Mechanosensation of raindrops via trichome hair-like cells evokes plant immune response

Raindrops are known to contain bacteria, fungi, and virus, and thus we hypothesized that plants may perceive raindrops as the emergency signal. However, if and how plants respond to raindrops is totally unknown. Transcriptome analysis of Arabidopsis leaves demonstrated that artificial raindrops unambiguously express genes related to disease resistance via mechanical stress. Genetic and bioimaging studies revealed that the mechanical stress induces Ca2+ waves and the expression of immune gene concentrically around trichomes. Furthermore, our results indicate that a member of the calmodulin binding transcription activator (CAMTA) is involved in the transcriptional changes induced by raindrops. We confirmed that trichome-mediated immunity significantly reduces the growth of virulent Pseudomonas syringae, concluding that trichomes are the sensors of mechano-stimuli to initiate the earliest layer of plant immunity.