Transcriptional profiling of Populus leaves with altered SA levels

Salicylic acid (SA) has long been implicated in plant responses to oxidative stress. A direct assessment of SA effects in planta has been difficult, as SA-overproducing Arabidopsis mutants are compromised in growth or other developmental processes. We now report that transgenic Populus expressing a bacterial bifunctional SA synthase accumulated two to three orders of magnitude more total SA than wild type without affecting growth. Microarray experiments were performed to gauge the transcriptional responses of young source leaves to SA manipulation and/or heat stress. Differentially expressed genes due to SA perturbation or temperature treatment were identified. Co-expression network analysis was performed to identify key driver genes in SA-modulated response. Wild-type and transgenic Populus expressing cytosolic Irp9 (Irp9 line I6), plastidic Irp9 (FD-Irp9 lines F10 and F52) or NahG (NahG line N31) genes were maintained in two identical growth chambers at normal (NT, 27°C/17°C, day/night) or high (HT, 35°C/25°C) temperatures. Young source leaves at leaf plastochron index (LPI) 5 were sampled one week after the start of heat stress. Two biological replicates were used.