Degradation of photosynthetic proteins in senescence-associated vacuoles of tobacco leaves

MARTÍNEZ DANA; COSTA LORENZA; OTEGUI MARISA; GUIAMET JUAN JOSÉ

Rosario, Argentina

Congreso; XLII Ruenión anual de la sociedad argentina de investigaciones en bioquímicas y biología celular (SAIB); 2006

Degradation of chloroplasts, the photosynthetic organelles of plants, is the hallmark of leaf senescence. However, the mechanism involved in 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 several dicots, and a few 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. 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) 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 senescence-associated vacuoles and degraded therein in senescing leaves.