3.4 Plant breeding and hybridization

Plant breeding and hybridization are vital techniques for improving crops. These methods combine genetic principles with practical techniques to create plants with desirable traits, enhancing yield, quality, and resilience.

Breeders use selection, hybridization, and genetic engineering to develop new varieties. These approaches aim to increase food security, adapt crops to changing environments, and meet consumer demands for better quality and nutrition.

Goals of plant breeding

• Plant breeding aims to develop new plant varieties with improved characteristics that benefit farmers, consumers, and the environment
• Involves the application of genetic principles and techniques to create plants with desirable traits
• Plays a crucial role in ensuring food security, enhancing crop productivity, and adapting to changing environmental conditions

Desirable traits

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• Focuses on identifying and selecting plants with specific characteristics that are beneficial for cultivation, consumption, or industrial use
• Includes traits such as improved flavor, enhanced nutritional content (high vitamin A in golden rice), and better storage quality (longer shelf life in tomatoes)
• Other desirable traits may include altered plant architecture (dwarf varieties of wheat), enhanced aesthetic appeal (novel flower colors in ornamental plants), and improved processing qualities (high oil content in sunflower)

Increased yield

• Aims to develop plant varieties that produce higher yields per unit area of land
• Involves selecting plants with increased number of fruits or grains per plant (high-yielding maize hybrids), larger fruit size (giant pumpkins), or higher biomass production (high-yielding sugarcane varieties)
• Increased yield helps to meet the growing demand for food, feed, and other plant-derived products while optimizing land use efficiency

Improved quality

• Focuses on enhancing the quality attributes of plant products to meet consumer preferences and market demands
• Includes improving the taste, texture, and appearance of fruits and vegetables (sweet and crispy apples)
• Aims to enhance the nutritional value of crops by increasing the content of essential nutrients (high-protein soybeans) or reducing anti-nutritional factors (low-phytate maize)
• Improves the processing qualities of crops for specific industrial applications (high-gluten wheat for bread making)

Disease resistance

• Develops plant varieties that are resistant to various diseases caused by pathogens such as fungi, bacteria, and viruses
• Incorporates genes or genetic regions that confer resistance to specific diseases (resistance to blast disease in rice)
• Helps to reduce crop losses, minimize the use of pesticides, and ensure stable crop production in disease-prone areas

Stress tolerance

• Aims to develop plant varieties that can withstand abiotic stresses such as drought, salinity, extreme temperatures, and nutrient deficiencies
• Involves identifying and incorporating genes or traits that enable plants to maintain growth and productivity under stress conditions (drought-tolerant maize)
• Enhances the resilience of crops to climate change and enables cultivation in marginal or stress-prone environments (salt-tolerant wheat for saline soils)

Genetic basis of plant breeding

• Plant breeding relies on the understanding and manipulation of the genetic basis of traits to develop improved plant varieties
• Involves the application of principles from different branches of genetics, including Mendelian genetics, quantitative genetics, and molecular genetics
• Utilizes the knowledge of inheritance patterns, gene interactions, and the relationship between genotype and phenotype to guide breeding strategies

Mendelian genetics

• Applies the principles of Mendelian inheritance, which describe the transmission of traits controlled by single genes
• Involves the study of dominant and recessive alleles, segregation of traits in offspring, and the independent assortment of genes
• Used in plant breeding to predict the inheritance of qualitative traits (flower color) and to develop pure breeding lines

Quantitative genetics

• Deals with the inheritance of complex traits that are controlled by multiple genes and influenced by environmental factors
• Involves the study of continuous variation, heritability, and the estimation of breeding values
• Utilizes statistical methods (ANOVA) to analyze the genetic basis of quantitative traits (yield) and to predict the response to selection

Molecular genetics

• Focuses on the structure, function, and manipulation of genes at the molecular level
• Involves the use of molecular markers (SNPs) to identify and select desirable alleles or genotypes
• Utilizes techniques such as marker-assisted selection (MAS) and genetic engineering to precisely introduce desired traits into plant varieties (Bt cotton)

Methods of plant breeding

• Plant breeding employs various methods to create new plant varieties with desired characteristics
• These methods involve the manipulation of plant genetic material through different approaches, such as selection, hybridization, mutation breeding, polyploidy, and genetic engineering
• The choice of breeding method depends on the breeding objectives, the reproductive biology of the plant species, and the available resources and technologies

Selection

• Involves the identification and propagation of plants with desirable traits from a diverse population
• Can be based on natural variation (landrace selection) or artificially induced variation (mutant selection)
• Includes mass selection (selecting best plants from a population), pure-line selection (selecting superior plants from self-pollinated crops), and clonal selection (selecting superior individuals from vegetatively propagated crops)

Hybridization

• Involves the crossing of two genetically distinct parents to produce offspring with a combination of traits from both parents
• Can be intraspecific (crossing within the same species) or interspecific (crossing between different species)
• Utilizes the principle of heterosis or hybrid vigor, where the hybrid offspring exhibit superior performance compared to the parents (hybrid maize)

Mutation breeding

• Involves the induction of genetic variation through the use of mutagenic agents such as radiation (gamma rays) or chemicals (ethyl methanesulfonate)
• Aims to create novel alleles or traits that are not present in the existing germplasm
• Requires extensive screening and selection to identify desirable mutants (semi-dwarf rice varieties)

Polyploidy

• Involves the manipulation of chromosome numbers to create plants with multiple sets of chromosomes
• Can occur naturally (autopolyploidy) or be induced through artificial means (colchicine treatment)
• Utilized to develop seedless fruits (triploid watermelon), enhance ornamental traits (larger flowers in roses), and restore fertility in interspecific hybrids (triticale)

Genetic engineering

• Involves the direct manipulation of genes using recombinant DNA technology
• Allows the introduction of specific genes from any organism into the plant genome
• Utilized to develop transgenic crops with enhanced traits such as herbicide resistance (Roundup Ready soybean), insect resistance (Bt cotton), and improved nutritional quality (Golden Rice)

Hybridization techniques

• Hybridization is a key method in plant breeding that involves the crossing of genetically distinct parents to create new plant varieties with desirable traits
• Various hybridization techniques are employed to achieve specific breeding objectives and overcome reproductive barriers
• These techniques include intraspecific hybridization, interspecific hybridization, backcrossing, and the utilization of heterosis or hybrid vigor

Intraspecific hybridization

• Involves the crossing of individuals within the same species
• Aims to combine desirable traits from different varieties or populations of the same species
• Widely used in self-pollinated crops (wheat) to create new pure lines and in cross-pollinated crops (maize) to develop hybrids

Interspecific hybridization

• Involves the crossing of individuals from different species within the same genus or closely related genera
• Aims to introgress desirable traits from wild relatives or related species into cultivated crops
• Requires techniques to overcome reproductive barriers such as embryo rescue (wheat-rye hybrids) or bridge crosses (potato-tomato hybrids)

Backcrossing

• Involves the repeated crossing of a hybrid with one of its parents or a genetically similar line
• Aims to transfer a specific trait (disease resistance gene) from a donor parent into a recipient parent while retaining the genetic background of the recipient parent
• Requires multiple generations of backcrossing and selection to recover the desired genotype (introgression of resistance genes in tomato)

Heterosis (hybrid vigor)

• Refers to the superior performance of hybrid offspring compared to their parents
• Exploits the genetic phenomenon where the combination of alleles from genetically diverse parents results in increased vigor, yield, or other desirable traits
• Widely utilized in the development of hybrid varieties in cross-pollinated crops (hybrid rice) and in the production of commercial seeds (hybrid maize)

Breeding strategies

• Breeding strategies are systematic approaches used in plant breeding to achieve specific breeding objectives
• These strategies involve the selection of suitable parents, the design of appropriate mating schemes, and the evaluation and selection of superior genotypes
• Different breeding strategies are employed based on the mode of reproduction, the genetic architecture of the traits, and the available resources

Pedigree method

• Involves the controlled crossing of selected parents and the subsequent selection of superior progeny over several generations
• Maintains a record of the ancestry or pedigree of each progeny line
• Allows for the identification and selection of desirable genotypes based on their phenotypic performance and genetic background
• Widely used in self-pollinated crops (wheat) for the development of pure line varieties

Bulk method

• Involves the crossing of selected parents and the growing of the resulting hybrid population in bulk for several generations without selection
• Relies on natural selection to eliminate inferior genotypes and increase the frequency of desirable alleles
• Requires less labor and record-keeping compared to the pedigree method
• Suitable for traits with high heritability and for developing varieties adapted to specific environments

Backcross method

• Involves the repeated crossing of a hybrid with one of its parents or a genetically similar line to transfer a specific trait into the recipient parent
• Aims to recover the genetic background of the recipient parent while retaining the desired trait from the donor parent
• Requires multiple generations of backcrossing and selection to achieve the desired genotype
• Used for the introgression of disease resistance genes, quality traits, or other desirable characteristics into elite cultivars

Recurrent selection

• Involves the repeated cycles of selection and intermating of superior individuals from a genetically diverse population
• Aims to gradually increase the frequency of favorable alleles and improve the overall performance of the population
• Can be based on phenotypic selection (mass selection), progeny testing (half-sib or full-sib selection), or marker-assisted selection
• Widely used in cross-pollinated crops (maize) for population improvement and the development of superior inbred lines

Challenges in plant breeding

• Plant breeding faces various challenges that can limit the efficiency and effectiveness of breeding programs
• These challenges arise from biological, technical, and environmental factors that influence the success of breeding efforts
• Addressing these challenges requires innovative approaches, advanced technologies, and a deep understanding of plant genetics and breeding principles

Incompatibility barriers

• Refers to the reproductive barriers that prevent successful hybridization between different species or genotypes
• Can be due to pre-zygotic barriers (pollen-stigma incompatibility) or post-zygotic barriers (embryo abortion or hybrid sterility)
• Requires specialized techniques such as embryo rescue, protoplast fusion, or bridge crosses to overcome these barriers
• Limits the gene pool available for breeding and restricts the introgression of desirable traits from wild relatives or distant species

Linkage drag

• Refers to the co-transfer of undesirable genes along with the desired gene during backcrossing or introgression
• Occurs due to the genetic linkage between the target gene and the nearby genes on the same chromosome
• Can result in the introduction of undesirable traits (reduced yield) or the loss of desirable traits (quality) in the recipient parent
• Requires extensive backcrossing, selection, and molecular markers to minimize linkage drag and recover the desired genotype

Inbreeding depression

• Refers to the reduction in vigor, fertility, or productivity of offspring resulting from the mating of closely related individuals
• Occurs due to the increased homozygosity of deleterious recessive alleles in the genome
• Can limit the development of inbred lines and the exploitation of hybrid vigor in cross-pollinated crops
• Requires the use of heterosis, outcrossing, or genetic diversity to mitigate the effects of inbreeding depression

Genotype vs environment interactions

• Refers to the differential performance of genotypes across different environmental conditions
• Arises from the complex interplay between the genetic makeup of plants and the environmental factors (climate, soil, management practices)
• Can lead to inconsistent performance of varieties across locations and years, making selection and recommendation challenging
• Requires multi-location trials, stability analysis, and the development of adapted varieties for specific target environments

Applications of plant breeding

• Plant breeding has diverse applications beyond crop improvement for food production
• Breeding efforts are directed towards developing plants for various purposes, including ornamental horticulture, medicinal uses, and biofuel production
• These applications demonstrate the versatility and importance of plant breeding in meeting the diverse needs of society and industry

Crop improvement

• Focuses on developing improved varieties of major food crops (rice, wheat, maize) to enhance food security and meet the growing global demand
• Aims to increase yield, nutritional quality, resistance to biotic and abiotic stresses, and adaptation to different agro-ecological conditions
• Utilizes various breeding methods and technologies to develop high-yielding hybrids, disease-resistant varieties, and climate-resilient crops

Ornamental plant breeding

• Involves the development of new varieties of ornamental plants (roses, chrysanthemums, orchids) with enhanced aesthetic traits and improved performance
• Focuses on breeding for novel flower colors, shapes, and fragrances, as well as improved plant architecture and disease resistance
• Utilizes techniques such as interspecific hybridization, polyploidy, and mutation breeding to create unique and visually appealing varieties for the horticulture industry

Medicinal plant breeding

• Aims to develop improved varieties of medicinal plants with higher yields of bioactive compounds and enhanced therapeutic properties
• Focuses on breeding for increased content of specific secondary metabolites (artemisinin in Artemisia annua), improved agronomic traits, and adaptation to cultivation conditions
• Utilizes marker-assisted selection, genetic engineering, and other advanced breeding techniques to accelerate the development of superior medicinal plant varieties

Biofuel crop breeding

• Focuses on developing improved varieties of biofuel crops (sugarcane, switchgrass, jatropha) for sustainable energy production
• Aims to increase biomass yield, improve conversion efficiency, and enhance adaptability to marginal lands and environmental stresses
• Utilizes breeding strategies such as hybridization, polyploidy, and genetic engineering to develop high-yielding and low-input biofuel crop varieties

Future prospects

• Plant breeding continues to evolve with the advancements in science and technology, offering new opportunities and solutions for crop improvement
• Future prospects in plant breeding involve the integration of cutting-edge technologies and approaches to accelerate breeding progress and address emerging challenges
• These prospects include marker-assisted selection, genomic selection, gene editing technologies, and sustainable plant breeding practices

Marker-assisted selection

• Involves the use of molecular markers (SNPs, SSRs) to select plants with desirable traits based on their genetic makeup
• Allows for the early and precise selection of superior genotypes without the need for extensive phenotypic evaluation
• Accelerates the breeding process by reducing the number of generations required to develop improved varieties
• Enables the pyramiding of multiple desirable traits (disease resistance genes) into a single genotype

Genomic selection

• Involves the use of genome-wide markers to predict the breeding value of individuals based on their genomic profile
• Utilizes statistical models to estimate the effect of each marker on the trait of interest and predict the performance of untested genotypes
• Allows for the selection of superior individuals without the need for phenotypic evaluation, reducing the time and cost of breeding
• Particularly useful for complex traits with low heritability and for accelerating the development of improved varieties in long-generation crops (fruit trees)

Gene editing technologies

• Involves the precise modification of plant genomes using tools such as CRISPR/Cas9, TALENs, and zinc-finger nucleases
• Allows for the targeted mutagenesis, insertion, or deletion of specific genes or regulatory elements
• Enables the rapid development of new plant varieties with desired traits (disease resistance, improved nutritional quality) without the introduction of foreign DNA
• Offers a more precise and efficient alternative to traditional genetic engineering and has the potential to revolutionize plant breeding

Sustainable plant breeding practices

• Focuses on developing plant varieties that are adapted to local conditions, require fewer inputs (water, fertilizers, pesticides), and promote biodiversity
• Involves the use of participatory breeding approaches, where farmers and local communities are actively involved in the breeding process
• Emphasizes the importance of conserving and utilizing plant genetic resources, including landraces and wild relatives, as sources of valuable traits
• Aims to develop resilient and sustainable cropping systems that can withstand the challenges posed by climate change and resource limitations