plant breeding and plant improvement

8/8/2019 Plant Breeding and Plant Improvement

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Plant Breeding and Plant Improvement

Conventional breeding methods are the most widely used for crop improvement. But in certain

situations these methods have to be supplemented with plant tissue culture techniques, either to

increase their efficiency or to achieve an objective not possible through conventional methods.

Embryo culture is now routinely used in recovery of hybrid plants from distant crosses. Someexamples are recovery' of hybrids from Hordeum vulgare x Agropyron repens and H. vulgare x

Triticum sp. In the case of Triticale, a rare hybrid between Triticum and Secale develops viableseeds. But most of the tetraploid and hexaploid wheat carry two dominant genes, Kr1 and Kr2,

which prevent seed development in crosses with Secale.

The hybrid seeds are minute, poorly developed and show very poor germination. By embryo

culture, 50-70% hybrid seedlings have been obtained. Hybrid seedlings from T. aestivum x H.

vulgare are not obtained. But in embryo culture when H. vulgare or T. aestivum (used as male) iscrossed with H. bulbosum (used as female), the chromosome complement of H. bulbosum is

eliminated from the developing embryo.

Most of the seedlings obtained from such crosses are haploid, having only one set of

chromosomes, either from the H. vulgare or the T. aestivum parent. Embryo culture is also usefulfor propagation of orchids, shortening the breeding cycle and overcoming seed dormancy.

In meristem culture the shoot apical meristem along with some surrounding tissue is grown in

vitro. This is used for clonal propagation and recovery of virus free plants and is potentiallyuseful in germplasm exchange and long term storage of germplasm through freeze preservation.

Anther and pollen culture has potential application in a plant breeding and plant improvement

program for the production of haploid as well as homozygous diploid plants. Year round rapidclonal propagation using plant tissue culture techniques has highlighted possibilities for new

plant improvement techniques. Protoplast culture and somatic hybridization is a promising linefor plant breeding and plant improvement techniques.

Another important approach is the mutation of tissue culture cells to produce a mutant line from

which plants can be raised. Production of a mutant line is highly desirable for plant breeding.Callus cells, produced either from a vegetative cell or reproductive tissues, can be subjected to a

range of mutagenic chemicals, e.g. N-nitroso-N-methyl urea, or ionizing radiations, e.g. gammarays.,

The hope is that such treatment will effect permanent changes in the DNA pattern of some cells.

Plants could be raised from the treated cultures and any mutant whole plants could be selectedfrom the population either by physical differences or by metabolic/biochemical differences.Biochemical mutants could be selected for disease resistance, resistance to phytotoxin,

improvement of nutritional quality, adaptation of plants to stress conditions, e.g. saline soils, andto increase the biosynthesis of plant products used for medicinal or industrial purposes.


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Modern plant breeding

Modern plant breeding uses techniques of molecular biology to select, or in the case of geneticmodification, to insert, desirable traits into plants.

Modern facilities in molecular biology has converted classical plant breeding to molecular plantbreeding

Steps of Plant Breeding

The following are the major activities of plant breeding;

1. Creation of variation2. Selection3. Evaluation4. Release5. Multiplication6. Distribution of the new variety

Marker assisted selection

Sometimes many different genes can influence a desirable trait in plant breeding. The use of

tools such as molecular markers orDNA fingerprinting can map thousands of genes. This allowsplant breeders to screen large populations of plants for those that possess the trait of interest. The

screening is based on the presence or absence of a certain gene as determined by laboratoryprocedures, rather than on the visual identification of the expressed trait in the plant.

Reverse Breeding and Doubled Haploids (DH)

A method for efficiently producing homozygous plants from a heterozygous starting plant, whichhas all desirable traits. This starting plant is induced to produce doubled haploid from haploid

cells, and later on creating homozygous/doubled haploid plants from those cells. While in naturaloffspring genetic recombination occurs and traits can be unlinked from each other, in doubled

haploid cells and in the resulting DH plants recombination is no longer an issue. There, arecombination between two corresponding chromosomes does not lead to un-linkage ofalleles or

traits, since it just leads to recombination with its identical copy. Thus, traits on one chromosomestay linked. Selecting those offspring having the desired set of chromosomes and crossing them

will result in a final F1 hybrid plant, having exactly the same set of chromosomes, genes andtraits as the starting hybrid plant. The homozygous parental lines can reconstitute the original

heterozygous plant by crossing, if desired even in a large quantity. An individual heterozygousplant can be converted into a heterozygous variety (F1 hybrid) without the necessity of

vegetative propagation but as the result of the cross of two homozygous/doubled haploid linesderived from the originally selected plant.patent


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Genetic modification

Genetic modification of plants is achieved by adding a specific gene or genes to a plant, or by

knocking down a gene with RNAi, to produce a desirablephenotype. The plants resulting fromadding a gene are often referred to as transgenic plants. If for genetic modification genes of the

species or of a crossable plant are used under control of their native promoter, then they arecalled cisgenic plants. Genetic modification can produce a plant with the desired trait or traitsfaster than classical breeding because the majority of the plant's genome is not altered.

To genetically modify a plant, a genetic construct must be designed so that the gene to be added

or removed will be expressed by the plant. To do this, apromoterto drive transcription and atermination sequence to stop transcription of the new gene, and the gene or genes of interest

must be introduced to the plant. A marker for the selection of transformed plants is also included.In the laboratory, antibiotic resistance is a commonly used marker: Plants that have been

successfully transformed will grow on media containing antibiotics; plants that have not beentransformed will die. In some instances markers for selection are removed bybackcrossing with

the parent plant prior to commercial release.

The construct can be inserted in the plant genome by genetic recombination using the bacteriaAgrobacterium tumefaciens orA. rhizogenes, or by direct methods like the gene gun or

microinjection. Using plant viruses to insert genetic constructs into plants is also a possibility,but the technique is limited by the host range of the virus. For example, Cauliflower mosaic virus

(CaMV) only infects cauliflowerand related species. Another limitation of viral vectors is thatthe virus is not usually passed on the progeny, so every plant has to be inoculated.

The majority of commercially released transgenic plants are currently limited to plants that haveintroduced resistance to insectpests and herbicides. Insect resistance is achieved through

incorporation of a gene fromBacillus thuringiensis (Bt) that encodes aprotein that is toxic tosome insects. For example, the cotton bollworm, a common cotton pest, feeds on Bt cotton it will

ingest the toxin and die. Herbicides usually work by binding to certain plant enzymes andinhibiting their action. The enzymes that the herbicide inhibits are known as the herbicides target

site. Herbicide resistance can be engineered into crops by expressing a version oftarget siteprotein that is not inhibited by the herbicide. This is the method used to produce glyphosate

resistant crop plants (See Glyphosate)

Genetic modification of plants that can producepharmaceuticals (and industrial chemicals),sometimes calledpharmacrops, is a rather radical new area of plant breeding.


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Domestication to Crop Improvement: Genetic Resources for Sorghum

and Saccharum (Andropogoneae)

1. Sally L. Dillon2,2. Frances M. Shapter1,3. Robert J. Henry1,*,4. Giovanni Cordeiro1,5. Liz Izquierdo1 and6. L. Slade Lee1

Oxford Journals,Life Sciences, Annals of BotanyVolume100, Issue5 Pp. 975-989.

The role of genomics in improving domesticatedS. bicolor

Sorghum bicolor, a diploid, has a relatively small genome (735 Mbp), which although larger than

rice (389 Mbp) is smaller than the other important cereals (wheat 16 900 Mbp, maize 2600Mbp). The last genome duplication event for the S. bicolorgenome seems to have occurred

much earlier than the divergence of the major cereal crops from a common ancestor (Paterson etal., 2004). Completion of the whole genome sequencing project in 2007 will exponentially

increase the sequence data available forSorghum and will provide valuable information oncereal domestication in the African continent, an event that appears to have occurred

independently of other continents though by similar reinforced selective pressures (Paterson etal., 2004). In a way, the sorghum genome sequencing will close a biographic triangle into the

knowledge of the polymorphism shared before the divergence of these important grasses andultimately in the understanding of the evolution in cereals crops between Africa, America and

Asia (Kresovich et al., 2005). The tenets of colinearity and microlinearity of grass genomesmean that our knowledge of other cereals and their evolutionary ties will also greatly improve.

Due to their economic and scientific value, cereal genomes have been studied over the last 15years using highly advanced technologies. The similarity at the DNA level makes it possible to

use comparative genetics to look for particular genes of unknown sequence between the genomeswith the aim of using that information to develop new varieties or discovering new genes that

could have a potential impact on traits that are of global importance (e.g. food quality, droughtresistance).

The genetic diversity existing within and between Australian Sorghum species was recently

evaluated using simple sequence repeats (SSRs) (Dillon et al., 2005). SSRs were sourced from

the cultivated S. bicolor(Brown et al., 1996; Taramino et al., 1997; Kong et al., 2000) todetermine diversity in these closely related taxa. This method has successfully evaluateddiversity in the related species of many crop groups (e.g. Peakall et al., 1998; Hernndez et al.,

2001; Chen et al., 2002; Scott et al., 2003; Gonzlez-Martnez et al., 2004; Sudupak, 2004). Thisevaluation of the Australian species has shown significantly higher levels of genetic diversity

both between (inter-) and within (intra-) species compared with the intra-specific diversity ofS.bicolorvarieties. The relatively high transfer rate ofS. bicolor-derived SSRs to the wild species


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and their high level of diversity suggests that these SSRs are an efficient, highly informativesource of molecular markers for the undomesticated Sorghum species.

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