25. Mutations affecting panicle architecture in rice 

Masayuki takahashi, Nobuhiro naGasawa and Yasuo nagato

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

 It is important to elucidate the genetic program of inflorescence development. Although in Arabidopsis and Antirrhinum, many inflorescence mutants have been isolated and used for genetic studies, genetic mechanisms of inflorescence formation are poorly understood in rice.

In this report, we describe in detail two recessive mutants showing altered panicle architecture. The two mutants exhibit similar phenotypes, e.g. increased but short primary rachis branches and malformed glumes (Fig. 1). Allelism tests revealed that the two mutations are allelic. As the mutants show abnormal panicle architecture which seems to be derived from modified phytomeric structure, this locus is designated as PANICLE PHYTOMER I (PAPI) and the two alleles as papl-I and papl-2, respectively. The two alleles, papl-I and papl-2, were identified among M2 populations of Taichung 65 and Kinmaze, respectively, mutagenized with MNU (N-methyl-N-nitrosourea).

Although the development of vegetative shoot is not affected by pap mutation, several defects are observed in the panicle of the mutants (Table 1, Fig. lB.C). The average length of primary rachis branches in papl-1 and papl-2 is much smaller than that in the wild type. Secondary and tertiary rachis branches are absent. On the other hand, the number of primary rachis branches is higher in papl-I and papl-2, but the panicle length is not affected. Hence, the rachis internode of pap mutants is shorter than that of wild type. Several primary rachis branches and caryopses in the mutants are frequently degenerated in the basal part of panicle. In the mutants, leaves are frequently developed from the basal nodes of the panicle which are thought to be degenerated at the early

98 Rice Genetics Newsletter Vol. 13

Fig. I. Panicles and caryopses of wild type and pap mutants. A, D: wild type, B,E: papl-l, andC,F: papl-2. Both papl-l and papl-2 show increased and short primary rachis branches and long empty and rudimentary glumes. In F, empty glume in palea side exhibits the homeotic conversion into palea. Bar=4cm in A-C and 2mm in D-F.
Table 1. Panicle phenotypes of pap mutants


Mutant Panicle length(cm) No. of caryopses per panicle No. of primary branches Length of primary branch(cm)
pap-l 18.5 123.0 15.0 3.7
pap-2 19.0 116.3 21.0 3.1
Taichung 65 19.1 94.5 9.0 6.6
Kinmaze 19.1 84.0 8.5 6.3

 
Research Notes 99
Table 2. Modification of empty and rudimentary glumes in pap mutants


Mutant No. of caryopses examined Wild type Elongation of glumes Conversion into palea or lemma
pap-l 228 147 78 3
pap-2 288 88 187 13

developmental stage in wild type. Although the severity of phenotypes varies slightly from year to year, pap1-2 consistently shows extreme phenotypes of panicle compared *with pap1-1.

Glumes of mutants exhibit various morphological abnormalities (Table 2, Fig. 1E, F). Empty glumes and rudimentary glumes in the two mutants are frequently elongated. Long empty glumes are observed in 35% of pap1-1 caryopses, whereas 70% of pap1-2 caryopses showed elongation. Not only the frequency of elongation, but also the degree of elongation in pap1-2 is much larger than that in papl-I. In some mutant caryopses, additional palea- or lemma-like organ is produced at the expense of empty glume. This organ is more often observed in palea side than in lemma side. In wild type caryopsis, lemma and palea have five and three vascular bundles, respectively, while empty glumes have only one. Additional palea-like organs have three or four vascular bundles, depending on the width. Trichome alignment of the additional organ is similar to that of original palea. These observations indicate that the mutation of PAP I gene causes the homeotic conversion of empty glume into palea or lemma.

The phenotype of pap mutation is different from the other mutants so far reported in rice such as Ur-I, Cl, lax and Dn-I (Murai and Izawa 1994). Also there is no similar mutation in other species. The abnormalities observed in pap panicles indicate that the wild type PAP I gene functions after reproductive transition. Since abnormalities are rarely found after the glume differentiation stage, PAP I gene would function mainly from the primary rachis branch initiation stage through palea and lemma initiation stage. Vegetative phytomer is composed of internode, leaf and lateral bud. Inflorescence phytomer is also formed by the homologous organs. In pap mutants, modifications are detected in all the components of inflorescence (panicle) phytomer: increased number of primary rachis branches (increase of phytomer), short rachis internode (short phytomer), elongation of neck leaf and glumes (elongation of inflorescence leaves) and the lack of secondary rachis branches (degeneration of lateral bud). Accordingly PAP I is considered to play important roles in the establishment of panicle architecture through regulating the development of panicle phytomer. (Gene symbol: New system)

References

Murai, M and M. Izawa, 1994. Effects of major genes controlling morphology on panicle in rice. Breed. Sci.

44:247-255.