DIVISION OF BIOLOGICAL REGULATION AND PHOTOBIOLOGY (ADJUNCT)

Plants respond to light, as an environmental factor to optimize development and regulate 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 elucidate the photoperception and the signal transduction pathways of photomorphogenesis.

I. Chloroplast relocation movement

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 since the gametophytes are very sensitive to light and the organization of the cells is very simple. We also use Arabidopsis mutants to identify the genes regulating chloroplast photo-relocation movement.

1- 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 significance of chloroplast avoidance movement, however, has been uncertain. We analyzed the influence of strong light using several mutants which are defective in avoidance movement in Arabidopsis. One of the mutants is phot2, defective in the blue-light photoreceptor phototropin. The other mutant is chup1 which is hypothesized to lack some part of the movement mechanism. Both mutants showed severe damage of leaves when irradiated with strong light for more than 10 hr. The reactive oxygene-scavenging capacity in these mutant leaves, especially the activities of scavenging enzymes, did not differ between mutant and wild-type plants. This suggests that the chloroplast avoidance response is a very important physiological response in plants under high light conditions.

1- 2 Adiantum phy3

1- 3 Adiantum phot2

Adiantum mutants which do not show chloroplast avoidance movement under strong blue light have been isolated. Gene analysis of these mutants and complimentation tests with PHOT2 gene reveal that phot2 is a photoreceptor of blue light-induced avoidance movement of chloroplasts in Adiantum, as in the case of Arabidopsis. Particle bombardment of modified PHOT2 genes into phot2 mutants show that LOV2 domain of phot2 functions as a chromophore binding domain but LOV1 domain does not. Gene deletions of PHOT2 from 3' region also show that an important domain of phot2 exists within the C-terminus.

II. Gene targeting and gene silencing

In order to elucidate the role of genes in Adiantum and rice whose function is unknown, we have tried to establish new methods for gene targeting in these organisms.

2- 1 Miniature transposable element

Miniature inverted-repeat transposable elements (MITE) have been found in wide range of organisms but active MITEs have not been identified. We found a new class of MITEs in rice and named them miniature Ping (mPing). mPing elements are activated in cells derived from anther culture and excise efficiently from original sites to reinsert into new loci. mPing-associated Ping element which has a putative transposase sequence was also found and shown to transpose within the rice genome.

List of publication:

Original articles

Imaizumi, T., Kadota, A., Hasebe, M., Wada, M. (2002) Cryptochrome light signals control development to suppress auxin sensitivity in the moss Physcomitrella patens. Plant Cell 14, 373-386.

Kasahara, M., Swartz, T. E., Olney, M. A., Onodera, A., Mochizuki, N., Fukuzawa, H., Asamizu, E., Tabata, S., Kanegae, H., Takano, M., Christie, J. M., Nagatani, A., Briggs, W. R. (2002) Photochemical properties of the flavin mononucleotide-binding domains of the phototropins from Arabidopsis, rice, and Chlamydomonas reinhardtii. Plant Physiol. 129, 762-773.

Kasahara, M., Kagawa, T., Oikawa, K., Suetsugu, N., Miyao, M., Wada, M. (2002) Chloroplast avoidance movement reduces photodamage in plants. Nature 420, 829-832.

Kikuchi, K., Terauchi, K., Wada, M., Hirano, H. (2003) The plant MITE mPing is mobilized in anther culture. Nature 421, 167-170.

Kawai, H., Kanegae, T., Christensen, S., Kiyosue, T., Sato, Y., Imaizumi, T., Kadota, A., Wada, M. (2003) Responses of ferns to red light are mediated by an unconventional photoreceptor. Nature 421, 287-290.

Sato, Y., Wada, M., Kadota, A. (2003) Accumulation response of chloroplasts induced by mechanical stimulation in bryophyte cells. Planta. (in press)

Review articles

Kagawa, T., Wada, M. (2002) Blue light-induced chloroplast relocation. Plant Cell Physiol. 43, 367-371.

Kagawa, T. (2003) The phototropin family as photoreceptors for blue light-induced chloroplast relocation. J. Plant Res. (in press)

Suetsugu, N., Wada, M. (2003) Cryptogam blue-light photoreceptors. Current Opinion of Plant Biology 6, 91-96 .

Sato, Y., Kadota, A., Wada, M. (2003) Chloroplast movement: dissection of events downstream of photo- and mechano-perception. J. Plant Res. (in press)

Wada, M., Kagawa, T., Sato, Y. (2003) Chloroplast movement. Annu. Rev. Plant Biol. (in press)

Figure. Phenotypes of plants exposed to continuous strong light. Wild-type, phot2-1, phot1-5 and chup1-2 mutant plants (4 weeks old) were irradiated with white light at 1400 mmol m-2 s-1 and were photographed before (0 hour) and after irradiation for 10, 22 and 31 hours. Leaves of phot2-1 and chup1-2 plants showed some signs of bleaching after 10 hours of treatment but were severely bleached and necrotic after 22 hours. In contrast, wild-type and phot1-5 plants withstood this light stress over the 31 hours time period of the treatment.