Gestörte Thylakoidbildung in Chloroplasten einer xantha-Mutante von Arabidopsis thaliana (L.) Heynh.

Summary

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.

Zusammenfassung

In den Palisadenzellen von Keim- und Rosettenblättern einer rezessiven xantha-Mutante von Arabidopsis thaliana fehlt den Plastiden die für normale, grüne Chloroplasten typische Schichtenstruktur. Licht- und elektronenmikroskopische Untersuchungen ergaben, daß sich dieser genabhängige Differenzierungsdefekt während der Plastidenentwicklung beim Übergang von den Tubuli des Prolamellarkörpers zu Thylakoiden manifestiert. Es entstehen aus dem persistierenden “Prolamellarkörper” nur einzelne abnorme Thylakoide, die zuweilen napfoder taschenartig verformt oder zu Magnograna vereinigt sein können. Besonders charakteristisch für die Mutante aber ist, daß solchen Thylakoiden häufig parallel laufende Züge einzelner Tubuli oder Fibrillen beidseitig dicht aufliegen. Auf Grund Dieser Anordnung wird die Möglichkeit eines Thylakoidwachstums durch laminalen Einbau von Tubuli erörtert.

In den Blättern der Mutante wurden nur Carotinoide, jedoch keine Chlorophylle festgestellt. Zugleich finden sich in den entwicklungsgehemmten Plastiden, neben Phytoferritin-Partikeln, zahlreiche osmiophile Globuli sowie Matrixeinschlüsse unbekannter Natur. Daher wird die Frage diskutiert, ob die beobachteten Störungen der Thylakoid-Differenzierung in der Mutante auf den Chlorophyllmangel oder auf eine blockierte Strukturproteid-Synthese zurückgehen.

Alle übrigen Zellorganelle sind in der Mutante ebenso gestaltet wie in der grünen Normalform. Besonders wird auf das Vorkommen von “dense bodies” hingewiesen und ihre Beziehung zu Sphärosomen erörtert.