Abstract：
Plant architecture is shaped by the production of new organs, most of which emerge postembryonically. This process includes the formation of new lateral branches along existing shoots. Current evidence supports a detached-meristem model as the cellular basis of lateral shoot initiation. In this model, a small number of undifferentiated cells are sampled from the periphery of the shoot apical meristem (SAM) to act as precursors for axillary buds, which eventually develop into new shoots. Repeated branching thus creates cellular bottlenecks (i.e. somatic drift) that affect how de novo (epi)genetic mutations propagate through the plant body during development. Somatic drift could be particularly relevant for stochastic DNA methylation gains and losses (i.e. spontaneous epimutations), as they have been shown to arise rapidly with each cell division. Here, we formalize a special case of the detached-meristem model, where precursor cells are randomly sampled from the SAM periphery in a way that maximizes cell lineage independence. We show that somatic drift during repeated branching gives rise to a mixture of cellular phylogenies within the SAM over time. This process is dependent on the number of branch points, the strength of drift as well as the epimutation rate. Our model predicts that cell-to-cell DNA methylation heterogeneity in the SAM converges to nonzero states during development, suggesting that epigenetic variation is an inherent property of the SAM cell population. Our insights have direct implications for empirical studies of somatic (epi)genomic diversity in long-lived perennial and clonal species using bulk or single-cell sequencing approaches.
摘要：
植物结构是由新器官的产生而形成的，其中大多数是后胚胎出现的。该过程包括沿着现有枝条形成新的侧枝。当前的证据支持分离的分生组织模型作为侧枝起始的细胞基础。在这个模型中,从茎尖分生组织（SAM）的外围取样少量未分化细胞，作为腋芽的前体，最终发展成新的芽。因此,重复分枝产生细胞瓶颈(即体细胞漂移),其影响从头(epi)基因突变如何在发育期间通过植物体传播。体细胞漂移可能是特别相关的随机DNA甲基化的增益和损失（即自发的表象），因为它们已经被证明会随着每次细胞分裂而迅速产生。这里,我们形式化了分离分生组织模型的特例，其中前体细胞以最大化细胞谱系独立性的方式从SAM外围随机取样。我们表明，随着时间的推移，重复分支过程中的体细胞漂移会导致SAM内细胞系统发育的混合。此过程取决于分支点的数量，漂移的强度以及表象率。我们的模型预测，SAM中的细胞间DNA甲基化异质性在发育过程中收敛到非零状态，这表明表观遗传变异是SAM细胞群体的固有特性。我们的见解对于使用批量或单细胞测序方法对长寿命多年生和克隆物种中的体细胞（epi）基因组多样性进行实证研究具有直接意义。
