The root architecture of Arabidopsis seedlings alters after inoculation with beneficial rhizobacteria in an auxin-dependent manner, resulting in longer root hairs. Surprisingly, Pečenková et al. find that pathogenic Pseudomonas syringae bacteria provoke a similar level of root hair growth stimulation starting several hours after the inoculation.

Model of stimulation of root hair growth by Psm/Pst and role of four phytohormonal/immunity sectors in this model.
Model of stimulation of root hair growth by Psm/Pst and role of four phytohormonal/immunity sectors in this model. Ethylene (ET) is of key importance in root hair growth in the developmental context. The other sectors, especially the jasmonic acid (JA) and PAD4 pathways, also have minor and indirect influences on the pathogen-related induction of root hair growth (A). When the most important sector, ethylene signalling, is missing (ein2), the phytohormonal misbalance allows salicylic acid (SA)/JA/PAD4 to inhibit root hair growth, which can be explained only by the possible negative control of ethylene over these three sectors (B).

Analyses of root hair lengths of several mutant lines indicate the roles of ethylene signalling and secretory machinery in eliciting root hair growth response. Neither of the known elicitor molecules was able to induce this reaction; however, the range of root hair response could be moderated by pre-treatment with the Flg22 peptide, butsalicylic and jasmonic acid pathways are not involved.