World wheat grain yields increased substantially in the 1960s and 1970s because farmers rapidly adopted the new varieties and cultivation methods of the so-called 'green revolution'. The new varieties are shorter, increase grain yield at the expense of straw biomass, and are more resistant to damage by wind and rain. These wheats are short because they respond abnormally to the plant growth hormone gibberellin. This reduced response to gibberellin is conferred by mutant dwarfing alleles at one of two Reduced height-1 (Rht-B1 and Rht-D1) loci. Here we show that Rht-B1/Rht-D1 and maize dwarf-8 (d8) are orthologues of the Arabidopsis Gibberellin Insensitive (GAI) gene. These genes encode proteins that resemble nuclear transcription factors and contain an SH2-like domain, indicating that phosphotyrosine may participate in gibberellin signalling. Six different orthologous dwarfing mutant alleles encode proteins that are altered in a conserved amino-terminal gibberellin signalling domain. Transgenic rice plants containing a mutant GAI allele give reduced responses to gibberellin and are dwarfed, indicating that mutant GAI orthologues could be used to increase yield in a wide range of crop species.

Genetic mapping of wheat, maize, and rice and other grass species with common DNA probes has revealed remarkable conservation of gene content and gene order over the 60 million years of radiation of Poaceae. The linear organization of genes in some nine different genomes differing in basic chromosome number from 5 to 12 and nuclear DNA amount from 400 to 6,000 Mb, can be described in terms of only 25 ''rice linkage blocks.'' The extent to which this intergenomic colinearity is confounded at the micro level by gene duplication and micro-rearrangements is still an open question. Nevertheless, it is clear that the elucidation of the organization of the economically important grasses with larger genomes, such as maize (2n ‫؍‬ 10, 4,500 Mb DNA), will, to a greater or lesser extent, be predicted from sequence analysis of smaller genomes such as rice, with only 400 Mb, which in turn may be greatly aided by knowledge of the entire sequence of Arabidopsis, which may be available as soon as the turn of the century. Comparative genetics will provide the key to unlock the genomic secrets of crop plants with bigger genomes than Homo sapiens.In the mid 1980s when restriction fragment length polymorphism (RFLP) technology was first applied to plants, the objectives of the early experiments-in tomato, Lycopersicon esculentum, by Steve Tanksley in New Mexico, in maize, Zea mays, by Tim Helentjaris in Utah, and ourselves in bread wheat, Triticum aestivum, in Cambridge-were no more ambitious than to produce a new generation of markers for use by breeders. In the race to build the first dense genetic maps, the early reports of synteny across genomes in 1988-between tomato and potato (1) and between the three diploid genomes of hexaploid wheat (2)-were interesting but not remarkable. Later the cross-genome comparisons became more compelling. These comparisons all employed hybridization-based mapping procedures, which, with variable stringency conditions, allowed the detection of similar but imperfectly matched DNA sequences. Large numbers of characterized DNA probes were still not available, and researchers of the day, therefore, used RFLP probes available in one species to create genetic maps in related genomes. Examples are the use of maize probes to map sorghum, Sorghum bicolor (3), and wheat probes to map rye, Secale cereale (4), and both studies revealed yet more colinearity. Nevertheless, it was not clear at the time that intergenomic synteny extends only to the genes themselves. Since the early 1990s alternative marker systems based on PCR have complemented RFLPs. However, cross-genome signals are only infrequently observed by using sequence-tagged sites or microsatellite primers because the sequences must match the template DNA precisely. In fact, had PCR been discovered 5 years earlier we may still have been ignorant about the conservation of gene order among plant species. A Consensus Grass MapSoon, wider comparisons between Tribes were reported. Ahn and Tanksley (5) showed the rice, Oryza sativa, and maize genomes...

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