ABA depolarizes guard cells in intact plants

 

 

ABA depolarizes guard cells in intact plants, through a transient activation of R- and S-type anion channels.

by Roelfsema M. R. G., Levchenko V., Hedrich R. (2004)

Julius-von-Sachs-Institut für Biowissenschaften, Lehrstuhl für Molekulare Pflanzenphysiologie und Biophysik, Universität Würzburg, Julius-von-Sachs-Platz 2, D-97082, Germany.

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in Plant Journal 37: 578–588. – doi:10.1111/j.1365-313X.2003.01985.x pmid:14756768 –

CrossRefMedlineWeb of ScienceGoogle Scholar –

https://www.ncbi.nlm.nih.gov/pubmed/14756768

Abstract

During drought, the plant hormone abscisic acid (ABA) induces rapid stomatal closure and in turn reduces transpiration. Stomatal closure is accompanied by large ion fluxes across the plasma membrane, carried by K+ and anion channels.

We recorded changes in the activity of these channels induced by ABA, for guard cells of intact Vicia faba plants. Guard cells in their natural environment were impaled with double-barrelled electrodes, and ABA was applied via the leaf surface. In 45 out of 85 cells tested, ABA triggered a transient depolarization of the plasma membrane. In these cells, the membrane potential partially recovered in the presence of ABA; however, a full recovery of the membrane potentials was only observed after removal of ABA.

Repetitive ABA responses could be evoked in single cells, but the magnitude of the response varied from one hormone application to the other. The transient depolarization correlated with the activation of anion channels, which peaked 5 min after introduction of the stimulus.

In guard cells with a moderate increase in plasma membrane conductance (DeltaG < 5 nS), ABA predominantly activated voltage-independent (slow (S)-type) anion channels. During strong responses (DeltaG > 5 nS), however, ABA activated voltage-dependent (rapid (R)-type) in addition to S-type anion channels.

We conclude that the combined activation of these two channel types leads to the transient depolarization of guard cells. The nature of this ABA response correlates with the transient extrusion of Cl- from guard cells and a rapid but confined reduction in stomatal aperture.

 

Cytoplasmic Ca(2+) concentrations in stomata

 

Stringent control of cytoplasmic Ca²⁺ in guard cells of intact plants compared to their counterparts in epidermal strips or guard cell protoplasts

by Levchenko V., Guinot D. R., Klein M.,

Roelfsema M. R. G.,

Hedrich R.,

Dietrich P. (2008)

in Protoplasma 233: 61–72 – doi: 10.1007/s00709-008-0307-x. –

[PubMed] – http://www.ncbi.nlm.nih.gov/pubmed/18648729

Abstract

Cytoplasmic calcium elevations, transients, and oscillations are thought to encode information that triggers a variety of physiological responses in plant cells. Yet Ca(2+) signals induced by a single stimulus vary, depending on the physiological state of the cell and experimental conditions.

We compared Ca(2+) homeostasis and stimulus-induced Ca(2+) signals in guard cells of intact plants, epidermal strips, and isolated protoplasts.

Single-cell ratiometric imaging with the Ca(2+)-sensitive dye Fura 2 was applied in combination with electrophysiological recordings. Guard cell protoplasts were loaded with Fura 2 via a patch pipette, revealing a cytoplasmic free Ca(2+) concentration of around 80 nM at -47 mV. Upon hyperpolarization of the plasma membrane to -107 mV, the Ca(2+) concentration increased to levels exceeding 400 nM.

Intact guard cells were able to maintain much lower cytoplasmic free Ca(2+) concentrations at hyperpolarized potentials, the average concentration at -100 mV was 183 and 90 nM in epidermal strips and intact plants, respectively. Further hyperpolarization of the plasma membrane to -160 mV induced a sustained rise of the guard cell cytoplasmic Ca(2+) concentration, which slowly returned to the prestimulus level in intact plants but not in epidermal strips.

Our results show that cytoplasmic Ca(2+) concentrations are stringently controlled in guard cells of intact plants but become increasingly more sensitive to changes in the plasma membrane potential in epidermal strips and isolated protoplasts.

Guard cell K+ content and K+-channel activity in poplar stomata

 

The poplar K⁺ channel KPT1 is associated with K⁺ uptake during stomatal opening and bud development.

by Langer K., Levchenko V., Fromm J., Geiger D., Steinmeyer R., Lautner S., Ache P., Hedrich R. (2004)

in Plant Journal 37: 828–838.

Wiley Online Library | PubMed | CAS |

Abstract

To gain insights into the performance of poplar guard cells, we have measured stomatal conductance and aperture, guard cell K+ content and K+-channel activity of the guard cell plasma membrane in intact poplar leaves.

In contrast to Arabidopsis, broad bean and tobacco grown under same conditions, poplar stomata operated just in the dynamic range – any change in conductance altered the rate of photosynthesis. In response to light, CO2 and abscisic acid (ABA), the stomatal opening velocity was two to five times faster than that measured for Arabidopsis thaliana, Nicotiana tabacum and Vicia faba.

When stomata opened, the K+ content of guard cells increased almost twofold, indicating that the very fast stomatal opening in this species is mediated via potassium uptake. Following impalement of single guard cells embedded in their natural environment of intact leaves with triple-barrelled microelectrodes, time-dependent inward and outward-rectifying K+-channel-mediated currents of large amplitude were recorded.

To analyse the molecular nature of genes encoding guard cell K+-uptake channels, we cloned K+-transporter Populus tremula (KPT)1 and functionally expressed this potassium channel in a K+-uptake-deficient Escherichia coli mutant.

In addition to guard cells, this K+-transporter gene was expressed in buds, where the KPT1 gene activity strongly correlated with bud break. Thus, KPT1 represents one of only few poplar genes associated with bud flush.