External jasmonic acid isoleucine mediates amplification of plant elicitor peptide receptor (PEPR) and jasmonate‐based immune signalling.

Jasmonic acid‐isoleucine (JA‐Ile) is a plant defence hormone whose cellular levels are elevated upon herbivory and regulate defence signalling. Despite their pivotal role, our understanding of the rapid cellular perception of bioactive JA‐Ile is limited. This study identifies cell type‐specific JA‐Ile‐induced Ca2+ signal and its role in self‐amplification and plant elicitor peptide receptor (PEPR)‐mediated signalling. Using the Ca2+ reporter, R‐GECO1 in Arabidopsis, we have characterized a monophasic and sustained JA‐Ile‐dependent Ca2+ signature in leaf epidermal cells. The rapid Ca2+ signal is independent of positive feedback by the JA‐Ile receptor, COI1 and the transporter, JAT1. Microarray analysis identified up‐regulation of receptors, PEPR1 and PEPR2 upon JA‐Ile treatment. The pepr1 pepr2 double mutant in R‐GECO1 background exhibits impaired external JA‐Ile induced Ca2+cyt elevation and impacts the canonical JA‐Ile responsive genes. JA responsive transcription factor, MYC2 binds to the G‐Box motif of PEPR1 and PEPR2 promoter and activates their expression upon JA‐Ile treatment and in myc2 mutant, this is reduced. External JA‐Ile amplifies AtPep‐PEPR pathway by increasing the AtPep precursor, PROPEP expression. Our work shows a previously unknown non‐canonical PEPR‐JA‐Ile‐Ca2+‐MYC2 signalling module through which plants sense JA‐Ile rapidly to amplify both AtPep‐PEPR and jasmonate signalling in undamaged cells. Summary statement: This work shows how plant hormone, jasmonoyl‐l‐isoleucine (JA‐Ile) is sensed by cells externally using Arabidopsis thaliana as a model system. Using calcium imaging, transcriptomics, hormone analysis we describe a non‐canonical PEPR‐JA‐Ile‐Ca2+‐MYC2 signalling module through which plants sense JA‐Ile rapidly to amplify both AtPep‐PEPR and jasmonate signalling in undamaged cells. [ABSTRACT FROM AUTHOR]