SummaryThe hyperaccumulation of zinc (Zn) and cadmium (Cd) is a constitutive property of the metallophyte Arabidopsis halleri. We therefore used Arabidopsis GeneChips to identify genes more active in roots of A. halleri as compared to A. thaliana under control conditions. The two genes showing highest expression in A. halleri roots relative to A. thaliana roots out of more than 8000 genes present on the chip encode a nicotianamine (NA) synthase and a putative Zn 2 uptake system.

No abstract

Two different thiol redox systems exist in plant chloroplasts, the ferredoxin-thioredoxin (Trx) system, which depends on ferredoxin reduced by the photosynthetic electron transport chain and, thus, on light, and the NADPH-dependent Trx reductase C (NTRC) system, which relies on NADPH and thus may be linked to sugar metabolism in the dark. Previous studies suggested, therefore, that the two different systems may have different functions in plants. We now report that there is a previously unrecognized functional redundancy of Trx f1 and NTRC in regulating photosynthetic metabolism and growth. In Arabidopsis (Arabidopsis thaliana) mutants, combined, but not single, deficiencies of Trx f1 and NTRC led to severe growth inhibition and perturbed light acclimation, accompanied by strong impairments of Calvin-Benson cycle activity and starch accumulation. Light activation of key enzymes of these pathways, fructose-1,6-bisphosphatase and ADP-glucose pyrophosphorylase, was almost completely abolished. The subsequent increase in NADPH-NADP + and ATP-ADP ratios led to increased nitrogen assimilation, NADP-malate dehydrogenase activation, and light vulnerability of photosystem I core proteins. In an additional approach, reporter studies show that Trx f1 and NTRC proteins are both colocalized in the same chloroplast substructure. Results provide genetic evidence that light-and NADPH-dependent thiol redox systems interact at the level of Trx f1 and NTRC to coordinately participate in the regulation of the Calvin-Benson cycle, starch metabolism, and growth in response to varying light conditions. Reversible disulfide bond formation between two Cys residues regulates structure and function of many proteins in diverse organisms (Cook and Hogg, 2013). Thiol disulfide exchange is controlled by thioredoxins (Trxs), which are small proteins containing a redoxactive disulfide group in their active site (Holmgren, 1985;Baumann and Juttner, 2002). The latter can be reduced to a dithiol by Trx reductases using NADPH or ferredoxin (Fdx) as electron donors. Due to their low redox midpoint potential, reduced Trxs are able to reductively cleave disulfide bonds in many target proteins and, thus, modulate their functions.Plants contain the most versatile Trx system found in all organisms with respect to the multiplicity of different isoforms and reduction systems (Buchanan and Balmer, 2005;Nikkanen and Rintamäki, 2014;Geigenberger and Fernie, 2014). The Arabidopsis (Arabidopsis thaliana) genome contains a complex family of Trxs, including up to 20 different isoforms grouped into seven subfamilies (Schürmann and Buchanan, 2008;Dietz and Pfannschmidt, 2011). Trxs f1-2, m1-4, x, y1-2, and z are located exclusively in the chloroplast, and Trx o is located exclusively in the mitochondria, while the eight Trx h representatives are distributed between the cytosol, nucleus, endoplasmic reticulum, and mitochondria (Meyer et al., 2012). The different Trxs can be reduced by two different redox systems, dependent on Fdx and Fdx-Trx reductase in the chlorop...

Arbuscular mycorrhiza (AM) symbioses contribute to global carbon cycles as plant hosts divert up to 20% of photosynthate to the obligate biotrophic fungi. Previous studies suggested carbohydrates as the only form of carbon transferred to the fungi.However, de novo fatty acid (FA) synthesis has not been observed in AM fungi in absence of the plant. In a forward genetic approach, we identified two Lotus japonicus mutants defective in AM-specific paralogs of lipid biosynthesis genes (KASI and GPAT6). These mutants perturb fungal development and accumulation of emblematic fungal 16:1ω5 FAs. Using isotopolog profiling we demonstrate that 13 C patterns of fungal FAs recapitulate those of wild-type hosts, indicating cross-kingdom lipid transfer from plants to fungi. This transfer of labelled FAs was not observed for the AM-specific lipid biosynthesis mutants. Thus, growth and development of beneficial AM fungi is not only fueled by sugars but depends on lipid transfer from plant hosts.

Large-scale metabolic profiling is expected to develop into an integral part of functional genomics and systems biology. The metabolome of a cell or an organism is chemically highly complex. Therefore, comprehensive biochemical phenotyping requires a multitude of analytical techniques. Here, we describe a profiling approach that combines separation by capillary liquid chromatography with the high resolution, high sensitivity, and high mass accuracy of quadrupole time-of-flight mass spectrometry. About 2,000 different mass signals can be detected in extracts of Arabidopsis roots and leaves. Many of these originate from Arabidopsis secondary metabolites. Detection based on retention times and exact masses is robust and reproducible. The dynamic range is sufficient for the quantification of metabolites. Assessment of the reproducibility of the analysis showed that biological variability exceeds technical variability. Tools were optimized or established for the automatic data deconvolution and data processing. Subtle differences between samples can be detected as tested with the chalcone synthase deficient tt4 mutant. The accuracy of time-of-flight mass analysis allows to calculate elemental compositions and to tentatively identify metabolites. In-source fragmentation and tandem mass spectrometry can be used to gain structural information. This approach has the potential to significantly contribute to establishing the metabolome of Arabidopsis and other model systems. The principles of separation and mass analysis of this technique, together with its sensitivity and resolving power, greatly expand the range of metabolic profiling.

Ralstonia solanacearum thrives in plant xylem vessels and causes bacterial wilt disease despite the low nutrient content of xylem sap. We found that R. solanacearum manipulates its host to increase nutrients in tomato xylem sap, enabling it to grow better in sap from infected plants than in sap from healthy plants. Untargeted GC/MS metabolomics identified 22 metabolites enriched in R. solanacearum-infected sap. Eight of these could serve as sole carbon or nitrogen sources for R. solanacearum. Putrescine, a polyamine that is not a sole carbon or nitrogen source for R. solanacearum, was enriched 76-fold to 37 µM in R. solanacearum-infected sap. R. solanacearum synthesized putrescine via a SpeC ornithine decarboxylase. A ΔspeC mutant required ≥ 15 µM exogenous putrescine to grow and could not grow alone in xylem even when plants were treated with putrescine. However, co-inoculation with wildtype rescued ΔspeC growth, indicating R. solanacearum produced and exported putrescine to xylem sap. Intriguingly, treating plants with putrescine before inoculation accelerated wilt symptom development and R. solanacearum growth and systemic spread. Xylem putrescine concentration was unchanged in putrescine-treated plants, so the exogenous putrescine likely accelerated disease indirectly by affecting host physiology. These results indicate that putrescine is a pathogen-produced virulence metabolite.

The enzyme ferrochelatase catalyses the formation of protoheme by inserting Fe(2+) into protoporphyrin IX. Although most organisms express only one ferrochelatase, all land plants analysed so far possess at least two ferrochelatase proteins. Analysis of publicly available expression data suggests that the two Arabidopsis thaliana ferrochelatases, FC1 and FC2, serve different functions, corroborating previous assumptions. Co-expression analysis of FC1 and FC2, together with microarray analyses, implies that fc1 and fc2 trigger different modes of plastid signalling in roots and leaves, respectively, and indicates that FC2 might be involved in stress responses. Thus, loss of FC2 increases resistance to salt and flagellin treatment. Whereas fc1 plants showed no obvious mutant phenotype, fc2 mutants formed abnormally small, pale green rosette leaves; were low in chlorophylls, carotenoids and several photosynthetic proteins; and their photosynthetic performance was impaired. These phenotypes are attenuated by growth in continuous light, in agreement with the finding that fc2 plants accumulate protochlorophyllide and display a fluorescent (flu) phenotype in the dark. In consequence we show that, contrary to earlier suggestions, FC2 produces heme not only for photosynthetic cytochromes, but also for proteins involved in stress responses, whereas the impairment of FC1 apparently interferes only marginally with stress responses.

Inactivation of thioredoxin