Correlations between surface area and volume in cell size and growth in Arabidopsis thaliana

Analyzing morphological parameters and growth at the cell level is crucial for a better understanding of organ development. The most popular approach for quantitative analyses of development relies on confocal imaging of an organ expressing a fluorescent membrane marker over several time points, and analyzing the confocal dataset to quantify changes in morphological parameters and growth rate. These analyses are commonly done on the surface, with the assumption that changes in the surface of a cell reflect faithfully changes of the whole, volumetric cell. However, this assumption has not yet been systematically and explicitly tested. It is also not clear how the correlation between areal and volumetric measurements would change over time. Here, we combined time-series live imaging and three-dimensional reconstruction to compare surface and volumetric size and growth of cotyledon and sepal epidermal cells in Arabidopsis thaliana. We found that on average, surface area is tightly correlated with volume in both cell size and growth, supporting the use of surface area as a good proxy for volume at the population level. However, cells with similar surface areas or surface growth can display substantial differences in volume and volumetric growth. This happens due to variation in cell thickness, which in turn is controlled by microtubules. These findings highlight limitations of surface-based metrics, and call for volumetric analyses if a more accurate assessment of cell parameters is needed.