Metabolic and Transcriptional Networks in Salicylic Acid-Hyperaccumulating Populus

The hormone salicylic acid (SA) regulates many aspects of plant growth, development and defense. Exogenous applications of SA have been shown to confer oxidative stress tolerance in agricultural crop species.  However, past efforts to increase endogenous SA levels have met with limited success, as SA overproduction in transgenic or mutant Arabidopsis leads to growth retardation. This limits not only the potential of SA manipulation for crop protection, but also the mechanistic investigation of SA response due to growth complications. By over-expressing a Yersinia bi-functional SA synthase, we showed that Populus can accumulate up to 1000-fold higher levels of SA and SA-conjugates without an apparent growth trade-off.  SA-hyperaccumulation induced partial stomatal closure and reduced photosynthesis. Network analysis identified metabolite and gene clusters associated with altered carbon inputs, phenylpropanoid homeostasis and redox regulation due to elevated SA.  The metabolic and transcriptional responses overlapped substantially with those of heat-stressed wild types. Potential drivers of the SA response included receptor-like kinases and WRKY transcription factors previously implicated in defense.  Orthologs of NPR1 and thioredoxins, key players of Arabidopsis SA signaling, were not represented in the network.  However, members of the expanded Populus nucleoredoxin family exhibited increased expression and increased network connectivity in response to elevated SA, implicating SA as a mediator of redox regulation.  Together, our data revealed a sustained, growth-compatible oxidative response in SA-hyperaccumulating Populus.  The work extends previous findings on SA-homeostatic differences between Arabidopsis and Populus, and provides experimental evidence that components of the SA- and/or redox-mediated signaling pathway differ as well.