Identification of the mechanism that regulates continuous tissue formation in plants

Most advances in Developmental Biology have focused on understanding how body patterns are established during embryogenesis. In animals, embryogenesis determines the main body axis that is present in adult organisms; however, plant embryos have very little resemblance with the final organisms they will become. Plant tissues must then be formed continuously, and therefore, its identity and functionality has to be determined throughout postembryonic development to ensure correct growth.

As opposed to animal cells, plant cells cannot move. Thus, plant cells remain in their initial position until a new cell is born. Newly formed cells then push older cells back creating a lineage that can be tracked by age or developmental time to study how a root is formed.

This study has identified the master regulators that organize root tissue formation, and in addition how these regulators interact with signals external to the tissue to establish a precise pattern. Thus cells can achieve their appropriate identities based on their position, to form tissues that make up the root and that in turn are required for its functionality. Roots can then correctly transport nutrients from soil to all plant organs.

Researches have observed that in plants lacking 3 of the factors identified, the ground tissue, although formed embryonically, was missing in adult plants, as the stem cell that gives rise to its lineage was also missing. This indicates that cell lineages in plants do not only depend on position as it was previously thought, but they are determined by specific endogenous factors, similarly to what is observed in animals. This is the first case in which a whole tissue is totally missing during postembryonic development; it is as we suddenly lost the liver or the heart, which are crucial for our body functions.

Research, which is in collaboration with Duke University (USA), shows that these five factors do not work in a linear fashion but cooperatively. It is a process that sets up tissue formation in three steps: first, specifying the stem cell and therefore, the lineage, secondly, perceiving the positional signal required for stem cell division (and which involves over-activation of these factors), and it finishes with the division itself that gives rise to the cortex and the endodermis. As these factors are transcriptional factors they activate gene expression conferring the correct cell identity. Therefore, this is a mechanism that allows tissues to continuously grow because it uses the same set of factors to start, grow and mature a tissue. This minimizes flaws and ensures developmental plasticity, as only cells expressing these factors can perceive the positional signal to generate endodermis and cortex.