Wheat Inflorescence Architecture

Abstract

The development of the wheat inflorescence, or spike, determines the number, size and shape of grain produced. Altering spike development has the potential to increase grain yield to support increasing global demands. Despite the importance of this specialized structure, little is known about the genes which underlie its development and thus contribute to grain output. Yield potential in wheat can be modified by numerous developmental outcomes. The number of vegetative branches, known as tillers determines the number of spikes per plant. Similarly, the number of nodes within the spike determines the number of grain bearing structures called spikelets. Increasing the number of spikelets per node in the form of branching further increases yield potential. Finally, the fertility of florets, which are the grain bearing units found within spikelets determine the number of grain set. Optimising these inflorescence structures is key for maximising yield potential in wheat. The important floral regulators, PPD-1 and FT positively regulate floral transition and influence the rate of floral development. Increased expression of PPD-1 leads to increases in FT expression and promotion of floral development. We have shown that loss-of-function mutations in both PPD-1 and FT contribute to a reduction in expression of floral meristem identity genes, a delay in flowering, an increase in nodes within the inflorescence and the production of a unique form of inflorescence branch called a paired spikelet. The important maize domestication gene TB1 is responsible for a reduction in vegetative branching and the single stem structure of modern maize. We show that increased copies of wheat TB1 reduce vegetative branches known as tillers as well as increase paired spikelet formation in wheat. We propose that TB1 interacts with FT in the inflorescence causing a similar reduction in meristem identity genes as we observed in FT and Ppd-1 mutants. The wheat domestication gene Q, an AP2 transcription factor, is one of only a handful of genes which is known to directly influence spike development in wheat. We have identified a novel gain-of-function mutation of Q which confers resistance to miR172, a microRNA that typically targets AP2 transcripts for degradation. We show that increased Q levels are associated with several phenotypes including, delayed flowering, the formation of ectopic florets in place of glumes, the formation of paired spikelets and a reduction in internode elongation throughout the plant. AP2 transcription factors are reported to delay flowering in other plant species. Increased expression of Q is therefore the likely cause of a delay in flowering time and the formation of paired spikelets not dissimilar to what we observed in loss-of-function mutations of FT and PPD-1. Taken together our results highlight a complex network of genes which regulate the number of grain producing units contributing to wheat yield. Many more genes regulating inflorescence development in wheat remain to be elucidated. With the recent release of the first wheat reference genome sequence, increased availability of reverse genetic resources and access to genome editing tools, it will be possible to identify novel gene function in wheat. Show more