03 Methods to Improve Crops
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Methods to genetically modify crops to improve vitamin A content and yield were described. Rice was genetically modified to produce beta-carotene in the grain through the insertion of genes from daffodil and bacteria. Genes could be directly inserted into rice cells using gene guns. Wheat was also discussed, having evolved from hybridization events to become a hexaploid crop. Polyploidy in plants can be induced to create hardier varieties. Maize breeding involved inbreeding, hybridization, and selection to develop high-yielding, uniform varieties with desirable traits.
03 Methods to Improve Crops
- 1. Methods to improve crops • Enhancing the vitamin A content of rice through genetic manipulation – Green parts of rice plants contain beta-carotene – vital precursor of vitamin A – No beta-carotene in grains – Vitamin A essential for operation of body’s immune system. Deficiency causes increased risk of infection, night-blindness and total blindness. – Genetically-modified strain of rice has been produced which stores significant levels of beta-carotene in the grains (golden rice) – contains genes from daffodil and a bacterium – General method used: incorporate genes into a bacterial plasmid and use bacteria (Agrobacterium tumefaciens) to carry genes into the plant cells – Method used for rice: genes are delivered directly into the cells using small μm-sized tungsten or gold bullets coated with DNA. The bullets are fired from a device that works similar to a shotgun (gene gun) ALBIO9700/2006JK
- 2. White rice and golden rice ALBIO9700/2006JK
- 3. Gene gun (Courtesy of Bio-Rad Laboratories) ALBIO9700/2006JK
- 5. Producing polyploids in wheat through hybridisation • Modern bread wheat (Triticum aestivum) is a hexaploid plant (6n) – result of hybridisation of several wild species of grasses (such sterile hybrids can only reproduce asexually) • Doubling of chromosome number appears to have occurred twice during the evolution of modern wheat – resulting in the formation of fertile polyploids from previously sterile hybrids • Formation of polyploids important in plants – animal polyploids are often not viable • Now it’s possible to induce the formation of polyploids by preventing spindle formation, using chemicals (e.g. colchicine) • Polyploids are generally more hardy and higher yielding than their parent species • Ancestors of wheat are small, not very robust and produce small ears of small seeds ALBIO9700/2006JK
- 6. Probable ancestral Chromosome Ploidy level Plant haploid number number domestic oat 7 42 6n peanut 10 40 4n sugar cane 10 80 8n banana 11 22, 33 2n, 3n white potato 12 48 4n tobacco 12 48 4n cotton 13 52 4n apple 17 34, 51 2n, 3n ALBIO9700/2006JK
- 8. Evolution of modern bread wheat ALBIO9700/2006JK
- 9. Producing vigorous, uniform maize through inbreeding and hybridisation • Through selection, inbreeding and hybridisation, growers have been able to produce varieties of maize that grow vigorously (therefore high yielding) and uniformly under the prevailing conditions (soil type, prevailing temperatures, rainfall, etc) • Assuming conditions remain similar year after year, farmers can continue to grow the same variety and expect to obtain a similar crop • Desirable characteristics: – High yielding – Disease resistant – Good quality in terms of market desirability – Vigorous growth under the prevailing conditions – Plants all grow to a similar height (easy harvesting) – Crops are all ready to harvest at the same time ALBIO9700/2006JK
- 10. • Plants that show desirable characteristics would be selected and self pollinated – pollen transferred to stigma – muslin bag placed around flower (ensure pollen from other plants does not reach stigma) – homozygous plants for desired characteristics will be produced if repeated for many generations • Inbreeding depression: loss of vigour and fertility, as well as a reduction in size and yield due to inbreeding • Inbred maize has very little variation (every plant having the same allele of every gene) • However, if 2 inbred lines are crossed, it will produce a hybrid that has a greater yield and is more vigorous than either of the parental lines (hybrid vigour) • This hybrid heterozygous for most genes, so deleterious recessive alleles are hidden – at the same time, it inherits the lack of variability from its parents • Such single cross hybridisation has been used for selective breeding since the early 1960’s to double the yield and to breed uniform, high yielding maize ALBIO9700/2006JK
- 11. Inbreeding depression ALBIO9700/2006JK
- 12. • Double cross hybridisation was used as a selective breeding tool to increase maize yields between 1920 and 1960 from 1.5 to 4 tonnes per hectare • In order to carry out the inbreeding or to carry out a cross to form a hybrid, pollen from a specific male parent must be used to fertilise a specific female parent – anthers are removed from some flowers which will form the female parent – pollen transferred from anthers of male parents flowers to stigmas of flowers without anthers – muslin bags are placed around fertilised flowers • Selection for measurable characteristics i.e. yield • Selection for disease or pest resistance • Seeds are grown and plants showing desirable characteristics are bred again – can be repeated for many generations ALBIO9700/2006JK
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