Newly discovered mechanism of plant hormone auxin acts the opposite way
Auxin accumulation at the inner bend of seedling leads to growth inhibition rather than stimulation as in other plant tissues
Increased levels of the auxin hormone usually promote cell growth in various plant tissues. Chinese scientists and researchers at the Institute of Science and Technology Austria (IST Austria) have now shown that in special areas of the plant, increased levels of auxins trigger a different gene expression pathway leading to growth inhibition. The discovery, published in the journal Nature, explains the formation of the typical elbow or apical hook that helps the seedling to break through the soil after germination.
Different concentrations of auxins mediate distinct developmental outcomes in different plant tissues. For example, the accumulation of auxin in stem tissues triggers a gene expression pathway that ultimately leads to an increase in cell elongation, which leads to stem growth. A growth scenario, which cannot be explained in a similar way, is however the development of the apical hook that the early plant forms to protect its delicate growth apex when it pierces the soil. In the cells of the inner elbow of the hook, i. e. the concave side, auxin accumulates; however, to take the form of a hook, the plant growth must grow less at the inner concave than on the outer convex side. Scientists have therefore found themselves in a paradoxical situation and have asked themselves the question: Can auxin do something contrary to what we know it does in other parts of plants?
One hormone - two different gene expressions
To solve the puzzle, the research group around Tongda Xu of the Chinese Academy of Sciences collaborated with Jií Friml, plant cell biologist at IST Austria, and Zuzana Gelová, her post-doc colleague. By testing various mutants of the model plant Arabidopsis thaliana, scientists were able to reveal a previously unknown gene expression pathway, triggered by the accumulation of auxins and leading to inhibition of growth on the concave side of the hook. Although the previously known pathway is located at the nucleus and involves the TIR1 receptor protein (Transport Inhibitor Response 1), this newly discovered pathway begins at the cell surface and involves another perceptual component, the transmembrane kinase (TMK1), whose function was unclear.
A paradox and TMK1 explained
In the newly discovered mechanism, the active auxin TMK1 on the cell surface and triggers the cleavage of the intracellular part of this protein. In the cell, the cleaved part of TMK1 interacts with specific transcriptional repressors. While auxin degrades similar repressive proteins in the nucleus-based TIR1 pathway to trigger gene expression leading to cell growth, it stabilizes repressors connected to the TMK1 pathway, causing growth inhibition rather than stimulation. Thus, TIR1 and TMK1 interact with different subsets of transcriptional proteins and thus facilitate auxin signaling through two different mechanisms, allowing the growth to occur on one side but not on the other. Co-author Jií Friml: "We have long wanted to understand how TMK1 works and if and how the accumulation of auxin can work in two different ways. Thanks to our perseverance and the major contributions of our Chinese colleagues, we now know both." From this point on, it would also be interesting for scientists to understand the complete repertoire of the development process beyond the apical hook controlled by this new auxin signaling pathway.
