Plasticity in phloem development

Plant & zo

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Plasticity in phloem development

Last week at a symposium, we were reminded by Antia Rodriguez-Villalon that in plants organogenesis does not stop after germination. In fact, plants keep producing new organs through their lives. While most of us think by organ formation in plants first about leaves or flowers, Antia Rodriguez-Villalon work actually focusses on vascular development in roots. Her main take home message was that vascular development is more plastic than we initial thought. And that this plasticity safeguards the development of a functional vascular system. So I was excited when this weekend I saw in a tweet about the latest article of her group describing this study.

protophloem - root
The radial organization of the vascular tissues in Arabidopsis roots where depicted phloem tissues are color coded (CC, companion cell; MSE, metaphloem sieve element; PPP, protophloem pole pericycle cells; PSE, protophloem sieve element). Copied from Gujas et al. 2019

Before I go into more details about her work we will take a short detour, about phloem development. In plants the fate of a cell is determined by its position. As such, we know the function of a cell by its position in the plant. In Arabidopsis roots the phloem pattern is made up of four cell types, and is well conserved. With the protophloem and the metaphloem sieve elements originating from a common stem cell, whereas the companion cell originates from a different stem cell. In effect, once the precursor cells to the protophloem and the companion cells get pushed outside the meristem region, these cells are in different but adjacent cell files. Cells that have a protophloem identity can be visualised with a protophloem marker.

Using this technique they looked at how the protophloem identity was affected in the mutant, cvp2 cvl1, which is severely compromised in protophloem development. Finding that protophloem identity was affected, they set out to determine the new identity of these affected cells. Surprisingly, these affected cells had a gene expression similar to that of companion cells. Investigating this a little further, by intentionally disrupting the protophloem development, showed that in case of disruption the protophloem identity switched from the protophloem cell-file to the companion cell cell-file. Giving the first hint of the plasticity of the protophloem development. This plasticity is restricted to a so called “plastic zone” in which the cells surrounded the protophloem are still in an uncommitted stage. Eventually, through growth, these uncommitted cells will be pushed out of the plastic zone and commit.

Further study to investigate how the plastic zone is regulated identified that RPK2 and CLE45 function to restrict protophloem identity to the protophloem position. In addition, CLE45 treatments prevented the plasticity of protophloem development. Suggesting that in the plastic zone, plants normally can modulate CLE45 perception at single cell level, enabling them to re-pattern to form a functional phloem pole upon positional cues.

protophloem plastic zone schematics
Schematic Overview of the Molecular Mechanisms Regulating PSE Identity and Phloem Patterning in Arabidopsis Roots. A longitudinal view (left) and radial view (middle) of the developmental trajectories of protophloem sieve elements (PSEs) and companion cells (CCs) within the root are represented. Previous to their entry into the plastic zone (surrounded in red), future PSE cells acquire their cell identity and enter into a proliferative phase, a process partially regulated by the activity of positive regulators (such as CVP2) and counteracted by negative regulators, such as CLE peptides. Within the plastic zone, PSE-surrounding elements are primed as phloem cells, but they still exhibit plastic identity and can switch their identity (red arrows) according to positional cues. Once PSE cells enter into differentiation process, RPK2 excludes PSE identity from PSE-surrounding cells, allowing these cells to commit to CC’s developmental trajectory. Copied from Gujas et al. 2019

This latest work of the group of Antia Rodriguez-Villalon showed us that phloem development has a back up plan for it worse case scenario. I look forward for them to find out more about how this is organised.

Literature

Gujas et al., A Reservoir of Pluripotent Phloem Cells Safeguards the Linear Developmental Trajectory of Protophloem Sieve Elements, Current Biology (2019), https://doi.org/10.1016/j.cub.2019.12.043

Published by Femke de Jong

A plant scientist who wants to let people know more about the wonders of plant science. Follow me at @plantandzo

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