ppGpp controlled by the Gac/Rsm regulatory pathway sustains biocontrol activity in Pseudomonas fluorescens CHA0.

In Pseudomonas fluorescens CHA0 and other fluorescent pseudomonads, the Gac/Rsm signal transduction pathway is instrumental for secondary metabolism and biocontrol of root pathogens via the expression of regulatory small RNAs (sRNAs). Furthermore, in strain CHA0, an imbalance in the Krebs cycle can affect the strain's ability to produce extracellular secondary metabolites, including biocontrol factors. Here, we report the metabolome of wild-type CHA0, a gacA-negative mutant, which has lost Gac/Rsm activities, and a retS-negative mutant, which shows strongly enhanced Gac/Rsm-dependent activities. Capillary electrophoresis-based metabolomic profiling revealed that the gacA and retS mutations had opposite effects on the intracellular levels of a number of central metabolites, suggesting that the Gac/Rsm pathway regulates not only secondary metabolism but also primary metabolism in strain CHA0. Among the regulated metabolites identified, the alarmone guanosine tetraphosphate (ppGpp) was characterized in detail by the construction of relA (for ppGpp synthase) and spoT (for ppGpp synthase/hydrolase) deletion mutants. In a relA spoT double mutant, ppGpp synthesis was completely abolished, the expression of Rsm sRNAs was attenuated, and physiological functions such as antibiotic production, root colonization, and plant protection were markedly diminished. Thus, ppGpp appears to be essential for sustaining epiphytic fitness and biocontrol activity of strain CHA0.

Use of aggregating brain cell cultures to study developmental effects of organophosphorus insecticides.

Aggregating brain cell cultures of fetal rat telencephalon can be grown in a chemically defined medium for extended periods of time. After a phase of intense mitotic activity, these three-dimensional cell cultures undergo extensive morphological differentiation, including synaptogenesis and myelination. To study the developmental toxicity of organophosphorus compounds (OP), aggregating brain cell cultures were treated with parathion. Protein content and cell type-specific enzyme activities were not affected up to a concentration of 10(5) M. Gliosis, characterized by an increased staining for glial fibrillary acidic protein (GFAP), was observed in immature and in differentiated cells. In contrast, uridine incorporation and myelin basic protein (MBP) immunoreactivity revealed strong differences in sensitivity between these two developmental stages. These results are in agreement with the view that in vivo the development-dependent toxicity is not only due to changes in hepatic detoxification, but also to age-related modifications in the susceptibility of the different populations of brain cells. Furthermore, they underline the usefulness of histotypic culture systems with a high developmental potential, such as aggregating brain cell cultures, and stress the importance of applying a large range of criteria for testing the developmental toxicity of potential neurotoxicants.

Contrasting growth responses in lamina and petiole during neighbor detection depend on differential auxin responsiveness rather than different auxin levels.

Foliar shade triggers rapid growth of specific structures that facilitate access of the plant to direct sunlight. In leaves of many plant species, this growth response is complex because, although shade triggers the elongation of petioles, it reduces the growth of the lamina. How the same external cue leads to these contrasting growth responses in different parts of the leaf is not understood. Using mutant analysis, pharmacological treatment and gene expression analyses, we investigated the role of PHYTOCHROME INTERACTING FACTOR7 (PIF7) and the growth-promoting hormone auxin in these contrasting leaf growth responses. Both petiole elongation and lamina growth reduction are dependent on PIF7. The induction of auxin production is both necessary and sufficient to induce opposite growth responses in petioles vs lamina. However, these contrasting growth responses are not caused by different auxin concentrations in the two leaf parts. Our work suggests that a transient increase in auxin levels triggers tissue-specific growth responses in different leaf parts. We provide evidence suggesting that this may be caused by the different sensitivity to auxin in the petiole vs the blade and by tissue-specific gene expression.

Rice perception of symbiotic arbuscular mycorrhizal fungi requires the karrikin receptor complex.

In terrestrial ecosystems, plants take up phosphate predominantly via association with arbuscular mycorrhizal fungi (AMF). We identified loss of responsiveness to AMF in the rice (Oryza sativa) mutant hebiba, reflected by the absence of physical contact and of characteristic transcriptional responses to fungal signals. Among the 26 genes deleted in hebiba, DWARF 14 LIKE is, the one responsible for loss of symbiosis . It encodes an alpha/beta-fold hydrolase, that is a component of an intracellular receptor complex involved in the detection of the smoke compound karrikin. Our finding reveals an unexpected plant recognition strategy for AMF and a previously unknown signaling link between symbiosis and plant development.