DIVISION OF BIOLOGICAL REGULATION AND PHOTOBIOLOGY (ADJUNCT)

Plants use light as an environmental factor which controls their development as well as their other physiological phenomena. Phytochrome and blue light receptors, such as cryptochrome and phototropin (phot), are the main photoreceptors for plant photomorphogenesis. The goal of our research is to clarify the signal transduction pathways of photomorphogenesis. One of our major subjects is chloroplast photo-relocation movement which is thought to be one of the simplest phenomena in this field. We use the fern Adiantum capillus-veneris and the moss Physcomitrella patens as model plants for our cell biological approach not only because the gametophytes are very sensitive to light, but also because the organization of the cells is very simple. We also use Arabidopsismutants to clarify the genes regulating chloroplast photo-relocation movement.

I. Cloning and characterization of blue-light photoreceptors

We have described many blue-light induced photomorphological responses in gametophytes of the fern Adiantum capillus-veneris.As the first step in understanding the molecular mechanisms of these blue-light responses, we are cloning and sequencing the genes of blue light receptors, and are studying intracellular distributions of the gene products and their function in Adiantumand Physcomitrella

1- 1 Cryptochromes

Cryptochrome functions were studied using CRY1aand CRY1bdouble mutant of Physcomitrella patens. It was revealed that blue light cryptochrome signals regulate many steps in moss development including induction of side branch of protonemata, and regulation of gametophyte induction and its development. In addition, the disruption of cryptochrome altered auxin response, including auxin-inducible genes. Cryptochrome disruptants were more sensitive to external auxin than wild type in a blue light-specific manner, suggesting that cryptochrome light signals repress auxin signals to control plant development.

1- 2 Phototropin

Phototropin (phot1) is another blue light photoreceptor isolated recently in higher plants, and is a flavin binding protein with light sensitive protein kinase activity. A cDNA of AdiantumPHOT2, a homologue of phototropin has been sequenced. Photocycle of FMN binding LOV domains of the phot1 and phot2 expressed in E. coli were studied.

II. Chloroplast relocation movement

2-1 Arabidopsis

Chloroplasts accumulate at the cell surface under weak light and escape from the cell surface to the anticlinal wall under strong light to optimize photosynthesis. The mechanism of chloroplast relocation, however, is not known. We screened several mutants from T-DNA tagging lines as well as EMS lines of Arabidopsis. By mutant analysis, we found last year that phot1 and phot2 were photoreceptors of chloroplast relocation movement induced by blue light. This year, we clarified that the phot1 and phot2 were also the photoreceptors redundantly working on stomatal opening in Arabidopsis, in collaboration with the Prof. Shimazaki's group of Kyushu University. Gene analysis of mutants defective in chloroplast accumulation response are also under way.

2-2 Adiantum

Adiantumphytochrome3 (PHY3) is a unique kimeric protein with a phytochrome structure in the N-terminal half and a phototropin structure in the C-terminal half. PHY3 gene analysis of EMS-induced rap (red light-induced aphototropic) mutants of Adiantumwhich do not show phototropic response and chloroplast photorelocation movement under red light revealed that five rap mutant lines tested have a mutation in the PHY3 gene. Moreover, over expression of PHY3 genes in a rapmutant rescued the red light-induced chloroplast movement, indicating that phy3 is the photoreceptor of this phenomenon. PHY3 is not yet known whether it works as a blue light photoreceptor.

Adiantum mutants which do not show chloroplast avoidance movement under strong blue light were isolated. Gene analysis of these mutants revealed that they have a mutation in Adiantum phot2 gene, suggesting that Adiantumalso use phot2 as a photoreceptor of blue light-induced avoidance response as in the case of Arabidopsis.

Figure 1 A scheme of chloroplast photorelocation movement in Arabidopsis thaliana. Strong blue light is absorbed by phototropin 2 (phot2) and chloroplasts move out from the light (avoidance movement) (Kagawa et al 2001). Under weak blue light condition, the blue light is absorbed by phot1 and phot2, and chloroplasts move towards the light irradiated area (accumulation movement) (Sakai et al 2001). The signal from photoreceptors to chloroplasts is not yet known.

List of publication:

Briggs, W.R. et al (2001) The phototropin family of photoreceptors. Plant Cell13:993-997.

Imaizumi, T. et al Cryptochrome light signals control development to suppress auxin sensitivity in the moss Physcomitrella patens. Plant Cell in press.

Kagawa, T. et al (2001) Arabidopsis NPL1: A phototropin homolog controlling the chloroplast high-light avoidance response. Science 291: 2138-2141.

Kinoshita, T. et al (2001) phot1 and phot2 mediate blue light regulation of stomatal opening. Nature414: 656-660.

Sakai, T. et al (2001) Arabidopsis nph1 and npl1: Blue-light receptors that mediate both phototropism and chloroplast relocation. Proc. Natl. Acad. Sci. USA98:6969-6974.

Schultz, T. F. et al (2001) A role for LKP2 in the circadian clock of Arabidopsis. Plant Cell13:2659-2670.