PAMP-triggered Genetic Reprogramming Involves Widespread Alternative Transcription Initiation and an Immediate Transcription Factor Wave

ABSTRACT Immune responses triggered by pathogen-associated molecular patterns (PAMPs) are key to pathogen defense, but drivers of the genetic reprogramming required to reach the immune state remain incompletely understood in plants. Here, we report a time-course study of the establishment of PAMP-triggered immunity (PTI) using cap analysis of gene expression (CAGE). Our results show that as much as 15% of all PAMP response genes display alternative transcription initiation. In several cases, use of alternative TSSs may be regulatory as it determines inclusion of target peptides or protein domains, or occurrence of upstream open reading frames (uORFs) in mRNA leader sequences. We also find that 60% of PAMP-response genes respond much earlier than previously thought. In particular, a previously unnoticed cluster of rapidly and transiently PAMP-induced genes is enriched in transcription factors whose functions, previously associated with biological processes as diverse as abiotic stress adaptation and stem cell activity, appear to converge on growth restriction. Furthermore, some examples of known potentiators of PTI, in one case under direct MAP kinase control, support the notion that the rapidly induced transcription factors could constitute direct links to PTI signaling pathways and drive gene expression changes underlying establishment of the immune state.