Organismal development comprises several more or less discrete stages, which we can recognize as phase changes. Thus, it is plausible that there exist genetic programs temporally regulating the developmental progress. The important role of heterochronic mutations in the development and evolution has long been suggested (Gould 1977). In recent years, it has been widely accepted that plants undergo phased development: embryogenesis - juvenile vegetative phase - adult vegetative phase - reproductive

phase. In contrast to the extensive studies on vegetative-reproductive phase change, the genetic mechanisms of juvenile-adult phase change in vegetative phase remain almost unknown (Poethig 1990). In this report, we describe three allelic mutations of rice, which are defective in juvenile-adult phase change.

We have identified three recessive allelic mutations, mori1-1, mori1-2 and mori1-3, from M2 populations of Kinmaze and Taichung 65 mutagenized with MNU. The mutants drastically modified the shoot architecture of rice. The most remarkable feature of mori1 plants was a rapid production of small leaves and short tillers (Fig. 1). The plant height of mori1 was about 5 cm even seven months after sowing. No reproductive growth was attained in mature mori1 plants (one year old) even when inductive short-day treatment was applied. Leaves at any position of seven-month-old mori1 plants were very

small in both blade and sheath, and the size and shape were comparable to those of the wild-type 2nd leaf (Fig. 2). In the wild-type stem, node and internode are differentiated in the adult stem where 4th or higher level leaves are inserted. However, in the juvenile stem where 1st through 3rd leaves are attached, nodes are not identified and vascular bundles are randomly oriented. The stem of mori1 seven months after sowing did not differentiate node and internode, and had randomly oriented vascular bundles, which were similar to the basal region of the wild-type stem where 2nd and 3rd leaves were inserted (Fig. 3). These structural characteristics of leaf and stem indicate that mori1 remains at the 2nd-leaf stage (juvenile phase) of the wild type.

Next, we examined the juvenility of shoot apical meristem (SAM) of mori1, since all

shoot traits are derived from the SAM. In the wild type, SAM becomes enlarged with the developmental progress, whereas the mori1 plants after germination maintained the SAM size as that of mature embryo. The plastochron (the elapsed time from the initiation of one leaf primordium to that of the next one) of mori1 after germination was short, being nearly equal to that of 2nd and 3rd leaves. Cell division activity in the SAM was examined by in situ hybridization probed with histone H4. The mori1 SAM one week after germination showed more hybridization signals than the wild-type SAM of the same age. The cell division activity in mori1 SAM was comparable to that of the wild-type embryo when 2nd and 3rd leaf primordia were being formed. Thus, the mori1 maintains the juvenile SAM throughout the development.

The mori1 plants were viable only when cultured asceptically on nutrient medium containing sucrose and became etiolated when transplanted to soil. Since mori1 plants could not grow in the nutrient medium lacking sucrose, it is suggested that mori1 plants are heterotrophic and show a low photosynthetic rate. Actually, the apparent photosynthetic rate in leaves of one-month-old mori1 plants was as low as that of the wild-type 2nd leaf, indicating that mori1 plants are heterotrophic as the 2nd-leaf stage of the wild type.

The above results indicate that mori1 is a heterochronic mutant which stays at juvenile phase (2nd leaf stage) and fails to become adult. Therefore, MORI1 plays a key role in the juvenile-adult phase change.

References

Gould, S. J., 1977. Ontogeny and Phylogeny,. Harvard Univ. Press, Cambridge.

Poethig, R. S., 1990. Phase change and regulation of shoot morphogenesis in plants. Science 250: 923-929.