The responses of plants to their environment are often dependent on the spatiotemporal dynamics of transcriptional regulation. While live-imaging tools have been used extensively to quantitatively capture rapid transcriptional dynamics in living animal cells, the lack of implementation of these technologies in plants has limited concomitant quantitative studies in this kingdom. Here, we applied the PP7 and MS2 RNA-labelling technologies for the quantitative imaging of RNA polymerase II activity dynamics in single cells of living plants as they respond to experimental treatments. Using this technology, we counted nascent RNA transcripts in real time in Nicotiana benthamiana (tobacco) and Arabidopsis thaliana. Examination of heat shock reporters revealed that plant tissues respond to external signals by modulating the proportion of cells that switch from an undetectable basal state to a high-transcription state, instead of modulating the rate of transcription across all cells in a graded fashion. This switch-like behaviour, combined with cell-to-cell variability in transcription rate, results in mRNA production variability spanning three orders of magnitude. We determined that cellular heterogeneity stems mainly from stochasticity intrinsic to individual alleles instead of variability in cellular composition. Together, our results demonstrate that it is now possible to quantitatively study the dynamics of transcriptional programs in single cells of living plants.

植物对其环境的反应通常取决于转录调控的时空动态。虽然实时成像工具已被广泛用于定量捕获活体动物细胞中的快速转录动态，但这些技术在植物中缺乏实施限制了该领域的同步定量研究。在这里，我们应用 PP7 和 MS2 RNA 标记技术对活植物单细胞中 RNA 聚合酶 II 活性动态进行定量成像，因为它们对实验处理有反应。利用这项技术，我们对烟草和拟南芥中的新生 RNA 转录本进行了实时计数。对热激记者的检查表明，植物组织通过调节从不可检测的基础状态转变为高转录状态的细胞比例来响应外部信号，而不是以分级方式调节所有细胞的转录速率。这种类似开关的行为，与细胞间转录率的变异性相结合，导致 mRNA 产生的变异性跨越三个数量级。我们确定细胞异质性主要源于个体等位基因固有的随机性，而不是细胞组成的变异性。总之，我们的结果表明，现在可以定量研究活植物单细胞中转录程序的动态。