The Circadian Clock Regulates Receptor-Mediated Immune Responses to a Herbivore-Associated Molecular Pattern

Plants activate induced defences through the recognition of molecular patterns. Like pathogen-associated molecular patterns, herbivore-associated molecular patterns (HAMPs) can be recognised by cell surface pattern recognition receptors, leading to defensive transcriptional changes in host plants. Herbivore-induced defensive outputs are regulated by the circadian clock, but the underlying molecular mechanisms remain unknown. To investigate how the plant circadian clock regulates transcriptional reprogramming of a specific HAMP-induced pathway, we characterised the daytime and nighttime transcriptional response to the caterpillar-derived HAMP peptide In11 in the legume crop cowpea (Vigna unguiculata). Using diel and free-running conditions, we found that daytime In11 elicitation resulted in stronger late-induced gene expression than nighttime. Plants with a conditional arrhythmic phenotype in constant light conditions lost time-of-day gated responses to In11 treatment, and this was associated with arrhythmic expression of circadian clock core transcription factor Late Elongated Hypocotyl VuLHY1 and VuLHY2. Reporter assays with VuLHY homologues indicated that they interact with the promoter of daytime In11-induced Kunitz Trypsin Inhibitor (VuKTI) via a canonical and a polymorphic CCA1/LHY binding site (CBS), consistent with a mechanism of direct regulation by circadian clock transcription factors. This study improves our understanding of the time-dependent mechanisms that regulate herbivore-induced gene expression.