7. PLASTOCHRON2 gene regulates the plastochron and the duration of vegetative phase
  T. KAWAKATSU, J-I. ITOH and Y. NAGATO

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

Plastochron (the rete of leaf initiation) and phyllotaxy (the spatial pattern of leaf initiation) are the fundamental regulators of plant architecture.

Previously, we have identified two recessive heterochronic mutants, plastochron1-1 (pla1-1) and pla1-2 in which plastochrones are shortened and primary rachis branches are converted into vegetative shoots (Itoh et al. 1998). Recently we have isolated another recessive mutant exhibiting similar phenotypes (rapid leaf emergence and abnormal panicle) from an M2 population of cv. Taichung 65 chemically mutagenized with N-methyl-N-nitrosourea. Since the allelism test revealed that this mutation was derived from an independent locus of PLA1, we named this mutant plastochron2 (pla2) (Fig. 1).

First, we measured the and rate of leaf emergence in pla2 and the wild type (Table1). The



rate of leaf emergence in pla2 was 1.7 days/leaf, about 1/3 that of the wild type. Since the rates of leaf emergence in two pla1 alleles were about half of their respective wild type, pla2 was assumed to be a severer mutant than pla1. As the developmental timing from leaf primordium initiation to emergence was not changed, the rapid leaf emergence means short plastochron. Following the emergence of the flag leaf, the pla2 plant produced several vegetative shoots instead of normal panicle. Further observation revealed that the primordia of primary rachis branches were converted into vegetative shoot, as in pla1 mutants. This means that in pla2 the vegetative development is maintained even after the onset of reproductive phase, and the vegetative and reproductive programs are co-operating. Thus, pla2 is a heterochronic mutant with prolonged vegetative phase.

The pla2 plant was small. Leaf blade and sheath were very short. The longest leaf of pla2 was 23.0 cm in length, about 1/4 that of the wild type. In addition, pla2 leaves were shorter than those of pla1. Accordingly, the rate of leaf emergence (plastochron)may be correlated with leaf size.

Leaves are differentiated from shoot apical meristem (SAM), and abnormalities in leaves and plastochron are considered to reflect the abnormality in the SAM. Then, we measured the size of SAM with a microscope equipped with Nomarski differential interference contrast optics (Fig. 2,Table 1). Both width and height of pla2 SAM were much larger than those of wild type. This agrees with previous studies that an enlarged SAM shortens the plastochron (Itoh et al. 1998). The SAM of pla2 was a little larger than that of pla1. Thus, the rate of leaf initiation is likely to be positively correlated with the size of SAM among pla1, pla2 and the wild type.

Our results suggest that the plastochron, leaf size and SAM size/shape are correlated significantly among pla mutants. This indicates that pla mutants are useful materials for addressing how SAM affects the initiation and growth of leaves.

To investigate the interaction between PLA1 and PLA2, we made double mutant pla1-2 pla2. In the F2 progenies of pla1-2xpla2, we were not able to distinguish the double mutant phenotype from the single mutant. F2 progenies were segregated into normal: pla1-2: pla2 = 83:28:40, indicating that the double mutant took the phenotype of pla2. Accordingly, PLA2 gene is epistatic to PLA1 gene.

In conclusion, PLA2 gene functions upstream of PLA1 gene to suppress the rapid production of leaf primordia and turn off the vegetative program at an appropriate time.

Reference

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.