Leaf Senescence in a Stay-Green Mutant of Arabidopsis thaliana: Disassembly Process of Photosystem I and II during Dark-Incubation

In this study the disassembly process of chlorophyII (ChI)protein complexes of a stay-green mutant (ore10 of Arabidopsis thaliana) was investigated during the dark incubation of detached leaves. During this dark-induced senescence (DIS), the Chi loss was delayed in the mutant, while the photochemical efficiency of photosystem II (PSII) or Fv/Fm was accelerated when compared with the wild type (WT) leaves. This indicates that the decrease in Fv/Fm is a separate process and not causally-linked to the degradation of Chi during DIS of Arabidopsis leaves. In the native green gel electrophoresis of the Chi-protein complexes, which was combined with an additional twodimensional SDS-PAGE analysis, the delayed senescence of this mutant was characterized by the appearance of an aggregate at 1 d or 2 d, as well as very stable light harvesting complex II (LHCII) trimers until 5 d after the start of DIS. The polypeptide composition of the aggregates varied during the whole DIS at 5 d. Dl protein appeared to be missing in the aggregates. This result supports the idea of a faster depletion of functional PSH in the mutants compared with WT, as suggested by the earlier reduction of Fv/Fm and the stable Chl a/b ratio in the mutants. At 5 d, the WT leaves also often showed aggregates, but the polypeptide composition was different from those of ore10. The results presented suggest that the formation of aggregates, or stable LHCII trimers in the stay-green mutants, is a way to structurally protect Chi-protein complexes from serious proteolytic degradation. Detailed disassembly processes of Chi-protein complexes in WT and ore10 mutants are discussed.