INVOLVEMENT OF SENESCENCE ASSOCIATED VACUOLES? IN THE DEGRADATION OF CHLOROPLASTIC PROTEINS IN TOBACCO LEAVES.

MARTINEZ DANA; COSTA M LORENZA; GUIAMÉT JUAN J

Chicago IL

Congreso; Plant Biology & Botany Congress; 2007

American Society of Plant Biology

Chloroplast disassembly and the breakdown of photosynthetic proteins characterize leaf senescence. The mechanism involved in the massive degradation of plastid proteins is poorly understood. We previously reported that a novel class of ?senescence-associated vacuoles? (SAVs) with high proteolytic activity accumulates in chloroplast-containing cells of senescing leaves of Arabidopsis and soybean (Otegui et al., 2005 Plant J. 41: 831-844). Using tobacco as a model plant, we found that the number of SAVs increases when chloroplast degradation rate is accelerated by treatment with ethephon. SAVs might be involved in chloroplast protein degradation. We used a transgenic line of tobacco expressing GFP fused to a chloroplast transit peptide (TP-GFP) to examine, by confocal microscopy, if chloroplast proteins are delivered to SAVs for degradation. While TP-GFP is correctly targeted to the plastid in non-senescent leaves, with no GFP signal outside chloroplasts, TP-GFP signal is readily detected in SAVs of senescing leaves. Spectral analysis of fluorescence emission confirms that the signal coming from SAVs corresponds to GFP. A SAV-enriched fraction was prepared by sucrose density centrifugation of a post-chloroplast supernatant. Isolated SAVs lacked the D1 protein of PSII, indicating that this fraction was free of chloroplasts or chloroplast fragments. On a protein basis, SAVs contained substantial amounts of the chloroplast-encoded, large subunit of Rubisco (RbcL). Levels of RbcL decreased in a time-dependent manner when SAVs were incubated at 30ºC for periods up to 4 hs, and this degradation was abolished by addition of protease inhibitors. Collectively, these data support the idea that SAVs are involved in the degradation of some chloroplast proteins during leaf senescence.