Transcriptome dynamics at Arabidopsis graft junctions reveal an intertissue recognition mechanism that activates vascular regeneration

The ability for cut tissues to join together and form a chimeric organism is a remarkable property of many plants, however, grafting is poorly characterized at the molecular level. To better understand this process we monitored genome-wide temporal and spatial gene expression changes in grafted Arabidopsis thaliana hypocotyls. Tissues above and below the graft rapidly developed an asymmetry such that many genes were more highly expressed on one side than the other. This asymmetry correlated with sugar responsive genes and we observed an accumulation of starch above the graft that decreased along with asymmetry once the sugar-transporting vascular tissues reconnected. Despite the initial starvation response below the graft, many genes associated with vascular formation were rapidly activated in grafted tissues but not in cut and separated tissues indicating that a recognition mechanism activated that was independent of functional vascular connections. Auxin which is transported cell-to-cell, had a rapidly elevated response that was symmetric, suggesting that auxin was perceived by the root within hours of tissue attachment to activate the vascular regeneration process. A subset of genes were expressed only in grafted tissues, indicating that wound healing proceeded via different mechanisms depending on the presence or absence of adjoining tissues. Such a recognition process could have broader relevance for tissue regeneration, inter-tissue communication and tissue fusion events.

许多植物都具备一项极为独特的特性：离体组织可相互接合并形成嵌合生物体（chimeric organism），但目前学界对植物嫁接过程的分子层面机制仍缺乏充分解析。为深入解析这一过程，本研究对嫁接拟南芥（Arabidopsis thaliana）下胚轴（hypocotyl）的全基因组时空基因表达变化进行了监测。研究发现，嫁接位点上下的组织会快速形成不对称性，即大量基因在单侧的表达水平显著高于另一侧。该不对称性与糖响应基因的表达模式显著相关；研究人员观察到，嫁接位点上方会出现淀粉积累，一旦负责运输糖分的维管组织重新连通，淀粉积累量便会随不对称性的减弱而逐步降低。尽管嫁接位点下方最初会出现饥饿响应，但嫁接组织中大量与维管形成相关的基因被快速激活，而离体且未接合的切割组织则无此变化，这表明存在一种不依赖功能性维管连接的识别机制被激活。细胞间运输的生长素（auxin）其响应信号会快速升高且呈现对称分布，这提示组织接合后的数小时内，根系即可感知生长素，从而启动维管再生过程。存在一类仅在嫁接组织中特异性表达的基因子集，这表明伤口愈合的具体机制会因相邻组织是否存在而有所差异。这类识别机制或许在组织再生、组织间通讯以及组织融合等生物学过程中具有更广泛的研究价值与相关性。