The agronomic and molecular characterisation of Rht8 in hexaploid wheat

Reduced height 8 (Rht8) is the main alternative to the GA-insensitive Rht alleles in hot and dry environments and reduces plant height without yield penalty. The potential of Rht8 in northern-European wheat breeding remains unclear. In the present study, near-isogenic lines contrasting for the Rht8/tall allele in the UK-adapted and photoperiod-sensitive variety Paragon were evaluated in trials with varying nitrogen fertiliser (N) treatments and water regimes across sites in the UK and Spain. Rht8 conferred a robust height reduction of 11% regardless of treatment and was more resistant to root-lodging at agronomically-relevant N levels. In the UK, the Rht8 NIL showed a 10% yield penalty due to concomitant reduction in grain number and spike number whereas grain weight and harvest index were not significantly different to the tall NIL. The yield penalty was abolished at low N and in irrigated conditions in the UK and Spain. This indicates the utility of Rht8 in reduced-input agriculture. Decreased spike length and constant spikelet number in Rht8 compacted spikes by 15% independent of environment. The genetic interval of Rht8 overlaps with the most recent mapping of the compactum gene on 2DS (Johnson et al., 2008) and future work with the markers found in this study is required to genetically dissect these loci. Rht8 had been previously fine-mapped to a 1.29 cM interval (Gasperini et al., 2012). Rht8 was further fine-mapped using an RNA-Seq enabled bulked segregant analysis method, as well as utilising SNP-platforms and emerging Triticeae genomic resources to identify molecular markers. Rht8 was reduced to a 1.015 cM genetic interval and syntenic intervals of 1.34 Mb on rice chromosome 4, 1.36 Mb on Brachypodium chromosome 5, 2.9 Mb on barley 2H and 4.25 Mb on Ae. tauschii 2D. Disruption to micro-collinearity was found with Brachypodium and rice, with better but imperfect collinearity with Ae. tauschii and barley. Rht8 was also anchored to a single IWGSC-2 POPSEQ bin and to a 2.3 cM region in the whole genome shotgun-ordered wheat scaffolds