Structural and physiological studies of a plastome cotton mutant with slow sorting out

Abstract

Structural and physiological investigations of a variegated plastome mutant of cotton (Gossyplum hirsutum L.) first found in 1964 were undertaken to determine the nature of the lesion in the mutant and possible reasons for the relatively long persistence of the variegation. Mutant plastids were much less structurally complex even at early developmental stages and large differences in chloroplast ribosome density were noted. Only single, or slightly doubled thylakoids were noted in the mature mutant plastid. At later stages the pigments from the mutant tissue were bleached and the thylakoid swelled to form small “plastid vacuoles.” Numerous mixed cells, single cells with both mutant and clearly wild-type plastids, were present in much of the mosaic tissue. However, unlike other plastome mutants with persistent mixed cells, no fusions between wild-type and mutant plastids were noted. We suggest that the persistence of mixed cells in this cotton mutant is due to the sorting out of both chloroplast DNA molecules as well as the two chloroplast types. Electrophoretic studies of chloroplast proteins indicate that many chloroplast-encoded and nuclearly encoded polypeptides are present relatively less in the mutant but polypeptides in the Mr range from 55–100 kD are present in increased amounts compared to the green chloroplasts. Although some cyclic electron flow through cytochrome f was noted in the mutant tissue, the fluorescence transients and the lack of grana stacks indicate a lack of photosystem II activity.