Multiple mechanisms of nitrate sensing by Arabidopsis nitrate transceptor NRT1.1.

In Arabidopsis the plasma membrane nitrate transceptor (transporter/receptor) NRT1.1 governs many physiological and developmental responses to nitrate. Alongside facilitating nitrate uptake, NRT1.1 regulates the expression levels of many nitrate assimilation pathway genes, modulates root system architecture, relieves seed dormancy and protects plants from ammonium toxicity. Here, we assess the functional and phenotypic consequences of point mutations in two key residues of NRT1.1 (P492 and T101). We show that the point mutations differentially affect several of the NRT1.1-dependent responses to nitrate, namely the repression of lateral root development at low nitrate concentrations, and the short-term upregulation of the nitrate-uptake gene NRT2.1, and its longer-term downregulation, at high nitrate concentrations. We also show that these mutations have differential effects on genome-wide gene expression. Our findings indicate that NRT1.1 activates four separate signalling mechanisms, which have independent structural bases in the protein. In particular, we present evidence to suggest that the phosphorylated and non-phosphorylated forms of NRT1.1 at T101 have distinct signalling functions, and that the nitrate-dependent regulation of root development depends on the phosphorylated form. Our findings add to the evidence that NRT1.1 is able to trigger independent signalling pathways in Arabidopsis in response to different environmental conditions.

A plant Shaker-like K+ channel switches between two distinct gating modes resulting in either inward-rectifying or "leak" current.

Among the Shaker-like plant potassium channels, AKT2 is remarkable because it mediates both instantaneous "leak-like" and time-dependent hyperpolarisation-activated currents. This unique gating behaviour has been analysed in Xenopus oocytes and in COS and Chinese hamster ovary cells. Whole-cell and single-channel data show that (i) AKT2 channels display two distinct gating modes, (ii) the gating of a given AKT2 channel can change from one mode to the other and (iii) this conversion is under the control of post-translational factor(s). This behaviour is strongly reminiscent of that of the KCNK2 channel, recently reported to be controlled by its phosphorylation state.

Guard cell inward K+ channel activity in arabidopsis involves expression of the twin channel subunits KAT1 and KAT2.

Stomatal opening, which controls gas exchanges between plants and the atmosphere, results from an increase in turgor of the two guard cells that surround the pore of the stoma. KAT1 was the only inward K(+) channel shown to be expressed in Arabidopsis guard cells, where it was proposed to mediate a K(+) influx that enables stomatal opening. We report that another Arabidopsis K(+) channel, KAT2, is expressed in guard cells. More than KAT1, KAT2 displays functional features resembling those of native inward K(+) channels in guard cells. Coexpression in Xenopus oocytes and two-hybrid experiments indicated that KAT1 and KAT2 can form heteromultimeric channels. The data indicate that KAT2 plays a crucial role in the stomatal opening machinery.

A shaker-like K(+) channel with weak rectification is expressed in both source and sink phloem tissues of Arabidopsis.

RNA gel blot and reverse transcription-polymerase chain reaction experiments were used to identify a single K(+) channel gene in Arabidopsis as expressed throughout the plant. Use of the beta-glucuronidase reporter gene revealed expression of this gene, AKT2/AKT3, in both source and sink phloem tissues. The AKT2/AKT3 gene corresponds to two previously identified cDNAs, AKT2 (reconstructed at its 5' end) and AKT3, the open reading frame of the latter being shorter at its 5' end than that of the former. Rapid amplification of cDNA ends with polymerase chain reaction and site-directed mutagenesis was performed to identify the initiation codon for AKT2 translation. All of the data are consistent with the hypothesis that the encoded polypeptide corresponds to the longest open reading frame previously identified (AKT2). Electrophysiological characterization (macroscopic and single-channel currents) of AKT2 in both Xenopus oocytes and COS cells revealed a unique gating mode and sensitivity to pH (weak inward rectification, inhibition, and increased rectification upon internal or external acidification), suggesting that AKT2 has enough functional plasticity to perform different functions in phloem tissue of source and sink organs. The plant stress hormone abscisic acid was shown to increase the amount of AKT2 transcript, suggesting a role for the AKT2 in the plant response to drought.

pH control of the plant outwardly-rectifying potassium channel SKOR.

SKOR, an Arabidopsis depolarisation-activated K+-selective channel, was expressed in Xenopus oocytes, and external and internal pH effects were analysed. Internal pH was manipulated by injections of alkaline or acidic solutions or by acid load from acetate-containing medium. An internal pH decrease from 7.4 to 7.2 induced a strong (ca. 80%) voltage-independent decrease of the macroscopic SKOR current, the macroscopic gating parameters and the single channel conductance remained unchanged. An external acidification from 7.4 to 6.4 had similar effects. It is proposed that pH changes regulate the number of channels available for activation. Sensitivity of SKOR activity to pH in the physiological range suggests that internal and external pH play a role in the regulation of K+ secretion into the xylem sap.

Evidence for a multi-ion pore behavior in the plant potassium channel KAT1.

KAT1 is a cloned voltage-gated K+ channel from the plant Arabidopsis thaliana L., which displays an inward rectification reminiscent of 'anomalous' rectification of the if pacemaker current recorded in animal cells. Macroscopic conductance of KAT1 expressed in Xenopus oocytes was 5-fold less in pure Rb+ solution than in pure K+ solution, and negligible in pure Na+ solution. Experiments in different K+/Na+ or K+/Rb+ mixtures revealed deviations from the principle of independence and notably two anomalous effects of the K+/Rb+ mole fraction (i.e., the ratio [K+]/([K+]+[Rb+])). First, the KAT1 deactivation time constant was both voltage- and mole fraction-dependent (a so-called 'foot in the door' effect was thus observed in KAT1 channel). Second, when plotted against the K+/Rb+ mole fraction, KAT1 conductance values passed through a minimum. This minimum is more important for two pore mutants of KAT1 (T259S and T260S) that displayed an increase in PRb/PK. These results are consistent with the idea that KAT1 conduction requires several ions to be present simultaneously within the pore. Therefore, this atypical 'green' member of the Shaker superfamily of K+ channels further shows itself to be an interesting model as well for permeation as for gating mechanism studies.

Identification and disruption of a plant shaker-like outward channel involved in K+ release into the xylem sap.

SKOR, a K+ channel identified in Arabidopsis, displays the typical hydrophobic core of the Shaker channel superfamily, a cyclic nucleotide-binding domain, and an ankyrin domain. Expression in Xenopus oocytes identified SKOR as the first member of the Shaker family in plants to be endowed with outwardly rectifying properties. SKOR expression is localized in root stelar tissues. A knockout mutant shows both lower shoot K+ content and lower xylem sap K+ concentration, indicating that SKOR is involved in K+ release into the xylem sap toward the shoots. SKOR expression is strongly inhibited by the stress phytohormone abscisic acid, supporting the hypothesis that control of K+ translocation toward the shoots is part of the plant response to water stress.

Toward a better understanding of the future of the solo medical practitioner in health care industry: a conceptual review.

Even a brief conceptual review of the current developments in the health care industry indicates that the future of independent medical practitioners is rather challenging. It may be necessary for these parties to pursue proactive and aggressive marketing strategies to be able to compete with the managed care organizations. Accordingly, this paper outlines some of the current trends in health care marketing as they relate to the ongoing changes to which solo medical practitioners need to respond. It is hoped that the review of the issues raised in this paper can provide an initial basis for a better understanding of some of the challenges to come up with more comprehensive and effective strategy decisions.

[The household and the small family, a methodological illusion of statistics and survey demography]. / Le menage et la famille restreinte, illusion methodologique de la statistique et de la demographie d'enquete.

The authors question the use of the concept of "households" in demographic analyses of the family. "Designed with reference to the Western family, is the concept still operational in radically different contexts, and in particular where extended families are dominant? The authors reply that it might be as long as it is handled with caution, knowledge of its limits and only as a practical collection concept. Wanting to force it to coincide with a social reality leads to methodological illusion." (SUMMARY IN ENG)

[Informal unions in sub-Saharan Africa: the example of the Congolese "second office"]. / Les unions informelles en Afrique au sud du Sahara: l'exemple du deuxieme bureau congolais.

The author analyzes an informal type of marital union known as the "second office", which is prevalent in countries such as the Congo, the Ivory Coast, and Zaire. "He points out...aspects of the phenomenon which pertain to tradition (polygamy) as well as to sociological aspects linked to modern lifestyles (affectivity in interpersonal relationships, preeminence of the father over the uncle in matrilineal kinship systems), to the urban way of life (greater feminine independence) and to essentially monetary economic relationships. The survey method was based on 'focus groups'." (SUMMARY IN ENG AND ITA)

[The family in Tunis: some statistical data from the Survey on Migration and Employment in Tunis (1972-1973)]. / La famille a Tunis: de quelques donnees statistiques issues de l'Enquete Migration et Emploi a Tunis (1972-1973)

PIP: The main characteristics of families in the city of Tunis, Tunisia, are examined using data from a 1972-1973 survey on migration and employment. Consideration is given to family type, origin, and size and to the migration and economic activity of the individuals concerned. (summary in ENG, SPA)^ieng

Migrants' health and adjustment to urban life--Senegal.

PIP: During 1970-1973, a study was done on the Serer, a Senegalese tribe known to have difficulty in adapting to new situations. The study was done by the World Health Organization and the University in Dakar to determine the health differences, if any, of those still living in their rural environment and those residing in the city. 262 adults were surveyed in the rural area of Niakhar and 225 Serer natives living in the urban area of Dakir were also surveyed. Questionnaires were prepared dealing with sociodemographic, biomedical, mental health, housing and hygiene data. It was found that urban dwellers tended to be younger than their rural counterparts. There was also a greater percentage of Serer women who live in the city. Education was higher among those who lived in Dakar with male urban dwellers having a 13% higher ratio than women. Serer men who live in the city tend to socialize more with other groups while women mixed poorly saying that other cultures had "doubtful morality." Women who moved to Dakir trusted modern medicine, while urban dwelling men believed both in modern and folk medicine. 3 areas of medical concern were studied: precursors to heart disease, anemia, and infectious diseases. Little difference was observed between rural and urban populations for precursors to heart disease. Anemia was common in the rural area for both sexes. More malaria, intestinal parasites and tuberculosis were found in rural households. In regard to mental health, depression and anxiety are approximately the same for urban and rural dwellers while psychiatric syndrome, cerebral syndromes and personality disorders do appear more frequently in urban dwellers. The authors discuss the practical implications of their study and suggest further research that can be done to extend their analysis.^ieng