A Model for the Development of the Rhizobial and Arbuscular Mycorrhizal Symbioses in Legumes and Its Use to Understand the Roles of Ethylene in the Establishment of these two Symbioses

We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis–outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants

Effects of long-term heavy metals contamination on soil microbial characteristics in calcareous agricultural lands (Saiss plain, North Morocco)

International audienceSoil is a natural resource and support for all economic activities. Its role is particularly crucial in agriculture. However, agro systems are vulnerable to all natural or anthropogenic disturbances such as erosion, organic or inorganic pollution. In this study, conducted in the region of Fez (Morocco), effects of irrigation with contaminated watercourses were investigated on soil heavy metal contents and microbiological characteristics, in comparison with a control soil. The total heavy metal concentrations of contaminated soil samples were 63.4, 201.2, 291.2 and 33.2 mg/kg for Cr, Cu, Zn, and Ni respectively. Quantitative analysis of soil microbial populations showed that certain groups of soil microbes (actinomycetes and fungi) were particularly sensitive to long-term contamination and revealed a strong negative correlation with Cr, Cu and Zn. While a notably higher number of cultivable bacteria was observed in the contaminated soil compared to the control

Effet des polyamines sur la réduction du chrome hexavalent par des souches bactériennes et leur résistance

Effect of polyamines on the reduction of hexavalent chromium by bacterial strains and their resistance. Polyamines are involved in several functions in bacteria. In this study, we examined the role of polyamines in hexavalent chromium (Cr[VI]) reduction by three bacterial strains isolated from sites contaminated by tannery effluents. The strains were identified as Serratia proteamaculans, Leucobacter chromiireducens and Brevibacterium frigoritolerans. The inhibition of polyamine synthesis by α-difluoromethylornithine (DFMO) caused a decrease in Cr(VI) tolerance in the bacterial isolates, indicating the role of endogenous polyamines in resistance to Cr(VI). The exogenous application of polyamines (putrescine, spermidine, cadaverine) was found to stimulate growth and Cr(VI) reduction by the bacterial strains in Luria-Bertani medium. The results show the importance of polyamines in response to heavy metal stresses, especially Cr(VI) toxicity

The effect of polyamines on endomycorrhizal infection of wild-type <i>Pisum sativum</i>, cv. Frisson (nod⁺myc⁺) and two mutants (nod^–myc⁺ and nod^–myc^–)

The effect of four polyamines, putrescine, cadaverine, spermidine and spermine, on arbuscular mycorrhizal (AM) infection by Glomus intraradices was tested on Pisum sativum, cv. Frisson (nod+myc+) and two isogenic mutants of this cultivar, P56 (nod–myc+) and P2 (nod–myc–). Polyamines were applied at 0 and 5.10–4 M as soil drenches. Endomycorrhizal infection parameters were measured 3 weeks after inoculation. Polyamine treatment significantly increased the frequency of mycorrhizal infection in the myc+ pea lines (cv. Frisson and P56) and the number of appressoria formed in the myc– line (P2). A positive correlation was found between polyamine chain length and their stimulation of fungal development. Results are discussed in relation to the possibility that polyamines may act as regulatory factors in plant-AM fungus interactions.</p

Impact du mode de gestion de la subéraie de la Maamora (Maroc) sur la diversité des champignons ectomycorhiziens associés à Quercus suber

The cork oak forest is an ecosystem playing a major role in Moroccan socio-economy and biodiversity conservation. However, this ecosystem is negatively impacted by extensive human-and climate-driven pressures, causing a strong decrease in its distribution and a worsening of the desertification processes. This study aims at characterising the impact of cork oak forest management on a major actor of its functioning, the ectomycorrhizal (EcM) fungal community associated with Quercus suber, and the determination of EcM bio-indicators. The EcM fungal community has been monitored during spring and winter seasons in two sites of the Moroccan Maamora forest, corresponding to a forest site either impacted by human activities or protected. A significant impact of cork oak forest management on the EcM fungal community has been revealed, with major differences during the summer season. The results confirmed the potential ecological significance of several EcM fungi (e.g., Cenococcum) in the sustainability of the cork oak forest functioning, but also the significant association of certain EcM fungi (Pachyphloeus, Russula, Tomentella) with a perturbation or a season, and consequently to the cork oak forest status or to climatic conditions, respectively. The development of study at the Mediterranean scale may improve the robustness of ecological models to predict the impact of global changes on this emblematic ecosystem of Mediterranean basin