Analysis of the Plant bos1 Mutant Highlights Necrosis as an Efficient Defence Mechanism during D. dadantii/Arabidospis thaliana Interaction

Dickeya dadantii is a broad host range phytopathogenic bacterium provoking soft rot disease on many plants including Arabidopsis. We showed that, after D. dadantii infection, the expression of the Arabidopsis BOS1 gene was specifically induced by the production of the bacterial PelB/C pectinases able to degrade pectin. This prompted us to analyze the interaction between the bos1 mutant and D. dadantii. The phenotype of the infected bos1 mutant is complex. Indeed, maceration symptoms occurred more rapidly in the bos1 mutant than in the wild type parent but at a later stage of infection, a necrosis developed around the inoculation site that provoked a halt in the progression of the maceration. This necrosis became systemic and spread throughout the whole plant, a phenotype reminiscent of that observed in some lesion mimic mutants. In accordance with the progression of maceration symptoms, bacterial population began to grow more rapidly in the bos1 mutant than in the wild type plant but, when necrosis appeared in the bos1 mutant, a reduction in bacterial population was observed. From the plant side, this complex interaction between D. dadantii and its host includes an early plant defence response that comprises reactive oxygen species (ROS) production accompanied by the reinforcement of the plant cell wall by protein cross-linking. At later timepoints, another plant defence is raised by the death of the plant cells surrounding the inoculation site. This plant cell death appears to constitute an efficient defence mechanism induced by D. dadantii during Arabidopsis infection

New ABA-Hypersensitive Arabidopsis Mutants Are Affected in Loci Mediating Responses to Water Deficit and Dickeya dadantii Infection

On water deficit, abscisic acid (ABA) induces stomata closure to reduce water loss by transpiration. To identify Arabidopsis thaliana mutants which transpire less on drought, infrared thermal imaging of leaf temperature has been used to screen for suppressors of an ABA-deficient mutant (aba3-1) cold-leaf phenotype. Three novel mutants, called hot ABA-deficiency suppressor (has), have been identified with hot-leaf phenotypes in the absence of the aba3 mutation. The defective genes imparted no apparent modification to ABA production on water deficit, were inherited recessively and enhanced ABA responses indicating that the proteins encoded are negative regulators of ABA signalling. All three mutants showed ABA-hypersensitive stomata closure and inhibition of root elongation with little modification of growth and development in non-stressed conditions. The has2 mutant also exhibited increased germination inhibition by ABA, while ABA-inducible gene expression was not modified on dehydration, indicating the mutated gene affects early ABA-signalling responses that do not modify transcript levels. In contrast, weak ABA-hypersensitivity relative to mutant developmental phenotypes suggests that HAS3 regulates drought responses by both ABA-dependent and independent pathways. has1 mutant phenotypes were only apparent on stress or ABA treatments, and included reduced water loss on rapid dehydration. The HAS1 locus thus has the required characteristics for a targeted approach to improving resistance to water deficit. In contrast to has2, has1 exhibited only minor changes in susceptibility to Dickeya dadantii despite similar ABA-hypersensitivity, indicating that crosstalk between ABA responses to this pathogen and drought stress can occur through more than one point in the signalling pathway

Caractérisation génétique et physiologique de mutants de Nicotiana plumbaginifolia affectés dans leur photomorphogenèse: étude du rôle des phytochromes dans la régulation du métabolisme hormonal

*INRA, Centre de Versailles Diffusion du document : INRA, Centre de Versailles Diplôme : Dr. d'Universit

Gram-negative phytopathogenic bacteria, all hemibiotrophs after all?

International audienceIrrespective of the bacteria considered, after an initial biotrophic phase, in which the bacteria avoid the elicitation of or suppress plant defences, a switch to necrotrophic stage, in which the bacteria actively kill plant cells, is observed. Both phases are required to induce disease although the length of time of each phase will vary between bacteria. The length of each phase will also vary for the same bacteria between experimental conditions, as the inoculum concentration, age of the plant and the inoculation procedure will influence the outcome of the interaction. Another difficulty arises because the switch between the initial biotrophic phase and the later necrotrophic phase cannot be visualised through microscopic examination, as it has been performed with fungi. This makes the necrotroph/biotroph classification highly hazardous for phytopathogenic bacteria. Furthermore, all phytopathogenic bacteria possess weapons to supress or avoid premature elicitation of plants defences, indicating they initially develop on living hosts, and weapons involved in cell death induction, probably resulting in nutrient acquisition. Therefore, phytopathogenic bacteria should always be considered as hemibiotrophs. Although the biotrophic phase has been thoroughly described for type III-dependent phytobacteria, it has been overlooked for type II-dependent soft rot pectinolytic bacteria, probably because the asymptomatic biotrophic phase is highly dependent on environmental conditions and is difficult to reproduce in laboratory conditions. Moreover, because the zig-zag model has been so powerful in understanding the biotrophic phase of type III-dependent phytobacteria, their necrotrophic phase is often underestimated. To achieve a better comprehension of plant-bacterial interactions, the hemibiotrophic nature of these interactions should be recognized

Identification of two loci involved in phytochrome expression in Nicotiana plumbaginifolia and lethality of the corresponding double mutant

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Characterization of three hormone mutants of Nicotiana plumbaginifolia: evidence for a common ABA deficiency

International audienc

In vitro morphogenic characteristics of phytochrome mutants in Nicotiana plumbaginifolia are modified and correlated to high indole-3-acetic acid levels

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Characterization of the early response of Arabidopsis thaliana to Dickeya dadantii infection using expression profiling.

To draw a global view of plant responses to interactions with the phytopathogenic enterobacterale Dickeya dadantii, a causal agent of soft rot diseases on many plant species, we analysed the early Arabidopsis responses to D. dadantii infection. We performed a genome-wide analysis of the Arabidopsis thaliana transcriptome during D. dadantii infection and conducted a genetic study of identified responses. A limited set of genes related to plant defence or interactions with the environment were induced at an early stage of infection, with an over-representation of genes involved in both the metabolism of indole glucosinolates (IGs) and the jasmonate (JA) defence pathway. Bacterial type I and type II secretion systems are required to trigger the induction of IG and JA-related genes while the type III secretion system appears to partially inhibit these defence pathways. Using Arabidopsis mutants impaired in JA biosynthesis or perception, we showed that induction of some IG metabolism genes was COI1-dependent but, surprisingly, JA-independent. Moreover, characterisation of D. dadantii disease progression in Arabidopsis mutants impaired in JA or IG pathways showed that JA triggers an efficient plant defence response that does not involve IGs. The induction of the IG pathway by bacterial pathogens has been reported several times in vitro. This study shows for the first time, that this induction does indeed occur in planta, but also that this line of defence is ineffective against D. dadantii infection, in contrast to its role to counteract herbivorous or fungal pathogen attacks

Multifunctionality is affected by interactions between green roof plant species, substrate depth, and substrate type

Summary Green roofs provide ecosystem services through evapotranspiration and nutrient cycling that depend, among others, on plant species, substrate type, and substrate depth. However, no study has assessed thoroughly how interactions between these factors alter ecosystem functions and multifunctionality of green roofs. We simulated some green roof conditions in a pot experiment. We planted 20 plant species from 10 genera and five families (Asteraceae, Caryophyllaceae, Crassulaceae, Fabaceae, and Poaceae) on two substrate types (natural vs. artificial) and two substrate depths (10 cm vs. 30 cm). As indicators of major ecosystem functions, we measured aboveground and belowground biomasses, foliar nitrogen and carbon content, foliar transpiration, substrate water retention, and dissolved organic carbon and nitrates in leachates. Interactions between substrate type and depth strongly affected ecosystem functions. Biomass production was increased in the artificial substrate and deeper substrates, as was water retention in most cases. In contrast, dissolved organic carbon leaching was higher in the artificial substrates. Except for the Fabaceae species, nitrate leaching was reduced in deep, natural soils. The highest transpiration rates were associated with natural soils. All functions were modulated by plant families or species. Plant effects differed according to the observed function and the type and depth of the substrate. Fabaceae species grown on natural soils had the most noticeable patterns, allowing high biomass production and high water retention but also high nitrate leaching from deep pots. No single combination of factors enhanced simultaneously all studied ecosystem functions, highlighting that soil-plant interactions induce trade-offs between ecosystem functions. Substrate type and depth interactions are major drivers for green roof multifunctionality. K E Y W O R D S ecosystem services, evapotranspiration, nitrogen and carbon cycles, soil-plant interactions, trade-offs, urban ecology, water retention 2358 | DUSZA et Al