Cytokinin Sensing in Bacteria

Although it has long been known that bacteria detect and react to plant chemicals to establish an interaction, the cellular signaling mechanisms involved in these perception processes have hitherto remained obscure. Some exciting recent advances in the field have described, for the first time, how some phytopathogenic bacteria sense the host plant hormones, cytokinins. These discoveries not only advance the understanding of cell signaling circuitries engaged in cytokinin sensing in non-plant organisms, but also increase our knowledge of the broad role of these ancient molecules in regulating intra- and interspecific communications.

Comparative quantitative proteomics of osmotic signal transduction mutants in Botrytis cinerea explain mutant phenotypes and highlight interaction with cAMP and Ca2+ signalling pathways

Signal transduction (ST) is essential for rapid adaptive responses to changing environmental conditions. It acts through rapid post-translational modifications of signalling proteins and downstream effectors that regulate the activity and/or subcellular localisation of target proteins, or the expression of downstream genes. We have performed a quantitative, comparative proteomics study of ST mutants in the phytopathogenic fungus Botrytis cinerea during axenic growth under non-stressed conditions to decipher the roles of two kinases of the hyper-osmolarity pathway in B. cinerea physiology. We studied the mutants of the sensor histidine kinase Bos1 and of the MAP kinase Sak1. Label-free shotgun proteomics detected 2425 proteins, 628 differentially abundant between mutants and wild-type, 270 common to both mutants, indicating independent and shared regulatory functions for both kinases. Gene ontology analysis showed significant changes in functional categories that may explain in vitro growth and virulence defects of both mutants (secondary metabolism enzymes, lytic enzymes, proteins linked to osmotic, oxidative and cell wall stress). The proteome data also highlight a new link between Sak1 MAPK, cAMP and Ca²⁺ signalling. This study reveals the potential of proteomic analyses of signal transduction mutants to decipher their biological functions. TEXT-VULGARISATION: The fungus Botrytis cinerea is responsible for grey mold disease of hundreds of plant species. During infection, the fungus has to face important changes of its environment. Adaptation to these changing environmental conditions involves proteins of such called signal transduction pathways that regulate the production, activity or localisation of cellular components, mainly proteins. While the components of such signal transduction pathways are well known, their role globally understood, the precise impact on protein production remains unknown. In this study we have analysed and compared the global protein content of two Botrytis cinerea signal transduction mutants - both avirulent - to the pathogenic parental strain. The data of 628 differential proteins between mutants and wild-type, showed significant changes in proteins related to plant infection (secondary metabolism enzymes, lytic enzymes, proteins linked to osmotic, oxidative and cell wall stress) that may explain the virulence defects of both mutants. Moreover, we observed intracellular accumulation of secreted proteins in one of the mutants suggesting a potential secretion defect.

Cyclooxygenases and lipoxygenases are used by the fungus Podospora anserina to repel nematodes

Oxylipins are secondary messengers used universally in the living world for communication and defense. The paradigm is that they are produced enzymatically for the eicosanoids and non-enzymatically for the isoprostanoids. They are supposed to be degraded into volatile organic compounds (VOCs) and to participate in aroma production. Some such chemicals composed of eight carbons are also envisoned as alternatives to fossil fuels. In fungi, oxylipins have been mostly studied in Aspergilli and shown to be involved in signalling asexual versus sexual development, mycotoxin production and interaction with the host for pathogenic species. Through targeted gene deletions of genes encoding oxylipin-producing enzymes and chemical analysis of oxylipins and volatile organic compounds, we show that in the distantly-related ascomycete Podospora anserina, isoprostanoids are likely produced enzymatically. We show the disappearance in the mutants lacking lipoxygenases and cyclooxygenases of the production of 10-hydroxy-octadecadienoic acid and that of 1-octen-3-ol, a common volatile compound. Importantly, this was correlated with the inability of the mutants to repel nematodes as efficiently as the wild type. Overall, our data show that in this fungus, oxylipins are not involved in signalling development but may rather be used directly or as precursors in the production of odors against potential agressors.

SIGNIFICANCE

We analyzse the role in inter-kingdom communication of lipoxygenase (lox) and cyclooxygenase (cox) genes in the model fungus Podospora anserina. Through chemical analysis we define the oxylipins and volatile organic compounds (VOCs)produce by wild type and mutants for cox and lox genes, We show that the COX and LOX genes are required for the production of some eight carbon VOCs. We show that COX and LOX genes are involved in the production of chemicals repelling nematodes. This role is very different from the ones previously evidenced in other fungi.

Phenylpyrroles: 30 Years, Two Molecules and (Nearly) No Resistance

Phenylpyrroles are chemical analogs of the natural antifungal compound pyrrolnitrin. Fenpiclonil, but mainly fludioxonil are registered against multiple fungal crop diseases since over 25 years for seed or foliar treatment. They have severe physiological impacts on the pathogen, including membrane hyperpolarization, changes in carbon metabolism and the accumulation of metabolites leading to hyphal swelling and burst. The selection and characterization of mutants resistant to phenylpyrroles have revealed that these fungicides activate the fungal osmotic signal transduction pathway through their perception by a typical fungal hybrid histidine kinase (HHK). The HHK is prone to point mutations that confer fungicide resistance and affect its sensor domain, composed of tandem repeats of HAMP motifs. Fludioxonil resistant mutants have been selected in many fungal species under laboratory conditions. Generally they present severe impacts on fitness parameters. Since only few cases of field resistance specific to phenylpyrroles have been reported one may suspect that the fitness penalty of phenylpyrrole resistance is the reason for the lack of field resistance.