incread proportion of elite line genome.B,Gene pyramiding.Genes/QTLs associated with different beneficial traits(blue,red,orange,green)are combined into the same genotype via crossing and lection.C,Pedigree breeding.Two individuals with desirable and complementary phenotypes are crosd;F1progeny are lf-pollinated tofix new,improved genotype combinations.D,Recurrent lection.A population of individuals(10in this example)gregate for two traits(red,blue),each of which is influenced by two major favorable QTLs.Intermating among individuals and lection for desirable phenotypes/genotypes increas the frequen-cies of favorable alleles at each locus.For this example,no individual in the initial population had all of the favorable alleles,but after recurrent lection half of the population posss the desired genotype.For hybridized crops,recurrent lec
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tion can be performed in parallel within two complementary populations to derive lines that are then crosd to form hybrids;this method is called reciprocal recurrent lection.
Moo and Mumm
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performance (Fig.1C).Recurrent lection aims to simultaneously increa the frequencies of favorable alleles at multiple loci in breeding populations through intermating of lected individuals (Fig.1D).For hy-bridized crops such as maize,recurrent lection may be extended to improve the performance of distinct complementary populations (e.g.heterotic groups)that are ud as parents to form superior hybrid combina-tions.This practice is referred to as reciprocal recurrent lection.
Quantitative genetic principles have been particu-larly powerful as the theoretical basis for both popu-lation improvement and methods of lecting and stabilizing desirable genotypes (Hallauer,2007).An important concept in quantitative genetics and plant breeding is genetic gain (D G),which is the predicted change in the mean value of a trait within a population that occurs with lection.Regardless of species,the trait of interest,or the breeding methods employed,D G rves as a simple universal expression for expected ge-netic improvement (Fehr,1987;Falconer and Mackay,1996).Figure 2shows the genetic gain equation and an expansion of its terms to fundamenta
l parameters of quantitative genetics.Though clearly an oversimplifi-cation of the advanced quantitative genetic principles employed in plant breeding,the genetic gain equation effectively relates the four core factors that influence breeding progress:the degree of phenotypic variation prent in the population (reprented by its SD ,s P ),the probability that a trait phenotype will be transmitted from parent to offspring (heritability,h 2),the propor-tion of the population lected as parents for the next generation (lection intensity,i ,expresd in units of SD from the mean),and the length of time necessary to complete a cycle of lection (L ).L is not only a function of how many generations are required to complete a
lection cycle,but also how quickly the generations can be completed and how many generations can be completed per year.
简短工作汇报It is clear that D G can be enhanced by increasing s P ,h 2
,or i ,and by decreasing L .Thus,the genetic gain equation provides a framework for comparing the predicted effectiveness of particular breeding strate-gies and is often ud as a guide to the judicious allocation of resources for achieving breeding objec-tives.When considered in the context of the genetic gain concept,molecular plant breeding offers powerful new approaches to overcome p
revious limitations in maximizing D G.The following ctions cite examples where molecular plant breeding positively impacts D G and each of its component variables.For brevity,we focus on examples from maize where molecular breed-ing is most advanced,and has now become the primary means to develop improved commercial hybrids.
Molecular Plant Breeding Expands Uful Genetic Diversity for Crop Improvement
The maximum potential for genetic gain is propor-tional to the phenotypic variation (s P )prent in the original source population and maintained in sub-quent cycles of lection.Phenotypic variation is positively associated with genetic diversity,yet also depends on environmental factors and the interactions between genotype and environment.Genetic diversity may be derived from breeding populations (either naturally occurring or synthetic),gregating progeny from a cross of lected parental lines,exotic materials that are not adapted to the target environment,wide interspecific cross,naturally occurring or induced mutations,the introduction of transgenic events,or combinations of the
sources.
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Figure 2.The genetic gain equation and its component variables.The top portion illustrates an idealized distribution showing the frequency of individuals within a breeding population (y axis)that exhibit various class of phenotypic values (x axis).Mean phenotypic value (m 0)of the original population (shown as entire area under the normal curve)and mean (m S )for the group of lected i
ndividuals (shaded in blue)are indicated.In this generalized example,trait improvement is achieved by lecting for a lower phenotypic ain moisture at harvest in maize.Components of variation (s 2)that contribute to the SD of the phenotypic distribution (s P )are indicated below the histogram.
Molecular Plant Breeding
However,not all phenotypic variation is equal.For example,the u of exotic germplasm has been ex-tremely successful for improving many crop species, but difficulties may be encountered through the intro-duction of undesirable alleles associated with lack of adaptation.The need for genetic diversity must be balanced by elite performance,becau choosing the best parents is key to maximizing the probability for successful improvement.In contrast,the expected increa in linkage diquilibrium among elite popu-lations derived from inten prior lection may also limit the creation of new genetic combinations for fu-ture gain.Intermating source populations for genetic recombination may overcome this problem,but delays cultivar development.
Molecular markers and more recently,high-through-put genome quencing efforts,have dramatically in-cread knowledge of and ability to characterize genetic diversity in the germplasm pool for esn
tially any crop species.Using maize as one example,surveys of molecular marker alleles and nucleotide quence variation have provided basic information about ge-netic diversity before and after domestication from its wild ancestor teosinte,among geographically distrib-uted landraces,and within historically elite germplasm (for review,e Cooper et al.,2004;Niebur et al.,2004; Buckler et al.,2006).This information enriches investi-gations of plant evolution and comparative genomics, contributes to our understanding of population struc-ture,provides empirical measures of genetic respons to lection,and also rves to identify and maintain rervoirs of genetic variability for future mining of beneficial alleles(McCouch,2004;Slade et al.,2005).In addition,knowledge of genetic relationships among germplasm sources may guide choice of parents for production of hybrids or improved Dudley et al.,1992;Collard and Mackill,2008).
While molecular markers and other genomic appli-cations have been highly successful in characterizing existing genetic variation within species,plant biotech-nology generates new genetic diversity that often ex-tends beyond species boundaries(Gepts,2002;Johnson and McCuddin,2008).Biotechnology enables access to genes heretofore not available through crossing and creates an esntially infinite pool of novel genetic variation.Genes may be acquired from existing ge-nomes spanning all kingdoms of life,or designed and asmbled de novo in the laborator
y.Both subtle and extreme examples of the power of transgenes to intro-duce novel phenotypic variation can be found in the three different transgenes developed for resistance to glyphosate herbicides in maize and other crops.The first glyphosate-tolerant maize hybrids ud a modi-fied version of the endogenous maize gene encoding 5-enol-pyruvylshikimate-3-P syntha(Spencer et al., 2000),which was followed later by events produced with a5-enol-pyruvylshikimate-3-P syntha gene iso-lated from Agrobacterium(Behr et al.,2004).More re-cently,a synthetic gene with enhanced glyphosate acetyltransfera activity was created via gene shuf-fling and lection in a microbial system(Castle et al., 2004).Each of the glyphosate-tolerant maize events also illustrates another benefit of biotechnology,where new combinations of regulatory he cauliflower mosaic virus35S and rice actin1promoters) may be ud to achieve optimal trait expression with respect to overall activity and tissue distribution rela-tive to what might be possible with endogenous genes (Heck et al.,2005).
Molecular Plant Breeding Increas Favorable
Gene Action
Quantitative genetics us the theoretical concept of heritability to quantify the proportion of phenoty
pic variation that is controlled by genotype.In practice, heritability is greatly influenced by the genetic archi-tecture of the trait of interest,which is described by the number of genes,the magnitude of their effects,and the type of gene action associated with phenotypes.Better knowledge of genetic architecture and favorable gene action(that which is more amenable to lection)often has the greatest impact on improving genetic gain.For the genetic gain formula,heritability(h2)is ud in its narrow n,reprenting the proportion of pheno-typic variation due to additive genetic effects(tho that reflect changes in allele dosage or allelic substitu-tions).Additive genetic effects are also referred to as the breeding value becau they are predictably trans-mitted to progeny.Deviations from additive effects are significant for many traits,and are partitioned into either dominance effects that reflect the interactions between different alleles at the same locus or epistatic effects resulting from interactions among different loci. Gene action and breeding values are characterized by progeny testing,where the phenotypes of individuals in a population are compared to their parents and siblings produced from either lf-pollination or out-crossing.
Previous efforts to develop large numbers of molec-ular markers,high density genetic maps,and appro-priately structured mapping populations have now made routine for many crop species the ability to simultaneously define gene action and breeding value at hundreds and often thousands of l
oci distributed relatively uniformly across entire genomes.The results from such mapping studies provide greatly improved estimates for the number of loci,allelic effects,and gene action controlling traits of interest.More importantly, genomic gments can be readily identified that show statistically significant associations with quantitative traits(quantitative trait loci[QTLs]).In addition to genetic mapping in families derived from biparental cross,new advances in association genetics with candidate genes and approaches that combine linkage diquilibrium analysis in families and populations (Holland,2007;Yu et al.,2008)further enhance power for QTL discovery.
Information about QTLs can be ud in a number of ways to increa heritability and favorable gene action.
7寸是多大Moo and Mumm
For traits exhibiting low to moderate heritability,such as grain yield,QTLs,and their associated molecular markers often account for a greater proportion of the additive genetic effects than the phenotype alone.Fur-thermore,knowledge of genetic architecture can be exploited to add or delete specific alleles that contribute to breeding value.When either genetic linkage or epistasis among loci with antagonistic effects on a trait limits genetic gain,QTL information can be ud to break the un
desirable allelic relationships. Success in using information about QTLs to increa genetic gain depends greatly on the magnitude of QTL effects,preci estimation of QTL positions,sta-bility of QTL effects across multiple environments,and whether QTLs are robust across relevant breeding germplasm.Prediction of QTL positions is enhanced by furtherfine mapping,which facilitates testing QTL effects and breeding values in additional popula-tions.When the density of obrved recombinations approaches the resolution of single genes,the causal genetic change for a QTL can be determined(for review, e Salvi and Tuberosa,2005;Yu and Buckler,2006;Belo´et al.,2008;Harjes et al.,2008).Molecular isolation of QTLs permits the development of perfect or functional molecular markers at the potential resolution of the fundamental unit of inheritance,the nucleotide,and dramatically increas the specificity and precision by which genetic effects are estimated and manipulated in breeding programs.
The u of transgenes can further simplify the genetic architecture for desirable traits,in ways that may be superior to or not possible even when perfect markers are available for robust QTLs of large effect.Transgenes typically condition strong genetic effects at operation-ally single loci,which also exhibit dominant gene action where only one copy of the event is needed for maximal trait expression in a hybrid cultivar.The features of transgenes can reduce complex quantitative improv
e-ment to a straightforward,often dramatic,solution. Excellent examples are provided by the expression in transgenic corn hybrids of incticidal toxin proteins from Bacillus thuringiensis(Bt)to reduce feeding dam-age by larvae of the European corn borer(Ostrinia nubilalis)or the corn rootworm beetle(Diabrotica spp.). Partial resistance in maize germplasm to the inct pests had been previously characterized as quantita-tively inherited traits with low heritability(Papst et al., 2004;Tollefson,2007),but the Bt transgenic events offer a simply inherited alternative that is efficiently manip-ulated in breeding programs.
By simplifying genetic architecture,transgenes may also permit disruption of allelic interactions between factors controlling the trait of interest and other im-portant performance characteristics.For example,em-ploying a transgenic source of inct a single locus Bt transgene)may facilitate lection for favorable alleles for yield improvement that are tightly linked in repulsion with endogenous genes for resis-tance to the same class of inct pests.In addition, transgenic events may be engineered to uncouple neg-ative pleiotropic effects from beneficial phenotypes conditioned by recessive mutations.This application is illustrated by the u of RNA interference to specifi-cally down-regulate zein ed storage protein gene expression(Segal et al.,2003;Huang et al.,2004).This strategy mimics the effects of the opaque2mutation on improving the amino acid profile of maize grai
n for animal feed,while circumventing the softer endo-sperm texture and susceptibility to fungal pathogens typically associated with opaque2.
Transgenic events can also be designed to intervene at key regulatory steps for entire metabolic or develop-mental pathways,such that gene action for the corre-sponding traits are largely inherited as single dominant factors that are less nsitive to environmental effects. Examples include the expression of a transcription factor that increas drought tolerance(Nelson et al., 2007),and altering the balance between levels of the GLOSSY15transcription factor relative to its repressor, microRNA172,to delayflowering time in maize hybrids (Lauter et al.,2005).
Biotechnology also facilitates the molecular stacking of transgenes that control a trait or suite of traits into a single locus haplotype defined by a transgenic event. Examples of such an approach include the initial Golden Rice(Ye et al.,2000),recently relead Yield-Guard VT Triple transgenic maize hybrids where her-bicide tolerance and multiple inct resistance traits are integrated as one genomic locus(www. /VTScience/Default.aspx),or the com-bination of transgenes that simultaneously increa synthesis and decrea catabolism of Lys in maize eds(Frizzi et al.,2008).Recent reports of improve-ments in gene targeting technology(Ow,2007)and the construction of meiotically transmissible plant mini-chromosomes(Carls
极限怎么求on et al.,2007;Yu et al.,2007)pave the way for introducing more traits with increasing complexity.With such advances,biotechnology is now poid to asmble uful genetic diversity from es-ntially any source into constructs that concentrate favorable gene action and maximize heritability for a greatly expanded t of traits.
In closing this ction about how molecular plant breeding increas favorable gene action,it is impor-tant to emphasize that QTL studies,when conducted with appropriate scale and precision to identify causal genes,reprent a powerful functional genomics ap-proach.The molecular cloning of QTLs has yielded novel insights about the biology of quantitative traits that were not likely to be discovered from the analysis of gene knockouts or overexpression strategies,in particular the impacts of regulatory variation on phe-notypic variation and Cong et al.,2002; Clark et al.,2006;Yan et al.,2004;Salvi et al.,2007). Furthermore,molecular markers,genomics,and bio-technology are now applied in an iterative network to exploit genetic diversity for crop improvement.Ge-nomic information enables the discovery of beneficial alleles via QTL mapping and cloning,followed by the u of information learned from the molecular char-
Molecular Plant Breeding
acterization of QTLs to design optimal transgenic strategies for crop improvement.
Molecular Plant Breeding Increas the Efficiency
of Selection
Conventional plant breeding that relies only on phenotypic lection has been historically effective. However,for some traits,phenotypic lection has made little progress due to challenges in measuring phenotypes or identifying individuals with the highest breeding value.The effects of environment,genotype by environment interaction,and measurement errors also contribute to obrved differences.Evaluation of genotypes in multiple environments with replicated designs allows better estimation of breeding values, but requires additional time and expen.For some traits,it may be necessary to sacrifice the individual to measure phenotypes,or trait expression may depend on variable environmental dia pressure)and the stage of ain quality can only be assd afterflowering).Further-more,plant breeders typically must simultaneously improve a suite of commercially valuable traits,which may limit gains from lection.Just as molecular plant breeding helps to expand genetic diversity,character-ize genetic architecture,and modify gene action,its methods can also be applied to increasing the effi-ciency of lection.
An extensive body of literature has considered the utility of molecular marker-assisted lection and itsfit with different breeding methods(Fig.1),with the reader being referred to a number of recent excellent reviews on this topic(Dekkers and Hospital,2002; Holland,2004;Johnson,2004;Varshney et al.,2006; Collard and Mackill,2008).Molecular marker geno-types that are either within genes or tightly linked to QTL influencing traits under lection can be em-ployed as a supplement to phenotypic obrvations in a lection index(Lande and Thompson,1990).In cas where genetic correlations are high,further effi-ciencies can be gained by substituting genotypic for phenotypic lection during some lection cycles, which can reduce phenotyping efforts and cycle times by permitting the u of off-ason nurries.Johnson (2004)summarized an early example of combining phenotypic data and molecular marker scores to in-crea lection gains for maize grain yield and resis-tance to European corn borer.An effective strategy to simultaneously modify multiple traits is the u of lection indices that consider multiple factors in choosing thefinal improved genotype.Eathington et al.(2007)recently reported on results obtained from the u of multiple trait indices and marker-assisted lection for nearly250unique corn breeding populations.U of molecular markers incread breeding efficiency approximately2-fold relative to phenotypic lection alone,with similar gains also obrved in soybean(Glycine max)and sunflower (Helianthus annuus)populations.
Marker-assisted lection can also significantly en-hance genetic gain for traits where the phenotype is difficult to evaluate becau of its expen or its de-pendence on specific environmental conditions.Mo-lecular markers may be ud to increa the probability of identifying truly superior genotypes,by focusing testing resources on genotypes with the greatest arly elimination of inferior genotypes), by decreasing the number of progeny that must be screened to recover a given level of gain,and by en-abling simultaneous improvement for traits that are negatively correlated(Knapp,1998).Successful exam-ples include resistance to soybean cyst nematode (Young,1999),resistance to cereal dias(for review, e Varshney et al.,2006),and drought tolerance in maize(Ribaut and Ragot,2007;Tuberosa et al.,2007). The efficiency of phenotypic lection for some complex traits can be enhanced by including physio-logical or biochemical phenotypes as condary traits, if the exhibit strong genetic correlations with the target trait and posss high heritability.Recent ad-vances in functional genomics permit the population-scale profiling of RNA abundance,protein levels and activities,and metabolites that are associated with important traits.In addition to molecular markers that tag DNA quence variation,such genetical genomics approaches may provide additional condary pheno-types as lection targets(Jann and Nap,2001; Johnson,2004),particularly for traits defined by re-spons to environmental,developmental,or physio-logical cues.
Marker-assisted lection also accelerates the de-ployment of transgenes in commercial cultivars.Typ-ically,this has been achieved through marker-assisted backcrossing.However,for future biotechnology im-provements such as tolerance to drought or nutrient limitation,forward breeding may be required to coop-timize transgene expression and genetic background becau endogenous genes and environmental factors may have the potential to influence the phenotypes re-sulting from transgenic modifications(Mumm,2007). Of cour,u of molecular markers could aid forward breeding efforts as well.Alternatively,discovery ef-forts for additional genes or QTLs that are necessary for dependable trait performance may suggest de-sign of new transgene constructs that stack primary transgenes with known genetic modifiers into cond-generation transgenic events.
INCREASING ADOPTION OF MOLECULAR
PLANT BREEDING
The adoption of molecular plant breeding ap-proaches has occurred at different rates among crop species and institutions engaged in crop improvement, due to the combined influence of scientific,economic, and sociological factors.Important early scientific barriers included the recalcitrance of cereal crop spe-cies to Agrobacterium-mediated transformation and
Moo and Mumm
