Sequence‐based mapping identifies a candidate transcription repressor underlying awn suppression at the B1 locus in wheat

Abstract

Summary Awns are stiff, hair‐like structures which grow from the lemmas of wheat (Triticum aestivum) and other grasses that contribute to photosynthesis and play a role in seed dispersal. Variation in awn length in domesticated wheat is controlled primarily by three major genes, most commonly the dominant awn suppressor Tipped1 (B1). This study identifies a transcription repressor responsible for awn inhibition at the B1 locus. Association mapping was combined with analysis in biparental populations to delimit B1 to a distal region of 5AL colocalized with QTL for number of spikelets per spike, kernel weight, kernel length, and test weight. Fine‐mapping located B1 to a region containing only two predicted genes, including C2H2 zinc finger transcriptional repressor TraesCS5A02G542800 upregulated in developing spikes of awnless individuals. Deletions encompassing this candidate gene were present in awned mutants of an awnless wheat. Sequence polymorphisms in the B1 coding region were not observed in diverse wheat germplasm whereas a nearby polymorphism was highly predictive of awn suppression. Transcriptional repression by B1 is the major determinant of awn suppression in global wheat germplasm. It is associated with increased number of spikelets per spike and decreased kernel size.

Dominant inhibition of awn development by a putative zinc‐finger transcriptional repressor expressed at the B1 locus in wheat

“…As demonstrated by the companion paper in this issue, DeWitt et al . (), and consistent with the previous studies in rice, barley and wheat discussed in the Introduction above, awn presence in STxGH promoted increases in grain size and in particular grain length (Fig. d,e).…”

“…Our identification of B1 parallels a companion paper in this issue by DeWitt et al . (), but whereas the present paper provides evidence for the repressor functionality of B1, DeWitt et al . () detail the intricate relationship of B1 and awn inhibition to grain length and spikelets per spike.…”

“…Consistent with the companion paper by DeWitt et al . (), TraesCS4D02G476700LC shared the highest homology to B1 in wheat. However, TRIUR3_34498‐P1 from T. urartu was identical in sequence to B1.…”

“…Taken together with the BSR‐seq fine‐mapping of B1 in durum wheat, it is proposed that TraesCS5A02G542800 , annotated as a C2H2 zinc finger encoding gene, is the B1 awn inhibitor gene – a statement also consistent with the companion paper by DeWitt et al . ().…”

“…However, the companion paper by DeWitt et al . () suggests that no polymorphism within this region was diagnostic and that, rather, a 30‐nt deletion 4 kb downstream of the gene was the most (but not entirely) diagnostic for awn inhibition. Thus, the functional mechanism for B1 expression and subsequent awn inhibition remains enigmatic and further experiments are required.…”