CHARACTERIZATION OF CHLOROPLAST DIVISION USING THE ARABIDOPSIS MUTANTARC5

ard is a chloroplast division mutant of Arabidopsis thaliana. To ident ify the role of ARC5 in the chloroplast replication process we have fo llowed the changes in arc5 chloroplasts during their perturbed divisio n. ARC5 does not affect proplastid division but functions at a later s tage in chloroplast development. Chloroplasts in developing mesophyll cells of arc5 leaves do not increase in number and all of the chloropl asts in mature leaf cells show a central constriction. Young ard chlor oplasts are capable of initiating the division process but fail to com plete daughter-plastid separation. Wild-type plastids increase in numb er to a mean of 121 after completing the division process, but in the mutant arc5 the approximately 13 plastids per cell are still centrally constricted but much enlarged. As the arc5 chloroplasts expand and el ongate without dividing, the internal thylakoid membrane structure bec omes flexed into an undulating ribbon. We conclude that the ARC5 gene is necessary for the completion of the last stage of chloroplast divis ion when the narrow isthmus breaks, causing the separation of the daug hter plastids.