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Modern crops are the result of intensive plant breeding and have many characteristics different from their wild ancestors. The process of plant breeding has resulted in increased yield and removal of undesirable characteristics, but with this is the risk of increased disease susceptibility of clonal populations and loss of biodiversity.
Plant breeding Methods
Selection has resulted in recognizable varieties or landraces of crops. These varieties are heterozygous for crops that cross pollinate, but homozygous for those (like wheat) which do not. Cross hybridization gives hybrid vigor. Cross pollination between lines can be achieved by removing the anthers from one line and pollinating with a second line.
Recently male sterile lines that do not make pollen have been produced. Desirable characteristics are introduced into plants by cross-fertilizing lines to produce hybrids. Back crossing allows a desirable trait to be introduced into an existing useful line.
Limits to conventional plant breeding
Plant breeding has been very successful in generating high-yielding crops. However, as well as being time consuming and labor intensive, it is also limited by natural barriers of pollination between species.
All the major crops world-wide are the result of a repetitive process of breeding, selection and further breeding to alter characteristics and improve yield. Alongside this process have come great increases in human population as agricultural productivity has attempted to meet the needs of that population. For some crops, the process has been carried out only recently (e.g. for oil-seed rape, Canola) while for others it has been going on for many thousands of years and in many civilizations. Wheat was domesticated first in the near-East. Breadwheat appears to have resulted from crossing primitive einkorn wheat with goatgrass to generate emmer wheat, which was crossed again with goatgrass to yield bread wheat. Corn (maize) was domesticated in Mesoamerica. One of its closest relatives is teosinte, which produces small, corn-like seed ears with hard outer husks. Rice was domesticated in Indo-China from a wild rice, Oryza rufipogon. Apart from yield, domestication has altered many characteristics of crop plants, including loss of dormancy from the seed and loss of dispersal mechanisms. There are also some negative effects of domestication; for instance, domesticated varieties are frequently demanding of nutrients and soil conditions. Intensive breeding programmes resulting in near clonal crops may result in loss of biodiversity and susceptibility to disease.
Selecting plants for desirable traits resulted in the development of identifiable varieties or landraces of crops, each with slightly different characteristics. As some species can cross-fertilize, their landraces tend to be heterozygous (genetically mixed) while those that do not normally cross-fertilize (like wheat) will form genetically pure (homozygous) lines.
Selection breeding is the process of choosing plants showing desirable characteristics and generating seed from them. It is straightforward if the crop is self-pollinated but produces increasingly inbred lines and may result in loss of yield. Cross-pollinators can also be bred in this way, though creation of a homozygous line will not be possible. Loss of yield in inbred lines can be overcome by deliberate cross hybridization to achieve what is known as hybrid vigor. This is achieved by removing the anthers from one line and planting it adjacent to a second line with anthers. All the pollination of the first line will then be by the second line, and the seeds produced by that line will be hybrids of the two lines. Some lines are naturally male sterile (i.e. do not produce viable pollen). These plants are very valuable in plant breeding, as the anthers do not have to be removed by hand. More recently, genetic engineering has been used to create male sterile lines, by linking ribonuclease gene expression with a promoter sequence controlling an anther-specific gene. Ribonuclease produced in the anthers degrades the messenger RNA (mRNA) they produce. This results in plants that do not make pollen.
Desirable characteristics are introduced into plants by cross breeding. Lines are cross-fertilized with others (or with wild ancestors or related species) to produce hybrids containing a mixture of characteristics from both parents, some useful and some not. To eliminate the undesirable characteristics and develop useful ones, back crossing is carried out. In this process, the progeny of a cross (for instance between a high-yielding strain and a disease-resistant but low yielding strain; Table 1) are repeatedly hybridized with the high-yielding line.
Conventional plant breeding has been very successful in generating the varieties of high-yield crops we use today. It is time consuming and labor intensive. It is also limited by the natural pollination barriers between species that mean desirable traits cannot be easily introduced from one species to another. Artificial mutagenesis (by X-rays or chemicals) has been used to generate new characteristics, together with tissue culture techniques . However, these have largely been replaced by genetic manipulation . This technique allows single characteristics to be transferred into a crop in a far more controlled and specific manner than was previously possible.
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