Iron status and root cell morphology of Arabidopsis thaliana as modified by a bacterial ferri-siderophore.

We previously provided evidence for the contribution of pyoverdine to the iron nutrition of Arabidopsis. In the present article, we further analyze the mechanisms and physiology of the adaptations underlying plant iron nutrition through Fe(III)-pyoverdine (Fe(III)-pvd). An integrated approach combining microscopy and nanoscale secondary ion mass spectrometry (NanoSIMS) on plant samples was adopted to localize pyoverdine in planta and assess the impact of this siderophore on the plant iron status and root cellular morphology. The results support a possible plant uptake mechanism of the Fe(III)-pvd complex by epidermal root cells via a non-reductive process associated with the presence of more vesicles. Pyoverdine was transported to the central cylinder via the symplastic and/or trans-cellular pathway(s), suggesting a possible root-to-shoot translocation. All these processes led to enhanced plant iron nutrition, as previously shown. Overall, these findings suggest that bacterial siderophores contribute to plant iron uptake and homeostasis.

Inhibition of broomrape germination by 2,4-diacetylphloroglucinol produced by environmental Pseudomonas.

Parasitic weeds such as broomrapes (Phelipanche ramosa and Orobanche cumana) cause severe damage to crops and their development must be controlled. Given that phloroglucinol compounds (PGCs) produced by environmental Pseudomonas could be toxic towards certain plants, we assessed the potential herbicidal effect of the bacterial model Pseudomonas ogarae F113, a PGCs-producing bacterium, on parasitic weed. By combining the use of a mutagenesis approach and of pure PGCs, we evaluated the in vitro effect of PGC-produced by P. ogarae F113 on broomrape germination and assessed the protective activity of a PGC-producing bacteria on oilseed rape (Brassica napus) against P. ramosa in non-sterile soils. We showed that the inhibition of the germination depends on the PGCs molecular structure and their concentrations as well as the broomrape species and pathovars. This inhibition caused by the PGCs is irreversible, causing a brown coloration of the broomrape seeds. The inoculation of PGCs-producing bacteria limited the broomrape infection of P. ramosa, without affecting the host growth. Moreover, elemental profiling analysis of oilseed rape revealed that neither F113 nor applied PGCs affected the nutrition capacity of the oilseed rape host. Our study expands the knowledge on plant-beneficial Pseudomonas as weed biocontrol agents and opens new avenues for the development of natural bioherbicides to enhance crop yield.

Importance of the Rhizosphere Microbiota in Iron Biofortification of Plants.

Increasing the iron content of plant products and iron assimilability represents a major issue for human nutrition and health. This is also a major challenge because iron is not readily available for plants in most cultivated soils despite its abundance in the Earth's crust. Iron biofortification is defined as the enhancement of the iron content in edible parts of plants. This biofortification aims to reach the objectives defined by world organizations for human nutrition and health while being environment friendly. A series of options has been proposed to enhance plant iron uptake and fight against hidden hunger, but they all show limitations. The present review addresses the potential of soil microorganisms to promote plant iron nutrition. Increasing knowledge on the plant microbiota and plant-microbe interactions related to the iron dynamics has highlighted a considerable contribution of microorganisms to plant iron uptake and homeostasis. The present overview of the state of the art sheds light on plant iron uptake and homeostasis, and on the contribution of plant-microorganism (plant-microbe and plant-plant-microbe) interactions to plant nutritition. It highlights the effects of microorganisms on the plant iron status and on the co-occurring mechanisms, and shows how this knowledge may be valued through genetic and agronomic approaches. We propose a change of paradigm based on a more holistic approach gathering plant and microbial traits mediating iron uptake. Then, we present the possible applications in plant breeding, based on plant traits mediating plant-microbe interactions involved in plant iron uptake and physiology.

Impact of Bacterial Siderophores on Iron Status and Ionome in Pea.

Including more grain legumes in cropping systems is important for the development of agroecological practices and the diversification of protein sources for human and animal consumption. Grain legume yield and quality is impacted by abiotic stresses resulting from fluctuating availabilities in essential nutrients such as iron deficiency chlorosis (IDC). Promoting plant iron nutrition could mitigate IDC that currently impedes legume cultivation in calcareous soils, and increase the iron content of legume seeds and its bioavailability. There is growing evidence that plant microbiota contribute to plant iron nutrition and might account for variations in the sensitivity of pea cultivars to iron deficiency and in fine to seed nutritional quality. Pyoverdine (pvd) siderophores synthesized by pseudomonads have been shown to promote iron nutrition in various plant species (Arabidopsis, clover and grasses). This study aimed to investigate the impact of three distinct ferripyoverdines (Fe-pvds) on iron status and the ionome of two pea cultivars (cv.) differing in their tolerance to IDC, (cv. S) being susceptible and (cv. T) tolerant. One pvd came from a pseudomonad strain isolated from the rhizosphere of cv. T (pvd1T), one from cv. S (pvd2S), and the third from a reference strain C7R12 (pvdC7R12). The results indicated that Fe-pvds differently impacted pea iron status and ionome, and that this impact varied both according to the pvd and the cultivar. Plant iron concentration was more increased by Fe-pvds in cv. T than in cv. S. Iron allocation within the plant was impacted by Fe-pvds in cv. T. Furthermore, Fe-pvds had the greatest favorable impact on iron nutrition in the cultivar from which the producing strain originated. This study evidences the impact of bacterial siderophores on pea iron status and pea ionome composition, and shows that this impact varies with the siderophore and host-plant cultivar, thereby emphasizing the specificity of these plant-microorganisms interactions. Our results support the possible contribution of pyoverdine-producing pseudomonads to differences in tolerance to IDC between pea cultivars. Indeed, the tolerant cv. T, as compared to the susceptible cv. S, benefited from bacterial siderophores for its iron nutrition to a greater extent.

Molecular response to nitrogen starvation by Frankia alni ACN14a revealed by transcriptomics and functional analysis with a fosmid library in Escherichia coli.

The transcriptome of Frankia alni strain ACN14a was compared between in vitro ammonium-replete (N-replete) and ammonium-free dinitrogen-fixing (N-fixing) conditions using DNA arrays. A Welch-test (p < 0.05) revealed significant upregulation of 252 genes under N-fixing vs. N-replete (fold-change (FC) ≥ 2), as well as significant downregulation of 48 other genes (FC ≤ 0.5). Interestingly, there were 104 Frankia genes upregulated in vitro that were also significantly upregulated in symbiosis with Alnus glutinosa, while the other 148 genes were not, showing that the physiology of in vitro fixation is markedly different from that under symbiotic conditions. In particular,in vitro fixing cells were seen to upregulate genes identified as coding for a nitrite reductase, and amidases that were not upregulated in symbiosis. Confirmatory assays for nitrite reductase showed that Frankia indeed reduced nitrite and used it as a nitrogen source. An Escherichia coli fosmid clone carrying the nirB region was able to grow better in the presence of 5 mM nitrite than without it, confirming the function of the genome region. The physiological pattern that emerges shows that Frankia undergoes nitrogen starvation that induces a molecular response different from that seen in symbiosis.