DEPARTMENT OF BIOENVIRONMENTAL RESEARCH II

Diversity of plant form is mostly attributable to variation of leaf and floral organs, which are modified, leaves. The leaf is the fundamental unit of the shoot system, which is composed with leaf and stem. So the leaf is the key organ for a full understanding of plant morphogenesis. However, the genetic control of development of these shapes had remained unclear. Recently, studies of leaf morphogenesis has been in a turning point, after our successful application of the techniques of developmental and molecular genetics to it, using model plants, Arabidopsis thaliana (L.) Heynh (Dengler and Tsukaya, 2001). Based on it, a new perspective on understanding of mechanisms for leaf morphogenesis is proposed (Tsukaya, in press).

Focusing on mechanisms that govern polarized growth of leaves in a model plant, Arabidopsis thaliana, we found that the two genes act independently to each other on the processes of polar growth of leaves: the AN gene regulates width of leaves and the ROT3 gene regulates length of leaves. The AN gene controls the width of leaf blades and the ROT3 gene controls length. Cloning of the AN gene revealed that the gene is a member of gene family found from animal kingdom (Kim et al., submitted). The ROT3 gene was cloned by us in 1998. Transgenic experiments proved that the ROT3 gene regulates leaf-length without affect on leaf-width via biosynthesis of steroids (Kim et al., 1999). In relation to it, we recently revealed that a steroid hormone, brassinosteroid, controls both proliferation and expansion of leaf cells (Nalaya et al., 2002). Apart from polar elongation, we identified the following genes involved in leaf expansion process. The AS1 and AS2 genes are needed for proportional growth of the leaf. Molecular and anatomical analysis of the as2mutant is now underway, in collaboration with a research team of Prof. Machida, Nagoya University (Endang et al., 2001). We also started analysis of blpmutation which strongly enhances the as2phenotype, in collaboration with a reaseach team of Prof. Nam, POSTECH, Korea.

On the other hand, we are trying to identify molecular mechanisms which distinguish developmental pathway of leaves from that of shoots. For such purposes, we introduced tropical plants having queer developmental program for leaf morphogenesis, namely, Chisocheton, Guarea and Monophyllaea, as materials for molecular studies.

In addition, we are interested in environmental adaptation of leaves, from view point of biodiversity (e.g.,Tsukaya and Tsuge, 2001; Tsukaya, in press). Leaf index, relative length of leaf to width, is also the most diverse factor of leaf shape, and is affected by environmental factors in some plants (Kuwabara et al., 2001). Are AN and ROT3 genes are involved in regulation of adaptive change of leaf index in natural condition? Are these genes the responsible for evolution of rheophytes? So called "Evo/Devo" study of leaf morphogenesis is also one of our research project in NIBB.

Publication List:

Dengler, N. and Tsukaya, H. (2001). Leaf morphogenesis in dicotyledons: current issues. Int. J. Plant Sci.162:459-464.

Kuwahara, A., Tsukaya, H., and Nagata, T. (2001) Identifca tion of factors that cause heterophylly in Ludwigia arcuata Walt. (Onagraceae) Plant Biology3:98-105.

Nakaya, M., Tsukaya, H., Murakami, N., and Kato, M. (2002) Brassinosteroids control the proliferation of the leaf cells in Arabidopsis thaliana. Plant Cell Physiol. 43: 239-244

Semiarti, E., Ueno, Y., Tsukaya, H., Iwakawa H., Machida C. and Machida, Y. (2001). The ASYMMETRIC LEAVES2 gene of Arabidopsis thaliana regulates formation of symmetric lamina, establishment of venation and repression of meristem-related homeobox genes in leaves. Development128:1771-1783.

Tsukaya, H. and Tsuge, T. (2001) Morphological adaptation of inflorescences in plants that bloom at low temperatures in early spring: the convergent evolution of "downy plants". Plant Biology3:536-543.

Tsukaya, H. (2002). Interpretation of mutants in leaf morphology: genetic evidence for a compensatory system in leaf morphogenesis that provides a new link between Cell and Organismal theory. Int. Rev. Cytol.217(in press)

Tsukaya, H. (2002) Optical and anatomical characteristics of bracts from the Chinese "Glasshouse" plant, Rheum alexandrae Batalin (Polygonaceae), in Yunnan, China. J. Plant Res.115: 59-64