Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 31

The science and technology component of modern plant breeding is rapidly expanding. Whereas a large number of science disciplines directly impact plant breeding, several are closely associated with it. These are plant breeding, genetics, agronomy, cytogenetics, molecular genetics, botany, plant physiology, biochemistry, plant pathology, entomology, statistics, and tissue culture. Knowledge of the first three disciplines is applied in all breeding programs. The technologies used in modern plant breeding are summarized in Table 1.2. These technologies are discussed in varying degrees in this book. The categorization is only approximate and generalized. Some of these tools are used to either generate variability directly or to transfer genes from one genetic background to another to create variability for breeding. Some technologies facilitate the breeding process through, for example, identifying individuals with the gene(s) of interest.

 GeneticsGenetics is the principal scientific basis of modern plant breeding. As previously indicated, plant breeding is about targeted genetic modification of plants. The science of genetics enables plant breeders to predict to varying extents the outcome of genetic manipulation of plants. The techniques and methods employed in breeding are determined based on the genetics of the trait of interest, regarding, for example, the number of genes coding for it and gene action. For example, the size of the segregating population to generate in order to have a chance of observing that unique plant with the desired combination of genes depends on the number of genes involved in the expression of the desired trait.

 BotanyPlant breeders need to understand the reproductive biology of their plants as well as their taxonomic attributes. They need to know if their plants to be hybridized are cross‐compatible, as well as know in fine detail about flowering habits, in order to design the most effective crossing program.

 Plant physiologyPhysiological processes underlie the various phenotypes we observe in plants. Genetic manipulation alters plant physiological performance, which in turn impacts the plant performance in terms of the desired economic product. Plant breeders manipulate plants for optimal physiological efficiency so that dry matter is effectively partitioned in favor of the economic yield. Plants respond to environmental factors, biotic (e.g. pathogens) and abiotic (e.g. temperature, moisture). These factors are sources of physiological stress when they occur at unfavorable levels. Plant breeders need to understand these stress relationships in order to develop cultivars that can resist them for enhanced productivity.

 AgronomyPlant breeders conduct their work in both controlled (greenhouse) and field environments. An understanding of agronomy (the art and science of producing crops and managing soils) will help the breeder to provide the appropriate cultural conditions for optimal plant growth and development for successful hybridization and selection in the field. An improved cultivar is only as good as its cultural environment. Without the proper nurturing, the genetic potential of an improved cultivar would not be realized. Sometimes, breeders need to modify the plant growing environment to identify individuals to advance in a breeding program to achieve an objective (e.g. withholding water in breeding for drought resistance).

 Pathology and entomologyDisease resistance breeding is a major plant breeding objective. Plant breeders need to understand the biology of the insect pest or pathogen against which resistance is being sought. The kind of cultivar to breed, the methods to use in breeding and evaluation all depend on the kind of pest or pathogen (e.g. its races or variability, pattern of spread, life cycle, and most suitable environment).

 StatisticsPlant breeders need to understand the principles of research design and analysis. This knowledge is essential for effectively designing field and laboratory studies (e.g. for heritability, inheritance of a trait, combining ability), and evaluating genotypes for cultivar release at the end of the breeding program. Familiarity with computers is important for record keeping and data manipulation. Statistics is indispensable to plant breeding programs. This is because the breeder often encounters situations in which predictions about outcomes, comparison of results, estimation of response to a treatment, and many more, need to be made. Genes are not expressed in a vacuum but in an environment with which they interact. Such interactions may cause certain outcomes to deviate from the expected. Statistics is needed to analyze the variance within a population to separate real genetic effects from environmental effects. Application of statistics in plant breeding can be as simple as finding the mean of a set of data, to complex estimates of variance and multivariate analysis.

 BiochemistryIn this era of biotechnology, plant breeders need to be familiar with the molecular basis of heredity. They need to be familiar with the procedures of plant genetic manipulation at the molecular level, including the development and use of molecular markers and gene transfer techniques.

Table 1.2 An operational classification of technologies of plant breeding.

Classical/traditional tools; e.g.	Common use of the technology/tool
Emasculation	Making a completer flower female; preparation for crossing
Hybridization	Crossing unidentical plants to transfer genes or achieve recombination
Wide crossing	Crossing of distantly related plants
Selection	The primary tool for discriminating among variability
Chromosome counting	Determination of ploidy characteristics
Chromosome doubling	Manipulating ploidy for fertility
Male sterility	To eliminate need for emasculation in hybridization
Triploidy	To achieve seedlessness
Linkage analysis	For determining association between genes
Statistical tools	For evaluation of germplasm
Relatively advanced tools
Mutagenesis	To induce mutations to create new variability
Tissue culture	For manipulating plants at the cellular or tissue level
Haploidy	Used for creating extremely homozygous diploid
Isozyme markers	To facilitate the selection process
In situ hybridization	Detect successful interspecific crossing
More sophisticated tools
DNA markers
RFLP	More effective than protein markers (isozymes)
RAPD	PCR‐based molecular marker
Advanced technology
Molecular markers	SSR, SNPs, ISSR, DART, etc.
Marker‐assisted selection	Facilitate the selection process
DNA sequencing, NGS	Ultimate physical map of an organism
Plant genomic analysis‐	Studying the totality of the genes of an organism
Bioinformatics	Computer‐based technology for prediction of biological function from DNA sequence data
Microarray analysis	To understand gene expression and for sequence identification
Primer design	For molecular analysis of plant genome
Plant transformation	For recombinant DNA work
OMICS technologies	For studying various aspects of the entire genome
Genome editing	For more efficient manipulation of the genome
Genome mapping	For more efficient gene discovery

Whereas the training of a modern plant breeder includes these courses and practical experiences in these and other disciplines, it is obvious that one cannot be an expert in all of them. Modern plant breeding is more of a team than a solo effort. A plant breeding team will usually have experts in all these key disciplines, each one contributing to the development and release of a successful cultivar. Increasingly important disciplines in plant breeding are computer science, for their role in bioinformatics, big data, and simulations.