Summary We analysed the effects of inactivation of the phytochrome genes cph1 and cph2 on the phototactic migration of the cyanobacterium Synechocystis sp. PCC 6803 under different light qualities. Motility towards white, green, red and far red light was not altered in several independently raised cph1, cph2 and cph1/cph2 double mutants. Blue light (λ = 400–500 nm) did not induce motility in wild‐type and cph1 mutant cells, whereas cph2 and cph1/cph2 double mutants moved towards the blue light. Inhibition of the photosynthetic electron transport by DCMU influenced the motility of cph2 mutants under blue light, but not under white, red and far red light. RNA blot hybridizations did not indicate an altered transcript accumulation of the pilin‐encoding pilA1 gene under blue light. We propose that the Cph2 protein is part of a light‐stimulated signal transduction chain inhibiting the movement of Synechocystis sp. PCC 6803 cells towards blue light.

The second cyanobacterial phytochrome Cph2 from Synechocystis sp. PCC 6803 was suggested as a part of a light-stimulated signal transduction chain inhibiting movement toward blue light. Cph2 has the two bilin binding sites, cysteine-129 and cysteine-1022, that might be involved in sensing of red/far-red and blue light, respectively. Here, we present data on wavelength dependence of the phototaxis inhibition under blue light, indicating that Cph2 itself is the photoreceptor for this blue light response. We found that inhibition of blue-light phototaxis in wild-type cells occurred below the transition point of about 470 nm. Substitution of cysteine-1022 with valine led to photomovement of the cells toward blue light (cph2(-) mutant phenotype). Analysis of mutants lacking cysteine-129 in the N-terminal chromophore binding domain indicated that this domain is also important for Cph2 function or folding of the protein. Furthermore, putative blue-light and phytochrome-like photoreceptors encoded by the Synechocystis sp. PCC 6803 genome were inactivated in wild-type and cph2 knockout mutant background. Our results suggest that none of these potential photoreceptors interfere with Cph2 function, although inactivation of taxD1 as well as slr1694 encoding a BLUF protein led to cells that reversed the direction of movement under blue light illumination in mutant strains of cph2.

The acclimation of leaf pigments and antioxidative systems along the vertical canopy light gradient was determined in mature European beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Matt.) Liebl.] trees in a mixed beech/oak forest in northwest Germany. The aim of the study was to investigate if the higher shade tolerance of beech, relative to oak, is correlated with characteristics of the leaf pigment composition and antioxidative systems. Both species showed a similar acclimative response to increasing light availability. Compared to shade leaves, leaves of the upper crown layer exhibited higher chlorophyll a to chlorophyll b ratios and higher contents of photoprotective carotenoids, ascorbate and glutathione per unit chlorophyll, and higher mean activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase. The comparison of beech and oak leaves sampled at crown positions with similar light environments revealed "shade-type" pigment characteristics for beech, relative to oak, whereas "sun-type" characteristics were found with respect to antioxidative systems. At a given average light availability level, leaves of F. sylvatica exhibited higher antioxidant contents and higher ascorbate peroxidase activities as compared to those of Qu. petraea. The data suggest that a higher antioxidative capacity is required in leaves of the more shade-tolerant species. The supposed higher production rate of radicals and reactive oxygen species (ROS) in beech can be explained by: (1) the lower energy utilizing capacity, as beech leaves show lower light-saturated rates of photosynthesis than oak leaves, and (2) by specific features of the photosynthetic apparatus as suggested by the pigment data. A higher ROS generation rate in beech versus oak leaves could be seen as a consequence of efficient photosynthesis under low light conditions. Therefore a high capacity of protection against oxidative stress might be a prerequisite of shade tolerance.

Inactivation of the genes for the cyanobacterial phytochromes cph1 and cph2 in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 affected the growth of the cells under certain light conditions. Differences in growth were detected by recording growth curves and in competition experiments. Mutation of cph1 and cph2 resulted in different effects. The cph1-mutant strains exhibited a reduced growth rate under far-red light (FRL), whereas the growth of the cph2-mutant strains was inhibited by red light (RL). The growth rate of a cph1- / cph2- double mutant was reduced under both RL and FRL. Furthermore, cph1-, cph2- as well as double-mutant strains showed impaired growth under high-light (HL) conditions. Acclimation of the photosynthetic apparatus of the mutants to RL, FRL and HL, as determined by pigment analysis, was similar to that of the wild type.

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