Coordination between the activity of ion transport systems in the root and photosynthesis in the shoot is a main feature of the integration of ion uptake in the whole plant. However, the mechanisms that ensure this coordination are largely unknown at the molecular level. Here, we show that the expression of five genes that encode root NO 3 ؊ , NH 4 ؉ , and SO 4 2 ؊ transporters in Arabidopsis is regulated diurnally and stimulated by sugar supply. We also provide evidence that one Pi and one K ؉ transporter also are sugar inducible. Sucrose, glucose, and fructose are able to induce expression of the ion transporter genes but not of the carboxylic acids malate and 2-oxoglutarate. For most genes investigated, induction by light and induction by sucrose are strongly correlated, indicating that they reflect the same regulatory mechanism (i.e., stimulation by photosynthates). The functional importance of this control is highlighted by the phenotype of the atnrt2 mutant of Arabidopsis. In this mutant, the deletion of the sugar-inducible NO 3 ؊ transporter gene AtNrt2.1 is associated with the loss of the regulation of high-affinity root NO 3 ؊ influx by light and sugar. None of the sugar analogs used (3-O -methylglucose, 2-deoxyglucose, and mannose) is able to mimic the inducing effect of sugars. In addition, none of the sugar-sensing mutants investigated ( rsr1-1 , sun6 , and gin1-1 ) is altered in the regulation of AtNrt2.1 expression. These results indicate that the induction of AtNrt2.1 expression by sugars is unrelated to the main signaling mechanisms documented for sugar sensing in plants, such as regulation by sucrose, hexose transport, and hexokinase (HXK) sensing activity. However, the stimulation of AtNrt2.1 transcript accumulation by sucrose and glucose is abolished in an antisense AtHXK1 line, suggesting that HXK catalytic activity and carbon metabolism downstream of the HXK step are crucial for the sugar regulation of AtNrt2.1 expression.

Cytokinesis requires membrane fusion during cleavage-furrow ingression in animals and cell plate formation in plants. In Arabidopsis, the Sec1 homologue KEULE (KEU) and the cytokinesis-specific syntaxin KNOLLE (KN) cooperate to promote vesicle fusion in the cell division plane. Here, we characterize AtSNAP33, an Arabidopsis homologue of the t-SNARE SNAP25, that was identified as a KN interactor in a yeast two-hybrid screen. AtSNAP33 is a ubiquitously expressed membrane-associated protein that accumulated at the plasma membrane and during cell division colocalized with KN at the forming cell plate. A T-DNA insertion in the AtSNAP33 gene caused loss of AtSNAP33 function, resulting in a lethal dwarf phenotype. atsnap33 plantlets gradually developed large necrotic lesions on cotyledons and rosette leaves, resembling pathogen-induced cellular responses, and eventually died before flowering. In addition, mutant seedlings displayed cytokinetic defects, and atsnap33 in combination with the cytokinesis mutant keu was embryo lethal. Analysis of the Arabidopsis genome revealed two further SNAP25-like proteins that also interacted with KN in the yeast two-hybrid assay. Our results suggest that AtSNAP33, the first SNAP25 homologue characterized in plants, is involved in diverse membrane fusion processes, including cell plate formation, and that AtSNAP33 function in cytokinesis may be replaced partially by other SNAP25 homologues.

). ² The ®rst two authors contributed equally to this work. SummaryRegulation of root N uptake by whole-plant signalling of N status was investigated at the molecular level in Arabidopsis thaliana plants through expression analysis of AtNrt2.1 and AtAmt1.1. These two genes encode starvation-induced high-af®nity NO 3 ± and NH 4 + transporters, respectively. Split-root experiments indicate that AtNrt2.1 expression is controlled by shoot-to-root signals of N demand. Together with 15 NO 3 ± in¯ux, the steady-state transcript level of this gene is increased in NO 3 ± -fed roots in response to N deprivation of another portion of the root system. Thus AtNrt2.1 is the ®rst identi®ed molecular target of the long-distance signalling informing the roots of the whole plant's N status. In contrast, AtAmt1.1 expression is predominantly dependent on the local N status of the roots, as it is mostly stimulated in the portion of the root system directly experiencing N starvation. The same behaviour was found for NH 4 + in¯ux, suggesting that the NH 4 + uptake system is much less ef®cient than the NO 3 ± uptake system, to compensate for a spatial restriction of N availability. Other major differences were found between the regulations of AtNrt2.1 and AtAmt1.1 expression. AtNrt2.1 is strongly upregulated by moderate level of N limitation, while AtAmt1.1 transcript level is markedly increased only under severe N de®ciency. Unlike AtNrt2.1, AtAmt1.1 expression is not stimulated in a nitrate reductasede®cient mutant after transfer to NO 3 ± as sole N source, indicating that NO 3 ± per se acts as a signal repressing transcription of AtAmt1.1. These results reveal two fundamentally different types of mechanism involved in the feedback regulation of root N acquisition by the N status of the plant.

The fusion of vesicles in the secretory pathway involves the interaction of t-soluble N-ethylmaleimide-sensitive factor attachment protein receptors (t-SNAREs) on the target membrane and v-SNAREs on the vesicle membrane. AtSNAP33 is an Arabidopsis homolog of the neuronal t-SNARE SNAP-25 involved in exocytosis and is localized at the cell plate and at the plasma membrane. In this paper, the expression of AtSNAP33 was analyzed after different biotic and abiotic stresses. The expression of AtSNAP33 increased after inoculation with the pathogens Plectosporium tabacinum and virulent and avirulent forms of Peronospora parasitica and Pseudomonas syringae pv tomato. The expression of PR1 transcripts encoding the secreted pathogenesis-related protein 1 also increased after inoculation with these pathogens and the expression of AtSNAP33 preceded or occurred at the same time as the expression of PR1. AtSNAP33 was also expressed in npr1 plants that do not express PR1 after pathogen inoculation as well as in cpr1 plants that overexpress PR1 in the absence of a pathogen. The level of AtSNAP33 decreased slightly in leaves inoculated with P. parasitica in the NahG plants, and eds5 and sid2 mutants that are unable to accumulate salicylic acid (SA) after pathogen inoculation, indicating a partial dependence on SA. AtSNAP33 was also expressed in systemic noninoculated leaves of plants inoculated with P. syringae. In contrast to the situation in infected leaves, the expression of AtSNAP33 in systemic leaves was fully SA dependent. Thus, the expression of AtSNAP33 after pathogen attack is regulated by SA-dependent and SA-independent pathways. Mechanical stimulation also led to an increase of AtSNAP33 transcripts.The plant secretory pathway comprises different organelles including the endoplasmic reticulum, the Golgi apparatus, the plasma membrane, and the vacuole. Proteins destined to the extracellular space or the vacuole enter the secretory pathway at the endoplasmic reticulum and are then transported through the Golgi apparatus. At the trans-Golgi network, secreted proteins are sorted from vacuolar proteins and packaged into secretory vesicles. Transport between the organelles of the secretory pathway occurs by budding of vesicles from a donor membrane and fusion with an acceptor membrane. Exocytosis is the fusion of secretory vesicles with the plasma membrane, permitting the release of their content outside the cell. The fusion of vesicles involves the interaction of vesicle (v)-soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptors (SNAREs) localized on the vesicle membrane and t-SNAREs localized on the target membrane (Rothman, 1996; Hay and Scheller, 1997). A four-helical bundle of SNAREs is formed. One helix of this SNARE complex is provided by a v-SNARE and three helices are provided by t-SNAREs, which always include a member of the syntaxin family contributing one helix. The remaining two helices are contributed from a single SNAP-25-like protein or from two separate t-SNARE light chains. Trimeric SN...

Carbohydrate-starved cultures of Lactococcus lactis subsp. lactis IL1403 showed enhanced resistance to heat, ethanol, acid, osmotic, and oxidative stresses. This cross-protection seems to be established progressively during the transitional growth phase, with maximum resistance occurring when cells enter the stationary phase. Chloramphenicol or rifamycin treatment does not abolish the development of a tolerant cell state but, on the contrary, seems to provoke this response in L. lactis subsp. lactis.