Transcriptome analysis reveals role for WRKY70 in early N-hydroxy-pipecolic acid signaling

N-hydroxy-pipecolic acid (NHP) is a mobile metabolite essential for inducing and amplifying systemic acquired resistance (SAR) following pathogen attack. Early phases of NHP signaling leading to immunity have remained elusive. Here we report the early transcriptional changes mediated by NHP and the role salicylic acid (SA) plays during this response. We show that distinct waves of expression within minutes to hours of NHP treatment include increased expression of WRKY transcription factors as the primary transcriptional response, followed by the induction of WRKY-regulated defense genes as the secondary response. The majority of genes induced by NHP within minutes were SA-dependent, whereas those induced within hours were SA-independent. These data suggest that NHP induces the primary transcriptional response in a low SA environment and new SA biosynthesis is dispensable for induction of the secondary transcriptional response. We demonstrate that WRKY70 is required for the induced expression of a set of genes defining some of the secondary transcriptional response, SAR protection, and NHP-dependent enhancement of ROS production in response to flagellin treatment. Taken together, our study highlights the key genes and pathways defining early NHP responses and a role for WRKY70 in the regulation of NHP-dependent transcription.