Controlling Organelle Positioning: A Novel Chloroplast Movement Protein

It has been recognized for more than a century that chloroplasts alter their distribution within cells depending on the external light conditions. Senn (1908) documented light-induced chloroplast movement in a number of higher plants and algae, and the phenomenon has long been accepted as a means of optimizing photosynthetic light absorption under changing light conditions. Thus, chloroplasts can be observed to move to positions that maximize photon absorption under low-fluence light (along the periclinal walls parallel to the leaf surface and perpendicular to the incident light) and, conversely, to move to positions that minimize photon absorption under high-fluence light (appressed along the anticlinal walls perpendicular to the leaf surface and parallel to the incident light). The movement away from areas of strong light is believed to offer protection against photooxidative damage. Kasahara et al. (2002) recently showed that Arabidopsis mutants defective in chloroplast high-light avoidance movement are more susceptible than wild-type plants to photoinhibition under high-light conditions, confirming the physiological importance of this phenomenon. The photoreceptors responsible for lightinduced chloroplast movement in higher plants are phototropins, the blue light receptors that also mediate phototropism. In some species of green algae, moss, and fern, chloroplast relocation apparently is controlled by red light as well as blue light, suggesting that phytochrome plays a role in this process (Kagawa and Wada, 2002). Interestingly, the photoreceptor for red light control of chloroplast relocation and phototropism in the fern Adantium capillus-veneris was identified recently as phytochrome3, which is a chimera protein of phytochrome and phototropin that has been found only in Adantium and a number of other ferns (Kawai et al., 2003). Arabidopsis encodes two phototropins, PHOT1 and PHOT2 (previously known as NPH1 and NPL1, respectively [Briggs et al., 2001]). PHOT1 is the primary photoreceptor that controls phototropism in low-intensity light (Huala et al., 1997), whereas PHOT2 is responsible for the light-avoidance relocation of chloroplasts under high light (Kagawa et al., 2001; Jarillo et al., 2001). Sakai et al. (2001) demonstrated that PHOT1 and PHOT2 actually have overlapping functions in both phototropic and chloroplast relocation responses in a fluence rate–dependent manner. PHOT1 appears to function in response to a broad range of blue light intensities, whereas PHOT2 functions principally in response to high-fluence-rate blue light. Thus, PHOT1 is the primary photoreceptor for chloroplast accumulation movements under low light, but only PHOT2 is responsible for the light avoidance movement in response to high light. PHOT1 also appears to be the primary photoreceptor control-