Extra protection
To prevent that everything and everyone can just entre plant roots have an extra lignin layer to protect their vascular tissue. But lots of plants have in addition to this protecting ring another barrier. Now a group of international researchers show in Nature Plants how plants are regulating this barrier.
Plant roots contain multiple cell layers. In the middle is the vascular tissue which transport nutrients and water to the rest of the plant. The first layer surrounding the vascular tissue is the endodermis, followed by one or multiple layers of cortex cells. Those are surrounded by a layer of exodermis cells and the epidermis. Some plants, like tale cress, are missing the exodermis layer.
To prevent that everything can access the vascular tissue the epidermis cells form a protective layer, the so called Casparian strip. This is a ring of polymerised lignin through the middle of epidermis cells. Shielding the vascular tissue from the outside world. But plants like tomato plants, that also have an exodermis, also have another barrier. Over this barrier is less known. That the researchers decided to change.
Plants regulate their polar lignin cap independent of their Casparian strip
First they analysed the exact location of this barrier. They did this through using a colouring that stains lignin. Using this the researchers noticed that the side of exodermis cells that border the epidermis have a thicker lignin layer. In older exodermis cells also the side of the exodermis cells, which border other exodermis cells also have a thicker lignin layer. But the sides that borders the cortex cells is free of this extra lignin. The researchers named this barrier the polar lignin cap.
Subsequently the researchers set out to find which genes regulated the forming of the polar lignin cap. Firstly they analysed mutants of genes known to be involved in forming the Casparian strip. But all these mutants still had a normal polar lignin cap.
The researchers then turned their attention to the genes that were activated in exodermis cells. Two genes stood out. The first was EXO1, a gene activator. Plants that produced more of this gene activator had a thinner polar lignin cap. While plants without EXO1 also had a thick lignin layer between the cortex and exodermis cells.
Just like the Casparian strip, the polar lignin cap is stopping ions from entering
The second gene was SCZ, a gene that helps cortex cells to give their identity. The roots of plants without SCZ have just as roots of EXO1 missing plants a thicker lignin barrier between the cortex and exodermis cells. It appears that in tomato plants EXO1 and SCZ are together responsible for the regulation of the polar lignin cap.
Lastly the researchers tested how well the polar lignin cap works. Hereby the analysed the ion uptake in plants missing either the Casparian strip or the polar lignin cap. Both barriers stop ions, but the Casparian strip a little better than the polar lignin cap.
The discovery that plants regulate the polar lignin cap separately from their Casparian strip gives opportunities. It suggests that it is possible for the plant to adjust their polar lignin cap to the demands of their environment.
Literature
Manzano, C., Morimoto, K.W., Shaar-Moshe, L. et al. Regulation and function of a polarly localized lignin barrier in the exodermis. Nat. Plants (2024). https://doi.org/10.1038/s41477-024-01864-z
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