ARC6, A FERTILE ARABIDOPSIS MUTANT WITH ONLY 2 MESOPHYLL CELL CHLOROPLASTS

A novel mutant of Arabidopsis thaliana, arc6 (accumulation and replica tion of chloroplasts), has been isolated from a transfer DNA-mutageniz ed population of Arabidopsis seedlings. ard has the most extreme are m utant phenotype we have yet described, with only one to three chloropl asts per leaf mesophyll cell compared to a mean of 83 in cells of the wild-type var Wassilewskija. The chloroplasts of arc6 are 20-fold larg er than wild-type chloroplasts. Chloroplast division is almost certain ly precluded in ard mesophyll cells, since chloroplast number per cell does not increase during mesophyll cell expansion. ard chloroplasts a re long and thin in cross-section and only one-half the width of wild- type chloroplasts and the arrangement of thylakoid membranes is largel y unaltered. arc6 segregates as a monogenic recessive nuclear mutation in a normal Mendelian manner and the arc6 phenotype is stably inherit ed for at least four generations. ard plants grow normally and are fer tile, although the rosette leaves appear curled and twisted. arc6 plan ts accumulate 70 to 75% of the biomass of wild type. The phenotype of this novel mutant is discussed in relation to the nature of the contro l of chloroplast division in leaf cells.