arc6, an extreme chloroplast division mutant of Arabidopsis also alters proplastid proliferation and morphology in shoot and root apices.

The arc6 (accumulation and replication of chloroplasts) mutant of Arabidopsis has only two greatly enlarged chloroplasts per mature leaf mesophyll cell compared with ninety chloroplasts per cell in the wild type. The mutation is a single nuclear gene and the plant phenotype is normal. Shoot and root apical meristems of arc6 plants have been examined to determine how early during plastid development the mutant arc6 phenotype can be recognised. In the cells of the arc6 apical meristem there are only two proplastids, which are larger than wild type with a highly variable morphology. In the cells of the leaf primordia where differentiation of proplastids to chloroplasts occurs arc6 plastids are larger and at a more advanced developmental stage than wild-type plastids. In arc6 root cells statoliths and other plastids also show grossly abnormal morphology and the statoliths are greatly increased in size. During arc6 stomatal guard cell development the perturbation in proplastid population dynamics affects plastid segregation and 30% of stomata lack plastids in one or both guard cells. Our evidence would suggest that ARC6 is expressed throughout the vegetative cells of the Arabidopsis seedling with major effects on both the proplastid phenotype and the proplastid population. ARC6 is the first gene to be identified in Arabidopsis which has a global effect on plastid development in cells arising from both the shoot and root meristems, and is of major importance in the nuclear control of plastid differentiation in higher plants.