8. 
Interaction of panicle kvelopment and internode elongation in rice
H. 
Sunohara1 S. Uozu2, H. Kitano2 and Y. NAGATO’
1) Graduate School of Agricukural and Life Sciences, University of Tokyo, Tokyo, 113-8657 Japan
2) Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, 464-8601 Japan

 
     Dwarfism in rice has been studied intensively. However, little is known about how panicle and internode interact. We have observed that mutants with aberrant panicles tend to exhibit modified cuhn length, especially shorter stature. Here we report the interaction between the panicle development and the internode elongation using panicle/flower mutants of rice.
     We used 14 panicle/flower mutants derived from Taichung 65 mutagenized with a chemical mutagen, N-methyl-N-nitrosourea. Three of them (foni, di-supi and ur2) were reported earlier, and the other 11 mutants designated by the strain name (fm) were newly identified. These mutants are classified into two groups by the stage at which the mutant phenotype is first observed in panicle. The first group shows abnormalities in the rachis and/or the rachis branches. Seven mutants, i.e. ur2, fm2O, fm23, fm24, fm6O, fm6l and fm62, belong to this group. Among them, four mutants, fm23, fmóO, fm6l and fm62, have short panicles. The second group has defects in flowers such as modified organ number and change in organ identity, and includes seven mutants,fonl, di-supi , fm2, fm4, fml4, fm47 and fm55. First we measured the internode lengths of the mutants and Taichung 65 as the wild type (Fig. 1). Most of the mutants had short stature. Two mutants (ur2 and fm6O) showed the increased number of elongated internodes. The internode elongation pattern in Fig. 1 indicates that the length of the uppermost internode is affected in the mutants, but the lengths of lower internodes are comparable to that of the wild type.
     We analyzed how the lengths of internodes are correlated with one another and how the panicle traits are related to the internode length (Table 1). The table shows that the length of an internode is positively correlated with that of the adjacent internode. The lengths of first and second internodes from top are correlated with each other, but not with those of the lower internodes. This suggests that the elongation of lower internodes is regulated independently of that of the upper ones.
     The panicle length was significantly correlated with the lengths of first and second internodes from top, but not with the lengths of lower internodes. Other panicle traits (the numbers of primary and secondary branches and the number of spikelets) showed a significant correlation with only the uppermost internode length. Thus the panicle development affects the elongation of upper internodes
     To analyze how the internode elongation pattern is associated with the type of mutants, principal component analysis was carried out based on the data of internode lengths (Fig. 2). It is clear that mutants defective in floral organs are distributed in the neighborhood of the wild type, whereas most mutants with abnormalities in the rachis and/or branches are positioned away from the wild type. These results indicate that the early defects in panicle development severely modify the internode elongation pattern.
     The present analyses show that the cuim length, especially the length of uppermost internode, is affected by the early developmental mode of panicle in rice. Although the mechanism by which panicle development affects the elongation of upper internodes is still unclear, the elucidation of early panicle development is useful for understanding of rice dwarfism.