CAB Reviews 2014 9, No. 043 Accelerated plant breeding B.P. Forster1*, B.J. Till1, A.M.A. Ghanim1, H.O.A. Huynh1, H. Burstmayr2,3 and P.D.S. Caligari4,5 Address: 1 Plant Breeding and Genetics Laboratory, Joint FAO/IAEA Division, IAEA Laboratories, A-2444 Seibersdorf, Austria. 2 University of Natural Resources and Life Sciences, Vienna, Austria. 3 Department for Agrobiotechnology (IFA-Tulln), Institute for Biotechnology in Plant Production, Konrad Lorenz Str. 20, A-3430 Tulln, Austria. 4 BioHybrids International Ltd, P.O. Box 2411, Earley, Reading RG6 5FY, UK. 5 Instituto de Ciencias Biolo´gicas, Universidad de Talca, 2 Norte 685, Talca, Chile. *Correspondence: B.P. Forster. Email: [email protected] Received: 22 April 2014 Accepted: 17 November 2014 doi: 10.1079/PAVSNNR20149043 The electronic version of this article is the definitive one. It is located here: http://www.cabi.org/cabreviews © The Author(s) 2015. This article is published under a Creative Commons Attribution 4.0 International License (CC BY 4.0) (Online ISSN 1749-8848) Abstract The time to develop new cultivars and introduce them into cultivation is an issue of major importance in plant breeding. This is because plant breeders have an urgent need to help provide solutions to feed a growing world population, while in parallel, time savings are linked to profit- ability. Plant breeding processes may in general be broken down into the following five key elements: (1) germplasm variation; (2) crossing; (3) generation of new genetic combinations; (4) screening and selection (identification and subsequent fixation of desired allelic combinations); and (5) line/cultivar development. Each of these has implications in relation to the time taken to breed a new cultivar; a brief introduction is given for each to highlight the obstacles that may be targeted in accelerating the breeding process. Specific techniques that provide a time advantage for these elements are then discussed. Some targets for enhancing the efficiency of plant breeding, e.g., the manipulation of meiotic recombination, have proven to be recalcitrant. However, other methods that create new genetic variation along with improvements in selection efficiency com- pensate to a large extent for this limitation. Progress in accelerating the plant breeding process continues by exploiting new emerging ideas in science and technology. Keywords: Accelerated breeding, Mutation breeding, GM breeding, Rapid generation cycling, Doubled haploidy, Marker-assisted selection, High-throughput screening. Review Methodology: Searched various websites including the http://worldmeters.info/worldpopulation http://www.census.gov/ popclock; FAO/IAEA database of mutant varieties, http://mvgs.iaea.org. FAO Yearbooks and AGROSTAT, BBC News. Used the following key words in searches: world population, global forecasts, accelerated breeding. Discussed contents with colleagues. Introduction – Issues and Drivers of Faster history, plant breeding has been a test bed for scientific Breeding Methods innovations, especially those in genetics, botany, physiol- ogy and biotechnology, thus plant breeding has capitalized The evolution of crop plants can be described as having on research in recombination, heritability, polyploidy, three phases: (1) gathering from the wild; (2) domestica- chromosome engineering, tissue culture, heterosis, gen- tion and agronomy and (3) plant breeding [1, 2]. The con- etic linkage mapping, molecular genetics, mutagenesis and version of wild plants to crop plants involves the selection transformation. Plant breeding has been a major factor in of suitable types, which developed in conjunction with increasing crop production [3–5]. Research and devel- suitable agronomic practices. Domestication and associ- opment achievements have added much to the toolbox, ated agronomy therefore played a significant and intimate but a major constraint and frustration that remains is the role in mankind’s early food security and thereby human time to generate de novo, a new cultivar for growers. evolution. However, this phase has been eclipsed by sub- Plants are grown for a wide range of uses, chiefly for sequent achievements in plant breeding. Throughout its food, feed, fibre and fuel, but also for specialized products http://www.cabi.org/cabreviews 2 CAB Reviews (e.g., medicines), and social amenity such as recreation pedigree inbreeding method (PIM) as described by Briggs and ornamentation. The global human population is ex- and Knowles in 1967 in their book: ‘Introduction to plant pected to grow from current estimates of 7.1–7.2 billion breeding’ [9]. This method has, and continues, to serve (http://worldmeters.info/worldpopulation http://www. plant breeders well. Modern time-saving methods are census.gov/popclock) to 9.1 billion in 2040 [6]. Plant then described and may be compared with PIM. This breeders face a daunting [5] task as crop production is not means that we will tend to concentrate our focus and keeping pace with demands; crop yields are currently examples on inbreeding species, but in reality the princi- increasing at about 1.3% per annum, about half the rate that ples apply to species with other breeding systems and is required [7]. The area of land under cultivation is not the reader can easily apply the possibilities for the expected to increase; it has remained static over the past methodologies to those species 50 years at about 660 million hectares (data from FAO Homozygous lines, commonly referred to as genetically Yearbooks), and there is even concern that agricultural pure lines, or breeding pure lines, are important goals in land is degrading. Future increases in crop production are many plant breeding programmes and may be developed therefore dependent upon greater yields per unit area of through PIM. Pure lines not only form the end product for land area and therefore crops with high yield potentials self-pollinating species of the breeding programmes (lines must be developed and developed quickly. At the same that may be developed into cultivars in many crops, time, climate change is providing a major uncertainty as to notably cereals), but are exploited as parental lines in F1 how it will limit or change agricultural productivity. For hybrid cultivar production (notably in maize and many example, in temperate crops significant yield reductions vegetable crops). New homozygous lines are typically de- are expected as temperatures rise by more than 4C; while veloped from crosses between two distinct parental lines similarly, in tropical regions a rise of 2C will cause major that are complementary for desired traits. This provides yield losses [8]. Temperature is only one aspect of climate an opportunity for re-assortment and recombination of change. Other direct effects include salinity, drought, genes and alleles through meiosis and thereby the pro- waterlogging and storms, all of which bring with them new duction of segregating populations in subsequent gen- pest and disease problems. Some of the effects of climate erations, as first described in the experiments of Mendel change may be mitigated by the introduction of cultivars (1822–1864 [15]). In crosses between two pure lines, adapted to the new conditions, or growing new crops segregation is observed first in the F2 generation as the F1 species, both of which solutions require rapid breeding. individuals are presumed to be genetically identical but This review does not intend to go into details of plant heterozygous for all loci by which the parents differ. The breeding methods or comparisons between them. There plant breeder attempts to make improvements either are several excellent reference books that discuss theory negatively, by rogueing out inferior plants, or positively, by and methods in plant breeding in a wide range crops (e.g., selecting the best, starting with the segregating F2 popu- [9–14]). Here we provide a general overview of techni- lation. The next objective is to develop pure lines from ques that can accelerate the plant breeding process, from the selections and in self-pollinating species this is tradi- parental choice to cultivar release. tionally done by repeated rounds of selfing in the PIM (Figure 1). Seed from individual selected plants are sown out as F3 family rows; this is repeated in the F4 and F5. Components of Plant Breeding From F5 to F6 there is a shift from single plant selection to single family selection. Seed of an individual family are Plant breeding processes may be broken down into the grown out in small plots in the F6 bulk harvested, and then following elements: (1) access to germplasm or creation grown in larger replicated plots in F7––F10 generations. of genetic variation, (2) crossing; (3) generation of At about the F7 stage the term ‘line’ replaces the term new genetic combinations (normally through meiosis); ‘family’ as there should be no visible variation at this stage (4) generation of segregating populations, (5) selection (plants within a line look the same). Multi-locational trials (fixation of desired alleles); and (6) line development. Each often take place from F8 to F10. Larger strip tests take of these components introduces a time component place in F11 and F12 generations to bulk seed for potential and any, or all, of which can be a bottleneck in the plant commercial release, since at this stage the selected lines breeding process. The relevance and limitation of each must enter national, regulatory trials to achieve official component are described while specific techniques that cultivar status. This normally includes passing DUS tests, overcome these limitations are then discussed. i.e., the line must be distinct, uniform and stable, in The terms ‘traditional plant breeding’ and ‘accelerated addition it must normally give superior yields compared plant breeding’ are difficult to define as continual im- with standard checks or possess other important traits provements have taken place in deliberate plant breeding (such as disease resistance, superior quality, etc.). If the since Mendel first established the genetic principles line passes ‘is accepted’ it may be officially released and involved, principles that provided a scientific basis for grown by farmers.