HOME :: CHAPTER 20 :: Essay 20.3
Green Revolution Genes
Peter Hedden, Rothamsted Research, Harpenden, Hertfordshire, UK
The Green Revolution
The term “Green Revolution” refers to the huge increases in grain yields after the 1960s, resulting from the introduction of new varieties of wheat and rice, particularly for use in the developing world. This development was a major factor in maintaining per capita food supplies worldwide in the late 20th Century despite a doubling in the world population during this time (Evans, 1998) and was recognised by the award in 1978 of the Nobel Peace prize to Norman Borlaug of the International Maize and Wheat Improvement Center (CIMMYT). Borlaug developed high yielding wheat varieties suitable for growing in sub-tropical and tropical climates. The higher grain yields were obtained in part through increased use of fertilisers and pesticides. However, the heavier grain caused the plants to become unstable and prone to lodging (falling over) in high winds and rain. Borlaug introduced dwarfing genes into wheat giving the plants a stronger, shorter stem that resisted lodging.
The advantages of using dwarfing genes with high-yielding varieties was soon recognised and most commercial wheat varieties contain such genes, in temperate as well as in sub-tropical regions. An unexpected additional benefit from these genes has been an increase in grain yield through an improvement in the ‘harvest index’ (the proportion of plant weight in the grain). This means that a greater proportion of the products of photosynthesis accumulates in the grains rather than in the leaves. Modern wheat varieties have a harvest index of over 50%, with a sharp increase since the introduction of the dwarfing genes (Evans, 1998). Furthermore, the extensive use of herbicides has meant that the wheat plant does not need to compete with weeds and is therefore not disadvantaged by its short stature.
At about the same time that the dwarf wheat varieties were being developed, there were programmes to breed dwarf rice varieties, which gave the same benefits as were obtained with wheat, namely resistance to lodging and a higher harvest index. There are two sub-species of cultivated rice: indica rice is grown mainly in tropical and sub-tropical regions and is particularly susceptible to lodging, while japonica rice is grown in temperate regions and is shorter than indica rice. Dwarf varieties were first developed for indica rice, but many of the commercial varieties of both sub-species now containing a dwarfing gene.
The Dwarfing Genes of Wheat
There are many genes associated with a semi-dwarf growth habit in wheat (Ellis et al., 2005). They are known as Reduced height (Rht) genes and many of them are dominant or semi-dominant, indicating that they actively inhibit growth through a so-called gain-of-function mutation. The identity and function of most of these genes are not known, but some have been found to prevent the action of gibberellins. Two genes in particular, Rht-B1b and Rht-D1b, are used in many commercial wheat varieties, some varieties containing both of these dwarfing gene, which have an additive effect on growth. The effects of these genes as well as the more severe dwarfing gene Rht-B1c are compared in Figure 1. Bread wheat, Triticum aestivum, is hexaploid, which means it has three genomes (A, B and D), these being derived from its three wild diploid ancestors. Each genome contains 14 chromosomes so that bread wheat has 42 chromosomes in total. The A, B and D genomes are very similar to each other and are described as homoeologous. Rht-B1b and Rht-D1b, which were formerly called Rht1 and Rht2, are corresponding (homoeologous) genes on the B and D genomes, respectively.
The Rht-B1b and Rht-D1b dwarfing genes were derived from Norin 10, which was a semi-dwarf variety bred in Japan and released in 1935 (Gale and Youssefian, 1985). Norin 10 was used in US breeding programs in the 1950s in order to improve lodging resistance in winter wheat, in which lodging was a serious problem when high rates of nitrogen fertilizers were applied. A selection from a cross of Norin 10 with the US variety Brevor was particularly promising and was used by Borlaug in the development of the ‘Green Revolution’ wheat varieties.
The wild-type (non-mutant) alleles of Rht-B1b and Rht-D1b (designated Rht-B1a and Rht-D1a, respectively) were isolated a few years ago (Peng et al., 1999) and shown to encode DELLA proteins, which are components of the GA signal transduction pathway. As explained in chapter 20, DELLA proteins act as repressors of plant growth, probably by interacting with and suppressing the activity of transcription factors. Gibberellins participate in a process that results in polyubiquitination of the DELLA proteins so that they are targeted for degradation by the 26S proteasome (reviewed in Thomas and Hedden, 2006). Thus GAs stimulate growth by removing a growth repressor. Peng et al. found that the Rht-B1b and Rht-D1b dwarfing alleles each contained a point mutation that introduced a stop codon into a conserved region known as the DELLA domain, which is present near the N-terminus of the protein. The presence of the DELLA domain was known to be important for GA-induced degradation. It has been shown from work with Arabidopsis thaliana that if the DELLA domain was not present the protein accumulated in the plant even when GAs were applied. The DELLA domain does not appear to be necessary for the growth-inhibiting activity of the protein so that accumulation of the mutant DELLA protein caused the plants to be dwarfed. Peng et al. proposed that in Rht-B1b and Rht-D1b, translation of the protein might re-start after the introduced stop codon to produce shorter proteins in which part of the DELLA domain has been lost. These proteins would then be resistant to GA-induced degradation. As yet, this proposal has not been confirmed experimentally and there are many uncertainties still to be resolved about the Rht genes. For example, two further dwarfing genes, Rht-B1c and Rht-D1c (formerly known as Rht3 and Rht10, respectively) produce a more severe dwarf phenotype than Rht-B1b and Rht-D1b. As their name indicates, Rht-B1c and Rht-D1c are allelic to Rht-B1b and Rht-D1b, respectively, but the mutations they contain and how these might result in more severe dwarfing have yet to be described.
As mentioned previously, an important consequence of the presence of the dwarfing genes is an increase in harvest index. This is due to the production of more grains, which, despite reduced grain size, results in a higher overall yield (Flintham et al., 1997). In the more severely dwarfed lines, such as Rht-B1c, higher grain numbers do not compensate for the smaller grains so that the yield is reduced. There is consequently a limit to the degree of dwarfing that can be applied.
The Dwarfing Genes of Rice
Only a single gene, semi-dwarf-1 (sd-1), is used widely for producing semi-dwarf rice, although there are many alleles of this gene. Unlike Rht wheat, sd-1 rice responds to applied GA, indicating that the plants are deficient in this hormone. The sd-1 gene was found by three different research groups to encode a mutant form of an enzyme involved in GA biosynthesis (reviewed in Hedden, 2003). The wild-type enzyme OsGA20ox2 is a GA 20-oxidase, which catalyzes three steps in the biosynthetic pathway (see Chapter 20). Rice plants lacking this enzyme are slightly reduced in height and there are no detrimental effects on grain yields. Gibberellin 20-oxidase, in common with the other enzymes that function late in the GA-biosynthetic pathway, is encoded by several different genes that show some tissue specificity in their expression. Therefore, these genes may be involved in the growth of different tissues, although in many cases two or more genes may share roles. This is because their expression patterns may overlap to some extent, but additionally, since GAs are mobile, the product of an enzyme active in one tissue can move to another tissue. In rice there are four GA 20-oxidase genes (Sakamoto et al., 2004) and, although OsGA20ox2 influences stem growth, one or more of the other genes is also involved in this process. Similarly, grain formation is normal in sd-1 mutants because other 20-oxidase genes are expressed in reproductive tissues (Sasaki et al., 2002).
The rice dwarfing gene arose as a spontaneous mutation in the Taiwanese indica strain woo-gen. The resulting strain dee-geo-woo-gen was used in breeding programs in the far-east to produce many of the high-yielding semi-dwarf cultivars grown today. Woo-gen and dee-geo-woo-gen are shown are in Figure 2. The sd-1 allele in dee-geo-woo-gen contains a 383-base-pair deletion, which introduces a stop codon so that a truncated, inactive enzyme would be produced. Remarkably, semi-dwarf lines produced independently by mutagenesis, such as the japonica varieties Calrose 76 and Reimei, were found also to contain mutations at the sd-1 locus. Thus, the ideal combination of short stature and high yields provided by sd-1 mutants has ensured that these alleles have been selected consistently despite the availability of numerous other dwarfing genes in rice, many of which affect GA biosynthesis.
|Fig. 2 Comparison of woo-gen (right) and dee-geo-woo-gen strains, the latter containing the sd1 mutation (courtesy of Drs Matsuoka and Ashikari). (Click image to enlarge.)|
Gibberellins and Agriculture
An overview of the GA signaling pathway, including biosynthesis and signal transduction, is shown in Figure 3, which indicates the blockages in this pathway present in the dwarf mutants discussed above. It is probably no coincidence that the mutations selected in wheat and rice for dwarfism both compromise this pathway since GAs control many of the processes, such as stem elongation and reproductive development, that are of relevance to agriculture. In the diploid species rice, the mutation causes loss of function of a GA-biosynthetic enzyme, whereas in wheat, which is hexaploid, loss-of-function mutations are unlikely to produce a phenotype since it would be necessary to obtain corresponding mutations in all three genomes. In the case of wheat, therefore, a gain-of-function mutation that interferes with GA signal transduction was selected. Although the dwarf mutants of wheat and rice have attracted most publicity because of the importance of these crops to global nutrition, selection for reduced height has been a common theme in agriculture. The selected lines invariably contain mutations in the GA signaling pathway. Another notable example is the le (length) mutation, which is present in most commercial dwarf varieties of garden pea. The le mutation, which reduces the height of the stem without affecting reproductive development, was famously used by Mendel in his classical studies on the nature of inheritance. The Le gene encodes a GA-biosynthetic enzyme, PsGA3ox1 (see Figure 3), which, in the le-1 allele, contains a single amino acid substitution that reduces enzyme activity (Lester et al., 1997; Martin et al., 1997).
|Figure 3 Simplified overview of GA-signaling pathway showing the sites of mutation in dwarf lines of wheat, rice and pea. (Click image to enlarge.)|
When suitable dwarf lines are not available it is common practice to control stem growth by treating with growth retardants, which inhibit enzymes in the GA-biosynthetic pathway (see Web Topic 20.1). In fact, Rht wheat is often treated with chlormequat (CCC) to obtain further dwarfing when necessary. However, the application of chemicals for such purposes, particularly to food crops may not be sustainable in the long term. An alternative strategy is to modify GA content by genetic engineering. This can be achieved by suppressing expression of biosynthetic genes or by increasing expression of genes that encode GA-deactivation enzymes, such as GA 2-oxidases. Limiting expression of the GA 2-oxidase (GA2ox) genes to specific tissues by the appropriate choice of promoters should ensure that there are no undesirable pleiotropic effects. The advantages of targeting expression in this way has been clearly illustrated for rice (Sakamoto et al., 2003). When a rice GA2ox gene was used with the actin promoter, which drives strong, constitutive expression, severely dwarfed plants were obtained and grain set was seriously affected. Sakamoto et al. (2003) coupled the 2-oxidase gene to the promoter of a GA-biosynthetic gene, OsGA3ox2, which is expressed only in vegetative tissues. In this way they ensured that GA deactivation occurred at the site of synthesis and only in the target tissue. The result was semi-dwarf plants with normal grain set, similar to those containing sd-1 dwarfing alleles. Such approaches provide an attractive option for introducing desirable traits into crop and ornamental species and could form the basis for future advances in agriculture.
Ellis, M. H., Rebetzke, G. J., Azanza, F., Richards, R. A., and Spielmeyer, W. (2005) Molecular mapping of gibberellin-responsive dwarfing genes in bread wheat. Theor. Appl. Genet. 111: 423–430.
Evans, L. T. (1998) Feeding the Ten Billion: Plant and Population Growth. Cambridge University Press, Cambridge, UK.
Flintham, J. E., Angus, W. J., and Gale, M. D. (1997) Heterosis, overdominance for grain yield, and alpha-amylase activity in F-1 hybrids between near-isogenic Rht dwarf and tall wheats. J. Agric. Sci. 129: 371–378.
Gale, M. D., and Youssefian, S. (1985) Dwarfing genes in wheat. In Progress in Plant Breeding, Russell G. E. (ed.), Butterworths, London, pp. 1–35.
Hedden, P. (2003) The genes of the Green Revolution. Trends Genet. 19: 5–9.
Lester, D. R., Ross, J. J., Davies, P. J., and Reid, J. B. (1997) Mendel′s stem length gene (Le) encodes a gibberellin 3β-hydroxylase. Plant Cell 9: 1435–1443.
Martin, D. N., Proebsting, W. M., and Hedden, P. (1997) Mendel′s dwarfing gene: cDNAs from the Le alleles and function of the expressed proteins. Proc. Natl. Acad. Sci. USA 94: 8907–8911.
Peng, J. R., Richards, D. E., Hartley, N. M., Murphy, G. P., Devos, K. M., Flintham, J. E., Beales, J., Fish, L. J., Worland, A. J., Pelica, F., Sudhakar, D., Christou, P., Snape, J. W., Gale, M. D., and Harberd, N. P. (1999) "Green revolution" genes encode mutant gibberellin response modulators. Nature 400: 256–261.
Sakamoto, T., Morinaka, Y., Ishiyama, K., Kobayashi, M., Itoh, H., Kayano, T., Iwahori, S., Matsuoka, M., and Tanaka, H. (2003) Genetic manipulation of gibberellin metabolism in transgenic rice. Nature Biotech. 21: 909–913.
Sakamoto, T., Miura, K., Itoh, H., Tatsumi, T., Ueguchi-Tanaka, M., Ishiyama, K., Kobayashi, M., Agrawal, G. K., Takeda, S., Abe, K., Miyao, A., Hirochika, H., Kitano, H., Ashikari, M., and Matsuoka, M. (2004) An overview of gibberellin metabolism enzyme genes and their related mutants in rice. Plant Physiol. 134: 1642–1653.
Sasaki, A., Ashikari, M., Ueguchi-Tanaka, M., Itoh, H., Nishimura, A., Swapan, D., Ishiyama, K., Saito, T., Kobayashi, M., Khush, G. S., Kitano, H., and Matsuoka, M. (2002) Green revolution: A mutant gibberellin-synthesis gene in rice—New insight into the rice variant that helped to avert famine over thirty years ago. Nature 416: 701–702.
Thomas, S. G., and Hedden, P. (2006) Gibberellin metabolism and signal transduction. In Plant Hormone Signaling. Hedden, P. and Thomas, S. G. (eds), Blackwell Publishing Ltd., Oxford, UK, pp. 147–184.
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