A new mechanism identified for how plant hormones promote wood formation

Researchers at the University of Helsinki have discovered a mechanism that allows plants to regulate the ratio of produced wood (i.e. xylem) and phloem. Both tissues are formed by the vascular cambium. The discovery is important for understanding the factors that regulate tree growth.

Earlier studies in Professor Ari Pekka Mähönen’s research group have shown that plant hormone auxin is important for positioning stem cells within the vascular cambium. When a stem cell divides, one of the daughters remains a stem cell while the other one differentiates. However, how vascular cambium decides whether a cell will differentiate into wood or phloem has been unclear.

In this study, Riikka Mäkilä, a doctoral researcher in Mähönen’s group, discovered that plant hormone gibberellic acid (GA) regulates the ratio of how much xylem and phloem the cambium is producing. Specifically, GA does this by promoting the transport and signalling of the plant hormone auxin in the vascular cambium. When plants have more GA, they increase their auxin transport and signalling, which makes the stem cell daughter differentiate into wood more often than phloem, thus increasing wood production.

The discovery is important for understanding the factors that regulate tree growth and may provide new applicable knowledge for tree breeding.

Global warming and rising carbon dioxide (CO2) levels are greatly impacting humans and nature. Sequestering carbon is an efficient way to slow down these effects. Plants, especially trees, can sequester CO2 and store it in their trunks as wood.

“Wood is produced by cell divisions in the vascular cambium, but this tissue can also make phloem. Thus, it is important to understand how the production of these tissues is balanced. Due to the long generation times and difficulties in tree genetic manipulations, researchers use Arabidopsis thaliana root as a model, as it has structures similar to a tree trunk. Later, the knowledge gained from Arabidopsis can be transferred to tree breeding,” Mäkilä says.

Original article:

Mäkilä, R., Wybouw, B., Smetana, O. et al. Gibberellins promote polar auxin transport to regulate stem cell fate decisions in cambium. Nat. Plants (2023). https://doi.org/10.1038/s41477-023-01360-w

The cambium consists of cells capable of dividing and growing. Cambial cells are located under the outer bark, and they produce both wood and phloem cells. The phloem layer is located between the cambium and the bark of the tree. Its function is to transport products of photosynthesis, such as sugars, from the leaves to the trunk and roots. The wood layer is the innermost layer of the plant stems and roots, and its function is to provide mechanical support and to transport water from the soil to the leaves.

This research is basic research, which is the foundation of all scientific research at the university. Basic research is the study of the phenomenon or activity of something, and therefore increases scientific understanding of the subject. Basic research does not lead directly to an application in everyday life, but it can lead to a scientific breakthrough.