The SHOOTLESS2 and SHOOTLESS1 genes are involved in both initiation and maintenance of the shoot apical meristem through regulating the number of indeterminate cells.

To characterize the SHL2 and SHL1 genes in detail, we analyzed three strains carrying weak alleles of SHL2, shl2-6, shl2-7, and shl2-8, and one weak allele of SHL1, shl1-3. In contrast to strong alleles, which result in lack of shoot meristem, strains bearing these weak alleles formed shoot meristem frequently during embryogenesis. In shl2-6 and shl2-7 mutants, the meristem was lost during seed development. Only the shl2-8 mutant could survive after germination, but it showed abnormal initiation pattern and morphology of leaves. In strains bearing the weak alleles, the shoot meristem was composed of a small number of indeterminate cells and ultimately converted into leaf primordium. The shl1-3 mutant showed phenotypes similar to those of shl2-8. Thus SHL2 and SHL1 are required for both initiation and maintenance of shoot meristem. In shl2 mutants, there was a positive correlation between the size of the expression domain of OSH1 representing the number of indeterminate cells, the frequency of shoot meristem initiation, and the duration of meristem survival. Thus the shoot meristem will not initiate in an "all-or-nothing" fashion, but is formed in various degrees depending on the strength of the alleles. Double-mutant analyses indicate that SHL2 functions upstream of SHO to establish proper organization of the shoot meristem.