9. PLASTOCHRON3 gene regulates leaf initiation rate and termination of vegetative phase

T. KAWAKATSU and Y. NAGATO

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

Leaves are generated from the shoot apical meristem (SAM) in constant phyllotaxy and plastochron patterns. These traits constitute major components of plant architecture. In regard to leaf initiation rate, we have reported two mutants with shortened plastochron, plastochron1 (pla1) and pla2 (Itoh et al. 1998, Kawakatsu et al. 2001). In addition to rapid leaf initiation, they also exhibited heterochronic phenotypes: late flowering and conversion of primary rachis branches to vegetative shoots. PLA1 encodes CYP78A11, a member of plant specific subfamily of cytochrome P450 (Miyoshi et al. 2004). Since pla1-2 pla2-1 double mutants showed more severe phenotypes than either single mutant, PLA1 and PLA2 are supposed to act redundantly in independent pathways. 

Recently we have identified another recessive mutant exhibiting similar phenotypes (shortened plastochron and heterochrony) from an M2 population of cv. Kinmaze mutagenized with N-methyl-N-nitrosourea (MNU). Since this mutation was mapped on a locus independent of PLA1 and PLA2, we named this mutant plastochron3 (pla3).

Although mature embryos of pla1 and pla2 were indistinguishable from those of wild type and had three leaves, pla3 had larger embryos than wild type (Fig. 1A, B), and four leaves were present in mature embryos. This suggests that pla3 initiates leaves more rapidly in the embryonic phase (Fig. 1C, D).

In the vegetative phase, pla3 showed abnormal leaf initiation pattern. pla3 initiated leaves more rapidly than wild type, pla1 and pla2: the plastochrons were 1.7, 5.0, 2.3, 1.8 days, respectively (Fig. 1E). Frequently, pla3 showed a fusion of two leaves that were formed from adjacent shoots (Fig. 1F). Because these shoots were positioned closely and comparable in size, the SAM of pla3 might divide during development.

In the reproductive phase, pla3 produced several vegetative shoots instead of normal panicles as pla1 and pla2 did (Fig. 2A). SEM analysis revealed that after primary rachis branch primordia were formed in spiral phyllotaxy, they were soon converted into vegetative shoots. The number of ectopic shoots was larger in pla3 than in pla1-4 and pla2-1. As in pla1 and pla2, pla3 plants were dwarf due to reduced internode elongation. The pattern of internode elongation of pla3 was similar to that of pla1, severely deviated from those of wild type and pla2. Plant height was reduced in the order of wild type, pla1, pla2 and pla3 (Fig. 2B). Interestingly, the extents of plastochron reduction and dwarfism are positively correlated among pla mutants.

Fig. 1. Phenotypes of mature embryo and seedlings in wild type and pla3. A: Mature wild-type seed. B:  Mature pla3 seed. C: Plumule of mature wild-type embryo. D: Plumule of mature pla3 embryo. E: Plants at 1 month after germination. F: Cross section of shoot apex in 1-week-old pla3 seedling. Numerals in C and D indicate leaf numbers. Arrow in F indicates the fusion of two leaves. Co: Coleoptile.

Fig. 2. Phenotypes of wild type and plas in reproductive phase. A: Panicles. B: Schematic representation of internode elongation pattern. Numerals in B represent the internode numbers counted from the top.

In summary, PLA3 gene is involved in various developmental events. PLA3 is a negative regulator of leaf initiation and is a terminator of the vegetative phase. In addition, a possibility is suggested that PLA3 is also associated with meristem maintenance. At present, it is to be revealed how these three PLASTOCHRON genes are related with one another. Double mutant analysis and expression analysis would be necessary to address the above subject. We are now trying to isolate PLA3 by map-based cloning. Identification of PLA3 gene will greatly assist our understanding on the regulation of plastochron in rice.

References

Itoh, J.-I., A. Hasegawa, H. Kitano and Y. Nagato, 1998. A recessive heterochronic mutation, plastochron1, shortens the plastochron and elongates the vegetative phase in rice. Plant Cell 10: 1511-1521.

Kawakatsu, T., J.-I. Itoh and Y. Nagato. 2001. PLASTOCHRON2 gene regulates the plastochron and the duration of vegetative phase. RGN 18: 21-24.

Miyoshi, K., B. O. Ahn, T. Kawakatsu, Y. Ito, J.-I. Itoh, Y. Nagato and N. Kurata, 2004. PLASTOCHRON1, a timekeeper of leaf initiation in rice, encodes cytochrome P450. Proc. Natl. Acad. Sci. USA 101: 875-880.