Abstract
Despite being composed of immobile cells, plants reorient along directional stimuli. The hormone auxin is redistributed in stimulated organs leading to differential growth and bending. Auxin application triggers rapid cell wall acidification and elongation of aerial organs of plants, but the molecular players mediating these effects are still controversial. Here we use genetically-encoded pH and auxin signaling sensors, pharmacological and genetic manipulations available for Arabidopsis etiolated hypocotyls to clarify how auxin is perceived and the downstream growth executed. We show that auxin-induced acidification occurs by local activation of H+-ATPases, which in the context of gravity response is restricted to the lower organ side. This auxin-stimulated acidification and growth require TIR1/AFB-Aux/IAA nuclear auxin perception. In addition, auxin-induced gene transcription and specifically SAUR proteins are crucial downstream mediators of this growth. Our study provides strong experimental support for the acid growth theory and clarified the contribution of the upstream auxin perception mechanisms.
Highlights
Plants support their bodies by a hydrostatic skeleton that consists of pressurized cells encased by a strong extracellular composite, the cell wall (Hofte, 2014)
Our results show that Small Auxin Up mRNAs (SAUR) proteins mediate the signal between nuclear auxin perception and auxin-induced growth execution, but it is important to note that the conclusion is based mainly on the overexpression of a stabilized version of the SAUR19 protein
We showed that after auxin application, nuclear auxin signaling, apoplast acidification and growth occur with a similar lag phase of approximately 20 min
Summary
Plants support their bodies by a hydrostatic skeleton that consists of pressurized cells encased by a strong extracellular composite, the cell wall (Hofte, 2014). This has several crucial consequences for plant multicellularity: (vascular) plant cells do not move, and they grow mostly symplastically (no sliding occurs; reviewed in Lintilhac, 2014). Auxin induces rapid (minutes to hours) cell elongation in the aerial organs. This feature was used to define and discover auxins in a coleoptile bending test By pumping protons into the apoplastic space, the PM is hyperpolarized, which drives
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