[Disturbed thylakoid-formation in chloroplasts of a xantha-mutant of Arabidopsis thaliana (L.) HEYNH].

A monohybrid recessive xantha-mutant of Arabidopsis thaliana was grown aseptically on mineral agar +2% sucrose up to maturity. The cotyledons and rosette leaves were non-green during all developmental stages, their yellow colour bleaching with age.Palisade cells of various leaves of mutant plants were investigated light- and electron microscopically. All of the organelles of these cells, except the plastids, had a normal appearance. Special attention was paid to the existence of "dense bodies", and their relationship to sphaerosomes and lysosomes was outlined.In the mutant plastids the typical thylakoid differentiation present in the chloroplasts of the green Arabidopsis plant is entirely lacking. Within the palisade cells the plastids are normal in number and size. Under the light microscope they appear as hyaline vesicles with usually two "primary grana". Their outline is characterized by numerous invaginations and protrusions; however, it is still uncertain to what extent this "amoeboidy" is only an artifact.The specific gene dependent block in the plastid development is apparently effective during the conversion of tubules of the prolamellar body to thylakoids. From the persistent "prolamellar bodies" in the mutant only single abnormal thylakoids originate. These often have a deformed cup- or ventricle-like shape and occasionally are piled up as magnograna. In a characteristic manner tubules and fibrils are closely associated in parallel rows with the outsides of the thylakoids. The fibrils having a mean diameter of 75 Å are connected with the tubules which are derived from the "prolamellar body". From this striking aggregation the possibility of a laminal fusion of tubules as a process of normal thylakoid growth is deduced.By pigment analysis of mutant leaves no chlorophyll but only carotenoids were detected. However, many osmiophilic globules as well as large bodies of unknown composition (proteinaceous?) appear in the plastid matrix. Therefore the question is discussed whether the disturbances of the thylakoid differentiation is due primarily to a blocked chlorophyll formation or to an inhibition of the synthesis of some structural membrane proteids.Phytoferritin particles which up to now have not been observed either in normal or in mutant plastids of Arabidopsis were frequently found in the plastid matrix of this xantha-mutant.