Inhibition of cell expansion by rapid ABP1-mediated auxin effect on microtubules

In roots and dark-grown hypocotyls of Arabidopsis thaliana , ABP1-mediated auxin signalling induces swift re-orientation of the microtubule cytoskeleton from transverse to longitudinal, thus inhibiting cell expansion. How auxin influences the plant cytoskeleton The plant hormone auxin controls many aspects of plant growth and development through the regulation of cell expansion, yet the molecular components that link auxin signalling to cytoskeletal dynamics during cell expansion have remained elusive. Jiri Friml and colleagues provide evidence that, in roots and the hypocotyls of Arabidopsis thaliana , auxin induces swift reorientation of the microtubule cytoskeleton from transversal to longitudinal, thus inhibiting cell expansion and promoting cell elongation. This fast auxin effect depends on the auxin binding protein 1 (ABP1), and requires various downstream signalling components including ROP6 GTPase, the ROP-interactive protein RIC1 and the microtubule-severing protein katanin. This signalling pathway also seems to mediate asymmetric growth during gravitropic response in which a plant changes direction of growth in response to gravity. The prominent and evolutionarily ancient role of the plant hormone auxin is the regulation of cell expansion 1 . Cell expansion requires ordered arrangement of the cytoskeleton 2 but molecular mechanisms underlying its regulation by signalling molecules including auxin are unknown. Here we show in the model plant Arabidopsis thaliana that in elongating cells exogenous application of auxin or redistribution of endogenous auxin induces very rapid microtubule re-orientation from transverse to longitudinal, coherent with the inhibition of cell expansion. This fast auxin effect requires auxin binding protein 1 (ABP1) and involves a contribution of downstream signalling components such as ROP6 GTPase, ROP-interactive protein RIC1 and the microtubule-severing protein katanin. These components are required for rapid auxin- and ABP1-mediated re-orientation of microtubules to regulate cell elongation in roots and dark-grown hypocotyls as well as asymmetric growth during gravitropic responses.