Microscopes and roots: investigating whether cell wall thickenings are phi (Φ) or “C” shaped

As children or students in biology class, we all tore plants and flowers apart just to see how they are built. Flowers are great fun and colourful and might seem more fun than roots. For scientists equipped with microscopes and lab equipment, discovering how roots are built is still fun. It also helps to answer countless fundamental questions about how plants work. 

Kováč and colleagues, mainly based at Comenius University in Bratislava, used nine different staining and microscopy methods to look at some unusual root cell thickening in Alpine pennycress (Noccaea caerulescens). The scientists discovered how this cell thickening is formed, where, what is the chemical composition of the cell walls and if this layer provides as a protective barrier.

Roots are complex structures, made up of many layers with different functions. Some secondary cell wall thickenings can be found in the root cortex in many plant species (e.g. apples, orchids, oilseed rape). As their formation resembles Greek letter phi (Φ), they are called phi thickenings. Whilst phi thickenings have been observed widely, in 2008, Zelko and colleagues found interesting cell wall thickenings in the root sections of Alpine pennycress, shaped like a half-moon or letter “C” and named them ‘peri-endodermal thickenings’ (PET).

After two decades of the first observation, the authors used nine different stainings including immuno-labelling, electron microscopy to understand the composition, formation and function of PET. The scientists collected seeds from a former mining site in Salzburg, Austria. They compared how different dyes (stains) are taken up by the Alpine pennycress compared to the well-researched thale cress (Arabidopsis thaliana) which has phi thickenings.

Microscopy showed that the PET starts to form 1-1.5 mm from the root-shoot junctions and never formed around the collar zone of lateral roots. PET contained phenolic components, lignin but not always pectin which is a characteristic of phi thickenings. When heavy metals (zinc and cadmium) were added to N.caerulescens and A. thaliana, PET acted as a barrier and less Zn and Cd were in the xylem vessels.

The authors write, “[W]e need to take into account that N. caerulescens is a heavy metal tolerant hyperaccumulating plant; therefore, we need to consider an additional role of PET in this species”. 

The study shows why scientists need to keep dissecting plants to understand how they work. These cell wall thickenings might be especially important to heavy metal tolerant species and should be investigated in other plants. Understanding how hyper-acculmulators work could help reclaim polluted sites, or even make recycling materials through phytomining a possibility.

You can also request the article from the authors via Researchgate.