26. Genes affecting shoot apical meristem and leaf primordia differentiation

Jun-ichi itoh, Hidemi kitano and Yasuo naoato

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

2) Department of Biology, Aichi University of Education, Kariya 448, Japan

The vegetative apical meristem gives rise to a distinct shoot form. Accordingly, key processes regulating shoot development should be localized in the shoot apical meristem. Although mutants affecting vegetative shoot development have been isolated and analyzed in Arabidopsis (Medford et al. 1992), the relationship between structure and activities of shoot apical meristem (SAM) is poorly understood. Several mutants showing abnormal shoot organization have been isolated in rice (Hong et al. 1995). Among them, three single recessive mutants at two loci, SHOOT ORGANIZATION I (SHO1) and SH02, show pleiotropic phenotypes at various stages (embryonic, juvenile and reproductive phases). After germination, plants homozygous for these mutations develop unusual seedlings which exhibit various abnormalities of shoot organization. However, no abnormality is detectable in the root. In addition, six mutants have been recently isolated, showing phenotypes indistinguishable from those of sho l and 5/102, although they have not yet been genetically characterized. The early shoot apex of sho1-l is flat, not dome-like. The plastochron is very short, and leaf primordia are formed at various positions around the shoot apex, resulting in a random phyllotaxis (Tamura et al. 1992). Phenotypes of shol-2 and sho2 are very similar to that of shol-l. In this report, we describe the relationship between the structure of SAM and differentiation of leaf primordia, using shol-l, shol-2, sho2 and the other six mutants.

Mutants and wild type seeds were sterilized in 0.8% sodium hypochlorite and germinated on filter papers in petri dish at 30°C. After one week, the number of leaves and leaf position in each seedling were directly measured under a dissecting microscope. In these mutant seedlings, shoot apex is not covered with young leaf primordia, and hence it is possible to specify the position and timing of leaf primordial emergence under a dissecting microscope. Then, these seedlings were Fixed in FAA, cleared with BB-4-1/2 solution, and observed with Nomarsky optics.

We measured four structural parameters (height, width, height/width ratio and volume) of SAM (Fig. 1, Table 1). The SAM height of the three sho mutants was about 2/3 of that of the wild type, but the width of mutant SAM was larger. Consequently, the height/width ratio in the mutants was smaller, resulting in flat shape of mutant SAMs. Many sho mutants had large SAM volume, which varied widely with individuals.

Next, we measured parameters on leaf primordial differentiation (plastochron, divergence angle and entropy of phyllotaxis) (Table 1). Plastochron was calculated from the number of leaf primordia produced during seven days after germination. All the three mutants show much shorter plastochron than the wild type. In contrast to the consistent 1/2 alternate phyllotaxis of the wild type, primordia of sho mutants were produced at random positions around the apex. The divergence angle of two successive leaf primordia and entropy of phyllotaxis were measured. The mean divergence angle

Research Notes 101
Table 1. Characteristics of shoot apical meristem (SAM) and leaf primordia in
sho mutants
Characters Wild type shol-l shol-2 sho2
SAM width (microm) 48.0 68.7 51.5 66.3
SAM height (microm) 36.8 23.3 24.3 24.5
Height/width 0.77 0.34 0.47 0.37
SAM volume (microm3) 10895 14512 8347 13523
Plastochron(day) 3.50 1.11 1.22 0.80
Divergence angle(°) 180.0 109.4 127.6 114.4
SAM volume (microm3) 0.0 1.03 0.65 0.93

 

Fig. 1. Shoot apical meristems of sho mutants observed with Nomarski optics.
A: wild type, B: shol-l, C: shol-2, D: shol. Bar = 50 microm.
of sho mutant seedlings was around 110°. To quantitatively evaluate the randomness of phyllotaxis, we radially divided the shoot apex into diagonally duplicated eight regions (A, B, C and D in the below), and defined the entropy of phyllotaxis (H) as follows: where fi is the number of leaf primordia in region i, and n is the total number of leaf primordia examined. In the present case, completely random phyllotaxis gives the maximum value of 1.34. The entropy of phyllotaxis in each mutant was about 1 in contrast to that of wild type being minimum 0.

The above results suggest that abnormal differentiation of leaf primordia is closely associated with the malformed SAM. Then the correlation analysis was made between the structural parameters and leaf primordial parameters using data on 50 seedlings (Table

 102 Rice Genetics Newsletter Vol. 13

Table 2. Correlations between the structural parameters and leaf primordial parameters


Parameter Plastochron Divergence angle Entropy of phyllotaxis
SAM width -0.228 -0.411** 0.392*
SAM height 0.537** 0.438** -0.579**
Height/width 0.618** 0.646* -0.732**
SAM volume 0.000 -0.210 0.134
*, **: significant at 5% and 1% level, respectively. 2). Among the four parameters of SAM structure, height and height/width ratio showed significant correlation with any parameters of leaf primordia (plastochron, divergence angle and entropy of phyllotaxis). The width was not correlated with plastochron. There was no correlation between SAM volume and any parameter of leaf primordia. In addition, since the three parameters of leaf primordia were significantly correlated with one another, these parameters were not independently regulated. In conclusion, SAM height and shape, not the size, are closely associated with the spatial and temporal regulation of leaf primordial differentiation. (Gene symbol: New system)

References

Hong, S.-K.., T. Aoki, H. Kitano, H. Satoh and Y. Nagato, 1995. Phenotypic diversity of 188 rice embryo

mutants. Dev. Genet. 16: 298-310. Medford, J.I., F.J. Behringer, J.D. Callos and K.A. Feidman, 1992. Normal and abnormal development in the Arabidopsis vegetative shoot apex. Plant Cell 4: 631 -643. Tamura, Y., H. Kitano, H. Satoh and Y. Nagato, 1992. A gene profoundly affecting shoot organization in the early phase of rice development. Plant Sci. 82: 91-99.