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

花序是小麦（Triticum sp.）植株最突出的部分。因此，自农业出现以来，花序结构一直处于不断的选择之中，并且由于其对潜在粮食产量的影响，在现代育种中经久不衰。野生小麦和驯化小麦等古代小麦表现出长矛形穗，而现在的小麦品种由于轴（穗轴）节间短而表现出短而紧凑的穗表型。以往的研究表明，小麦穗形向致密化的进化主要是游离脱粒和不脆性等关键驯化性状的间接选择所致。然而，轴生长本身对籽粒产量组成部分发育的影响尚不清楚。轴本质上是生殖发育的骨干；它不仅携带所有的花的结构，所有的供应（即水，营养，同化物）的花的生长和发育也必须通过它。因此，它的解剖特征和图案至少在一定程度上决定了粒数和粒重等基本农艺性状。在此，我们建议使用硬粒小麦分化过程中选择的历史位点（约7000-7500年前）的自然变异来研究轴生长变异的发育起源，以及这反过来如何影响粮食产量潜力。利用正向遗传方法，我们在小麦4A染色体长臂上定位了这个调节穗长约3.5 Mbp的轴特异位点。表型分析表明，与该位点的野生emmer等位基因相关的穗状花序产生更长的轴节间，但同样更宽的轴节。有趣的是，我们发现轴节间长度与每个小穗发育的粒数呈正相关。我们假设整个轴的生长包括轴维管束数量或直径的增加，之前的研究表明，维管束数量或直径的增加可以通过增加花同化物的供应和籽粒的形成来促进小花的育性。这个想法似乎非常令人兴奋，并可能揭示野生二聚体丰富的等位基因库，形成小麦花序结构。因此，在本研究项目中，我们将重点研究小麦轴生长、同化物质分配和小花发育之间的发育遗传学联系，以发现新的粒数决定机制。

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.