Molecular mechanisms underlying cross-talk phenomena in hormone-regulated plant defense responses in Arabidopsis thaliana
Final Report Abstract
The plant hormone salicylic acid (SA) is essential for defense responses against biotrophic pathogens. At the same time, it triggers increased susceptibility of plants against necrotrophic attackers by suppressing part of the jasmonic acid (JA)/ethylene (ET) defense response. Within the framework of this DFG-funded project we show that this diseasepromoting SA effect is abolished in plants lacking the three related bZIP transcription factors TGA2, TGA5 and TGA6 (class-II TGAs). Microarray analysis unraveled that expression of all those ET-induced genes that are subject to the negative effect of SA depend on class-II TGAs. Most likely, all these genes are also regulated by the AP2/ERF transcription factor ORA59. The ORA59 promoter contains an essential TGA binding site and is associated in vivo with class-II TGAs. Their positive function on the ORA59 promoter is abrogated by SA through a still unknown mechanism (Zander et al., 2014). Still, further TGA-dependent SA-suppressible mechanisms that operate downstream of the ORA59 promoter have to be postulated. We found in the second yet unpublished part of the funded work that this additional regulation is not due to a posttranscriptional mechanism that leads to decreased ORA59 levels as suggested by van der Does et al., 2013. We also think that the recent publication on the specific role of ANAC032 as a modulator of the JA/SA cross-talk has to be revisited (Allu et al., 2016). Still, members of the ATAF-clade of NAC transcription factors might function as SA-induced TGA-dependent negative regulators of PDF1.2 expression. Since the JA/ET-induced marker gene PDF1.2 is not a representative member of all those genes that are subject to the SA cross-talk, generalizations derived from analysis of PDF1.2 expression have to be avoided. Moreover, variability in the behavior of this hormonal network requires at least ten independent experiments. Even van der Does pointed out that the negative effect of SA on ORA59 protein levels occurred only in three out of five experiments. It would have been essential to test, whether this frequency is reproducible with the next five experiments. Since we consider only reproducible data as relevant for publication, we cannot yet deliver. Work finalizing our study is ongoing.
