The genetic and physical analysis of mutants of chloroplast number and size in Arabidopsis thaliana.

Abstract

The genetic control of chloroplast division in Arabidopsis thaliana was investigated by the study of ARC genes. Seven arc mutants which displayed altered mesophyll cell chloroplast number and size compared to wild type were isolated from populations of Arabidopsis mutagenised by T-DNA or transposon insertion. The arc mutants represent alleles of six independent nuclear loci and are stably inherited in a normal Mendelian manner. The mean chloroplast number per mesophyll cell in arc mutants and wild type varies between 2 and 120 chloroplasts per cell. The size of arc mutant chloroplasts varies between half the size to 50 fold larger than wild type. Chloroplast number and chloroplast size varies inversely in the Arabidopsis mesophyll cell so that total chloroplast cover per cell is similar between wild type and arc mutants. The arc mutants are fully fertile and only one mutant, arc6, displays any significant whole plant mutant phenotype. The chloroplasts of arc5 do not normally divide but expand to a plan area six fold larger than wild type. Almost all arc5 chloroplasts are constricted around the centre of the plastid, indicating that chloroplast division has been attempted but not completed. The comparison of the chloroplast phenotype of arc5 to wild type has suggested several physical constraints to chloroplast division in Arabidopsis mesophyll cells. arc11, which has highly variable chloroplast number and size per mesophyll cell, is tagged by the insertion of a transposon, indicated by reversion of the arc11 mutant phenotype to wild type. Fragments of the putative ARC11 gene have been amplified by IPCR, cloned and sequenced. The arc6 mutant which affects both proplastid and chloroplast division may have as few as a single chloroplast per cell. arc6 is not tagged, requiring a map-based strategy to isolate ARC6. RFLP analysis using the ARMS and CAPS mapping sets indicated that ARC6 is located on chromosome 5, between markers m247 and DPR.