Three levels of heterogeneity - growth of Arabidopsis leaf epidermis.

Background: Growth of leaf epidermis is symplastic, i.e. cell contacts are preserved, but strongly heterogeneous because of variation in cell identity and differentiation status. Heterogeneous growth requires coordination to allow a predictable leaf shape to be formed. Here we investigate growth heterogeneity and coordination using expanding leaf epidermis of Arabidopsis (Arabidopsis thaliana) as a model system. We first analyse relationships between heterogeneity of epidermis growth at different scales: from subcellular via cellular to tissue scale. Next, based on the heterogeneity analysis, we verify the hypothesis that growth heterogeneity and coordination are affected by microtubule defects in mor1-1 mutant treated with high temperature.

Results: Using microbeads labelling and original protocols to compute growth we show high growth heterogeneity of epidermal pavement cells at cellular and subcellular scales, and influence of stomata lineage on pavement cells growth. However, despite this high growth variation between cells (cellular scale) and within cells (subcellular scale), growth coordination is revealed at the tissue (supracellular) scale by the pattern of streamlines that follow subcellular growth directions. In leaf epidermis of mor1-1 plants treated with high temperature we reveal local disturbances in growth coordination, which are related to contact changes between cells that are likely a consequence of aberrant cytokinesis and reduction of cell adhesion. Otherwise, the growth pattern in high-temperature treated mor1-1 is similar to the non-treated mor1-1 and wild type. The high temperature treatment results only in a tendency to increase growth heterogeneity in the mutant more than in wild type grown in the same conditions.

Conclusions: Overall, our study reveals high heterogeneity of growth within and between cells of leaf epidermis. Despite this heterogeneity, a defined supracellular growth pattern exists that changes in time. Our analysis shows only a weak and likely indirect influence of defective microtubules on leaf epidermis growth.