The sensing of oxygen deprivation in bacteria, fungi, metazoa and plants involves multiple sensors and signal transduction pathways. Cellular responses result in a reprogramming of gene expression and metabolic processes that enhance transient survival and can enable long-term tolerance to sub-optimal oxygen levels. The mechanism of sensing can involve molecules that directly bind or react with oxygen (direct sensing), or recognition of altered cellular homeostasis (indirect sensing). The growing knowledge of the activation of genes in response to oxygen deprivation has provided additional information on the response and acclimation processes. Conservation of calcium fluxes and reactive oxygen species as second messengers in signal transduction pathways in metazoa and plants may reflect the elemental importance of rapid sensing of cellular restriction in oxygen by aerobic organisms.

Mitogen-activated protein kinases (MPKs) are regulated by diverse stresses with a reactive oxygen species (ROS) component. Here, we report the rapid and transient activation of MPK3, MPK4 and MPK6 upon oxygen deprivation as well as reoxygenation in seedlings of Arabidopsis thaliana. MPK activation peaked within 2 h of oxygen deprivation and again at a higher magnitude within 5 min of reoxygenation. MPK6 was the predominant kinase regulated by oxygen availability in both aerial and root tissue, except in mpk6 mutants, which displayed compensatory activation of MPK3. A universal consequence of oxygen deprivation in eukaryotes is inhibition of the terminal step of the mitochondrial electron transport chain (mETC). We demonstrate that treatment of seedlings with the mETC inhibitors antimycin A and potassium cyanide under normoxia promotes transient MPK6 and MPK3 activation. Confocal imaging of seedlings provided evidence that both oxygen deprivation and mETC inhibitors stimulate mitochondria-associated ROS production. We found that seedling survival of prolonged oxygen deprivation was improved in transgenics that ectopically overexpress MPK3, MPK4 and MPK6, but the induction of mRNAs associated with low oxygen acclimation responses were not markedly altered in MPK6 overexpression lines or mpk6 loss-of-function mutants. However, distinctions in MPK6 activation potential were correlated with other differences in mRNAs accumulation. Our findings suggest that oxygen deprivation and reoxygenation trigger mitochondrial ROS production to activate MPK signaling, which in turn regulate reversible processes that aid survival of transient oxygen deprivation.

California Small White beans were found to contain about 1% phytate, of which 70% is in water-soluble form. Incubation of presoaked beans in water at 60°C for 10 hr lowered their phytate content by 90%. Incubation in pH 5.5 buffer at 50°C for 24 hr lowered it by 62%. When leaching was eliminated by incubating in water-saturated air at 60°C overnight, 33% of the phytate was destroyed by hydrolysis. Similar treatments were effective with mung beans, lima beans, and wheat. The study suggests that destruction of heat-sensitive celI membranes leads to the potentiation of phytase.

Transposon tagging with modified maize Ds-GUS constructs was used to isolate genes induced by oxygen deprivation in Arabidopsis thaliana. Seedlings of 800 gene-trap (DsG) and 600 enhancer-trap (DsE) lines were grown on vertically positioned plates for 1 week, oxygen deprived for up to 24 h and stained for GUS activity. Oxygen deprivation induced intricate patterns of gene expression in seedlings of 65 lines. The insertion site and phenotypes of 15 lines were examined. Surprisingly, none of the insertions were into genes that encode known anaerobic polypeptides. Insertions were identified within or adjacent to genes encoding proteins of regulatory, enzymatic, mitochondrial protein import and unknown function, as well as adjacent to genes encoding a putative receptor-like kinase and putative sensor-histidine kinase. Four lines had significantly lower ADH activity after 24 h of oxygen deprivation and three of these showed reduced stress tolerance. Two lines with wild-type levels of ADH were low-oxygen intolerant. Paradoxically, several lines had significantly higher ADH activity after 12 h of oxygen deprivation but reduced stress tolerance. Caffeine treatment, which increased ADH specific activity in wild-type seedlings under aerobic conditions, was sufficient to increase GUS staining in seven of the 15 lines, providing evidence that these genes may be regulated by cytosolic calcium levels. These results demonstrate the effectiveness of the Ds-GUS tagging system in the identification of genes that are regulated in response to oxygen deprivation and a calcium second messenger.