Oligogalacturonide-auxin antagonism does not require post-transcriptional gene silencing or stabilization of auxin response repressors in Arabidopsis

Alpha 1-4-linked oligogalacturonides (OGs) derived from plant cell walls are a class of damageassociated molecular patterns (DAMPs) and well-known elicitors of the plant immune response. Early transcript changes induced by OGs largely overlap those induced by flg22, a peptide derived from bacterial flagellin, a well-characterized microbe-associated molecular pattern (MAMP), although responses diverge over time. OGs also regulate growth and development of plant cells and organs, due to an auxin-antagonistic activity. The molecular basis of this antagonism is still unknown. Here we show that, in Arabidopsis, OGs inhibit adventitious root formation induced by auxin in leaf explants as well as the expression of several auxin-responsive genes. Genetic, biochemical and pharmacological experiments indicate that inhibition of auxin responses by OGs does not require ethylene, jasmonic acid and salicylic acid signaling and is independent of AtrbohD-mediated ROS production. Free IAA levels are not noticeably altered by OGs. Notably, OG- as well as flg22-auxin antagonism does not involve any of the following mechanisms: i) stabilization of auxin-response repressors; ii) decreased levels of auxin receptor transcripts through the action of microRNAs. Our results suggest that OGs and flg22 antagonize auxin responses independently of Aux/IAA repressor stabilization and of post-transcriptional gene silencing.