Miniature Transport Engineers: In its vegetative phase, the slime moldPhysarum polycephalum“slimes” its way through the world seeking food. As it explores, it links previously found food sources with tubular structures.Teroet al.(p.439) report that if food sources are deposited on a plate in a pattern corresponding in miniature to the positions of the cities that surround Tokyo, the resulting network ofPhysarumtubules that develops is rather similar in structure to the railroad network that connects the Japanese cities. A model was produced that describes the adaptive network development displayed by the slime mold. This biologically inspired model might provide insight into how to implement properties like resistance of transport systems to local failures into similar human-designed systems.

Tight control of cellular redox homeostasis is essential for protection against oxidative damage and for maintenance of normal metabolism as well as redox signaling events. Under oxidative stress conditions, the tripeptide glutathione can switch from its reduced form (GSH) to oxidized glutathione disulfide (GSSG), and thus, forms an important cellular redox buffer. GSSG is normally reduced to GSH by 2 glutathione reductase (GR) isoforms encoded in the Arabidopsis genome, cytosolic GR1 and GR2 dual-targeted to chloroplasts and mitochondria. Measurements of total GR activity in leaf extracts of wild-type and 2 gr1 deletion mutants revealed that ≈65% of the total GR activity is attributed to GR1, whereas ≈35% is contributed by GR2. Despite the lack of a large share in total GR activity, gr1 mutants do not show any informative phenotype, even under stress conditions, and thus, the physiological impact of GR1 remains obscure. To elucidate its role in plants, glutathione-specific redox-sensitive GFP was used to dynamically measure the glutathione redox potential ( E _GSH ) in the cytosol. Using this tool, it is shown that E _GSH in gr1 mutants is significantly shifted toward more oxidizing conditions. Surprisingly, dynamic reduction of GSSG formed during induced oxidative stress in gr1 mutants is still possible, although significantly delayed compared with wild-type plants. We infer that there is functional redundancy in this critical pathway. Integrated biochemical and genetic assays identify the NADPH-dependent thioredoxin system as a backup system for GR1. Deletion of both, NADPH-dependent thioredoxin reductase A and GR1, prevents survival due to a pollen lethal phenotype.

We provide evidence that the tripeptide thiol glutathione (GSH) participates in the regulation of cell division in the apical meristem of Arabidopsis roots. Exogenous application of micromolar concentrations of GSH raised the number of meristematic cells undergoing mitosis, while depletion of GSH had the opposite effect. A role for endogenous GSH in the control of cell proliferation is also provided by mapping of GSH levels in the root meristem using the GSH-specific dye monochlorobimane and confocal laser scanning microscopy. High levels of GSH were associated with the epidermal and cortical initials and markedly lower levels in the quiescent center. The mechanisms controlling cell division could also be triggered by other reducing agents: ascorbic acid and dithiothreitol. Our data also reveal significant plasticity in the relationship between the trichoblast cell length and the hair it subtends in response to alterations in intracellular redox homeostasis. While mechanisms that control trichoblast elongation are influenced by nonspecific redox couples, root hair tip growth has a more specific requirement for sulfhydryl groups. The responses we describe here may represent the extremes of redox control of root plasticity and would allow the root to maintain exploration of the soil under adverse conditions with minimal cell divisions and root hair production or capitalize on a favorable environment by production of numerous long hairs. Redox sensing of the environment and subsequent redox-dependent modulation of growth and development may be crucial components in the strategies plants have evolved for survival in a fluctuating environment.

The role of cytosolic Ca2+ in signal transduction in stomatal guard cells of Commelina communis was investigated using fluorescence ratio imaging and photometry. By changing extracellular K+, extracellular Ca2+, or treatment with Br-A23187, substantive increases in cytosolic Ca2+ to over 1 micromolar accompanied stomatal closure. The increase in Ca2+ was highest in the cytoplasm around the vacuole and the nucleus. Similar increases were observed when the cells were pretreated with ethyleneglycol-bis-(o-aminoethyl)tetraacetic acid or the channel blocker La3+, together with the closing stimuli. This suggests that a second messenger system operates between the plasma membrane and Ca2+-sequestering organelle(s). The endogenous growth regulator abscisic acid elevated cytosolic Ca2+ levels in a minority of cells investigated, even though stomatal closure always occurred. Ca2+-dependent and Ca2+-independent transduction pathways linking abscisic acid perception to stomatal closure are thus indicated.