While the most common plant cell response to pathogens is growth arrest, root hair cells can perform the opposite reaction.
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.
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.