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 Arab idopsis has only two greatly enlarged chloroplasts per mature leaf mes ophyll cell compared with ninety chloroplasts per cell in the wild typ e. The mutation is a single nuclear gene and the plant phenotype is no rmal. Shoot and root apical meristems of arc6 plants have been examine d to determine how early during plastid development the mutant arc6 ph enotype can be recognised. In the cells of the arc6 apical meristem th ere are only two proplastids, which are larger than wild type with a h ighly variable morphology. In the cells of the leaf primordia where di fferentiation of proplastids to chloroplasts occurs arc6 plastids are larger and at a more advanced developmental stage than wild-type plast ids, In arc6 root cells statoliths and other plastids also show grossl y abnormal morphology and the statoliths are greatly increased in size , During arc6 stomatal guard cell development the perturbation in prop lastid population dynamics affects plastid segregation and 30% of stom ata lack plastids in one or both guard cells, Our evidence would sugge st that ARC6 is expressed throughout the vegetative cells of the Arabi dopsis seedling with major effects on both the proplastid phenotype an d the proplastid population, ARC6 is the first gene to be identified i n Arabidopsis which has a global effect on plastid development in cell s arising from both the shoot and root meristems, and is of major impo rtance in the nuclear control of plastid differentiation in higher pla nts.