Rht-B1b and Rht-D1b (formerly Rht1 and Rht2) dwarfing genes have been used extensively since the Green Revolution to reduce height and increase yield in hexaploid wheat (Triticum aestivum L.). They have been used in the development of hundreds of modern cultivars. Semi-dwarf wheat varieties containing these GA-insensitive dwarfing genes generally yield more grain than their tall Rht-B1a and Rht-D1a (formerly rht1 and rht2) counterparts. However, Rht-B1b and Rht-D1b are better adapted to high-input environments and can reduce plant performance under adverse conditions such as heat or drought stress. In such environments, Rht8, a GA-sensitive dwarfing gene, could prove beneficial. This study aimed to assess agronomic performance of Rht8 compared to Rht-B1b and Rht-D1b using near-isogenic lines of the six possible genotypes (Rht-B1b, Rht-B1a, Rht-D1b, Rht-D1a, Rht8, and rht8) in four different spring wheat backgrounds. Field trials were conducted during the 2008 and 2009 growing seasons in Bozeman and Kalispell, MT. Traits measured included plant height, coleoptile length, stem solidness, days to heading, yield, flag leaf characteristics, harvest index, and others. Measurements were taken from field trials except coleoptile length, which was grown in a temperature and light controlled growth chamber. Rht genotype had a significant effect on plant height, coleoptile length, and yield. Height reductions of 20.5%, 22.4%, and 10.1% were observed for Rht-B1b, Rht-D1b, and Rht8, respectively. Rht8 showed no significant effect on coleoptile length, while Rht-B1b and Rht-D1b exhibited reductions in coleoptile length of 21.3% and 22.8%. Rht-B1b and Rht-D1b both showed a 6.1% and 14.1%. increase in yield potential. In contrast, Rht8 showed a yield decrease of 5.3%. Of most interest to plant breeders are those traits that effect yield. Yield is a complex, multi-genic trait that is affected by many plant characteristics and environmental factors. Because of the quantitative nature of yield, it is impacted by not only the presence of dwarfing genes, but also genetic background and environmental variables. This study suggests that Rht-B1b and Rht-D1b are superior, in terms of yield potential, to Rht8. However, additional evaluation of Rht8 in differing environments is needed to elucidate its precise effects on yield and other agronomic traits.

Finding alternate dwarfing gene system by modulating auxin distribution, a 'true' wheat ortholog (TaABCB19) of the Arabidopsis ABCB19 has shown to be involved in regulating plant height via modulating auxin transport in the stem. True ortholog of AtPGP19 has been identified in various monocots and dicots species. The smallest genomic copy of PGP19 is present in barley and quite largely in Soybean, the variation in gene size ranged from7496 bp to 6628 bp. The gene structure showed similarities between monocots and dicots, with highly conservative exonic region 1 and 8. The TaABCB19 RNAi plant resulted in reduced height up to 48%, and more than doubled spikelet number. This studies furthers the scientific understanding of the genetic relationship of plant height with other growth related traits for selecting and combining desirable traits for wheat improvement.