Distinctive Features of the Functioning of Lactic Acid and the Alcohol Fermentation Enzyme in Sorghum and Pea Leaves under the Conditions of Oxygen Deficiency

Dynamic changes in the enzymatic activity of lactate dehydrogenase, L-lactate-cytochrome-c-coxidoreductase, and alcohol dehydrogenase were analyzed in the seedlings of sorghum, a plant resistant to oxygen deficiency, and peas, a plant vulnerable to oxygen deficiency, cultivated under the conditions of flooding. The subcellular localization and isoform composition of the enzymes were characterized. The mechanism underlying the adaptive reaction of cell metabolism was devised from analysis of the results obtained. The reaction is supposed to involve lactate dehydrogenase and L-lactate-cytochrome-c-oxidoreductase enzymes that suppress cytoplasm acidification in sorghum and pea roots in the case of flooding.     

Computer simulation in template-directed oligonucleotide synthesis

Summary A computer simulation (KINSIM) modeling up to 33 competing reactions was used in order to investigate the product distribution in a template-directed oligonucleotide synthesis as a function of time and concentration of the reactants. The study is focused on the poly(C)-directed elongation reaction of an oligoguanylate (a 7-mer is chosen) with guanosine 5-monophosphate-2-methylimidazolide (2-MeImpG), the activated monomer. It is known that theelongation of oligoguanylates to form oligomeric products such as 8-mer, 9-mer, 10-mer, etc., is in competition with (1) thedimerization and further oligomerization reaction of 2-MeImpG that leads to the formation of dimers and short oligomers, and (2) thehydrolysis of 2-MeImpG that forms inactive guanosine 5-monophosphate, 5-GMP. Experimentally determined rate constants for the above three processes at 37°C and pH 7.95 were used in the simulation; the initial concentrations of 2-MeImpG, [M]_o, and of the oligoguanylate primer, [7-mer]_o, were varied, and KINSIM calculated the distribution of products as a function of time until equilibration was reached, i.e., when all the activated monomer has been consumed. In order to sort out how strongly the elongation reaction may be affected by the competing hydrolysis and dimerization, we also simulated the idealized situation in which these competing reactions do not occur. Simulation of the idealized system suggests that (1) the fraction of [7-mer]_o that has reacted as well as the product distribution after equilibration do not depend on the absolute concentrations of the reactants, but only on their ratio, [M]_o/[7-mer]_o; (2) the rate of elongation is proportional to [7-mer]_o and not to [M]_o; and (3) as the [M]_o/[7-mer]_o ratio increases longer oligomers are formed. The results of the computer simulation with the experimental system, i.e., elongation in the presence of both hydrolysis and dimerization, are similar to the ones obtained with the idealized system as long as dimerization and hydrolysis are not responsible for consuming a substantial fraction of 2-MeImpG.     

Influence of light on the apoplastic ph in microwounded cells of <Emphasis Type="Italic">Chara corallina</Emphasis>

Microscopic wounding of plant cell walls by pathogens or by feeding insects triggers the defense responses, including a sharp rise in pH at the cell surface (pH_o). Using internodal cells of Chara corallina Klein ex Willd., we show here that the elevated pH_o in the area of cell wall microincision decreases in darkness and increases on illumination. These pH_o changes occurred specifically in cell areas affected by microincision and were lacking in intact areas with active pHotosynthesis (acid zones). Localized illumination of a remote cell region located upstream the cytoplasmic flow at a 1.5-mm distance from the analyzed area also caused a transient increase in pH_o in the area of microwounding but had no such effect in unwounded cell regions having weakly acidic pH_o. Apparently, the increase in pH_o after wounding is mediated by a metabolite released from illuminated chloroplasts, which is transported with the cytoplasmic flow for long distances. The transient pH_o increase in the area of cell wall incision after illumination of a distant cell region coincided with a temporal increase in chlorophyll fluorescence F’. This implies the concurrent influence of the transported reductant (presumably NADH) on light emission of chloroplasts and on the H⁺ flow across the plasmalemma. We suppose that the alkalinization of cell surface in the area of microincision arises from H⁺ consumption in the apoplast in association with the transmembrane electron transport from cytoplasmic reducing equivalents to molecular oxygen.     

Dynamics of Cellular Responses to Radiation

Understanding the consequences of exposure to low dose ionizing radiation is an important public health concern. While the risk of low dose radiation has been estimated by extrapolation from data at higher doses according to the linear non-threshold model, it has become clear that cellular responses can be very different at low compared to high radiation doses. Important phenomena in this respect include radioadaptive responses as well as low-dose hyper-radiosensitivity (HRS) and increased radioresistance (IRR). With radioadaptive responses, low dose exposure can protect against subsequent challenges, and two mechanisms have been suggested: an intracellular mechanism, inducing cellular changes as a result of the priming radiation, and induction of a protected state by inter-cellular communication. We use mathematical models to examine the effect of these mechanisms on cellular responses to low dose radiation. We find that the intracellular mechanism can account for the occurrence of radioadaptive responses. Interestingly, the same mechanism can also explain the existence of the HRS and IRR phenomena, and successfully describe experimentally observed dose-response relationships for a variety of cell types. This indicates that different, seemingly unrelated, low dose phenomena might be connected and driven by common core processes. With respect to the inter-cellular communication mechanism, we find that it can also account for the occurrence of radioadaptive responses, indicating redundancy in this respect. The model, however, also suggests that the communication mechanism can be vital for the long term survival of cell populations that are continuously exposed to relatively low levels of radiation, which cannot be achieved with the intracellular mechanism in our model. Experimental tests to address our model predictions are proposed.     

Pectin methylesterase-generated methanol may be involved in tobacco leaf growth

Plant leaves undergo a sink-source modification of intercellular macromolecular transport during the transition from carbon import to carbon export. After assessing the role of metabolite signaling in gene regulation in Nicotiana tabacum sink and source leaves, we observed increased pectin methylesterase (PME)-mediated methanol generation in immature leaves. Using suppression subtractive hybridization (SSH), we identified a number of genes whose activity changes from sink to source leaves. The most abundant SSH-identified genes appeared to be sensitive to methanol. We hypothesize that tobacco leaf maturation and the sink-source transition are accompanied by a change in mRNA levels of genes that function in methanol-dependent cell signaling.