Root Damage and Immune Responses at Cellular Resolution

The study of the molecular mechanisms that underlie plant damage and immune responses is among the most active and advanced areas of plant research. Much of the paradigmatic models in this field, however, have been developed working on aerial tissues. Roots might have significantly different ways of integrating and responding to stimuli, such as cellular damage and perception of microbe-associated molecular patterns (MAMP). We have previously developed transcriptional, live-imaging reporters for stress and immune responses, covering ethylene (ET), jasmonic acid (JA), salicylic acid (SA) and MAMP responsive genes. Using single cell laser ablations, we show that such a damage of root cells leads to local surface depolarisation, ROS induction and Calcium increases, yet only causes a transcriptional response to ET, but not to JA, in contrast to laser-induced damage of leaf cells. Since roots are able to produce and respond to JA, this indicates that the translation of cell damage into jasmonate production does not occur in roots. Using our MAMP-responsive markers, we confirmed that root responses to flg22 are restricted to the late division/root elongation/early differentiation zone. We could demonstrate that this is due to absence of receptor, since expressing the flg22 receptor in non-responsive tissues was sufficient to install responsiveness in those cell layers in most cases. We show that laser-induced cell ablation can “unlock” MAMP-responsiveness in neighboring cells, associated with upregulation of MAMP receptor. Neither damage nor MAMP treatment alone were able to induce a MAMP response, suggesting that root cells perceive and combine cellular damage and MAMPs as two distinct signals. A similar “unlocking” of responses was observed in cortical cells, surrounding an emerging lateral root, although root emergence is not consistently associated with cell death and might be due to other factors, such as lateral root-produced peptides, small molecule signals or mechanical/cell wall stresses.