Plants, like animals, employ hormones as messengers, which coordinate growth and regulate how they react to the environment. One of these plant hormones, auxin, regulates nearly all aspects of plant behavior and development, for example phototropism, root growth and fruit growth. Depending on the context, auxin elicits a range of responses such as cell polarization or division. In this week's edition of Science a team of researchers including Jiři Friml from IST Austria and led by Zhenbiao Yang of the University of California, Riverside, report finding the molecular mechanism by which the plant hormone auxin affects the organization of the cell's inner skeletons.
Auxin is a remarkable molecule, impinging on a variety of plant responses in growth and development. How auxin can play such a range of roles is as yet unexplained, though auxin may activate distinct signaling systems in different contexts and so convey different signals for different responses. For example, a nuclear receptor pathway modulates gene transcription in response to auxin. Auxin binding protein 1 (ABP1) has been proposed to act independently of this nuclear pathway, regulating responses at the plasma membrane and in the cytoplasm. ABP1 was discovered nearly 40 years ago, but how it transmits the auxin signal and regulates responses remained unclear to date. In their Science publication, the researchers show that at the cell surface ABP1 interacts with transmembrane kinases (TMKs). In genetic variants of Arabidopsis in which TMKs are mutated, pathways regulated by ABP1 are impaired such as the characteristic arrangement of pavement cells.
TMKs and ABP1 are also both required for the activation of ROP GTPases, which regulate the organization of the cell's inner skeleton. This cytoskeleton is disrupted when TMKs are mutated, as filamentous actin does not localize correctly and cortical microtubules are disorganized. The researchers show that all of TMK1 as well as around a quarter of ABP1 can be found at the plasma membrane. In the presence of auxin, TMK1 and ABP1 bind to each other. The researchers propose that secreted ABP1 binds to TMK1 at the plasma membrane in response to extracellular auxin, and signal to ROP GTPases which affect the cytoskeleton.
TMK1 is at least one of the long-sought docking proteins of ABP1, which couple extracellular auxin and its perception by ABP1 to downstream cytoplasmic events. Solving the mystery of cell surface-cytoplasmic auxin perception, this research opens up a new horizon in auxin biology.
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