6. A mutant showing pleiotropic abnormalities in root, shoot and flower development in rice
  T.SUMIKURA and Y. NAGATO

Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, 113-8657 Japan

In higher plants, proper development requires the coordinated activity of a number of genes. Some of them are expressed throughout the life cycle, and regulate a variety of developmental events. Here, we describe a recessive mutant, showing pleiotropic abnormalities in root, shoot and flower development.

We have identified a recessive mutant, fish bone (fib) from M2 population of rice cv. Taichung 65 mutagenized with N-methyil-N-nitroso urea. The fib plants were small, and showed peculiar shape and abnormal pattern of internode elongation (Fig. 1A). The leaves were very short and narrow, and frequently lacked midrib causing them to droop. The leaf blade was bent largely at the lamina joint, in contrast to the slight bending in the wild type (Fig. 1A). The cross-section of shoot apex exhibited several abnormalities in leaf primordia. The phyllotaxy was frequently deviated from the normal 1/2 alternate. In a severe case, two successive leaf primordia developed from the same side (Fig. 1C). In the wild type, two margins of leaf primordia overlapped in a regular orientation, that is, a lateral region surrounding the specific side of shoot apical meristem (SAM) always forms outside. In fib plants, this regular



pattern was disrupted (Fig. 1C). Since leaf development is closely related to the organization of SAM (Itoh et al. 2000), we examined the size and shape of SAM. Although the height of fib SAM was almost normal, the width was smaller than that of wild type, resulting in slender SAM. The number of vascular bundles in leaf primordia was reduced to about half of that in the wild type (Fig. 1D). In addition, the regular orientation of vascular bundles was disrupted in the stem, suggesting FIB gene regulate vascular bundle development.

The fib plants showed defects in root development. The number of lateral roots and crown roots were severely reduced (Fig. 2B). In addition, the seminal root of fib plants was defective in gravitropism. When seedlings were grown vertically for three days after germination and then horizontally gravi-stimulated, wild-type roots quickly showed gravitropic curvature (Fig. 2C), but fib roots failed to respond the gravitropic stimulus (Fig. 2D).

In the reproductive phase, the fib mutant produced abnormal flowers. The wild-type flower consists of two lodicules, six stamens and one pistil, covered by lemma and palea. Lodicules of the fib flower were sometimes converted into stamens or mixed organs of lodicule and stamen. The number of stamens in their own whorl varied from four to seven. These results indicate that FIB gene regulates both floral organ identity and number.

The fib plants showed dwarfism. The suppression of internode elongation occurred in all internodes, conspicuous in the upper-most internode.

To summarize, the fib mutant showed a large number of defects in many developmental events throughout the life cycle: the initiation pattern and development of leaves, internode elongation, SAM shape, vascular bundle development, lateral and crown root formation, gravitropism and the number and identity of floral organs. This implies that the FIB gene plays a fundamental role In rice development. Furthermore, abnormalities in plant height, leaf blade bending, lateral root formation, vascular bundle development, gravitropism and so on suggest that plant hormone such as auxin would be involved in fib mutant.

Reference

Itoh, J.-I., H. Kitano, M. Matsuoka and Y. Nagato, 2000. SHOOT ORGANIZATION genes regulate shoot apical meristem organization and the pattern of leaf primordium initiation in rice. Plant Cell 12: 2161-2174.