Some properties of Proton Pumping ATPases at the Plasma Membrane and Tonoplast of Guard Cells

Systems dynamic modeling of the stomatal guard cell predicts emergent behaviors in transport, signaling, and volume control

The dynamics of stomatal movements and their consequences for photosynthesis and transpirational water loss have long been incorporated into mathematical models, but none have been developed from the bottom up that are widely applicable in predicting stomatal behavior at a cellular level. We previously established a systems dynamic model incorporating explicitly the wealth of biophysical and kinetic knowledge available for guard cell transport, signaling, and homeostasis. Here we describe the behavior of the model in response to experimentally documented changes in primary pump activities and malate (Mal) synthesis imposed over a diurnal cycle. We show that the model successfully recapitulates the cyclic variations in H⁺, K⁺, Cl⁻, and Mal concentrations in the cytosol and vacuole known for guard cells. It also yields a number of unexpected and counterintuitive outputs. Among these, we report a diurnal elevation in cytosolic-free Ca²⁺ concentration and an exchange of vacuolar Cl⁻ with Mal, both of which find substantiation in the literature but had previously been suggested to require additional and complex levels of regulation. These findings highlight the true predictive power of the OnGuard model in providing a framework for systems analysis of stomatal guard cells, and they demonstrate the utility of the OnGuard software and HoTSig library in exploring fundamental problems in cellular physiology and homeostasis.

Fusicoccin- and light-induced activation and in vivo phosphorylation of phosphoenolpyruvate carboxylase in vicia guard cell protoplasts

The in vivo regulation of phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31), was studied in purified guard cell protoplasts (GCPs) of Vicia faba L. Incubation of GCPs with fusicoccin (FC) led to the rapid activation of PEPCase and reduced its sensitivity towards the feedback-inhibitor malate. This was accompanied by an increase in the phosphorylation state of the enzyme. Additionally, PEPCase could be transiently activated by white light. Activation and phosphorylation of PEPCase upon illumination were dependent on the presence of potassium in the incubation medium. Treatment of GCPs with inhibitors of H(+)-ATPases, and with abscisic acid (ABA) suppressed the activation of PEPCase in a concentration-dependent manner. Treatment of protoplasts with butyrate also led to PEPCase activation, suggesting a role for the cytosolic pH (pH(cyt)) in the signal transduction process. The presented data indicate that guard cell PEPCase is regulated by reversible phosphorylation of at least one isoform and elucidate first components of the signaling pathway.

pH, abscisic acid and the integration of metabolism in plants under stressed and non-stressed conditions: cellular responses to stress and their implication for plant water relations

A paradigm for the response of plants to stress is presented which suggests that plants move towards a state of minimal metabolic activity as a stress intensifies and remain in that state until that stress is relieved. The paradigm is based on the proposition that cells that interface with the transpiration stream employ variations on the following theme to move towards that state. Tension on the apoplastic water opens a mechanosensitive Ca2+ channel, a response that is augmented by apoplastic ABA. The resulting elevated cytoplasmic Ca2+ deactivates a plasmalemma H+/ATPase and also activates a K(+)-H+ symport. The inflow of K+ and H+ depolarizes the membrane and renders the apoplast less acidic, the protons being removed to the vacuole and the K+ ions being re-exported via the K+ outward rectifying channel. The onset of darkness in guard and mesophyll cells deactivates the plasmalemma H+/ATPase and then the events outlined above ensue except that these cells do not appear to utilize either Ca2+ or ABA during these changes. In stressed cells it is proposed that elevated cytoplasmic Ca2+ activates the release of an ABA precursor from a stored form. ABA is then released in the apoplast after export of the precursor if the activity of the K(+)-H+ symport has brought the apoplastic pH close to 7.0. It is proposed that aquaporins in the xylem parenchyma and mesophyll cells are opened by elevated cytoplasmic Ca2+ when the water potential of the transpiration stream is high so that water can be stored in the 'xylem parenchyma reservoir'. The water in this reservoir is then used to increase the water potential in the transpiration stream when the water column is under tension and to help repair embolisms by a mechanism that resembles stomatal closure.