New Dominant Dwarfing Gene (Dwf2) in Barley

D.E. Falk
Crop Science Dept, O A C
University of Guelph
Guelph, Ontario, Canada N1G 2W1

A single dwarf tiller was observed on a spontaneously doubled haploid plant of H930-36 regenerated from anther culture. This tiller was about 50% of the height of the rest of the tillers and had very short, dark green leaves. The rachis internodes were shortened and the seed was smaller than that on the other tillers. Seed harvested from this tiller was planted out and produced eight dwarf plants and three normal height plants. The dwarfs resembled the original tiller and were quite slow growing and very late heading; they tillered profusely. The non-dwarf plants resembled the parent variety in all respects. A progeny test of the non-dwarf plants revealed that they were all homozygous for normal height. Gibberellic acid (GA3) was applied to the dwarf plants with no discernable results; they remained very dark green and the leaves and internodes showed no differences in elongation when compared to untreated plants.

The parent line, H930-36 was a doubled haploid produced via the Bulbosum method from a cross of Klages/Mata. The line H930-36 was being used for anther culture because of its good response in producing a high frequency of embryos directly when cultured; these embryos predominantly gave rise to green plantlets. This line was very stable and produced very few off-types in anther culture. It was concluded that the dwarf plants were the result of a single, spontaneous, somaclonal mutation and that the new mutant allele was dominant in conditioning the dwarf phenotype.

Genetic studies with this mutant, designated Dwf2 (see gene description this issue) have been difficult because of the slow growth and small stature of the plants. They tend to be very prostrate in habit and the heads do not exsert well from the sheath. Growing out progeny from a selfed dwarf has given 25 dwarf plants and 14 non-dwarfs. This is a satisfactory fit to a 3:1 ratio expected for a single locus with two alleles, the mutant allele giving the dominant dwarf phenotype. The dwarf plants have all been from very slow germinating seeds; some of them seem to die at a very early stage and so the dwarf plants may not all establish successfully. Several crosses were successfully made using dwarf plants as males onto normal height male sterile plants (msg6+sex1). All F1 progenies segregated for the dwarf phenotype, indicating that the dwarfs used in crossing had been heterozygous. There were a total of 20 dwarf and 25 non-dwarf F1 plants. The F1 dwarf plants were all taller than the dwarfs in the H930-36 background. They were also much earlier in heading and had more upright growth habit. Gibberellic acid was applied to these known heterozygotes with no effect. It is hoped that the dwarfs will be more vigourous and easier to manage in the new backgrounds.

Several mature tillers of dwarfs and non-dwarfs in the H930-36 background and the heterozygotes were dissected to determine internode length and total height (Figure 1.). These were also compared to dwarf plants homozygous for dwf1 (Falk and Kasha, 1982) with and without exogenously applied 100 ppm GA3 and plants heterozygous for dwf1. The untreated dwf1 plants had no discernable internode elongation while GA3-treated plants were nearly normal in total height and in the length of the upper three internodes (see Figure 1). Although the normal (non-dwarf) H930-36 is much shorter than the Perth background of the dwf1 mutant, the upper two internodes contributed virtually the same proportion of the total height in both backgrounds (57.6% and 58%, respectively). The Dwf2 plants, both in the H930-36 background and the F1's, had approximately 80% of their total height in the upper two internodes. All of the internodes of the Dwf2 plants were shortened relative to the parental line, however the lower internodes were shortened to a greater degree than the upper two. Whether the greater height of the known heterozygotes (F1's) is due to heterozygousity per se (eg Dwf2dwf2 may show partial dominance) or if it is due to the female parent being much taller than H930-36 is unknown at this time. Comparisons in the segregating generations of the crosses should reveal the true nature of heterozygotes and homozygous dwarfs (eg Dwf2dwf2 vs Dwf2Dwf2). The difficulty of working with the dwarfs in the original background has precluded such studies to date. A few F2 progeny grown from an early ripening head of one F1 plant indicates that the dwarf is probably independent of the n locus on chromosome 1 and the sex1 locus on chromosome 6. Further information should soon be available from other segregating progenies.

The Dwf2 dwarf in barley has several characteristics in common with the Rht3 dwarf in wheat (Zeven, 1969). They are both very short with dominant to semi-dominant gene expression, both have short, dark green leaves and neither responds to exogenously applied GA3. The Dwf2 is very different from the dwf1 dwarf and from the semi dwarfs that have been reported in barley. This mutant should be of interest to researchers working on plant growth hormones and general physiology. It may be particularly useful to pathologists working on obligate pathogens such as leaf rust and powdery mildew as a host plant for maintaining cultures under chimneys for much longer periods between subcultures. The dwf1 has proven to be useful for this, but is more difficult to manage as the dwarf plants do not normally flower and produce seed. Dwf2 should make an interesting morphological marker gene for traditional genetic studies. It is not likely to be of value in commercial cultivars as it is too extreme in dwarfness. Field trials of lines with this gene will be conducted as soon as appropriate materials are available. Limited amounts of seed for research purposes should be available from the author in the near future.

References

Zeven, AC. 1969. Tom Pouce Blanc and Tom Pouce Barbu Rouge, two Triticum aestivum sources of very short straw. Wheat Inform. Serv. 29:8-9.

Falk, DE and KJ Kasha. 1982. Vegetative dwarf. BGN:12:106.


Figure 1. Node heights of normal and dwarf barleys. Dwarf1(dwf1) is DS98 background and Dwarf2(Dwf2) is H930-36 background.