Rubisco activity in guard cells and stomatal opening

Photo credit: Google

Pisum sativum – Garden peas

Rubisco activity in guard cells compared with the solute requirement for stomatal opening.

by Reckmann U., Scheibe R., Raschke K. (1990)

Udo Reckmann, Renate Scheibe, Klaus Raschke

in Plant Physiol. 92, 246–253. – doi: 10.1104/pp.92.1.246 –

PubMed Abstract | CrossRef Full Text | Google Scholar –

http://www.plantphysiol.org/content/92/1/246

Abstract

We investigated whether the reductive pentose phosphate path in guard cells of Pisum sativum had the capacity to contribute significantly to the production of osmotica during stomatal opening in the light.

Amounts of ribulose 1,5-bisphophate carboxylase/oxygenase (Rubisco) were determined by the [¹⁴C]carboxyarabinitol bisphosphate assay. A guard cell contained about 1.2 and a mesophyll cell about 324 picograms of the enzyme; the ratio was 1:270.

The specific activities of Rubisco in guard cells and in mesophyll cells were equal; there was no indication of a specific inhibitor of Rubisco in guard cells. Rubisco activity was 115 femtomol per guard-cell protoplast and hour. This value was different from zero with a probability of 0.99.

After exposure of guard-cell protoplasts to ¹⁴CO₂ for 2 seconds in the light, about one-half of the radioactivity was in phosphorylated compounds and <10% in malate.

Guard cells in epidermal strips produced a different labelling pattern; in the light, <10% of the label was in phosphorylated compounds and about 60% in malate. The rate of solute accumulation in intact guard cells was estimated to have been 900 femto-osmol per cell and hour. If Rubisco operated at full capacity in guard cells, and hexoses were produced as osmotica, solutes could be supplied at a rate of 19 femto-osmol per cell and hour, which would constitute 2% of the estimated requirement.

The capacity of guard-cell Rubisco to meet the solute requirement for stomatal opening in leaves of Pisum sativum is insignificant.

Release of malate during stomatal closure

 

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Commelina communis (Asiatic Dayflower)

Release of malate from epidermal strips during stomatal closure. 

by Van Kirk C. A., Raschke K. (1978)

Carol A. Van Kirk, Klaus Raschke

in Plant Physiology 61, 474–475 – DOI: https://doi.org/10.1104/pp.61.3.474 –

CrossRef |PubMed |CAS | –

http://www.plantphysiol.org/content/61/3/474

Abstract

Isolated epidermal strips of Vicia faba and Commelina communis release malate into their bathing medium when stomata close. This release was largest (about 0.6 of the initial malate content) when epidermal strips of C. communis were floated on 10⁻⁵ M (±)-abscisic acid.

Alternation of anion conductance in stomata

 

Alternation of the slow with the quick anion conductance in whole guard cells effected by external malate.

by Raschke K. (2003)

Klaus Raschke

in Planta 217:651–657 – doi:10.1007/s00425-003-1034-3 – 

CrossRef Medline Google Scholar – 

https://link.springer.com/article/10.1007%2Fs00425-003-1034-3

Abstract

In previous investigations two anion conductances were discovered in guard-cell protoplasts: the quickly activating anion conductance (QUAC, R-type) and the slowly activating anion conductance (SLAC, S-type).

In this investigation, effects of malate on the two anion conductances were tested in whole guard cells of Vicia faba L. by the use of the discontinuous single-electrode voltage-clamp method.

Application of 1-s voltage ramps proved that QUAC displayed the malate shift of the activation threshold toward hyperpolarization also in complete guard cells. The sensitivity of SLAC to external malate was determined by responses to voltage pulses of 20 s duration at Cl⁻ concentrations of 0.1, 3 or 50 mM. At no voltage were the currents measured at the end of the pulses in the presence and absence of malate significantly different from each other; the current–voltage relationship of SLAC appeared not to be affected by malate.

However, in 32% of the cells exposed to malate, current activation in response to voltage steps occurred within 0.1 s, faster than was typical for SLAC, and activation was followed by inactivation with a half-time similar to 10 s: SLAC apparently had changed to QUAC.

Simultaneously, the free-running membrane voltage depolarized at 0.1 mM Cl⁻, did not change at 3 mM Cl⁻ and polarized at 50 mM Cl⁻, indicating that activation of QUAC increased the membrane conductance for anions and thereby drove the membrane voltage toward the equilibrium voltage of Cl⁻. The malate-induced changes were fully reversible at Cl⁻ concentrations of 0.1 and 3 mM.

These results reinforce the proposition that SLAC and QUAC represented two switching modes of the same anion channel (however, they do not suffice as proof); they also show that this interconvertibility can enable guard cells to control their membrane voltage rapidly.

Simultaneous requirement of CO2, ABA for stomatal closing

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Xanthium strumarium L

Simultaneous requirement of carbon dioxide and abscisic acid for stomatal closing in Xanthium strumarium L

by Raschke K. (1975)

• Klaus Raschke

in Planta 125:243–259.- doi:10.1007/BF00385601 – 

CrossRefWeb of ScienceGoogle Scholar – 

http://link.springer.com/article/10.1007%2FBF00385601

Summary

Open stomata of detached leaves of Xanthium strumarium L. closed only when carbon dioxide and abscisic acid (ABA) were presented simultaneously. Three parameters of stomatal closing were determined after additions of ABA to the irrigation water of detached leaves, while the leaves were exposed to various CO₂ concentrations ([CO₂]s) in the air;

a) the delay between addition of ABA and a reduction of stomatal conductance by 5%,

b) the velocity of stomatal closing, and

c) the new conductance.

Changes in all three parameters showed that stomatal responses to ABA were enhanced by CO₂; this effect followed saturation kinetics. Half saturation occurred at an estimated [CO₂] in the stomatal pore of 200 μl l^-1.

With respect to ABA, stomata responded in normal air with half their maximal amplitude at [ABA]s between 10^-6 and 10^-5 M(+-)-ABA. The amounts of ABA taken up by the leaves during the delay increased with a power <1 (on the average, 0.67) of the [ABA] in the transpiration stream. The minimal amount of ABA found to produce a stomatal response was about 1 pmol of (+-)-ABA per cm²leaf area, almost two orders of magnitude smaller than the original content of the leaves in ABA indicating that most of the endogenous ABA was in a compartment isolated from the guard cells.

An interaction between stomatal responses to CO₂ and ABA was also found in Gossypium hirsutum L. and Commelina communis L.; it was however much weaker than in X. strumarium.

Based on earlier findings and on the results of this investigation it is suggested that stomata close if the cytoplasm of the guard cells contains much malate and H⁺. The acid content in turn is determined by the relative rates of production of malic acid (from endogenous as well as exogenous CO₂) and its removal (by transport of the anion into the vacuole and exchange of the H⁺ for K⁺ with the environment of the guard cells). The simultaneous requirement of CO₂ and ABA for stomatal closure leads to the inference that ABA inhibits the expulsion of H⁺ from guard cells.