The ‘Miracle Wheat’ Homoeo-Allele, bht-B1, Affects Spike-Branching in Tetraploid Wheat

The wheat genome carries related genomes derived from interspecific hybridizations events of progenitors establishing a polyploid genome structure. Polyploidy in wheat is therefore associated with several gene variants originating from homoeologous chromosomes. These so called homoeo-alleles can be silenced or active thereby contributing to gene dosage effects. We previously identified Branched Head (TtBH-A1) on chromosome 2AS as the tetraploid wheat ortholog of ZmBd1, OsFZP, BdMOS1 and HvCOM2, for being the major gene controlling spike-branching. TtBH-A1, which encodes an AP2/ERF Transcription Factor, is involved in suppressing branch and supernumerary spikelet (SS) formation in wheat. By studying 145 Recombinant Inbred Lines (RILs) developed from a cross between a durum wheat variety Bellaroi (standard spike) and TRI 19165 (Branched spike or ‘Miracle Wheat’), we found that the 2B copy of TtBH (i.e. TtBH-B1) from Bellaroi is involved in functionally compensating the loss-of-function of the mutant allele at bh^t-A1 leading to a reduced or in some cases even complete abolition of SS formation. Unlike in hexaploid wheat, where WFZP-B1 is reported to be lowly expressed, TtBH-B1 in tetraploid wheat is therefore involved in the same functional role as its functional homoeo-allele, TtBH-A1. On the contrary, recombinants carrying both homoeo-alleles (bh^t-A1 and bh^t-B1) from ‘Miracle Wheat’ always tend to have strong branching and SS formation, indicating that both of these putative mutant homoeo-alleles are less or non-functional. Since loss-of-function in TtBH/WFZP lead to fertile supernumerary spikelets, TtBH/WFZP can be further manipulated for increasing grain yield in wheat.