Floral success depends on its backbone—first insights into rachis growth and development and its implications on wheat floret fertility and grain number

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The inflorescence is the most prominent part of the wheat (Triticum sp.) plant. As such, inflorescence architecture has been under constant selection since the dawn of agriculture and is enduring in modern breeding due to its the impact on potential grain yield. While ancient wheat forms like wild and domesticated emmer display a long spear-shaped spike, current wheat varieties exhibit a short and compact spike phenotype due to short rachis (spike axis) internodes. Previous studies have indicated that the evolution of wheat spike shape towards compactness mainly results from indirect selection of key domestication traits like free-threshing and non-brittleness. However, the effect of rachis growth per se on the development of grain yield components remains elusive. The rachis is essentially the backbone for reproductive development; it not only carries all floral structures, all supply (i.e., water, nutrients, assimilates) for floral growth and development must also pass through it. Therefore, its anatomical features and patterning determine, at least to some degree, essential agronomical traits like grain number and grain weight. Here, we propose to use natural variation in a historical locus selected during the divergence of durum wheat (~7000-7500 years ago) to study the developmental origin of variation in rachis growth and how this, in turn, affects grain yield potential. Using a forward genetic approach, we have mapped this rachis-specific locus regulating spike length to ~3.5 Mbp on the long arm of wheat chromosome 4A. Phenotypic analysis showed that spikes, associated with the wild emmer allele at this locus, produce longer rachis internodes but similarly broader rachis nodes.Interestingly, we found a positive correlation between rachis internode length and the number of grains developed in each spikelet. We hypothesize that overall rachis growth includes increased rachis vascular bundle numbers or diameters, previously shown to promote floret fertility by increasing floral assimilate supply and grain set. This idea appears very exciting and may uncover the rich allelic repertoire of wild emmer for shaping wheat inflorescence architecture. Therefore, in the proposed research project, we will focus on the developmental genetics of this promising, unexplored link between rachis growth, assimilate allocation and floret development to discover novel grain number determination mechanisms in wheat.