Genetic modiWcation of nitrogen metabolism via bacterial NADPH-dependent glutamate dehydrogenase (GDH; E.C.4.1.2.1) favorably alters growth and metabolism of C3 plants. The aim of this study was to examine the eVect of expression of GDH in the cytoplasmic compartment of Zea mays cells. The gdhA gene from Escherichia coli , that encoded a NADPH-GDH, was ligated to the ubiquitin promoter that incorporated the Wrst intron enhancer and used to transform Z. mays cv. 'H99' embryo cultures by biolistics. R0-R3 generations included selfed inbreds, back-crossed inbreds, and hybrids with B73 derivatives. The lines with the highest GDH speciWc activity produced infertile R0 plants. The highest speciWc activity of GDH from the fertile Z. mays plants was suYcient to alter phenotypes. Plant damage caused by the phosphinothricin in gluphosinate-type herbicides, glutamine synthetase (GS; EC 6.1.3.2) inhibitors, was less pronounced in Z. mays plants with gdhA pat than in gusA pat plants. Germination and grain biomass production were increased in gdhA transgenic plants in the Weld during seasons with signiWcant water deWcits but not over all locations. Water deWcit tolerance under controlled conditions was increased. Crops modiWed with gdhA may have value in semi-arid locations.

To investigate the contribution of root cytosolic glutamine synthetase (GS) activity in plant biomass production, two different approaches were conducted using the model legume Lotus japonicus. In the first series of experiments, it was found that overexpressing GS activity in roots of transgenic plants leads to a decrease in plant biomass production. Using (15)N labelling it was shown that this decrease is likely to be due to a lower nitrate uptake accompanied by a redistribution to the shoots of the newly absorbed nitrogen which cannot be reduced due to the lack of nitrate reductase activity in this organ. In the second series of experiments, the relationship between plant growth and root GS activity was analysed using a series of recombinant inbred lines issued from the crossing of two different Lotus ecotypes, Gifu and Funakura. It was confirmed that a negative relationship exists between root GS expression and plant biomass production in both the two parental lines and their progeny. Statistical analysis allowed it to be estimated that at least 13% of plant growth variation can be accounted for by variation in GS activity.

At cholinergic synapses, acetylcholinesterase (AChE) is critical for ensuring normal synaptic transmission. However, little is known about how this enzyme is maintained and regulated in vivo. In this work, we demonstrate that the dissociation of fluorescently-tagged fasciculin 2 (a specific and selective peptide inhibitor of AChE) from AChE is extremely slow. This fluorescent probe was used to study the removal and insertion of AChE at individual synapses of living adult mice. After a one-time blockade of AChEs with fluorescent fasciculin 2, AChEs are removed from synapses initially at a faster rate (t1 ⁄ 2 of ϳ3 days) and later at a slower rate (t1 ⁄ 2 of ϳ12 days). Most of the removed AChEs are replaced by newly inserted AChEs over time. However, when AChEs are continuously blocked with fasciculin 2, the removal rate increases substantially (t1 ⁄ 2 of ϳ12 h), and most of the lost AChEs are not replaced by newly inserted AChE. Furthermore, complete one-time inactivation of AChE activity significantly increases the removal of postsynaptic nicotinic acetylcholine receptors (AChRs). Finally, time lapse imaging reveals that synaptic AChEs and AChRs that are removed from synapses are co-localized in the same pool after being internalized. These results demonstrate a remarkable AChE dynamism and argue for a potential link between AChE function and postsynaptic receptor lifetime.

In chicory, we examined how NO 3 A supply aected NO 3 A uptake, N partitioning between shoot and root and N accumulation in the tuberized root throughout the vegetative period. Plants were grown at two NO 3 A concentrations: 0.6 and 3 mM. We used 15 N-labelling/ chase experiments for the quanti®cation of N¯uxes between shoot and root and for determining whether N stored in the tuberized root originates from N remobilized from the shoot or from recently absorbed NO 3A . The rate of 15 NO 3 A uptake was decreased by low NO 3 A availability at all stages of growth. In young plants (10±55 days after sowing; DAS), in both NO 3 A treatments the leaves were the strongest sink for 15 N. In mature (tuberizing) plants, (55±115 DAS), the rate of 15 NO 3 A uptake increased as well as the amount of exogenous N allocated to the root. In N-limited plants, N allocation to the tuberized root relied essentially on recent N absorption, while in N-replete plants, N remobilized from the shoot contributed more to N-reserve accumulation in the root. In senescing plants (115±170 DAS) the rate of 15 NO 3 A uptake decreased mainly in N-replete plants whereas it remained almost unchanged in N-limited plants. In both NO 3 A treatments the tuberized root was the strongest sink for recently absorbed N. Remobilization of previously absorbed N from shoot to tuberized root increased greatly in N-limited plants, whereas it increased slightly in N-replete plants. As a consequence, accumulation of the N-storage compounds vegetative storage protein (VSP) and arginine was delayed until later in the vegetative period in N-limited plants. Our results show that although the dynamics of N storage was aected by NO 3 A supply, the ®nal content of total N, VSP and arginine in roots was almost the same in N-limited and N-replete plants. This indicates that chicory is able to build up a store of available N-reserves, even when plants are grown on low N. We also suggest that in tuberized roots there is a maximal capacity for N accumulation, which was reached earlier (soon after 100 DAS) in N-replete plants. This hypothesis is supported by the fact that in N-replete plants despite NO 3 A availability, N accumulation ceased and signi®cant amounts of N were lost due to N eux.Abbreviations: DAS = days after sowing; 2-D = two-dimensional; VSP = vegetative storage protein Correspondence to: A.M. Limami; Fax: 33 (1) 30 83 30 96