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The Reduced Height Gene (Rht)

Margaret Boulton, JIC

The 'Green Revolution' that improved worldwide wheat yields from the 1960s was based upon a combination of new varieties and the increased use of fertilisers and pesticides. An important feature of the new varieties was reduced height: the semi-dwarf plants had increased yield, as less of the plants' energy was wasted on producing straw and more went into the harvested grain. The shorter plants were also stronger and more capable of bearing the increased yields without lodging (falling over) in wind and rain. We know now that these dwarf varieties of wheat carried a gene (called "Reduced Height" or Rht) that made them unresponsive to the plant growth hormone, gibberellin, which normally increases stem height

B x S plots   Recombinant doubled haploid lines of the Beaver x Soissons derived population showing large height variation due to joint segregation of dwarfing genes at both the Rht-B1 and Rht-D1 loci.

Changes in climate, agricultural practise and possible future restrictions in the use of fertilisers and growth-regulating chemicals mean that wheat varieties containing this gene may no longer be optimised for efficient production. Part of this project, a close collaboration between Rothamsted and the John Innes Centre, is thus aimed at identifying novel wheat genes that affect height and yield. Analysis of existing wheat cultivars suggests that these contain a range of alleles that affect height and might be useful if properly characterized. Our systematic studies should allow us to identify varieties, or mutants, with novel growth modifying characteristics. We hypothesize that at least some of these novel genes will, like Rht, be in the GA signalling pathway.

Mercia Rht3 tgn versus soil   Establishing conditions for assessing the affect of salt on shoot and root growth and plant survival in lines of wheat (Triticum aestivum var Mercia) near isogenic for reduced height (Rht) genes.

(Left) A soil-free Terragreen:sand growth medium has been identified that is suitable for wheat cultivation and allows better resolution of root morphology compared to soil-containing composts. Images were taken after 9 weeks growth in soil (top) or soil-free (lower) medium.

(Below) Assessment of survival of Mercia lines near-isogenic for Rht alleles. Alleles tested were wild type, Rht1, Rht2, Rht3, Rht8, Rht10, Rht12. Salt concentrations were 0 NaCl (back row) or 250 mM NaCl (front row). Plants were grown in a controlled environment.
Mercia Rht isolines + & -  salt

These genes could then be used to produce new dwarf varieties, either alone or in combination with the existing Rht alleles. This may also reduce or abolish the need for chemical growth retardants, widely used on cereals in the UK, which work by blocking the production of gibberellin.

We have also found that in the model plant, Arabidopsis, dwarfing genes orthologous to those used in wheat protect plants against environmental stresses. This factor may become even more important as climate change results in increasingly unpredictable weather patterns. A second aim of this project will therefore be to test a range of dwarfing genes for their effectiveness in protecting plants from stresses such as drought, salinity, heat and cold that can reduce yield and impact farmersí income in the UK and worldwide. Our studies will be wide-ranging, using mutant populations, Rht isolines and elite varieties and we will characterise the responses of the allelic variants in controlled environments and in the field. This will allow stresses to be introduced at defined developmental stages as well as under less predictable weather conditions. Finally, an important aim of this project is that the new dwarfing genes that we discover should be taken up by plant breeders to develop new commercial varieties. We will therefore ensure that any novel genes that confer improved growth properties or stress resistance will be transferred to modern varieties. This will allow them to be passed on to commercial wheat breeders in order to develop the next generation of bread wheats.

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