DEGRADATION OF PHOTOSYNTHETIC PROTEINS IN SENESCENCE-ASSOCIATED VACUOLES OF TOBACCO LEAVES

D. E. MARTÍNEZ; LORENZA COSTA; MARISA S. OTEGUI; JUAN J GUIAMET

Pinamar

Congreso; SAIB XLII Reunión anual; 2006

SAIB

Degradation of chloroplasts, the photosynthetic organelles of plants, is the hallmark of leaf senescence. However, the mechanism underlying the breakdown of chloroplast proteins is poorly understood. Previously, we discovered a novel class of ?senescence-associated vacuoles? (SAVs) with high proteolytic activity in senescing leaves of Arabidopsis, soybean and tobacco. Several observations suggest that SAVs might be involved in the degradation of photosynthetic proteins. Roughly, the abundance of SAVs in a cell correlates with rates of chloroplast degradation in tobacco. The number of SAVs decreases in cells were senescence is slowed down by treatment with the plant hormone cytokinin, whereas acceleration of senescence with ethylene greatly increases the number of SAVs. Immunolocalization experiments show the chloroplast-encoded large subunit of Rubisco (RbcL) is re-located to SAVs in senescing leaves. RbcL is also detected in Western blots of isolated SAVs. Autodigestion experiments with isolated SAVs incubated at pH 5.5 (the estimated lumenal pH of SAVs) show that RbcL is degraded by SAV proteases. These data strongly support the idea that chloroplast proteins are transferred to SAVs and degraded there during senescence of leaves.