GGD Thesis Defense Seminar – Frances Clark – Roeder Lab – Tuesday October 17, 2023 – 1pm – 404 Plant Science

“Mechanisms of Cell Size Patterning in the Epidermis of Arabidopsis thaliana”

Cellular patterning in development is the process by which initially identical cells differentiate into distinct cell types. Patterning is essential for multicellular organisms to split tasks between different cells to achieve complex functions as an organ. The ancestors of animals and plants developed multicellularity independently, so it is important to study cellular patterning in both kingdoms of life. Here, we focus on studying cell patterning mechanisms in the plant Arabidopsis thaliana. Specifically, we seek to understand how epidermal pavement cells of different sizes are patterned during development.

Sepal giant cells are very large, highly endoreduplicated cells interspersed between much smaller cells on the sepal epidermis. Giant cell localization differs between sepals but giant cell number is fairly consistent. Several genes involved in embryonic epidermal specification have been found to also control giant cell formation, including ATML1 which encodes a class IV homeodomain leucine zipper transcription factor. ATML1 protein fluctuates within epidermal nuclei of developing sepals and high concentrations reached in the G2 phase of the cell cycle is strongly correlated with giant cell fate specification. We find that the receptor-like kinase ALE2 functions upstream of the transcription factor ATML1 by affecting its protein activity and preventing it from properly increasing transcription of the CDK inhibitor LGO at the right time and to the right level for giant cell specification. Thus, we find that cell-cell signaling is necessary to sensitize cells of the developing sepal epidermis to endogenous ATML1 concentration peaks.

We find that PDF2, the paralogue of ATML1, also fluctuates in individual nuclei of the developing sepal epidermis and that its fluctuations are sometimes very correlated with ATML1 fluctuations but at times are not correlated at all. Like ATML1, increased PDF2 concentration promotes giant cell differentiation. The fact that ATML1 and PDF2 fluctuations are sometimes very correlated suggests either that they regulate the expression of one another or that they are responding to the same variations in cellular environment.

The Arabidopsis thaliana leaf epidermis also has a range of cell sizes and ploidies. We find that the same genetic pathway controlling cell size in the sepal also controls cell size in the leaf. Along with collaborators Gauthier Weissbart and Pau Formosa-Jordan at the Max Planck Institute for Plant Breeding Research, we reveal an underlying order to the spatial arrangement of leaf giant cells that initially appear randomly distributed. We find that giant leaf cells are scattered nonrandomly and touch one another more than would be expected by chance. We show that this spatial pattern can be produced by a model in which giant cells are specified by stochastic fluctuations of ATML1.