The malate requirement for stomatal opening in the light (malate dehydrogenase in guard cells)

 

 

Malate dehydrogenase in guard cells of Pisum sativum

by Scheibe R., Rechmann U., Hedrich R., Raschke K. (1990)

Renate ScheibeUdo ReckmannRainer Hedrich, Klaus Raschke,

Lehrstuhl für Pflanzenphysiologie, Universität Bayreuth, Universitätsstr. 30, 8580 Bayreuth, Federal Republic of Germany
Pflanzenphysiologisches Institut und Botanischer Garten der Universität Göttingen, Untere Karspüle 2, 3400 Göttingen, Federal Republic of Germany

 

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in Plant Physiol. 93: 1358-1364 – DOI: https://doi.org/10.1104/pp.93.4.1358

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1062680/

Abstract

Guard cell protoplasts of Pisum sativum show considerable NADP-dependent malate dehydrogenase (MDH) activity in darkness which can be enhanced severalfold by illumination or treatment with dithiothreitol (DTT). The question arose whether guard cells possess an NADP-MDH different from that present in the chloroplasts of the mesophyll (which is inactive in darkness or in the absence of DTT).

MDH activities were determined in extracts of isolated protoplasts from mesophyll and epidermis, and in mechanically prepared epidermal pieces (with guard cells as the only living cells and no interference from proteases originating from the cell wall digesting enzymes).

Guard cells possessed NAD-dependent MDHs of high activity and incomplete exclusion of NADP as a coenzyme. This NADP-dependent activity of the NAD-MDH(s) could not be stimulated by DTT or, inferentially, by light. The DTT- (and light-) dependent NADP-MDH represented 0.05% of the total protein of the guard cells and had a specific activity of 0.1 unit per milligram protein; both values are in the same range as the corresponding ones of the mesophyll cells.

Agreement was also found in the extent of light activation, in subunit molecular weight, immunological cross-reactions, and in the behavior on an ion exchange column. The activity of the chloroplastic NADP-MDH in guard cells barely suffices to meet the malate requirement for stomatal opening in the light.

It is therefore likely that NAD-MDHs residing in other compartments of the guard cells supplement the activity of the chloroplastic NADP-MDH particularly during stomatal opening in darkness.

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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 ReckmannRenate ScheibeKlaus 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.