Guard-cell apoplastic sucrose concentration--a link between leaf photosynthesis and stomatal aperture size in the apoplastic phloem loader Vicia faba L.

In broad bean (Vicia faba L.), an apoplastic phloem loader, the sucrose concentration increases up to approximately 2 mM in the leaf apoplast and up to approximately 150 mM in the guard-cell apoplast during the photoperiod. This high concentration in the guard-cell apoplast results from transpiration and is sufficient osmotically to reduce stomatal aperture size by up to 3 microm or approximately 25% of the maximum aperture size. In this paper, we investigated a parallel and required role for high bulk-leaf apoplastic sucrose concentration, which correlates with high photosynthesis rate. An empirically determined combination of lowered light intensity and lowered CO(2) concentration reduced the photosynthesis rate to nominally one-fifth of the control value without a significant change in transpiration. This reduction in photosynthesis caused the sucrose concentration in the leaf apoplast--the immediate source pool for guard cells--to decrease by 70% (to 0.4 mM). In turn, sucrose concentration in the guard-cell apoplast decreased by approximately 80% (to approximately 40 mM). These results complete the required evidence for a non-exclusive, transpiration-linked, photosynthesis-dependent passive mechanism for the modulation of stomatal aperture size. In an ancillary investigation, hexoses in the bulk-leaf apoplast decreased when photosynthesis was lowered, but their concentrations in the guard-cell apoplast of control plants indicated that their osmotic contribution was negligible.

Guard cell apoplastic photosynthate accumulation corresponds to a phloem-loading mechanism.

Apoplastic phloem loaders have an apoplastic step in the movement of the translocated sugar, prototypically sucrose, from the mesophyll to the companion cell-sieve tube element complex. In these plants, leaf apoplastic sucrose becomes concentrated in the guard cell wall to nominally 150 mM by transpiration during the photoperiod. This concentration of external sucrose is sufficient to diminish stomatal aperture size in an isolated system and to regulate expression of certain genes. In contrast to apoplastic phloem loaders and at the other extreme, strict symplastic phloem loaders lack an apoplastic step in phloem loading and mostly transport raffinose family oligosaccharides (RFOs), which are at low concentrations in the leaf apoplast. Here, the effects of the phloem-loading mechanism and associated phenomena on the immediate environment of guard cells are reported. As a first step, carbohydrate analyses of phloem exudates confirmed basil (Ocimum basilicum L. cv. Minimum) as a symplastic phloem-loading species. Then, aspects of stomatal physiology of basil were characterized to establish this plant as a symplastic phloem-loading model species for guard cell research. [(14)C]Mannitol fed via the cut petiole accumulated around guard cells, indicating a continuous leaf apoplast. The (RFO+sucrose+hexoses) concentrations in the leaf apoplast were low, <0.3 mM. Neither RFOs (<10 mM), sucrose, nor hexoses (all, P >0.2) were detectable in the guard cell wall. Thus, differences in phloem-loading mechanisms predict differences in the in planta regulatory environment of guard cells.

Dyspepsia and its overlap with irritable bowel syndrome.

Patients with functional dyspepsia and the irritable bowel syndrome are commonly seen in both primary care and gastroenterology subspecialty settings. Although functional dyspepsia and the irritable bowel syndrome can occur separately, recent research suggests that they often appear together as an overlap syndrome and thus may represent different portions of a unifying spectrum of disease. Despite their widespread prevalence, the pathogenesis of these disorders is not well established but may include impaired gastric emptying, gastric dysrhythmias, hypersensitivity (to acid exposure and to stretch), and Helicobacter pylori infection. Once other disorders in the differential diagnosis have been excluded, treatment of patients with functional dyspepsia, irritable bowel syndrome, and the overlap syndrome without alarm signs underscores current prevailing pathophysiologies and is generally empiric and symptom based. It is hoped that management of these disorders will become more targeted and efficacious as our understanding of them improves.

Impaired fatty acid oxidation as a cause of liver disease associated with hyperemesis gravidarum.

Hyperemesis gravidarum (HG) is the most severe form of illness within the spectrum of nausea and vomiting of pregnancy. Liver disease, usually consisting of mild serum transaminase elevation, occurs in almost 50% of patients with HG. While multiple risk factors have been proposed, the etiology and underlying mechanism of maternal liver disease associated with HG remains unclear. In this report, we hypothesize that impairment of mitochondrial fatty acid oxidation (FAO) plays a role in the pathogenesis of maternal liver disease associated with HG. We hypothesize that women heterozygous for FAO defects develop HG associated with liver disease while carrying fetuses with FAO defects due to accumulation of fatty acids in placenta and subsequent generation of reactive oxygen species. Alternatively, it is possible that starvation leading to peripheral lipolysis and increased load of fatty acids in maternal-fetal circulation, combined with reduced capacity of the mitochondria to oxidize fatty acids in mothers heterozygous for FAO defects, can also cause HG and liver injury while carrying non-affected fetuses. The rationale for this hypothesis is discussed.

Requirements for activation of the signal-transduction network that leads to regulatory phosphorylation of leaf guard-cell phosphoenolpyruvate carboxylase during fusicoccin-stimulated stomatal opening.

Leaves regulate gas exchange through control of stomata in the epidermis. Stomatal aperture increases when the flanking guard cells accumulate K+ or other osmolytes. K+ accumulation is stoichiometric with H+ extrusion, which is compensated for by phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31)-mediated malate synthesis. Plant PEPCs are regulated allosterically and by phosphorylation. Aspects of the signal-transduction network that control the PEPC phosphorylation state in guard cells are reported here. Guard cells were preloaded with [32P]orthophosphate (32Pi); then stomata were incubated with fusicoccin (FC), which activates the guard-cell plasma membrane H+-ATPase. [32P]PEPC was assessed by immunoprecipitation, electrophoresis, immunoblotting, and autoradiography. In -FC controls, stomatal size, guard-cell malate, and [32P]PEPC were low; maximum values for these parameters were observed in the presence of FC after a 90-min incubation and persisted for an additional 90 min. This high steady-state phosphorylation status resulted from continuous phosphorylation and dephosphorylation, even after the malate-accumulation phase. PEPC phosphorylation was diminished by approximately 80% when K+ uptake was associated with Cl- uptake and was essentially abolished when stomatal opening was sucrose--rather than K+--dependent. Finally, alkalinization by NH4+ in the presence of K+ did not cause PEPC phosphorylation (as it does in C4 plants). As discussed, a role for cytoplasmic protons cannot be completely excluded by this result. In summary, activation of the plasma membrane H+-ATPase was essential, but not sufficient, to cause phosphorylation of guard-cell PEPC. Network components downstream of the H+-ATPase influence the phosphorylation state of this PEPC isoform.