Systemic defense signaling in Austrian pine

In plants, the induction of systemic defenses is essential for sustaining structural and physiological homeostasis during repeated attacks by various pests and pathogens. Plant hormones regulate numerous systemic responses; however, relatively little is known about their role in the expression of systemic induced resistance (SIR) against necrotrophic fungi, particularly in trees. Based on an established pathosystem comprised of Austrian pine and Diplodia pinea, we examined the timing of local and systemic accumulation of plant growth and stress hormones and systemic accumulation of transcripts, including those encoding proteins associated with hormone production, signal propagation, and local and systemic defense responses. Differentially expressed genes were enriched for those involved in pattern recognition, defense, and various transcription factors related to jasmonic acid (JA), abscisic acid (ABA), auxin, and gibberellin signaling pathways. The spatiotemporal accumulation of hormones and their precursors and conjugates, including ABA, JA, methyl jasmonate (MeJA), jasmonoyl isoleucine, and 12-oxophytodieonic acid, aligned with the transcriptomic data. Collectively, our results indicate that early ABA signaling and sustained JA signaling, along with mobile MeJA signaling, mediate the full activation of SIR following infection of Austrian pine by D. pinea.