Quorum sensing is a bacterial communication mechanism that controls genes, enabling bacteria to live as communities, such as biofilms. Homoserine lactone (HSL) molecules function as quorum-sensing signals for Gram-negative bacteria. Plants also produce previously unidentified compounds that affect quorum sensing. We identified rosmarinic acid as a plant-derived compound that functioned as an HSL mimic. In vitro assays showed that rosmarinic acid bound to the quorum-sensing regulator RhlR of Pseudomonas aeruginosa PAO1 and competed with the bacterial ligand N-butanoyl-homoserine lactone (C4-HSL). Furthermore, rosmarinic acid stimulated a greater increase in RhlR-mediated transcription in vitro than that of C4-HSL. In P. aeruginosa, rosmarinic acid induced quorum sensing–dependent gene expression and increased biofilm formation and the production of the virulence factors pyocyanin and elastase. Because P. aeruginosa PAO1 infection induces rosmarinic acid secretion from plant roots, our results indicate that rosmarinic acid secretion is a plant defense mechanism to stimulate a premature quorum-sensing response. P. aeruginosa is a ubiquitous pathogen that infects plants and animals; therefore, identification of rosmarinic acid as an inducer of premature quorum-sensing responses may be useful in agriculture and inform human therapeutic strategies.
Andrés Corral-Lugo, Abdelali Daddaoua, Alvaro Ortega, Manuel Espinosa-Urgel, and Tino Krell.
Science Signaling 05 Jan 2016. Vol. 9, Issue 409, pp. ra1. DOI: 10.1126/scisignal.aaa8271
A single lineage of Nicotiana benthamiana is widely used as amodel plant and has been instrumental in making revolutionarydiscoveries about RNA interference (RNAi), viral defenceand vaccine production. It is peerless in its susceptibility toviruses and its amenability in transiently expressing transgenes.These unparalleled characteristics have been associatedboth positively and negatively with a disruptive insertionin the RNA-dependent RNA polymerase 1 gene, Rdr1. For aplant so routinely used in research, the origin, diversity andevolution of the species, and the basis of its unusual abilities,have been relatively unexplored. Here, by comparison with wild accessions from across the spectrum of the species’ natural distribution, we show that the laboratory strain of N. benthamiana is an extremophile originating from a population that has retained a mutation in Rdr1 for ∼0.8 Myr and thereby traded its defence capacity for early vigour and survival in the extreme habitat of central Australia. Reconstituting Rdr1 activity in this isolate provided protection. Silencing the functional allele in a wild strain rendered it hypersusceptible and was associated with a doubling of seed size and enhanced early growth rate. These findings open the way to a deeper understanding of the delicate balance between protection and vigour.
Julia Bally, Kenlee Nakasugi, Fangzhi Jia, Hyungtaek Jung, Simon Y.W. Ho, MeiWong, ChloeM. Paul, Fatima Naim, Craig C. Wood, Ross N. Crowhurst, Roger P. Hellens, James L. Dale and Peter M. Waterhouse.
Nature Plants 1, Article number: 15165 (2015) doi:10.1038/nplants.2015.165
Thousands of putative biosynthetic genes in Arabidopsis thaliana have no known function, which suggests that there are numerous molecules contributing to plant fitness that have not yet been discovered. Prime among these uncharacterized genes are cytochromes P450 upregulated in response to pathogens. Here we start with a single pathogen-induced P450, CYP82C2, and use a combination of untargeted metabolomics and coexpression analysis to uncover the complete biosynthetic pathway to 4-hydroxyindole-3-carbonyl nitrile (4-OH-ICN), a previously unknown Arabidopsis metabolite. This metabolite harbours cyanogenic functionality that is unprecedented in plants and exceedingly rare in nature; furthermore, the aryl cyanohydrin intermediate in the 4-OH-ICN pathway reveals a latent capacity for cyanogenic glucoside biosynthesisin Arabidopsis. By expressing 4-OH-ICN biosynthetic enzymes in Saccharomyces cerevisiae and Nicotiana benthamiana, we reconstitute the complete pathway in vitro and in vivo and validate the functions of its enzymes. Arabidopsis 4-OH-ICN pathway mutants show increased susceptibility to the bacterial pathogen Pseudomonas syringae, consistent with a role in inducible pathogen defence. Arabidopsis has been the pre-eminent model system for studying the role of small molecules in plant innate immunity; our results uncover a new branch of indole metabolism distinct from the canonical camalexin pathway, and support a role for this pathway in the Arabidopsis defence response. These results establish a more complete framework for understanding how the model plant Arabidopsis uses small molecules in pathogen defence.
Jakub Rajniak, Brenden Barco, Nicole K. Clay and Elizabeth S. Sattely