Unusual thylakoid structures appearing during degradation of the photosynthetic apparatus in chloroplasts

Abstract

Two types of thylakoid degradation are described, both including leaf bleaching (i.e. chlorophyll degradation) and destruction of the photosynthetic membranes. The first type of degradation is characterized by coiling of single thylakoids and by cup-shape stacking of grana thylakoids. This type of thylakoid degradation was observed in leaves of the aurea varieties (conditional mutants) of several trees and shrubs. Leaf bleaching and thylakoid damage required strong sun-illumination and was repairable by shading, although only in young, still growing leaves. A structurally similar type of membrane damage, although not repairable, was detected in wheat and bean leaves treated with cadmium, followed by sun-exposure. The formation of interthylakoidally located membrane coils was not always dependent on strong light; they were also found in plastids of senescing leaves of Sophora japonica. A second type of thylakoid degradation appeared as the result of irreversible damage: leaf bleaching induced by treatment with amitrole, followed by sun-exposure. The damage began with tight stacking of long straight grana thylakoids. Later, on plane and perpendicularly oriented sections of these grana thylakoids, osmiophilic particles with diameters around 8 nm became visible. These were arranged in hexagonal arrays, with an approximate spacing of 12 nm. The possible origin of these regular structures is discussed. Structurally similar substructures in tightly stacked grana thylakoids were also detected in plastids of the subepidermal cells of Cucurbita pepo var. ovifera and pyriformis during fruit ripening, i.e. during the chloroplast-chromoplast transition.