We have used rice and Arabidopsis mutants to study how terpenoid hormones regulate growth and development in plants. Our work in Arabidopsis has shown that gibberellins play an important role in growth responses to environmental signals, such as light and temperature. In collaboration with other research groups, we uncovered new gibberellin deactivation mechanisms, including epoxidation by a cytochrome P450 monooxygenase in rice and methylation by methyltransferases in Arabidopsis. Previous studies using shoot branching mutants of pea, Arabidopsis and rice have suggested the involvement of a new hormonal signal in inhibiting shoot branching in these species. Recently, we set out to reveal the chemical identity of this new hormone using a series of rice branching mutants. We found that strigolactones, a group of terpenoid lactones that were initially discovered as germination stimulants of root parasitic plants, act as the novel shoot branching inhibitor or its biosynthetic precursors.

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Cyanamide (NH_2CN) has been synthesized for over a hundred years for agricultural and industrial uses. This compound was isolated from Vicia villosa subsp. varia, which was guided by a plant growth inhibitory activity on lettuce seedlings. A large proportion of the inhibitory activity in the crude extract of V. villosa subsp. varia was explained by the presence of cyanamide. This was the first isolation of cyanamide from natural sources. To demonstrate that the cyanamide was of natural origin, [^<15>N] nitrate was administered to V. villosa subsp. varia seedlings. The isotopic ratio of ^<15>N in the cyanamide isolated from the plants was higher than that of the cyanamide extracted from the seedlings grown in the presence of a natural N source, indicating that this plant biosynthesizes cyanamide. We established a direct quantitative determination method of cyanamide by stable isotope dilution gas chromatographymass spectrometry (GCMS) using selected ion monitoring (SIM) mode. We investigated 553 species of higher plants but have so far found only three species containing cyanamide at detectable levels; V. villosa subsp. varia, V. cracca, and Robinia pseudo-acacia. The distribution of the plants that accumulate natural cyanamide appears to be highly limited.

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In angiosperms, approximately 1%, ca, 4,500 species are parasitic. Parasitic plants can be classified as to their photosynthesis status. Hemiparasites retain photosynthetic ability. On the other hand, holoparasites lack photosynthetic ability and obtain all their carbon from their hosts. In these parasitic species, selective pressure to maintain photosynthetic ability had been released. Photoresponses of plants which are important to optimize photosynthesis for green plants are not necessary in holoparasites. Photoresponses are also involved in the regulation of plant development such as germination process. Studies on photoresponses of parasitic plants revealed that the pattern of their photoresponses had been modified for adaptation to its parasitic life cycle. Molecular analysis of their red/far-red light photoreceptor phytochrome also demonstrated that the alteration of thier amino acid sequences and associated molecular function.

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Plants have to adapt to drastic changes in environmental conditions because of their immortality. They have to cope with various types of environmental stresses, such as starvation, oxidative stress, drought stress, and invasion by phytopathogens, during their differentiation, development and aging processes. The germination and growth of seeds in nitrogen-poor soil or in the shade, for instance, indicates that plants have mechanisms for coping with nitrogen- and carbon-starvation. Under such nutrient-poor conditions the bulk degradation and recycling of macromolecules is integral to the ability of plants to adapt to their environment. Autophagy is the major system responsible for the degradation of organelles, fragmented organelles and cytosolic macromolecules in the vacuole and therefore is assumed to be an extremely important function in plants. Here we briefly describe early studies of plant autophagy, summarize the recent studies on molecular functions of ATG genes and plant autophagy-monitoring systems, and finally introduce the speculated role of autophagy in plants.

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[in Japanese]

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Gibberellin (GA) is a crucial plant hormone involved in regulating plant growth and development. GA signaling is controlled by the nuclear repressor DELLA proteins. Once plant cells receive GA molecules, the DELLA proteins are rapidly degraded through an ubiquitin-proteasome pathway resulting GA responses. In this process, the GA receptor GIBBERELLIN INSENSITIVE DWARF1 (GID1) binds two conserved sequences, DELLA and VHYNP, of the DELLA protein in a GA dependent manner and triggers DELLA protein recognition and ubiqutination by the E3 ubiquitin ligase, SCF^<SLEEPY1 (SLY1)> Crystal structures of the GA-GID1-DELLA complex revealed how GID1 discriminates active or inactive GA molecules and recognizes DELLA proteins in a GA dependent manner. GA is caught in the GA binding pocket of GID1 by forming hydrogen bond networks via three conserved polar groups in the active GA molecules and induces conformational changes in the GID1 N-terminal region to form a hydrophobic molecular surface for DELLA protein binding. DELLA protein interacts with the GID1 N-terminal surface, forming a helix bundle with a thumb-like helix from the random coil structure. The series of structural changes will trigger the DELLA protein recognition and subsequent ubiqutination by the SCF^<SLY1>.

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Plants are characterized by mineral nutrition, which require 17 essential elements for their growth. Among them, 14 are minerals. These minerals are taken up by the roots, translocated from the roots to the shoots and finally distributed to different cells for functions. A large number of transporters is required for these transport processes. On the other hand, plants are exposed to toxic minerals, therefore detoxification of these minerals by plants at different cellular levels is also required for survival. Herein, we reviewed recent progresses in identification of transporters involved in uptake, translocation, and distribution of essential minerals and of transporters involved in the detoxification of toxic minerals, focusing on iron (Fe), silicon (Si) and aluminum (Al), which represent essential, beneficial and toxic minerals, respectively.

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We found that the heavy ion beam is an excellent tool for mutation breeding. There are the advantage of ion beam mutagenesis, low dose with high survival rates, high mutations rates and a wide variation. The irradiation treatment given to the various plant materials is short, only seconds or a few minutes, but is enough to induce mutation. A new variety can be obtained by selecting a mutant with a modification to the target characteristic while retaining the existing valuable ones. The time span for breeding can be shortened significantly to two to three yeas. We already put 18 new cultivars on the market since 2002.

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Bryophyte diversity is very important for the development of bio-sorption systems of water contaminated with metals. Funaria hygrometrica Hedw. is one of the useful species and so-called 'ash-tolerance moss' and their colonies grow on waste area (e.g. final landfill, smelter). Funaria moss-filtration technology was developed on a novel conceptual bio-sorption system using the protonemal cells of the moss F. hygrometrica. The technology was checked for removal rate of lead from a solution (100μM PbCl_2) by X-ray fluorescence micro-analyzer (XRF), inductively coupled plasma mass spectrometry (ICP-MS) and bioassay test. The technology was also checked for gold adsorption from a solution (100μM AuCl). Moreover, we develop on the bio-sorption system using a novel mutant cells which created by ion-beam irradiation technology. We found optimum irradiation dose condition of four kinds of heavy-ion beam to get a lot of mutants. We established an original screening system with XRF and cluster analysis for phyto-mining.

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[in Japanese]

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