Giant cells: from random initiation to nonrandom pattern

Leaf epidermal cells have very different sizes and ploidy levels (the number of chromosome sets), but what actually controls their size and how these cells become organized within the tissue remains unclear. A collaborative and interdisciplinary study involving imaging, quantitative analyses, and modeling has been published in PLOS Biology by the groups of Pau Formosa-Jordan at the Max Planck Institute for Plant Breeding Research and Adrienne Roeder at Cornell University, which finally brings the pieces together.
 

Previous work from the Roeder group showed that the ATML1 transcription factor is important for the specification of very large and scattered cells in the sepal epidermis called giant cells. Recently, a quantitative comparison of leaves and sepals of different genotypes related to the ATML1 pathway revealed that similar cell phenotypes are present in the leaves and sepals, indicating that the same ATML1 pathway that specifies giant cells in sepals also regulates cell size in leaves (see Fig. 1). Although leaf cells vary considerably in size, this new work shows that part of this variability is the result of the specification of giant cells in the leaf. The groups then addressed how these giant cells form and how their scattered pattern arises. Previous work proposed that giant cells are specified randomly. According to this hypothesis, fluctuations in ATML1 concentration trigger giant cell fate specification, without the need for cell-to-cell communication. This is a very different mechanism from how stomata or trichomes are patterned, which relies on cell–cell communication through biochemical signals.

To test whether giant cells are randomly organized, quantitative image analysis and statistics were used to compare real tissues with computationally randomized tissues. Unexpectedly, giant cells were found to be more clustered than expected by chance in mature leaves and sepals (see Fig. 2). This was intriguing and led to the question of how a random and cell-autonomous mechanism of cell specification can produce a pattern that is subsequently not random.

In development, spatial patterns usually involve either direct cell-to-cell communication or long-range diffusible signals. Here, neither of these is needed to reproduce the experimental results, so the authors came up with the hypothesis that because giant cells are specified very early in organ growth, the final spatial pattern is also impacted by the dynamics of tissue growth.

The teams tracked giant cells both in real sepals and in simulations and compared early- and late-stage tissues. Early in development, giant cells were positioned more randomly, but they later appeared more clustered when considering the whole tissue. This was not due to cell movement (cells do not move with respect to each other in plants) or to the formation of new giant cells. Instead, it was due to a cell proliferation effect; when cells surrounding giant cells divide, the initial random pattern becomes a non-random and clustered pattern of giant cells. Therefore, the observed pattern tells us a story not only about how cells form, but also about how the tissue has grown and how cells divide.

In conclusion, this study shows that giant cells are common to both leaves and sepals, and are regulated by the same genes. Moreover, their scattered distribution pattern is shaped not just by genes, but also by tissue growth. What appears random at first can become more ordered over time. Gauthier Weissbart, the co-first author who performed the computational part of the work, said, “We often think of the emergence of order as arising from self-organization or cell–cell interactions, but here, order emerges from random cell fate decisions amplified by the geometry of tissue growth. This is a subtle example of how patterns can arise from the interplay of simple rules at different levels”. And Pau Formosa-Jordan added, “Cellular patterns of different cell types are present in many other plant and non-plant tissues, and I hope our work can inspire new research to decipher how signaling combined with growth results in ordered or disordered patterns across systems.“

References

1. F. K. Clark, et al., A common pathway controls cell size in the sepal and leaf epidermis leading to a nonrandom pattern of giant cells. PLoS Biol 23, e3003469 (2025).
2. H. M. Meyer, et al., Fluctuations of the transcription factor ATML1 generate the pattern of giant cells in the Arabidopsis sepal. Elife 6, e19131 (2017).
3. A. H. K. Roeder, et al., Variability in the Control of Cell Division Underlies Sepal Epidermal Patterning in Arabidopsis thaliana. PLoS Biol 8, e1000367 (2010).
4. A. H. K. Roeder, A. Cunha, C. K. Ohno, E. M. Meyerowitz, Cell cycle regulates cell type in the Arabidopsis sepal. Development 4427, 4416–4427 (2012).