Juvenile roots

---

Juvenile roots

Plants, just like humans, go through different life stages. After germination they start off as a seedling, and subsequently grow into a mature plant which, when the circumstances are right, flowers and forms seeds. These phases are good to distinguish for the above ground part of the plant. And we know how a plant regulates the transition between the phases. Less clear is the question if roots have different life phases as well, and how a plant regulates that transition. Now a group of American researchers with help from researchers out the Netherlands and England found out.

In shoots and leaves micro RNAs (miRNAs) decide if seedling or adult genes are turned on. The juvenile miRNA miRNA156 represses the adult genes, while the mature miRNA172 represses the juvenile genes. And miRNA156 also repress miRNA172. But while a plant matures the amount of miRNA156 reduces , and slowly miRNA172 can turn on the adult genes. At least that is how the plants regulates the transition above ground.

To check if miRNA156 is also below ground promoting youth, the researchers studied roots of plants that make more miRNA156. They noticed that when there was more miRNA156, the roots were longer and more branched. In just germinated seedlings, with normal levels of miRNA156, miRNA156 was found in the whole root. But when the plant gets older, miRNA156 disappears from the root tip, where the growth centre of the root is located. Contrasting, in roots of plants with more miRNA156, miRNA156 stays longer in the root tip.

---

Also in roots represses miRNA156 the adult genes

---

To find out how this influences root growth and branching the researchers analysed the expression of growth region gene PLETHORA 2. This gene is needed to prevent specialisation of stem cells in the growth region. The researchers noticed that under normal circumstances the amount of PLETHORA2 is more or less constant, only around the third day after germination the amount of PLETHORA 2 temporally increases. This temporally increase of PLETHORA 2 allows the growth region to expand, and for thicker roots. This expansion, so observed the researchers, stops as soon as the levels of PLETHORA 2 return to their normal values.

But in presence of more miRNA156 the researchers observed that the amount of PLETHORA 2 is lower. Not only that, only round the fourth day after germinating the amount of PLETHORA 2 temporally increased, but to lower levels than it normally would. This not only affected the size of the growth region, but also the number of root branches.

Subsequently the researchers analysed if the increased number of root branches was a direct effect of reduced levels of PLETHORA 2. Plants with less PLETHORA 2 had more root branches that the control plants. In contrast plants with more PLETHORA 2 the researchers observed less root branches. Suggesting that the amount of PLETHORA 2 indeed influences the number of root branches.

Altogether, this suggests that also in roots miRNA156 is repressing adult genes, like PLETHORA 2. And can we in roots distinguish different life phases of a plant. As the plant matures it gets thicker roots, and less root branches. Ready for a new phase of its life.

Literature

Marta J. Laskowski, Helene C. Tiley, Yiling Fang, Anabel Epstein, Yuyang Fu, Roberto Ramos, Thomas J. Drummond, Renze Heidstra, Priyanka Bhakhri, Tobias I. Baskin, Ottoline Leyser (2022) The miR156 juvenility factor and PLETHORA 2 form a regulatory network and influence timing of meristem growth and lateral root emergence. Development 149 (21): dev199871. doi: https://doi.org/10.1242/dev.199871