A GENETIC-ANALYSIS OF CHLOROPLAST DIVISION AND EXPANSION IN ARABIDOPSIS-THALIANA

A nuclear recessive mutant of Arabidopsis thaliana, arc5, has been iso lated in which there is no significant increase in chloroplast number during leaf mesophyll cell expansion and in which there are only 13 ch loroplasts per mesophyll cell compared with 121 in wild-type cells. Ma ture arc5 chloroplasts in fully expanded mesophyll cells are 6-fold la rger than in wild-type cells. A large proportion of arc5 chloroplasts also show some degree of central constriction, suggesting that the mut ation has prevented the completion of the chloroplast division process . To examine the interaction of are loci, a double mutant was construc ted between arc1, a mutant possessing many small chloroplasts, and arc 5. A second double mutant was also constructed between arc3, a previou sly discovered mutant also possessing few targe chloroplasts per cell, and arc1. Analysis of these double mutants shows that chloroplast num ber per mesophyll cell is greater when arc5 and arc3 mutations are exp ressed in the arc1 background than when expressed alone. The cell-spec ific nature of are mutants was also analyzed. The phenotypic traits ch aracteristic of arc3 and arc5 are a reduction in chloroplast number an d an increase in chloroplast size in mesophyll cells: these changes ar e also observed in reduced form in the epidermal and guard cell chloro plasts of arc3 and arc5 plants. Analysis of parenchyma sheath cell chl oroplasts suggests that in leaves of arc1 plants the normal developmen tal distinction between mesophyll and parenchyma sheath chloroplasts i s perturbed. The relevance of these findings to the analysis of the co ntrol of chloroplast division in mesophyll cells is discussed.