In photosynthetic cells, chloroplasts migrate towards illuminated sites to optimize photosynthesis and move away from excessively illuminated areas to protect the photosynthetic machinery. Although this movement of chloroplasts in response to light has been known for over a century, the photoreceptor mediating this process has not been identified. The Arabidopsis gene NPL1 (ref. 2) is a paralogue of the NPH1 gene, which encodes phototropin, a photoreceptor for phototropic bending. Here we show that NPL1 is required for chloroplast relocation induced by blue light. A loss-of-function npl1 mutant showed no chloroplast avoidance response in strong blue light, whereas the accumulation of chloroplasts in weak light was normal. These results indicate that NPL1 may function as a photoreceptor mediating chloroplast relocation.

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We present an open source Java application for analysis of force curves and images recorded with the Atomic Force Microscope. AtomicJ supports a wide range of contact mechanics models and implements procedures that reduce the influence of deviations from the contact model. It generates maps of mechanical properties, including maps of Young's modulus, adhesion force, and sample height. It can also calculate stacks, which reveal how sample's response to deformation changes with indentation depth. AtomicJ analyzes force curves concurrently on multiple threads, which allows for high speed of analysis. It runs on all popular operating systems, including Windows, Linux, and Macintosh.

Chloroplasts of Arabidopsis fhaliana move in response to blue light. Sensitivity to light and the range of fluence rates to which the chloroplasts respond were found to be comparable to those of other higher plants studied. We investigated typical chloroplast distributions in Arabidopsis grown under three different light conditions: standard-light conditions, similar to natural light intensities; weaklight intensities, close to the compensation point of photosynthesis; and strong-light intensities, close to the saturation of the lightresponse curve of photosynthesis. We observed a striking difference in chloroplast arrangement in darkness between plants grown under weak-and strong-light conditions. There was a slight difference after weak-light pretreatment, and the arrangements of chloroplasts after strong-light pretreatment in both plant groups were very similar. These results support the ecological significance of chloroplast movements.Chloroplast movement is a phenomenon commonly observed throughout the plant kingdom (see reviews: Zurzycki, 1980; Haupt and Scheurelein, 1990;Wada et al., 1993). In most plants studied so far, the movement is controlled by a blue UV-absorbing system (Zurzycki, 1980; Galland and Senger, 1980). Only in a few cases are red light and phytochrome also involved: Mougeotia (Haupt, 1959), Adiantum (Yatsuhashi et al., 1985), and Mesotaenium (Haupt and Thiele, 1961). Chloroplast rearrangements in cells are induced and maintained by irradiation and depend on light direction, wavelength, and irradiance.There are two extreme chloroplast positions: (a) face position (low fluence rate arrangement), with chloroplasts at the cell walls perpendicular to light; and (b) profile position (high fluence rate arrangement), with chloroplasts at the walls parallel to light. In D, the chloroplasts are distributed either randomly around a11 of the cell walls, or their position depends on local factors inside the cell (Haupt and Scheuerlein, 1990). These three arrangements are typical of species with multichloroplast cells like Funaria, Lemna, and Tradescantia. There are some differences in chloroplast movement in plants with cells containing one large chloroplast (e.g. Hormidium, Mougeotia, and Mesotaenium) or for coenocytes (eg. Vaucheria) (see review: Schonbohm, 1980). Although movement patterns differ in their detail in various species, the common result of these patterns is greater exposure of chloroplasts to WL and reduced exposure under SL conditions.The conventional interpretation of the ecological role of chloroplast movements is that they result in optimizing light utilization in photosynthesis. However, little is actually known about the significance of chloroplast movements, and only a few studies have been devoted to this problem. Zurzycki (1955) was the first to ask if there was a correlation between the level of photosynthesis and chloroplast response. He carried out experiments on the green alga Mougeotia sp., the moss Funaria hygrometrica, and the duckweed Lemna trisulca, a11 of which hav...

Light and sugars are fundamental elements of plant metabolism and play signaling roles in many processes. They are also critical factors determining the condition of plants cultured in vitro. The aim of this work was to investigate the simultaneous influence of irradiance and sugar content in the medium on the growth and photosynthetic apparatus condition of Arabidopsis thaliana in vitro. Plants were grown on media containing 1 or 3% of sucrose or glucose at three irradiances: 25, 100, and 250 lmol m-2 s-1 (weak, medium, and strong light). Media without sugar were used for control plants. Plant growth parameters were measured and the following physiological processes were investigated: photosynthesis, blue light-induced chloroplast relocations, and xanthophyll cycle activity. The expression of genes related to these processes was analyzed. The presence of sugar in the medium was found to be essential for the growth of Arabidopsis in vitro. Weak light significantly limited growth and the capacity to acclimate to changing light conditions. Strong light was a source of stress in some cases. Contrary to earlier reports, exogenous sugars showed a positive effect on photosynthesis. At higher concentration they acted as photoprotectants, overcoming the negative influence of strong light on photosynthesis and the xanthophyll cycle. The expression of all investigated genes was influenced by irradiance and sugar presence. In many cases differential effects of sugar type and concentration could be observed. The specific effects of some irradiance/sugar concentration combinations point to possible interactions between sugar-and light-induced signaling pathways.

Abstract. The presence of calcium is essential for chloroplast movement induced by blue light in Lemna trisulca L. The regulatory role of calcium was confirmed by the inhibition of chloroplast movement by cytochalasin B and trifluoperazine. The calcium concentration in tissues was modified by ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA), the calcium ionophore A23187 and La 3 § Only a long period of incubation (12h) in EGTA or La ~+ caused distrubances in chloroplast movement. This indicates that calcium influx is not essential for chloroplast movement. Those conditions that dramatically changed the internal calcium concentration, either applications of calcium ionophore A23187 and EGTA, or ionophore and La 3+, markedly decreased the amplitude of response to blue-light pulses. This demonstrates that disturbances of chloroplast movement are observable only when internal stores of calcium are affected by Ca2 § We suggest that the calcium involved in blue-light-induced chloroplast movement is derived from intracellular stores. The addition of Mg 2+ to EGTA buffer counteracted its effect, indicating that Mg 2 § as well as Ca 2+, might possibly be involved in chloroplast movement.

Chloroplast movements are among the mechanisms allowing plants to cope with changes in their environment. Chloroplasts accumulate at illuminated cell areas under weak light while they avoid areas exposed to strong light. These directional responses may be controlled by blue and/or red light, depending on the plant group. In terrestrial angiosperms only the blue light perceived by phototropins is active. The last decade has seen a rapid development of studies on the mechanism of directional chloroplast movements, which started with an identification of the photoreceptors. A forward genetic approach has been used to identify the components which control chloroplast movements. This review summarizes the current state of research into the signalling pathways which lead to chloroplast responses. First, the molecular properties of phototropins are presented, followed by a characterization both of proteins which are active downstream of phototropins and of secondary messengers. Finally, cross-talk between light signalling involved in chloroplast movements and other signalling pathways is discussed.

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