Differential effects of ambient or diminished CO2 and O2 levels on thylakoid membrane structure in light‐stressed plants

Over‐reduction of the photosynthetic electron transport chain may severely damage the photosynthetic apparatus as well as other constituents of the chloroplast and the cell. Here, we exposed Arabidopsis leaves to saturating light either under normal atmospheric conditions or under CO₂‐ and O₂‐limiting conditions, which greatly increase excitation and electron pressures by draining terminal electron acceptors. The two treatments were found to have very different, often opposing, effects on the structure of the thylakoid membranes, including the width of the granal lumenal compartment. Modulation of the latter is proposed to be related to movements of ions across the thylakoid membrane, which alter the relative osmolarity of the lumen and stroma and affect the partitioning of the proton motive force into its electrical and osmotic components. The resulting changes in thylakoid organization and lumenal width should facilitate the repair of photodamaged photosystem II complexes in response to light stress under ambient conditions, but are expected to inhibit the repair cycle when the light stress occurs concurrently with CO₂ and O₂ depletion. Under the latter conditions, the changes in thylakoid structure are predicted to complement other processes that restrict the flow of electrons into the high‐potential chain, thus moderating the production of deleterious reactive oxygen species at photosystem I.     

Carbohydrate metabolism in Nerine sarniensis bulbs developing in liquid culture

Anatomical and physiological changes accompanying enhanced Nerine sarniensis cv. Salmon Supreme bulb growth in vitro were examined. Small bulbs, 2–3 mm in diameter, grown in vitro on a semi-solid medium were subcultured to liquid medium with elevated levels of sucrose (Suc) and inorganic phosphate. Bulbs' fresh and dry weights, carbohydrate contents and the activities of enzymes related to carbohydrate metabolism were determined at different stages of bulb development. Starch was the dominant storage carbohydrate in these bulbs, and the leaf bases parenchyma cells were the principal storage tissue. During the first month of bulb growth, only small changes in starch content were detected. However, an increase in starch level was observed at later stages of development. The activity of ADP-glucose pyrophosphorylase (EC 2.7.7.27), a key enzyme of starch synthesis, increased just before the increase in starch accumulation. Sucrose was the dominant soluble sugar in the bulbs, only traces of glucose and fructose were detected. The activity of alkaline invertase (INV, EC 3.2.1.26) was higher than that of acid INV during the growth period. Sucrose synthase (EC 2.4.1.13) exhibited the highest Suc-degrading activity during bulb growth. Suc was hydrolyzed in the medium by the cell wall bound acid INV during the growing period. The results are discussed in relation to enhanced nerine bulb growth and development in vitro.     

Evaluating soil seed banks of phosphate mining restoration in the hyper-arid Negev desert

Ecological systems are severely damaged by the anthropogenic disturbance of mining. Phosphate open-pit mining fields cover over 200 km² of the Negev desert, Israel. However, the effects of the ongoing mine site restoration efforts on the plant community have not been studied. Plants and their seed banks have a major role in ecosystem processes and restoration. In this study, we focused on three mining sites, restored in different years, along Zin River valley. We compared the plant community of restored mining plots within these areas to adjacent natural plots. We asked whether plant community germination potential from the soil seed bank differs between the restored plots and the adjacent natural plots within a mining site. We hypothesized that: (1) there is a lack of seed bank in the restored plots; (2) the altered soil composition at the restored plots inhibits germination. We used soil samples collected from the different mining sites and set up greenhouse experiments. One experiment compared natural and restored areas with different soil treatments. In another experiment, we added native seeds to test their germination potential on restored soil. Our results indicate that lack of seed bank is the major limiting factor hindering germination and not the composition of the soil after restoration. Our findings shed light on the constraints of seed bank establishment in post-mining areas of hyper-arid regions. We suggest considering active restoration practices to facilitate natural dispersal and improve seed bank establishment.     

Fate of lipoprotein lipase taken up by the rat liver. Evidence for a conformational change with loss of catalytic activity

When isolated rat livers were perfused with medium containing lipoprotein lipase, 40-60% was taken up during a single passage. This value was similar for lipoprotein lipase derived from culture medium of rat preadipocytes, and for lipoprotein lipase purified from bovine milk. It was also, similar, irrespective of the lipoprotein lipase concentration, at least up to 1 microgram/ml. Immediately following its uptake by the liver, a large fraction of the lipoprotein lipase could be released by heparin, but the magnitude of this fraction decreased with time. The enzyme lost its catalytic activity rather rapidly, but its degradation to acid-soluble products, or to larger fragments, was much slower. On heparin-agarose chromatography, the enzyme taken up by the liver eluted at a lower salt concentration than the original lipoprotein lipase preparation. This change in affinity for heparin suggests that the originally dimeric lipoprotein lipase had dissociated into monomers, in analogy to the findings in model experiments. It is suggested that the initial uptake of lipoprotein lipase occurs by binding to a polyanion at the liver cell surface. This is followed by endocytosis and dissociation of the enzyme from its heparan sulfate-like binding site. Acidification of the endosome may cause a conformational change in the lipase molecule with dissociation to inactive monomers, preceding ultimate proteolytic degradation.     

Factors influencing the production of hardened glaucous carnation plantlets in vitro

Medium type, its water status and the relative humidity in the culture vessel modified carnation leaf development in vitro. Carnation shoot apices cultured on liquid or on 0.8% agar solidified media developed into plantlets having succulent and translucent leaves which are not transplantable to non-aseptic conditions. Increasing the agar and/or sucrose concentration in the medium as well as decreasing the relative humidity in the culture vessel by a desiccant promoted glaucous leaf production. Increased water status (H₂O and relative humidity) increased shoot proliferation and translucency of leaves. Decreased water status reduced shoot proliferation but induced the formation of glaucous leaves. The culture of apices for 5–6 days on liquid medium prior to their sub-culture to 1.5% agar medium improved shoot proliferation and normal leaf development. An agar slant prevented the submergence of apices in water accumulating on the medium and thus reduced leaf translucency. Survival was further increased by the transfer of plantlets in uncapped culture vessels to a desiccator for 1–2 weeks prior to transplanting to soil.     

Bile salts promote the absorption of insulin from the rat colon

The absorption of insulin mixed with sodium deoxycholate (DOC) or sodium cholate from the rectal mucosa of diabetic and non-diabetic rats was measured by the effect on blood glucose levels. Blood sugar was lowere by 50% one hour after administration of 12 u soluble insulin mixed with 1–10 mg/ml DOC, or 2–20 mg/ml sodium cholate. There was a linear correlation between the reduction in blood glucose and the amount of insulin administered (1–64 units) when mixed with 5 mg/ml DOC. Radioimmuno-assay of plasma insulin showed an increase from 16.2 μu/ml to 3335 muuu/ml after rectal administration of 12 u soluble insulin. We conclude that insulin when mixed with bile salts can be absorbed by the intestine to reach the circulation in a biologically active form.     

Expression of Cell Cycle-Related Genes During Neuronal Apoptosis: Is there a Distinct Pattern?

An emerging hypothesis considers the process of neuronal apoptosis as a consequence of unscheduled and unsynchronized induction of cell cycle mediators. Induction of several cell cycle genes precedes neuronal apoptosis and may be involved in determination of cell fate. We have now characterized changes in expression of cell cycle genes during apoptosis induced by oxidative stress in chick post-mitotic sympathetic neurons. Induction of cyclin B occurred prior to the commitment of neurons to both dopamine- and peroxide-triggered apoptosis. Both the neuronal death and the rise in cyclin B were inhibited by antioxidant treatment, suggesting a functional role for cyclin B induction during neuronal apoptosis. Induction of the cyclin dependent kinase CDK5 protein coincided with the time point when neurons were irreversibly committed to die. Expression of other cell cycle mediators such as cyclin D1 and the cyclin dependent kinases CDC2 and CDK2 was undetected and not induced by exposure to oxidative stress. Comparative analysis of the profile of cell cycle mediators induced during neuronal apoptosis of different neuronal cell populations revealed no distinct pattern of events. There are no cell cycle stage-specific mediators that are ultimately stimulated during neuronal apoptosis, suggesting that multiple pathways of re-activating the dormant cell-cycle, converge to determine entry into apoptosis. Nevertheless, the existence of some cell cycle mediators, that were not reported so far to be induced in post mitotic neurons during oxidative stress, substantiate them as part of the strong differentiating forces.