Light perception and the role of the xanthophyll cycle in blue-light-dependent chloroplast movements in Lemna trisulca L.

In most higher plants, chloroplasts move towards the periclinal cell walls in weak blue light (WBL) to increase light harvesting for photosynthesis, a nd towards the anticlinal walls as an escape reaction, thus avoiding photo- damage in strong blue light (SBL). The photo- receptor(s) triggering these responses have not yet been identified. In this study, the role of zeaxanth in as a blue-light photoreceptor in chloroplast movements was investigated. Time-lapse 3D confocal imaging in Lemna trisulca showed that individual ch loroplasts responded to local illumination when one half of the cell was tr eated with light of different intensity or spectral quality to that receive d by the other half, or was maintained in darkness. Thus the complete signa l perception, transduction and effector system has a high degree of spatial resolution and is consistent with localization of part of the transduction chain in the chloroplasts. Turnover of xanthophylls was determined using H PLC, and a parallel increase was observed between zeaxanthin and chloroplas t movements in SBL. Ascorbate stimulated both a transient increase in zeaxa nthin levels and chloroplast movement to profile in physiological darkness. Conversely, dithiothreitol blocked zeaxanthin production and responses to SBL and, to a lesser extent, WBL. Norflurazon preferentially inhibited SBL- dependent chloroplast movements. Increases in zeaxanthin were also observed in strong red light (SRL) when no directional chloroplast movements occurr ed. Thus it appears that a combination of zeaxanthin and blue light is requ ired to trigger responses. Blue light can cause cis-trans isomerization of xanthophylls, thus photo-isomerization may be a critical link in the signal transduction pathway.