Nitrate reductase, the fr-st enzyme in nitrate assimilation, is located at the crossroad of two energyconsuming pathways: nitrate assimilation and carbon fixation. Light, which regulates the expression of many higher-plant carbon fixation genes, also regulates nitrate reductase gene expression. Located in the cytosol, nitrate reductase obtains its reductant not from photosynthesis but from carbohydrate catabolism. This relationship prompted us to investigate the indirect role that light might play, via photosynthesis, in the regulation of nitrate reductase gene expression. We show that sucrose can replace light in eliciting an increase of nitrate reductase mRNA accumulation in dark-adapted green Arabidopsis plants. We show further that sucrose alone is sufficient for the full expression of nitrate reductase genes in etiolated Arabidopsis plants. Finally, using a reporter gene, we show that a 2.7-kilobase region of 5' flanking sequence of the nitrate reductase gene is sufficient to confer the light or the sucrose response.

SummaryThe geminivirus tomato golden mosaic virus (TGMV) replicates in nuclei and expresses genes from high copy number DNA episomes. The authors used TGMV as a vector to determine whether episomal DNA can cause silencing of homologous, chromosomal genes. Two markers were used to assess silencing: (1) the sulfur allele (su) of magnesium chelatase, an enzyme required for chlorophyll formation; and (2) the firefly luciferase gene (luc). Various portions of both marker genes were inserted into TGMV in place of the coat protein open-reading frame and the constructs were introduced into intact plants using particle bombardment. When TGMV vectors carrying fragments of su (TGMV::su) were introduced into leaves of wild-type Nicotiana benthamiana, circular, yellow spots with an area of several hundred cells formed after 3-5 days. Systemic movement of TGMV::su subsequently produced variegated leaf and stem tissue. Fragments that caused silencing included a 786 bp 5Ј fragment of the 1392 bp su cDNA in sense and anti-sense orientation, and a 403 bp 3Ј fragment. TGMV::su-induced silencing was propagated through tissue culture, along with the viral episome, but was not retained through meiosis. Systemic downregulation of a constitutively expressed luciferase transgene in plants was achieved following infection with TGMV vectors carrying a 623 bp portion of luc in sense or anti-sense orientation. These results establish that homologous DNA sequences localized in nuclear episomes can modulate the expression of active chromosomal genes.

Root-knot nematodes are obligate plant parasites that induce development of an elaborate feeding site during root infection. Feeding-site formation results from a complex interaction between the pathogen and the host plant in which the nematode alters patterns of plant gene expression within the cells destined to become the feeding site. Expression of TobRB7, a gene expressed only in tobacco roots, is induced during feeding site development. The cis-acting sequences that mediate induction by the nematode are separate from those that control normal root-specific expression. Reporter transgenes driven by the nematode-responsive promoter sequences exhibit expression exclusively in the developing feeding site.

The differential regulation of the two nitrate reductase (NR, EC 1.6.6.1) genes of Arabidopsis thaliana L. Heynh was examined. cDNAs corresponding to each of the NR genes (NR1 and NR2) were used to measure changes in the steady-state levels of NR mRNA in response to nitrate, light, circadian rhythm, and tissue specificity. Although nitrate-induction kinetics of the two genes are very similar, NRI is expressed in the absence of nitrate at a higher basal level than NR2. Nitrate induction is transient both in the roots and leaves, however the kinetics are different: the induction and decline in the roots precede that in the leaves. Light induces the expression of each of the genes with significantly different kinetics: NR2 reached saturation more rapidly than did NR1. Both genes showed similar diurnal patterns of circadian rhythm, with NR2 mRNA accumulating earlier in the morning.

Mannitol is the most abundant sugar alcohol in nature, occurring in bacteria, fungi, lichens, and many species of vascular plants. Celery (Apium graveolens L.), a plant that forms mannitol photosynthetically, has high photosynthetic rates thought to result from intrinsic differences in the biosynthesis of hexitols vs. sugars. Celery also exhibits high salt tolerance due to the function of mannitol as an osmoprotectant. A mannitol catabolic enzyme that oxidizes mannitol to mannose (mannitol dehydrogenase, MTD) has been identified. In celery plants, MTD Mannitol is a six-carbon noncyclic sugar alcohol found in diverse organisms ranging from bacteria to higher plants. Mannitol is present in more than 100 species of higher plants, where it can be a significant portion of the soluble carbohydrate (1-3). For instance, celery (Apium graveolens) translocates up to 50% of its photoassimilate as mannitol, with the remainder being sucrose (4). Both translocated carbohydrates are assimilated during growth of nonphotosynthetic heterotrophic (i.e., sink) tissues. Other postulated roles for mannitol include carbon storage, free radical scavenging, and osmoprotection (4-7).The use of mannitol as a photoassimilate and translocated carbohydrate is reported to be advantageous to the plant in several ways. Celery, a C3 plant, has carbon fixation rates equivalent to those of many C4 plants (8). This may result from both increased NADP/NADPH turnover compared to plants that exclusively form sugars and from the additional cytosolic sink for photosynthetically fixed CO2 provided by mannitol synthesis (7,9,10). In addition to the increased carbon fixation that accompanies mannitol biosynthesis, the initial step of mannitol utilization generates NADH, thus giving a higher net ATP yield than the catabolism of an equal amount of sucrose (7). Finally, mannitol-producing plants also exhibit a high degree of salt tolerance due to the function of mannitol as an osmoregulator and compatible solute (6,11,12). Celery plants grown in hydroponic culture with a salinity equivalent to 30% that of sea water show dry weight gains equal to plants grown at normal nutrient levels (12). In addition, tobacco that was genetically engineered to synthesize mannitol through the introduction of the Escherichia coli NAD-dependent mannitol-1-phosphate dehydrogenase acquired significant salt tolerance (6).Metabolite pool sizes in plants are usually determined by relative rates of synthesis and utilization. The isolation and characterization of a plant NAD-dependent mannitol dehydrogenase (MTD), the enzyme responsible for the oxidation of mannitol to mannose in celery, was reported by our laboratory (13). MTD is a monomeric mannitol:mannose 1-oxidoreductase with a molecular mass of "40 kDa (13,14). In celery plants, the expression of MTD is highly regulated. MTD activity is highest in young actively growing root tips, is also high in young rapidly growing (sink) leaves, but is not detected in mature photosynthetic (source) leaves. Extractable MTD activity ...