Abstract
α-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.
Highlights
Dipartimento di Biologia e Biotecnologie “C. Darwin,” Istituto Pasteur-Fondazione Cenci Bolognetti, Sapienza Universitadi Roma, 00185 Rome, Italy 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
Arabidopsis leaf explants cultured for 15 d on basal medium containing IAA developed adventitious roots, which emerged in a polar manner at the cut proximal edge of the explants
Histochemical analysis showed that GUS activity was localized at the tip of the root apex in untreated seedlings and, upon treatment with IAA, extended to the entire root apical meristem and across the division zone, up to the differentiation zone; GUS activity appeared at the sites of emergence of lateral root primordia
Summary
Dipartimento di Biologia e Biotecnologie “C. Darwin,” Istituto Pasteur-Fondazione Cenci Bolognetti, Sapienza Universitadi Roma, 00185 Rome, Italy 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. The observations that defense responses induced by a Phytophthora glucan preparation are inhibited by auxin in protoplasts (Leguay and Jouanneau, 1987) and that auxin-induced growth is competitively inhibited by elicitor-active oligogalacturonides (OGs) in pea (Pisum sativum) stem segments (Branca et al, 1988) provided, to our knowledge, the first evidence of an antagonistic action between this hormone and elici-. OGs and flg[22] activate defense responses effective against the microbial pathogens Botrytis cinerea and Pseudomonas syringae, respectively, independently of SA, ET, and JA (Zipfel et al, 2004; Ferrari et al, 2007). Both elicitors trigger a fast and transient response characterized by activation of early stages of multiple defense signaling pathways. SA-dependent secretory pathway genes and PR1 expression are substantially induced only by flg[22] (Denoux et al, 2008)
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