Arabidopsis thaliana ASN2 encoding asparagine synthetase is involved in the control of nitrogen assimilation and export during vegetative growth

We investigated the function of ASN2, one of the three genes encoding asparagine synthetase (EC 6.3.5.4), which is the most highly expressed in vegetative leaves of Arabidopsis thaliana. Expression of ASN2 and parallel higher asparagine content in darkness suggest that leaf metabolism involves ASN2 for asparagine synthesis. In asn2‐1 knockout and asn2‐2 knockdown lines, ASN2 disruption caused a defective growth phenotype and ammonium accumulation. The asn2 mutant leaves displayed a depleted asparagine and an accumulation of alanine, GABA, pyruvate and fumarate, indicating an alanine formation from pyruvate through the GABA shunt to consume excess ammonium in the absence of asparagine synthesis. By contrast, asparagine did not contribute to photorespiratory nitrogen recycle as photosynthetic net CO₂ assimilation was not significantly different between lines under both 21 and 2% O₂. ASN2 was found in phloem companion cells by in situ hybridization and immunolocalization. Moreover, lack of asparagine in asn2 phloem sap and lowered ¹⁵N flux to sinks, accompanied by the delayed yellowing (senescence) of asn2 leaves, in the absence of asparagine support a specific role of asparagine in phloem loading and nitrogen reallocation. We conclude that ASN2 is essential for nitrogen assimilation, distribution and remobilization (via the phloem) within the plant.     

Ultrastructure of laevigate hilate spores in sporangia and spore masses from the Upper Silurian and Lower Devonian of the Welsh Borderland

Spore masses and isolated sporangia, containing laevigate hilate cryptospores attributable to the dispersed taxon Laevolancis divellomedia sensu lato, have been recovered on bulk maceration of Upper Silurian (Pridoli) and Lower Devonian (Lochkovian) deposits from the Welsh Borderland. Detailed morphological, anatomical and ultrastructural analysis, using light microscope, scanning electron microscope and transmission electron microscope techniques, reveals subtle differences between the specimens and they can be grouped into five distinct types. The different groups are distinguished principally by using sporangia-spore mass characteristics, presence or absence of extra-exosporal material and nature of spore-wall ultrastructure. Of the groups, one has a uniformly homogeneous exospore and the other four groups have a bilayered exospore. In the former the spores lack extra-exosporal material and occur in a discoidal sporangium. Of the bilayered groups, two have exospores of homogeneous composition but with the two layers differing in electron density. They occur in discoidal sporangia and spore masses and are distinguished on the presence or absence of extra-exosporal material and differences in the widths of the two layers. Finally, two bilayered groups possess a lamellate inner layer, but vary in presumed sporangial shape. Elongate sporangia have spores with concentric continuous lamellae, lacking further ultrastructure. In contrast, spores from a discoidal spore mass have white-line-centred, presumably tripartite, lamellae which are laterally discontinuous, overlapping and irregularly spaced. These findings, which suggest that morphologically similar spores were produced by a number of plant taxa, have important implications regarding the assessment of early land-plant diversity. The affinities of hilate cryptospore-producing plants are unknown and problematic, particularly as no extant non-angiosperm plants produce dyads, other than through meiotic irregularity, and spore-sporangial characters have no exact counterpart in coeval plants. Studies of specimens with in situ hilate cryptospores suggest that they derive from rhyniophytoids, i.e. plants that resemble the simplest of vascular plants but lack evidence of vascular tissue, although hilate cryptospore-containing examples show no axial branching. It might be argued, based on evidence from spore wall ultrastructure, that some of the plants have more in common with lycopsids and filicopsids than bryophytes, a surprising finding bearing in mind the stratigraphic distribution of hilate cryptospores-dyads and inferences that the producers were bryophyte-like. Detailed studies of wall structure in the hilate cryptospores permit consideration of spore wall development. It is suggested that extra-exosporal material derives from a tapetum and is thus produced by the diploid sporophyte. The white-line-centred lamellae in a single specimen provide the earliest evidence for the presence of such structures in early land plant spores and provide further evidence that sporopollenin deposition on such structures is the most primitive mode of sporopollenin deposition among land plants.     

Juvenile hormone regulation of structural changes and DNA synthesis in the follicular epithelium of Leucophaea maderae

Juvenile hormone (JH I) stimulates specific morphological and biochemical changes in the follicular epithelium surrounding the terminal oöcytes in Leucophaea maderae. These include extracellular and intracellular structural changes, increased rates of follicle cell DNA synthesis, and elevated follicle cell DNA concentrations.Using females decapitated 24 hr after ecdysis, we have shown that JH I injections stimulate the following structural changes in the follicular epithelium: the appearance of channels between adjacent follicle cells and of spaces between the follicular epithelium and the maturing oöcyte; an increase in follicle cell size; the development of an extensive rough endoplasmic reticulum system; and an enlarged nucleus within each follicle cell. No increase in the number of follicle cells surrounding the developing terminal follicles is found in 7-day JH I-treated females, although the terminal follicles are almost twice as long as those in untreated females.In addition, we have demonstrated that JH stimulates the following biochemical events in the ovary: a 3.5 fold increase in thymidine incorporation into follicle cell DNA, with no subsequent transfer of such DNA to the developing oöcyte, and a 1.4 fold increase in ovarian DNA in 7-day JH-treated females. These data indicated that JH stimulates follicle cell DNA synthesis. The absence of any corresponding division of follicle cells suggests that JH I may induce polyploidy in follicle cells.Extended exposure of decapitated females to JH I does not result in complete ovarian maturation. Although fat bodies in the treated insects continue to display an increasing rate of vitellogenin synthesis, DNA synthesis in the terminal follicles declines rapidly after day 9, and the terminal follicles ultimately degenerate.     

Developmental-stage-dependent adenine transport in Neurospora crassa.

Although germinated conidia of Neurospora crassa transport adenine through two different systems, only one of these, namely, the general purine transport system, which transports adenine, hypoxanthine, guanine, and 6-methylpurine, is present in freshly harvested conidia of the wild type. The second system develops during germination. The latter system can transport adenine and 6-methylpurine. Time course and kinetic studies of adenine transport in freshly harvested conidia of an ad-8 mutant indicated that, in contrast to the wild type, the general purine transport activity is very low in this strain and that the second adenine transport system is possibly present in the ungerminated conidia. A study of adenine and hypoxanthine uptake in ad-8 and ad-4 mutants, both of which cannot utilize hypoxanthine for growth, isolated that the two transport systems may be under different metabolic controls.     

Effect of i.c.v. infusion of the alpha-MSH agonist MTII on meal patterns in male rats following nicotine withdrawal

The present study explored the role of endogenous alpha-MSH in the alteration of meal patterns induced by nicotine (NIC) withdrawal. Male Sprague Dawley rats bearing third ventricle cannulas were placed in computerized food intake monitors. On days 1-21, the rats were given 4 mg/kg/day of NIC or saline (SAL) in four equal i.p. doses during the dark period. NIC suppressed (P < 0.05) food intake only during the first week. The normalization of food intake occurred when the reduced meal size of the NIC injected rats was countered by an increase in meal number. Despite the normalization of 24-h food intake, body weight in NIC rats was decreased (P < 0.05) for 21 days. On day 22, the rats were divided into 4 groups (n's = 7-8 each) and injected into the third ventricle with various doses of the alpha-MSH agonist MTII or artificial cerebrospinal fluid (aCSF): SAL + aCSF, SAL + MTII, NIC + aCSF, NIC + MTII. Infusion of MTII (30 ng/rat) suppressed (P < 0.01) dark phase food intake in both groups, but the NIC + MTII group ate (P < 0.05) more than the SAL + MTII group. Meal number during the dark phase was suppressed by MTII, but the NIC + MTII group took significantly more meals that the SAL + MTII group. Infusion of MTII suppressed meal size in SAL and NIC treated rats, but this effect was attenuated in NIC treated rats. All meal parameters normalized by the day after i.c.v. infusion. These data indicate that NIC treatment differentially affects the neural controls of meal number and meal size and attenuates the suppression by MTII of meal number and meal size.