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.

The introduction of semi-dwarf wheat and rice varieties since the 1960s contributed to unprecedented yield gains by improving lodging tolerance and harvest index in high-input environments. In wheat, Rht-B1b and Rht-D1b (encoding DELLA proteins) provided the main genetic source for the short stature of modern varieties cultivated worldwide. Despite the yield advantages in optimal conditions, undesirable pleiotropic effects (e.g. poor seedling vigor) associated with these alleles can cause yield loss in adverse environments such as dry/hot climates. Therefore, searching alternative height genes and dwarfing alleles has been an important goal in wheat breeding programs. This dissertation explores natural and induced variation to identify new genetic resources for modulating plant height in wheat. In Chapter 1, I describe the identification of a quantitative trait locus (QHt.ucw-6AS) for plant height in a hexaploid wheat recombinant inbred line population, and its precise mapping as a Mendelian gene officially designated as Rht25. Rht25 confers a mild dwarfing effect (~50% of Rht-B1b and Rht-D1b), exhibits pleiotropic effects on coleoptile length, heading time, and spike development, and presents a potential dwarfing source for fine-tuning wheat plant height. In Chapter 2, I describe a mapping-by-exome-sequencing approach and use it to identify candidate causal mutations associated with increased height in a tetraploid wheat mutant population. The proposed strategy provides a cost-efficient and rapid mapping procedure to identify causative induced mutations associated with a target phenotype in species with large genomes. In Chapter 3, I describe the transcriptomic changes induced by a missense mutation (E529K) in the GRAS functional domain of the RHT-B1b protein causing increased height, and provide distinct sets of potential GA-response genes regulated directly or indirectly by DELLA in a highly tissue-specific manner.

The papers herein are volume 1 of the proceedings of the 11th International Wheat Genetics Symposium, held in Brisbane, Australia, in 2008. The series presents the science of the genetic sciences applied to bread and durum wheats and other species.