We studied the response of the photosynthetic alpha-proteobacterium Rhodobacter sphaeroides to iron limitation in order to get first insights into the underlying mechanisms and the link between iron metabolism and oxidative stress. Our data reveal the production of elevated levels of reactive oxygen species upon iron limitation, nevertheless the response to iron limitation shows clear differences to the oxidative stress response of R. sphaeroides. While most genes of the oxidative stress response were not induced by iron limitation, we observed an upregulation of the alternative sigma factor RpoE, which has a main role in the regulation of the defence to singlet oxygen. Deletion of the Fur orthologue RSP_2494, which was designated Mur as a result of a proposed regulatory role in manganese metabolism, revealed that this protein is involved in regulation of the iron metabolism in R. sphaeroides. One predicted target of Fur/Mur is the sit operon encoding a Mn(2+) /Fe(2+) transport system. The basal level of sitA was higher in a fur/mur deletion strain compared with the wild type, which is in agreement with a repressor function of the Fur/Mur protein. In addition, we could also demonstrate a function of the Fur/Mur protein in manganese homeostasis.

In Rhizobia the Irr protein is an important regulator for iron-dependent gene expression. We studied the role of the Irr homolog RSP_3179 in the photosynthetic alpha-proteobacterium Rhodobacter sphaeroides. While Irr had little effect on growth under iron-limiting or non-limiting conditions its deletion resulted in increased resistance to hydrogen peroxide and singlet oxygen. This correlates with an elevated expression of katE for catalase in the Irr mutant compared to the wild type under non-stress conditions. Transcriptome studies revealed that Irr affects the expression of genes for iron metabolism, but also has some influence on genes involved in stress response, citric acid cycle, oxidative phosphorylation, transport, and photosynthesis. Most genes showed higher expression levels in the wild type than in the mutant under normal growth conditions indicating an activator function of Irr. Irr was however not required to activate genes of the iron metabolism in response to iron limitation, which showed even stronger induction in the absence of Irr. This was also true for genes mbfA and ccpA, which were verified as direct targets for Irr. Our results suggest that in R. sphaeroides Irr diminishes the strong induction of genes for iron metabolism under iron starvation.

Under unfavorable growth conditions, bacteria enter stationary phase and can maintain cell viability over prolonged periods with no increase in cell number. To obtain insights into the regulatory mechanisms that allow bacteria to resume growth when conditions become favorable again (outgrowth), we performed global transcriptome analyses at different stages of growth for the alphaproteobacterium Rhodobacter sphaeroides. The majority of genes were not differentially expressed across growth phases. After a short stationary phase (about 20 h after growth starts to slow down), only 7% of the genes showed altered expression (fold change of Ͼ1.6 or less than Ϫ1.6, corresponding to a log 2 fold change of Ͼ0.65 or less than Ϫ0.65, respectively) compared to expression at exponential phase. Outgrowth induced a distinct response in gene expression which was strongly influenced by the length of the preceding stationary phase. After a long stationary phase (about 64 h after growth starts to slow down), a much larger number of genes (15.1%) was induced in outgrowth than after a short stationary phase (1.7%). Many of those genes are known members of the RpoHI/RpoHII regulons and have established functions in stress responses. A main effect of RpoHI on the transcriptome in outgrowth after a long stationary phase was confirmed. Growth experiments with mutant strains further support an important function in outgrowth after prolonged stationary phase for the RpoHI and RpoHII sigma factors.IMPORTANCE In natural environments, the growth of bacteria is limited mostly by lack of nutrients or other unfavorable conditions. It is important for bacterial populations to efficiently resume growth after being in stationary phase, which may last for long periods. Most previous studies on growth-phase-dependent gene expression did not address outgrowth after stationary phase. This study on growth-phase-dependent gene regulation in a model alphaproteobacterium reveals, for the first time, that the length of the stationary phase strongly impacts the transcriptome during outgrowth. The alternative sigma factors RpoHI and RpoHII, which are important regulators of stress responses in alphaproteobacteria, play a major role during outgrowth following prolonged stationary phase. These findings provide the first insight into the regulatory mechanisms enabling efficient outgrowth.

Metal homeostasis is important in all living cells in order to provide sufficient amounts of metal ions for biological processes but to prevent toxic effects by excess amounts. Here we show that the gene product of RSP_2889 of the facultatively photosynthetic bacterium Rhodobacter sphaeroides is homologous to CueR, a regulator of copper metabolism in Escherichia coli and other bacteria. CueR binds to the promoter regions of genes for a copper-translocating ATPase and for a copper chaperone and is responsible for their high expression when cells are exposed to elevated levels of copper ions. While deletion of RSP_2889 has no significant effect on copper resistance, expression from a low-copy-number plasmid mediates increased sensitivity to copper.

A holistic multi-actor approach towards the design of new tomato varieties and management practices to improve yield and quality in the face of climate change From 2016-03-01 to 2020-02-29, ongoing project | TomGEM Website Project details Objective The world demographic growth and global climate change are major challenges for human society,hence the need to design new strategies for maintaining high crop yield in unprecedented environmental conditions.The objective of TomGEM is to design new strategies aiming to maintain high yields of fruit and vegetables at harsh temperature conditions, using tomato as a reference fleshy fruit crop.As yield is a complex trait depending on successful completion of different steps of reproductive organ development, including flower differentiation and efficient flower fertilization,TomGEM will use trans-disciplinary approaches to investigate the impact of high temperature on these developmental processes.The core of the project deals with mining and phenotyping a vast range of genetic resources to identify cultivars/genotypes displaying yield stability and to uncover loci/genes controlling flower initiation,pollen fertility and fruit set.Moreover,since high yield and elevated temperatures can be detrimental to quality traits,TomGEM will also tackle the fruit quality issue.The goal is to provide new targets and novel strategies to foster breeding of new tomato cultivars with improved yield.The main strength of TomGEM resides in the use of unique and unexplored genetic resources available to members of the consortium.It gathers expert academic researchers and private actors committed to implement a multi-actor approach based on demand driven innovation.Tomato producers and breeders are strongly involved from design to implementation of the project and until the dissemination of results.TomGEM will provide new targets and novel strategies to foster the breeding of new tomato cultivars with improved yield under suboptimal temperature conditions.TomGEM will translate scientific insights into practical strategies for better handling of interactions between genotype,environment and management to offer holistic solutions to the challenge of increasing food quality and productivity.