GENETIC ENHANCEMENT OF PLANTS

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1. GENETIC ENHANCEMENT OF PLANTS. INTRODUCTION


The genetic improvement of plants consists in creating new varieties from existing varieties (genetic diversity) . This gene transfer is done by directed crosses and selection of the best plants from these crosses (this requires knowledge of reproduction methods ). Other means of creating successful varieties exist including mutagenesis, protoplast fusion, transgenesis and somatic variations.
Genetic improvement of plants is the process by which humans modify a given plant species by expelling the previously existing genetic diversity. By tapping into diversity, humans recombine genes by several methods including directed crosses. He then practices a selection (sorting) and a multiplication of the plant material carrying the agronomic traits desired. The listing of the improved varieties precedes the marketing of the final products.

Historical references of genetic improvement of plants:
 1676 . Discovery of the role of sex organs in plants by Millington-Grew.
 1880 . Visualization of chromosomes by Strasburger-Boveri, and evidence of their involvement in cell division.
 1900 . Enforcement of Mendel’s Laws on Heredity. His work on the crossing of two pea varieties defines the basic rules of genetics. This is the birth of plant breeding.
 1902. Discovery of the totipotency of plant cells by Haberland. A plant tissue is able to regenerate a plant.
– 1908 . Discovery of the interest of hybrids by Shull on corn. The crossing of two lines makes it possible to obtain a hybrid that exploits heterosis.
– 1911 . Genetic linkage concept by Morgan. It demonstrates that genes are linearly placed on chromosomes and that, when located on the same chromosome, they are transmitted to the offspring as a single unit. They are said to be linked
– 1930 generation of variety by mutagenic treatments (X-rays)
– 1935 . First partial genetic map of maize by Emerson.
– 1950 . First techniques of in vitro culture . This is the vegetative propagation technique developed by Morel and Martin on the potato.
– 1953 . Description of the double helix structure of DNA by Watson and Crick.
– 1960 . Discovery of the genetic code by Crick, Nirenberg, Mathaeri and Ochoa, beginning of the green revolution
– 1961.Illustration of the principles of analysis of the loci involved in the variation of quantitative characters, by Thoday
– 1964 . First cultures of male sex cells in Datura innoxia , by Guha and Maheshwari. They pave the way for the production of haploid plants.
– 1965 . Discovery of restriction enzymes by Aber, Smith and Nathans. These proteins cut DNA at particular sites.
– 1975 . Description of the method of Southern, the name of its inventor. The principle of the technique relies on hybridization of the DNA with a labeled DNA probe.
– 1977. Discovery of gene transfer by Agrobacteria, pathogenic soil bacteria of many plant species, by Schell. He showed that the virulence of these bacteria is due to a transfer of genes from the bacterium to the plant cells.
– 1978 . First protoplast melting by Melchers. They allow to partially cross the barrier between species.
– 1983 . Development of the PCR by Karry Mullis and first transgenic tobacco obtained at the same time by a Belgian team and an American team
– 1985 First variety of wheat resulting from the technique of haplodiploïdation
– 2000: Sequencing of the genome on a brassicaceae ( Arabidopsis thaliana )
– 2002 : Sequencing of the genome of the rice

Domestication of plants and beginning of agriculture
Since the beginning of agriculture, the farmers keep, at each generation, the seeds of the most beautiful plants, in order to replant them the following year.


Keeping the best seeds gradually leads to an improvement of the cultivated species. The domestication of wild varieties is accompanied by a selection of useful traits such as:
– Size of consumable parts (seeds, tubers)
– Flavor
– Resistance to stress
– Other traits

The evolution of current techniques and knowledge has allowed plant breeders to increase their efficiency, save time on vegetative cycles and to have tools of measurement and analysis in the field and in the laboratory (biology, biochemistry, statistics , biotechnologies) to confirm their choice.

2. GENETIC IMPROVEMENT OF PLANTS. EXAMPLES OF ACHIEVEMENTS

Rapeseed ( Brassica napus ), called canola in Canada, is a crucifer grown primarily for its seeds, which contain about 50% of a good quality nutritional oil (high in unsaturated fatty acids). After extraction of oil, the rest of the seed (cake), rich in protein (40% of the dry matter) is used in animal feed. There are also fast growing forage varieties used for green feeding, grazing or silage. Rapeseed oil also has industrial applications such as the manufacture of fuel (esterification with methanol).

The glucosinolate called sulfur glycosides or ‘ thioglycosides ‘ are organic compounds responsible for the bitter taste or pungent many common foods such as mustard, radishes, watercress, cauliflower. In rapeseed, glucosinolate degradation products were responsible for significant physiological inappetence and physiological disorders in cattle

Rapeseed has a double genome since it comes from the natural crossing of a cabbage and a shuttle .


3. VARIETAL CREATION. PRE-REQUIRED FOR THE BIRTH OF NEW VARIETIES


The creation of variability (and at the same time creation of new varieties) can be achieved by:
– Intra-specific or interspecific directed crosses for the improvement of current species or the creation of new species or possibly by in vitro culture of immature embryos. Several studies refer to trials of interspecific crosses .
– Mutagenesis by physical agents (X-rays, gamma) or chemical (MSE) on seeds, meristems, pollen.
– Frequent somatic changes in in vitro culture practicesfrom fragments of differentiated organs, isolated cells or protoplasts.
– Fusion of protoplasts (isolated cells without pectocellulosic wall).
– Transgenesis concerning gene transfer by genetic engineering.
The modification of a genotype can be done on a qualitative scale by changing the nature of the genes that control the desired traits (color, resistance to diseases, ..) and by acting on their assembly. It can be practiced on a quantitative scale by modifying the dosage of genetic information by increasing or decreasing the number of chromosomes of a species.

3.1. Variety creation by intraspecific or interspecific directed crosses

The genes of interest that will be introduced into a given species, are sought in a neighboring variety of the same species. The greater the genetic variability in a species, the better the chance to find the gene of interest. In the opposite case, it is possible to use spawners of neighboring species or even of similar genera. Also, desired alleles can be created by mutations.
Choice of the parent carrying the quality gene
The parent may be intraspecific or interspecific.
– 
Intraspecific genus : teosinte ( Euchena mexicana ) is the wild ancestor of cultivated maize ( Zea mays ).
– 
Interspecific genus : Tomato, with low intraspecific variability, is enhanced by interspecific spawners, particularly for the introduction of disease resistance.

The Tm-2 gene for resistance to tobacco mosaic and insect resistance genes were introduced into the cultivated tomato ( Lycopersicum esculentum ) from the neighboring species; L. peruvianum and L. hirsutum , respectively.
In interspecific crosses, natural barriers prevent the complete development of the embryo. To remedy this situation, it is practice after fertilization an early harvest of embryos to put them in culture on a nutritious artificial medium. This in vitro culture technique is called interspecific embryo rescue.
Before the maturation phase of the seed, the embryo is removed, then
transplanted and grown on an artificial medium rich in sugar, allowing the regeneration of a new plant.
Gene transfer by directed crosses is related to the reproductive (autogamy, allogamy) patterns of plants.

3.2. Variety creation by mutagenesis

– Gene mutations (qualitative)
Gene mutations (allelic) are point mutations that modify the nucleotides of the DNA of a gene. These mutations are at the origin of the richness of the allelic forms.
– 
Chromosome mutations (quantitative)
They appear in the karyotype (detectable cytologically) and can alter the structure of a chromosome or the number of chromosomes.
Structural alterations of chromosomes. They make it possible to:
1 / break links between unfavorable genes and favorable genes,
2 / Associate a gene of interest with a ‘marker gene’ to identify the first. Example in barley: associate the male sterility gene with the chlorophyll-free gene.

Variations in the number of chromosomes (ploidy): genomic mutations
Euploidy : regular modification of the number (haploid, diploid, tetraploid, …)
Aneuploidy : Existence of a number of chromosomes. Example of trisomy 2n + 1, tetrasomy 2n + 2, monosomy 2n -1
There are also cytoplasmic mutations that generally exist in angiosperms, transmission and localization in chloroplasts and mitochondria transported by the mother. They are highlighted by reciprocal crosses. Ex: Cytoplasmic male-sterility.
Mutagenic agents:
Physical mutagens (ionizing radiation), chemical mutagenic agents (eg colchicine).

3.3. Creation of varieties by somatic modifications

In plants with limited gene mixing (vegetative multiplication), clones sometimes appear which differ from the mother plant. These individuals are called somatic variants . This has led to several varieties of potato and apple, in addition to pink grapefruit and navel orange.
The somatic variations are frequent in the following situations:
– In vitro culture from fragments of differentiated organs (with several subcultures).
– Culture of isolated cells or protoplasts
The somatic variants can carry positive traits (vigor, juvenility, precocity, resistance, ..) for the enhancers.

The determinism of somatic variations produced during regeneration remains complex (nuclear and extra-nuclear genetic information). The passage through the cal or microcal stageduring the regeneration, would be destabilizing and would cause these modifications.

3.4. Varietal creation by protoplast fusion (somatic hybridization)

The term protoplast means a plant cell unclasped from its skeletal wall. It appears in the form of a spherical cell, limited by its plasma membrane. The protoplast preparation technique was only really developed in the 1960s, when cell wall degrading enzymes were purified and used in this biotechnology.
Protoplasts are able to fuse to give cells with double chromosomal socks. If different varieties, species or genera are to be used somatic hybrids will be used . See the example of the citrus genre , subject of S5 review, Marrakech, 2011-2012 .

3.5. Varietal creation by transgenesis

The transgenesis (obtaining GMO ) is to be transferred to a plant a gene whose expression indicates a determined character. The biological origin of the genes used is variable. It is possible to express a gene from another plant species or another organism (bacterium, fungus) by the transcription and translation machinery of the cells of the host plant. This allows for a significant expansion of genetic resources. The latest biotechnological advances allow us to introduce several genes at a time. This is of great importance in the context of improving drought tolerance which is polygenic.

Dr. Kimberly Seltzer

Postdoctoral Scholar, UC Berkeley Research Assistant, MIT

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