title: Imaging the Photosystem I/Photosystem II chlorophyll ratio inside the leaf.
abstract: Oxygenic photosynthesis is driven by photosystems I (PSI) and II (PSII). In plants the number of chlorophylls of PSI versus PSII is adjusted to the light irradiance spectrum. On a timescale of days, this is regulated at the level of protein concentration. Instead, on a timescale of minutes, it is regulated by the dynamic association of light-harvesting complex II with either PSI or PSII. Thus far very diverse values have been reported for the PSI/PSII chlorophyll ratio, ranging from 0.54 to 1.4. The methods used require the isolation of chloroplasts and are time consuming. We present a fluorescence lifetime imaging approach that quantifies the PSI/PSII Chl ratio of chloroplasts directly in their natural leaf environment. In wild type Arabidopsis thaliana plants, grown under white light, the PSI/PSII chlorophyll ratio appeared to be 0.99±0.09 at the adaxial side and 0.83±0.05 at the abaxial side of the leaf. When these plants were acclimated to far red light for several days the PSI/PSII chlorophyll ratio decreased by more than a factor of 3 to compensate for the ineffective far red light absorption of PSII. This shows how plants optimize their light-harvesting capacity to the specific light conditions they encounter. Zooming in on single chloroplasts inside the leaf allowed to study the grana/stroma membrane network and their PSI/PSII chlorophyll ratios. The developed method will be useful to study dynamic processes in chloroplasts in intact leaves which involve changes in the grana and the stroma membranes such as state transitions.
authors.affiliation: Laboratory of Biophysics, Wageningen University, P.O. Box 8128, 6700 ET Wageningen, The Netherlands. Electronic address: emilie.wientjes@wur.nl.
authors.forename: Emilie
authors.initials: E
authors.lastname: Wientjes
authors.affiliation: Laboratory of Biophysics, Wageningen University, P.O. Box 8128, 6700 ET Wageningen, The Netherlands.
authors.forename: John
authors.initials: J
authors.lastname: Philippi
authors.affiliation: Laboratory of Biochemistry, Wageningen University, P.O. Box 8128, 6700 ET Wageningen, The Netherlands; MicroSpectroscopy Centre, Wageningen University, P.O. Box 8128, 6700 ET Wageningen, The Netherlands.
authors.forename: Jan Willem
authors.initials: JW
authors.lastname: Borst
authors.affiliation: Laboratory of Biophysics, Wageningen University, P.O. Box 8128, 6700 ET Wageningen, The Netherlands; MicroSpectroscopy Centre, Wageningen University, P.O. Box 8128, 6700 ET Wageningen, The Netherlands.
authors.forename: Herbert
authors.initials: H
authors.lastname: van Amerongen
mesh_terms: D002734:Chlorophyll / Q000737:chemistry / Q000378:metabolism*; D002736:Chloroplasts / Q000737:chemistry / Q000378:metabolism*; D008027:Light; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism; D010788:Photosynthesis; D045331:Photosystem I Protein Complex / Q000737:chemistry / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000378:metabolism*; D018515:Plant Leaves / Q000737:chemistry / Q000378:metabolism; D013050:Spectrometry, Fluorescence
chemical_list: D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D010100:Oxygen
keywords: Fluorescence lifetime imaging; LHCII; Light-harvesting complex II; Microscopy