Reviewer #1 (Public Review):
FLC is a gene involved in cold-dependent induction of flowering, as prolonged cold exposure leads to a progressive decrease in the level of this floral repressor as a result of a digital switch from an ON to an OFF state occurring asynchronously in cell populations. In this work, the authors analyze the contribution of analog and digital regulation to FLC expression in the absence of cold exposure. To do so, they use a genetic trick to be able to perform this analysis in the wild-type Ler ecotype where the molecular tools are available to do such an analysis. In Ler, an activator of FLC is missing due to a natural mutation and FLC expression is repressed during vegetative development by a pathway called the autonomous repressive pathway, allowing for a rapid transition to flowering. The authors used two mutant allele in one component of the autonomous pathway, the FCA gene. In the strongest allele, FLC is highly expressed and the plant are late flowering while in the weaker allele FLC shows a weaker expression and the plant requires an intermediate time between Ler and the strong fca allele to flower.
The authors demonstrate that the expression levels of the FLC gene vary quantitatively in the three genetic background they use (Ler and two fca alleles), and that mutating FCA leads to an analog increase in FLC expression. The quantifications performed by the authors indicate that increased level of FLC correlate with a decrease in the proportion of cells that can switch OFF FLC, with the strong fca allele showing a negligible amount of cells that can switch OFF FLC. The authors further measure the half-life of FLC mRNA and FLC protein, and show that FLC expression switch from ON to OFF is a one-way-switching. They used these data to build a computational model of the regulation of FLC expression and show that the model can reproduce the dynamics of FLC protein level at the cell population level in a time-course with measurement at 7, 15 and 21 days after sowing. Taken together their work suggest that, at least in the weak fca mutant, a combination of analog and digital regulation of transcription explains the population-wide dynamics of FLC expression. The authors propose that this regulation could be explained by high level of transcription of FLC preventing the digital switch, as a result of the short half-lives of FLC mRNA and FLC protein.
The finding of this work are potentially of wide interest to understanding transcriptional regulation by providing a functional link between the digital and analog mode of regulation of gene expression. However, the evidence of a link between expression levels resulting from analog regulation and the digital regulation are only partly supported by correlations from cell population-wide analysis of FLC expression. The authors did not provide experiments to more directly test that higher level of transcription could indeed prevent the OFF switch of FLC. It is likely but not shown that the ON to OFF switch of FLC is regulated similarly in the absence of cold exposure (this study) and upon cold exposure. Also, in their model, the authors used the assumption that FLC switches off at division but they do not test this important assumption. Finally it is unclear whether this combination of analog and digital regulation is relevant to FLC regulation in wild-type plants or is only relevant to the laboratory-induced mutants studied in this work.