Plant Cell http://doi.org/bhng (2016)

The acid growth theory, present in many textbooks, proposes that auxin induces the efflux of protons from plant cells into the surrounding apoplast by a non-transcriptional mechanism. The resulting local acidification activates hydrolases and expansins that loosen the plant cell wall and thereby allow the cells to elongate, a typical auxin response. David Villalobos from the University of Lausanne and colleagues use new genetic tools to show that this may not be true in roots.

To untangle cell division and elongation, which are both affected by auxin, the authors used two mutants of the model grass Brachypodium that have higher auxin content and elongation in roots, but normal meristematic activity. Root transcriptomes revealed that auxin signalling genes are largely buffered against auxin level variations, but cell wall remodelling factors are upregulated, and a chemical analysis indicated reduced abundance or complexity of cell wall arabinogalactans. Despite high auxin levels, proton excretion was stable. Furthermore, in both Brachypodium and Arabidopsis, growing roots in an acidic medium did not modify root cell expansion, whereas artificially activating the proton pump inhibited cell elongation.

Despite possible timescale differences, these new results reinforce previous observations that apoplastic acidification is not linked to cell expansion in roots, and that the auxin effect is mostly transcriptional. This study also highlights the fact that the universality of old theories needs to be reassessed with modern molecular and genetic tools.