REPRODUCTION AND RECRUITMENT OF NUCULOMA TENUIS (BIVALVIA: NUCULOIDA) FROM LOCH ETIVE, SCOTLAND

A population of the protobranch bivalve Nuculoma tenuis at adepth of c. 54 m in Loch Etive, West Scotland, was sampled monthlyfrom September 1986 to October 1988. The density of N. tenuisin the samples, and the relative proportions of adults and postlarvae,varied markedly from month to month suggesting patchiness atthe scale of the sampling. There was evidence for spatial segregationof adults and postlarvae. A seasonal reproductive cycle occurred,with a synchronised spawnout in winter; the exact timing appearingto vary in successive years by up to 2 months. Despite markedlyseasonal spawning, no recruitment peak was evident in shell-lengthfrequencies, and benthic postlarvae were present throughoutthe year. This corroborates findings from an earlier laboratorystudy, suggesting a prolonged phase of meiobenthic developmentin this species. (Received 1 February 1995; accepted 15 March 1995)     

Evidence for K+ channel control in Vicia guard cells coupled by G-proteins to a 7TMS receptor mimetic

To explore the involvement of a class of seven-trans-membrane-span (7TMS) receptors in cellular signalling, a synthetic analogue (mas7) of the amphipathic tetradecapeptide mastoparan was used to mimic hormonal stimulus in guard cells of Vicia faba. The ability for mas7 to substitute for an activated receptor complex was assayed by the effect on guard cell ion channel activities in the absence of any hormonal stimulus. Currents carried by inward-(I_K,in) and outward-(I_K,out) rectifying potassium channels were determined under voltage clamp conditions before, during, and after exposure to mas7. The dominant effect of mas7 was to inactivate I_K,in within 30 sec of application. By contrast, I_K,out was largely unaffected under these conditions. The effect of mas7 on I_K,in was both concentration- and voltage-dependent. At any one clamp voltage, mas7 inactivation showed Michaelian behaviour, with a mean K_i of 0.05 ± 0.02 µM at ?240 mV. Increasing mas7 concentration also shifted the voltage for half-maximal activation of the current negative, with 0.5 µM mas7 effecting a ?13 ± 2 mV displacement and lengthening the halftime for activation of the current by up to threefold. By contrast, the non-amphipathic analogue of mas7, masCP, had no appreciable effect on the steady-state current or its activation kinetics; nor was the poly-cation polylysine able to substitute for mas7 in its action on the K⁺ channels. Application of the non-hydrolysable analogue of GDP, GDP-β-S, either by iontophoresis or by diffusion from the microelectrode, effectively blocked mas7-induced inactivation of I_K,in. These, and additional results provide in vivo evidence for the involvement of G-protein-linked 7TMS receptors in the regulation of membrane transport in a higher plant cell.     

Oxidative stress responses in transgenic tobacco containing altered levels of glutathione reductase activity

A pea glutathione reductase cDNA was expressed in tobacco. Three classes of construct were used which gave a range of elevated levels of glutathione reductase (GR) activity in the cytosol (GR32), chloroplasts (GR36), or in both chloroplasts and mitochondria (GR46). In some transgenic progeny (T₂) from self-fertilized GR32 and GR36 primary transformants, having approximately twofold elevation of GR activity as compared with recessive siblings, there was an amelioration of the effect on leaf discs of up to 15 µM paraquat. However, lines with similarly elevated levels of GR activity showed no decreased sensitivity to the herbicide. None of the GR32 and GR36 lines was less sensitive to ozone. Conversely, T₂ progeny of GR46 lines, with greater than 4.5-fold elevations of GR activity, showed no reduced sensitivity to paraquat but two out of four of these lines were less sensitive to ozone fumigation. The differential response to stress co-segregated with the presence of the transgene but there was no relationship between the degree of stress response and the level of GR activity. There was an elevation in the total glutathione pool in all lines showing increased GR activity but there was no change in the ratio of oxidized to reduced glutathione. These results demonstrate that the mechanisms of protection against ozone and paraquat are different although both can be mediated by elevated GR activity.