Abiotic Stress and Agronomic Traits. Dwarfing Genes

References

1. The transfer and history of 'reduced height genes' (Rht) in wheat from Japan to Europe. Borojevic K, Borojevic K. In: Journal of Heredity, 2005, 96:455-459.

Wheat is the main crop and often a strategic crop in many European countries. From a historical perspective, we describe the transfer of "reduced height genes" (Rht genes) from Japanese wheat varieties to wheat varieties in Europe and their influence on the increase of the total wheat production in the last century. Historic pathways of Rht genes were influenced directly or indirectly by wheat breeders exchanging seed samples and by some governments importing large quantities of wheat during historically critical periods for their countries.

2. The genes of the Green Revolution. Hedden, P. In: Trends in Genetics, 2003, 19:5-9.

The spectacular increases in wheat and rice yields during the 'Green Revolution', were enabled by the introduction of dwarfing traits into the plants. Now, identification of the genes responsible for these traits shows that they interfere with the action or production of the gibberellin (GA) plant hormones. We knew that the wheat Rht genes encode growth repressors that are normally suppressed by GA, and recent work shows that the rice sd1 gene encodes a defective enzyme in the GA-biosynthetic pathway.

3. Distribution of microsatellite alleles linked to Rht8 dwarfing gene in wheat. Ahmad M, Sorrells ME. In: Euphytica, 2002, 123:235-240.

A wheat microsatellite locus, Xgwm 261, whose 192-bp allele closely linked to the dwarfing gene Rht8, on chromosome 2D, was used to screen 71 wheat cultivars from 13 countries to assess the variation at this locus. Screening of this wheat collection showed that a 165-bp allele and a 174-bp allele were the most frequent. None of the New Zealand cultivars possessed a 192-bp allele specific to Rht8, while only one cultivar from the US produced this important allele. The frequency of a 192-bp allele among these wheat cultivars was 5.63%. The highest allele frequency was observed for a 174-bp fragment (52.11%) followed by a 165-bp fragment (26.76%). The only durum wheat 'Cham 1', did not show any amplification due to the absence of D genome. Four new novel alleles, 180-bp, 198-bp, 200-bp and 204-bp present in the US and New Zealand wheat cultivars are reported.

4. Covariation for microsatellite marker alleles associated with Rht8 and coleoptile length in winter wheat. Bai G, Das MK, Carver BF, Xu X, Krenzer EG. In: Crop Science, 2004, 44:1187-1194.

Wheat (Triticum aestivum L.) cultivars with greater coleoptile elongation are preferred in low-precipitation dryland regions and in early-planted management systems of the Great Plains, but the presence of GA3 (gibberellin)-insensitive dwarfing genes tends to restrict coleoptile elongation. The agronomic value of Rht8 and the discovery of its diagnostic microsatellite marker, Xgwm 261, have accelerated breeders' interest in Rht8 as an alternative dwarfing gene. Our objectives were to determine allelic distributions at the marker locus in contemporary samples of hard winter and soft red winter wheat relative to samples of Chinese accessions from a Rht8-rich geographic region, and to compare coleoptile elongation in the presence or absence of Rht8 determined by the Xgwm 261 marker. The 165-bp (primarily hard winter wheats) and the 174-bp (primarily soft red winter wheats) alleles of Xgwm 261 were most frequent. About 8% of all U.S. accessions carried the 192-bp allele diagnostic for Rht8, compared with 64% of the Chinese accessions. Coleoptile length varied among accessions from 4.4 to 11.4 cm. Frequency distributions for 192- and non-192-bp genotypes showed no advantage of the 192-bp allele to coleoptile elongation. None of the 192-bp genotypes from the Great Plains showed greater coleoptile length than 'TAM 107', a hard red winter cultivar without Rht8 often chosen over contemporary cultivars for its greater emergence capacity with deeper seed placement. Since coleoptile elongation may be controlled by several quantitative trait loci, identifying only the presence of 192-bp allele of Xgwm 261 may be misleading if the primary motivation for its deployment is to increase coleoptile length in a semidwarf plant type.

5.Agronomic performance of Rht alleles in a spring wheat population across a range of moisture levels. Butler JD, Byrne PF, Mohammadi V, Chapman PL, Haley SD. In: Crop Science, 2005, 45:939-947.

Reduced height alleles at the Rht-B1 and Rht-D1 loci have been widely incorporated into wheat (Triticum aestivum L.) cultivars with the intent of improving partitioning of assimilates to grain. Although generally effective at increasing yield in high yield environments, their effects under heat and drought stress have been variable. We undertook this study to evaluate the effects of the Rht-B1b and Rht-D1b dwarfing alleles in a recombinant inbred line (RIL) spring wheat population under a range of soil moisture conditions. Rht-B1 and Rht-D1 genotypes of 140 RILs derived from a cross between 'Kauz' and MTRWA116 were determined by polymerase chain reactions (PCR). The population was evaluated for yield and agronomic traits in four Colorado environments under fully irrigated, partially irrigated, and rainfed conditions in 2001 and 2002. Lines with both dwarfing alleles were significantly (P < 0.01) shorter, lower yielding, and later heading in all environments compared with lines with one or no dwarfing allele. Lines with both tall alleles performed equal to or better (P < 0.05) than all other classes for grain yield, test weight, and kernel weight in all environments. Among lines with a single dwarfing allele, those with Rht-B1b on average outyielded those with Rht-D1b in the fully irrigated environment (5432 versus 4993 kg ha^-1, P < 0.05), but elsewhere their yields did not differ (P > 0.05). Desirable values for most traits occurred across a relatively wide range of plant heights, with the best performing lines either shorter lines in the tall class or taller lines in the semidwarf classes.

6. Identification and characterization of quantitative trait loci related to lodging resistance and associated traits in bread wheat. Verma V, Worland AJ, Sayers EJ, Fish L, Caligari PDS, Snape JW. In: Plant Breeding, 2005, 124:234-241.

Lodging is a major constraint to increasing yield in many crops, but is of particular importance in the small-grained cereals. This study investigated the genetic control of lodging and component traits in wheat through the detection of underlying quantitative trait loci (QTL). The analysis was based on the identification of genomic regions which affect various traits related to lodging resistance in a population of 96 doubled haploid lines of the cross 'Milan' x 'Catbird', mapped using 126 microsatellite markers. Although major genes related to plant height (Rht genes) were responsible for increasing lodging resistance in this cross, several other traits independent of plant height were shown to be important such as root and shoot traits, and various components of plant yield. Yield components such as grain number and weight were shown to be an indicator of plant susceptibility to lodging. QTL for lodging and associated traits were found on chromosomes 1B, 1D, 2B, 2D, 4B, 4D, 6D and 7D. QTL for yield and associated traits were identified on chromosomes 1B, 1D, 2A, 2B, 2D, 4D and 6A.

7. 'Perfect' markers for the Rht-B1b and Rht-D1b dwarfing genes in wheat. Ellis MH, Spielmeyer W, Gale KR, Rebetzke GJ, Richards RA. In: Theoretical and Applied Genetics, 2002, 105:1038-1042.

PCR-based markers were developed to detect the point mutations responsible for the two major semidwarfing genes Rht-B1b (Rht1) and Rht-D1b (Rht2) in wheat. These markers were validated by testing 19 wheat varieties of known Rht genotype. They included Rht-B1b and Rht-D1b dwarfs, double-mutant varieties and tall wheats. These were correctly genotyped with the Rht-B1b and Rht-D1b-specific primers, as well as markers specific for the tall alleles Rht-B1a and Rht-D1a. Using a family of doubled-haploid lines segregating for Rht-B1b and Rht-D1b, the markers were mapped to the expected homoeologous regions of chromosomes 4B and 4D, respectively. Both markers were strongly correlated with a reduction in height, accounting for 23% (Rht-B1b) and 44% (Rht-D1b) of the phenotypic variance in the population. These markers will have utility in marker-assisted selection of the Rht-B1b and Rht-D1b genes in wheat breeding programs.

8. Genetic analysis of the dwarfing gene (Rht8) in wheat. Part I. Molecular mapping of Rht8 on the short arm of chromosome 2D of bread wheat (Triticum aestivum L.). Korzun V, Roder MS, Ganal MW, Worland AJ, Law CN. In: Theoretical and Applied Genetics, 1998, 96:1104-1109.

Two sets of single chromosome recombinant lines comparing 2D chromosomes from the wheat varieties 'Ciano 67' and 'Mara' with the common 2D chromosome of 'Cappelle-Desprez' in a 'Cappelle- Desprez' background were used to detect a diagnostic wheat microsatellite marker for the dwarfing gene Rht8. The genetic linkage maps place the wheat microsatellite marker WMS 261 0.6 cM distal to Rht8 on the short arm of chromosome 2D. By PCR analysis the WMS 261 alleles of 'Mara', 'Cappelle-Desprez' and 'Ciano 67' could be distinguished by di¤erent fragment sizes of 192 bp, 174 bp and 165 bp, respectively. A screen of over 100 international varieties of wheat showed that the three allelic variants were all widespread. It also demonstrated that a limited number of varieties carried novel WMS 261 variants of over 200 bp. Following classification of the individual recombinant lines for allelic variants at the WMS 261 locus it was possible to attribute a 7- to 8-cm reduction in plant height with the WMS 261-192-bp allele compared to the WMS 261-174-bp allele in the set of recombinant lines comparing 2D chromosomes of 'Mara' and 'Cappelle-Desprez'. A height reduction of around 3 cm was detected between the WMS 261-174-bp allele and the WMS 261-165-bp allele in the recombinant lines comparing 2D chromosomes of 'Cappelle-Desprez' and 'Ciano 67'.