Due to the presence of a cell wall, plant cells are fixed within their tissue context and cannot move relative to each other during development. Plants thus need to rely on directed cell elongation and cell division to generate a full three-dimensional (3D) structure. Intrinsic polarity cues and cellular communication provide spatial information to plant cells and establishes their position relative to the tissue context and the axis of growth. This framework allows cells to integrate available information and orient their divisions in such a way that structured growth becomes possible. Controlling cell division orientations relative to the tissue axis is therefore the fundamental basis for 3D growth. Plants utilise two main types of cell division to allow directional growth: anticlinal and periclinal cell divisions. Anticlinal cell divisions (AD) generate more cells within a certain cell file (perpendicular to the tissue axis) and are thus one of the main drivers for longitudinal growth in the mitotically active regions of the plant, called meristems. This division type is obviously not sufficient, as it would only generate filamentous structures. In order to create a 3D structure, plants use formative or periclinal divisions (PD) that generate additional cell files (parallel to the tissue axis). This results in radial growth and the formation of new organs. It is clear that a very precise control of cell division orientation is crucial to allow normal 3D growth to occur. Indeed, excessive activity of factors triggering PD in plants result in strong radial expansion in for example root tissues. This illustrates the need for cells to divide in a particular orientation at a precise moment in development. Therefore, understanding the mechanisms that control cell proliferation and cell division orientation is a key question in developmental biology.

division orientation

Vascular tissues have the specific ability to undergo a tremendous amount of these oriented divisions and the additional cell files created this way generate almost all of the tissues that make up wood in trees, are crucial for source-to-sink transport throughout the plant and make up many edible structures such as fruits, roots and tubers. The plant vascular system is vital for transport of water, sugars and nutrients throughout the plant. Moreover, the evolution of an efficient fluid conducting system has allowed plants to grow beyond the size of non-vascular mosses. Despite the clear importance of this vascular proliferation for agricultural purposes and for our fundamental understanding of 3D growth, very little is known about how plant cells control the orientation of their cell divisions. 

Vascular tissue development depends (like most crucial developmental processes in plants) on large and complex regulatory networks, whereby genetic, hormonal and environmental cues must be integrated and interpreted by the appropriate developmental output. We have been identifying and describing several transcriptional regulators controlling this process in recent years, but we are only at the beginning of uncovering the whole regulatory network responsible for vascular cell proliferation and cell division orientation. We tackle this research problem using both forward and reverse genetics approaches and combining different omics technologies including single cell/nucleus RNA-sequencing and chemical biology.