Sensory transduction of blue light in guard cells

Our understanding of the sensory transduction cascade for blue-light-stimulated stomatal opening has been significantly increased by recent findings (Fig. 1Fig. 1). The absorption of blue photons by the carotenoid zeaxanthin, found in the antenna bed of the guard cell chloroplast, initiates the sensory transducing cascade. Excitation of zeaxanthin by blue light causes a photochemical reaction, presumably a trans to cis isomerization17xReversal of blue light-stimulated stomatal opening by green light. Frechilla, S. et al. Plant Cell Physiol. 2000; 41: 171–176Crossref | PubMedSee all References17, which is the first step in the cascade. Several unknown steps transmit this signal across the chloroplast envelope and activate a serine/threonine kinase that phosphorylates the C-terminus of the H+ ATPase and activates the enzyme1xBlue light activates the plasma membrane H+-ATPase by phosphorylation of the C-terminus in stomatal guard cells. Kinoshita, T. and Shimazaki, K. EMBO J. 1999; 18: 5548–5558Crossref | PubMed | Scopus (236)See all References1. A 14-3-3 protein binds to the phosphorylated H+ ATPase and stabilizes it1xBlue light activates the plasma membrane H+-ATPase by phosphorylation of the C-terminus in stomatal guard cells. Kinoshita, T. and Shimazaki, K. EMBO J. 1999; 18: 5548–5558Crossref | PubMed | Scopus (236)See all References1. The extent of proton pumping by the H+ ATPase depends on the extent of phosphorylation1xBlue light activates the plasma membrane H+-ATPase by phosphorylation of the C-terminus in stomatal guard cells. Kinoshita, T. and Shimazaki, K. EMBO J. 1999; 18: 5548–5558Crossref | PubMed | Scopus (236)See all References1, which in turn is determined by the zeaxanthin concentration in the guard cell chloroplast and the number of incident blue photons1xBlue light activates the plasma membrane H+-ATPase by phosphorylation of the C-terminus in stomatal guard cells. Kinoshita, T. and Shimazaki, K. EMBO J. 1999; 18: 5548–5558Crossref | PubMed | Scopus (236)See all References, 18xRole of zeaxanthin in blue light photoreception and the modulation of light–CO2 interactions in guard cells. Zeiger, E. and Zhu, J. J. Exp. Bot. 1998; 49: 433–442CrossrefSee all References. Proton pumping drives solute uptake and stomatal opening. Dephosphorylation dissociates the 14-3-3 protein and deactivates the H+ ATPase.Fig. 1Sensory transduction of blue light in guard cells. Blue light is sensed by the carotenoid, zeaxanthin in the antenna bed of the guard cell chloroplast. Violaxanthin and zeaxanthin are the major constituents of the xanthophyll cycle. The Arabidopsis npq1 mutant has a defective violaxanthin de-epoxidase and cannot accumulate zeaxanthin. Zeaxanthin concentration depends on the pH of the chloroplast lumen, which is modulated by rates of electron transport at the thylakoid membrane and consumption of ATP and NADPH by CO2 fixation in the Calvin cycle18xRole of zeaxanthin in blue light photoreception and the modulation of light–CO2 interactions in guard cells. Zeiger, E. and Zhu, J. J. Exp. Bot. 1998; 49: 433–442CrossrefSee all References18. The regulation of zeaxanthin concentrations by rates of CO2 fixation at the guard cell chloroplast provides guard cells with a CO2-sensing mechanism18xRole of zeaxanthin in blue light photoreception and the modulation of light–CO2 interactions in guard cells. Zeiger, E. and Zhu, J. J. Exp. Bot. 1998; 49: 433–442CrossrefSee all References18. The magnitude of the response to blue light depends on zeaxanthin concentration and the number of absorbed blue photons. The cascade is initiated by the excitation of zeaxanthin by blue light, and the signal is transmitted to the cytoplasm, where it activates a serine/threonine kinase. The kinase phosphorylates the C-terminus of the H+ ATPase and activates the enzyme. A 14-3-3 protein binds to the phosphorylated ATPase and stabilizes it. Dephosphorylation dissociates the 14-3-3 protein and deactivates the H+ ATPase (Ref. 1xBlue light activates the plasma membrane H+-ATPase by phosphorylation of the C-terminus in stomatal guard cells. Kinoshita, T. and Shimazaki, K. EMBO J. 1999; 18: 5548–5558Crossref | PubMed | Scopus (236)See all ReferencesRef. 1).View Large Image | Download PowerPoint SlideSome of the critical areas for future research include:•The characterization of the blue light-dependent zeaxanthin photochemistry and identification of the involved apoprotein(s) in the antenna bed of the guard cell chloroplast.•Identification of the second messenger communicating the signal across the chloroplast envelope.•Characterization of the transducing steps activating the serine/threonine kinase.•Identification of the enzyme that dephosphorylates the ATPase.