Stromal protein degradation is incomplete in Arabidopsis thaliana autophagy mutants undergoing natural senescence.

Abstract

Degradation of highly abundant stromal proteins plays an important role in the nitrogen economy of the plant during senescence. Lines of evidence supporting proteolysis within the chloroplast and outside the chloroplast have been reported. Two extra-plastidic degradation pathways, chlorophagy and Rubisco Containing Bodies, rely on cytoplasmic autophagy. In this work, levels of three stromal proteins (Rubisco large subunit, chloroplast glutamine synthetase and Rubisco activase) and one thylakoid protein (the major light harvesting complex protein of photosystem II) were measured during natural senescence in WT and in two autophagy T-DNA insertion mutants (atg5 and atg7). Thylakoid-localized protein decreased similarly in all genotypes, but stromal protein degradation was incomplete in the two atg mutants. In addition, degradation of two stromal proteins was observed in chloroplasts isolated from mid-senescence leaves. These data suggest that autophagy does contribute to the complete proteolysis of stromal proteins, but does not play a major degenerative role. In addition, support for in organello degradation is provided.