Oligogalacturonide-Auxin Antagonism Does Not Require Posttranscriptional Gene Silencing or Stabilization of Auxin Response Repressors in Arabidopsis  

Abstract

a-1-4-Linked oligogalacturonides (OGs) derived from plant cell walls are a class of damage-associated molecular patterns 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, 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 (Arabidopsis thaliana), 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 RESPIRATORY BURST OXIDASE HOMOLOGUE D-mediated reactive oxygen species production. Free indole-3-acetic acid levels are not noticeably altered by OGs. Notably, OG-as well as flg22-auxin antagonism does not involve any of the following mechanisms: (1) stabilization of auxin-response repressors; (2) decreased levels of auxin receptor transcripts through the action of microRNAs. Our results suggest that OGs and flg22 antagonize auxin responses independently of Aux/Indole-3-Acetic Acid repressor stabilization and of posttranscriptional gene silencing.

The Arabidopsis NUCLEUS- AND PHRAGMOPLAST-LOCALIZED KINASE1-Related Protein Kinases Are Required for Elicitor-Induced Oxidative Burst and Immunity

“…This is in agreement with previous observations showing that the antagonism mediated by OGs is independent of the accumulation of extracellular hydrogen peroxide (Bellincampi et al, 2000;Savatin et al, 2011). Notably, the loss of ANP function reveals that the inhibition of auxin-induced gene expression can be uncoupled from the activation of defense response and suggests that the former effect is mediated by an ANP-independent signal transduction pathway.…”

“…Therefore, ANPs have been proposed as a molecular link between oxidative stress and auxin signal transduction. However, we have previously shown that double knockout (KO) anp mutants exhibit a normal auxin/ OG antagonism (Savatin et al, 2011), thus providing no support to this conclusion. Our work left unanswered whether this was due to the presence of a functional third ANP member or to a lack of involvement of ANPs in elicitor/auxin antagonism and, in general, in the response to OGs, because a main role of ANP1 in response to flg22 had been previously ruled out (Asai et al, 2002).…”

“…The mechanistic basis of the elicitor/auxin antagonism remains obscure. So far, this antagonism has been shown to be independent of salicylic acid, jasmonic acid, and ethylene and does not involve the silencing of TRANSPORT INHIBITOR RESPONSE1/AUXIN F-BOX genes nor miR393 activity or posttranscriptional gene silencing (Savatin et al, 2011).…”

“…In fact, previous data show that (1) the normal auxin-OG antagonism behavior of the anp double KO mutants (Wassilewskija [Ws] background) may be due to a redundant role of ANPs and the presence of a functional third member (Savatin et al, 2011) and (2) an anp triple mutant could not be obtained by crossing (Krysan et al, 2002). Triple mutants were constructed by down-regulating, in an inducible manner, the expression of the third gene family member in both the double mutants anp1 anp2 and anp2 anp3.…”

“…For example, flg22 and OGs generate an extracellular oxidative burst mediated by RESPIRATORY BURST OXIDASE HOMOLOG D (RbohD) and induce protection against the necrotrophic fungus Botrytis cinerea independently of the ethylene, jasmonic acid, and salicylic acid pathways and of the RbohD-mediated production of reactive oxygen species (ROS; Zhang et al, 2007;Galletti et al, 2008). The inhibition of auxin responses is another feature shared by PAMPs and DAMPs (Savatin et al, 2011); in the case of OGs, the inhibition of auxin responses has been described as a true antagonism (Branca et al, 1988;Bellincampi et al, 1993;Savatin et al, 2011). On the other hand, microarray analyses indicate that late responses to the two classes of elicitors are considerably different .…”