27. DL regulates both leaf and pistil development in rice Nobuhiro naqasawa, Masahiro miyoshi, Yoshio sano and Yasuo nagato

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

2) Faculty of Agriculture, Hokkaido University, Sapporo 060, Japan

 Recently, genetic and molecular mechanisms of flower development have been intensively studied in two dicotyledonous species, Arabidopsis thaliana and Antirrhinum majus. In monocotyledons, however, genetic regulation of flower development has not been well investigated. Rice flowers have a very different architecture from those of the two model dicot species. In this report, we describe the morphological and genetic analysis of mutants which show homeotic conversion of pistil into stamens and the loss of midrib in rice.

Four rice mutations of DROOPING LEAF (DL) gene affecting midrib formation and pistil development were analyzed. At DL locus, drooping leaf-1 (dl-I) was first reported as a spontaneous mutation that was originated in "Tarebaine" (H0788) (Iwata and Omura

Research Notes

1971), and was designated as dl-I (H0788) in this report. The dl-I gene was introduced into Taichung 65 through several backcrosses, and designated as dl-I (T65). The origin of drooping leaf-2 (dl-2) mutation is unknown. We have identified two new mutations, drooping leaf-superman1 (dl-sup1) and drooping leaf-superman2 (dl-sup2), from M2 populations of Taichung 65 and Kinmaze, respectively, which were mutagenized with MNU (N-methyl-N-nitrosourea).

All the mutations caused drooping leaves lacking midrib and depending on the alleles, produced flowers with varying degrees of abnormality (Table 1, Fig. 1 ). In the weakest mutation dl-2, most of the flowers were normal except producing three stigmas at a low frequency (Fig. 1D). Although more than half of dl-I (H0788) flowers were normal, about 40% of flowers produced three or four stigmas and/or cell mass on the styles (Table 1, Fig. 1B). The expression of dl-I varied depending on genetic background. In Taichung 65 background, pistil development was severely affected than in H0788 background. In dl-I (T65), the increase in stigma number was observed in about 60 % of flowers, and the transformation of pistil into stamens or the production of ectopic stamens was detected in nearly 10% of flowers (Table 1, Fig. 1C). Two strong alleles, dl-supl and dl-sup2, caused complete transformation of pistil into stamens (Fig, 1E, F). In these mutants, after the formation of six normal stamens, many ectopic stamens were differentiated in the region of pistil in alternate phyllotaxis. Flowers of dl-supl and dl-sup2 also showed abnormal morphology of palea that developed longer than the lemma. Transheterozygous plants among the four mutations exhibit intermediate phenotype

Table 1. Frequency of pistil abnormalities in dl mutants


Mutants No. of flowers examined Normal More than two stigmas Two pistils Extrastamens Staminoid with pistil' pistil^2 Homeotic transformation^3
DL 50 50 0 0 0 0 0
dl-l(H0788) 65 36 27 0 1 1 0
dl-l(T65) 82 20 51 2 6 1 2
dl-2 43 40 3 0 0 0 0
dl-supl 50 0 0 0 0 0 50
dl-sup2 39 0 0 0 0 0 39
dl-2/dl-l(T65) 51 19 21 1 4 3  3
dl-l(T65)/dl-supl 101 17 35 9 15 8 17
dl-sup2/dl-supl 8 0 0 0 0 0 8
  1. Hxtrastamens are produced around normal pistil. 
  2. Staminoid pistil is produced instead of original pistil or in the Extrastamens are frequently produced. 3. Pistil is completely transformed to stamens.
 Staminoid pistil is not formed. vicinity of normal pistil.

 

104 Rice Genetics Newsletter Vol. 13

Fig.l. Phenotypes of dl flowers. (A):wild-type flower, (B): dl-1(H0788) flower with three stigmas, (C): dl-l(T65) flower with a pistil producing cell mass and one ectopic stamen, (D): dl-2 normal flower, (E): dl-supl flower in which pistil is homeotically converted into stamens, and (F): dl-sup2 flower in which pistil is homeotically converted into stamens. Bar=l microm.

Fig. 2. Phenotypes of transheterozygous dl mutants.

(A): dl-2/dl-l(T65) flower with four stigmas and cell mass on the style, (B): dl-l(T65)/dl-supl flower with a pistil producing five stigmas and underdeveloped stigmatic papilae, (C): dl-1 (T65)/dl-supl flower with two pistils. Bar=0.5 microm.
Research Notes 105
or that of stronger alleles. Transheterozygous plants, dl-2/dl-1(T65), showed abnormalities in pistils as well as drooping leaves. Flowers with three or four stigmas were more frequently observed in the heterozygote than in dl-2 (Table 1, Fig. 2A). Although the identity of pistil was not affected in dl-2, staminoid pistils and complete homeotic conversion of pistil into stamens were observed in dl-2/dl-l(T65) at a frequency comparable with that of dl-1(T65). Therefore, in this transheterozygous plant, stronger allele, dl-l(T65), is dominant over the weaker allele. Another transheterozygous plant, dl-l(T65)/dl-sup1, showed a very low seed fertility (lower than 1%). This mutant produced various abnormal pistils. Pistils with increased number of stigmas and underdeveloped stigmatic papilae were frequently observed (Fig. 2B). Sometimes, cell masses, which were observed in dl-1(T65) but not in dl-supl, emerged on the style. In addition, staminoid pistils were more frequently produced than in dl-1(T65), in which underdeveloped anther was differentiated on the style. Homeotic transformation of pistil into stamens was more frequently observed in dl-l(T65)/dl-sup1 than in dl-I (T65), but less frequently than in dl-supl (Table 1). Two pistils, one original and one ovuleless extrapistil, were produced in 9% of flowers (Table 1, Fig. 2C). The extrapistil was apparently pistil-like, but the surface of central portion resembled that of the filament of stamen. Therefore, this extrapistil is considered to be a mix of stamen and pistil. These results indicate that phenotypes of this transheterozygous plant is intermediate between the two single mutants. Thus the phenotypes of transheterozygous plants vary with the combination of alleles. The morphological abnormality in pistil development would depend on the amount of DL gene activity.

The function of DL may be separable into two; the formation of midrib and the specification of pistil. Similar mutations have been identified in other grass species, barley (Tsuchiya 1962), pearl millet (Rao et al. 1988), and Panicum maximum (Fladung et al. 1991), and in rice, all of which affect both midrib formation and pistil development. Accordingly, these loci play very important roles in plant development via midrib formation and pistil development. Since homologous mutations pleio-tropically affecting both leaf midrib and pistil development have not been reported in dicotyledons, the above genes are unique to grasses (monocots). Although it is unclear how single locus regulates two developmentally-unrelated organs, midrib and pistil, it is obvious that these genes have been conserved in many grasses. (Gene symbol: New system)

References

Fladung. M., G.Bossinger, G.W. Roeb and F. Salamini, 1991. Correlated alterations in leaf and flower morphology and rate of leaf photosynthesis in a midribless (mbl) mutant of Panicum maximum Jacq. Planta 184:356-361.

lwata, N. and T. Omura, 1971. Linkage analysis by reciprocal translocation method in rice plants (Oryza sativa L.). II. Linkage groups corresponding to the chromosomes 5,6,8.9.10 and 11. Sci. Bull. Fac. Agr.Kyushu Univ. 25: 137-153. (in Japanese with English summary)

 Rao, S.A., M.H. Mengesha and C.R. Reddy, 1988. Characteristics, inheritance, and allelic relationships of midribless mutants in pearl millet. J. Hered. 79: 18-20. Tsuchiya, T., 1962. Radiation breeding in two-rowed barley. Seiken Ziho 14: 21-34.