Recent studies have suggested that bacterial volatiles play an important role in bacterial-plant interactions. However, few reports of bacterial species that produce plant growth modulating volatiles have been published, raising the question whether this is just an anecdotal phenomenon. To address this question, we performed a large screen of strains originating from the soil for volatile-mediated effects on Arabidopsis thaliana. All of the 42 strains tested showed significant volatile-mediated plant growth modulation, with effects ranging from plant death to a sixfold increase in plant biomass. The effects of bacterial volatiles were highly dependent on the cultivation medium and the inoculum quantity. GC-MS analysis of the tested strains revealed over 130 bacterial volatile compounds. Indole, 1-hexanol and pentadecane were selected for further studies because they appeared to promote plant growth. None of these compounds triggered a typical defence response, using production of ethylene and of reactive oxygen species (ROS) as read-outs. However, when plants were challenged with the flg-22 epitope of bacterial flagellin, a prototypical elicitor of defence responses, additional exposure to the volatiles reduced the flg-22-induced production of ethylene and ROS in a dose-dependent manner, suggesting that bacterial volatiles may act as effectors to inhibit the plant's defence response.

In Arabidopsis thaliana, the endogenous danger peptides, AtPeps, have been associated with plant defences reminiscent of those induced in pattern-triggered immunity. AtPeps are perceived by two homologous receptor kinases, PEPR1 and PEPR2, and are encoded in the C termini of the PROPEP precursors. Here, we report that, contrary to the seemingly redundant AtPeps, the PROPEPs fall at least into two distinct groups. As revealed by promoter-β-glucuronidase studies, expression patterns of PROPEP1-3, -5, and -8 partially overlapped and correlated with those of the PEPR1 and -2 receptors, whereas those of PROPEP4 and -7 did not share any similarities with the former. Moreover, bi-clustering analysis indicated an association of PROPEP1, -2, and -3 with plant defence, whereas PROPEP5 expression was related to patterns of plant reproduction. In addition, at the protein level, PROPEPs appeared to be distinct. PROPEP3::YFP (fused to yellow fluorescent protein) was present in the cytosol, but, in contrast to previous predictions, PROPEP1::YFP and PROPEP6::YFP localized to the tonoplast. Together with the expression patterns, this could point to potentially non-redundant roles among the members of the PROPEP family. By contrast, their derived AtPeps, including the newly reported AtPep8, when applied exogenously, provoked activation of defence-related responses in a similar manner, suggesting a high level of functional redundancy between the AtPeps. Taken together, our findings reveal an apparent antagonism between AtPep redundancy and PROPEP variability, and indicate new roles for PROPEPs besides plant immunity.

The plant's innate immune system detects potential biotic threats through recognition of microbe-associated molecular patterns (MAMPs) or danger-associated molecular patterns (DAMPs) by pattern recognition receptors (PRR). A central regulator of pattern-triggered immunity (PTI) is the BRI1-associated kinase 1 (BAK1), which undergoes complex formation with PRR upon ligand binding. Although viral patterns inducing PTI are well known from animal systems, nothing similar has been reported for plants. Rather, antiviral defense in plants is thought to be mediated by post-transcriptional gene silencing of viral RNA or through effector-triggered immunity, i.e., recognition of virus-specific effectors by resistance proteins. Nevertheless, infection by compatible viruses can also lead to the induction of defense gene expression, indicating that plants may also recognize viruses through PTI. Here, we show that PTI, or at least the presence of the regulator BAK1, is important for antiviral defense of Arabidopsis plants. Arabidopsis bak1 mutants show increased susceptibility to three different RNA viruses during compatible interactions. Furthermore, crude viral extracts but not purified virions induce several PTI marker responses in a BAK1-dependent manner. Overall, we conclude that BAK1-dependent PTI contributes to antiviral resistance in plants.

HighlightPlant elicitor peptides (Peps) co-evolved with their receptors, resulting in interfamily incompatibility of Pep recognition. In contrast, operation of defence pathways by Pep receptors is conserved within the flowering plants.

The endogenous Arabidopsis (Arabidopsis thaliana) peptides, AtPeps, elicit an innate immune response reminiscent of patterntriggered immunity. Detection of various danger signals, including microbe-associated molecular patterns (MAMPs), leads to elevated transcription of PROPEPs, the AtPep precursors, and PEPRs, the AtPep receptors. It has been hypothesized that AtPeps are involved in enhancing pattern-triggered immunity. Following this idea, we analyzed the relationship between MAMP-and AtPep-elicited signaling. We found that the perception of MAMPs enhanced a subsequent AtPep-triggered production of reactive oxygen species (ROS). Intriguingly, other components of AtPep-triggered immunity like Ca 2+ influx, mitogen-activated protein kinase phosphorylation, ethylene production, and expression of early defense genes, as well as ROS-activated genes, remained unchanged. By contrast, treatment with methyl jasmonate promoted an increase of all analyzed AtPep-triggered responses. We positively correlated the intensities of generic AtPep-triggered responses with the abundance of the two AtPep receptors by generating constitutively expressing PEPR1 and PEPR2 transgenic lines and by analyzing pepr1 and pepr2 mutants. Further, we show that enhanced, as well as basal, ROS production triggered by AtPeps is absent in the double mutant of the respiratory burst oxidase homologs D and F (rbohD rbohF). We present evidence that the enhancement of AtPep-triggered ROS is not based on changes in the ROS detoxification machinery and is independent of mitogen-activated protein kinase and Ca 2+ signaling pathways. Taken together, these results indicate an additional level of regulation besides receptor abundance for the RbohD/RbohF-dependent production of AtPep-elicited ROS, which is specifically operated by MAMP-triggered pathways.