Senescence-associated vacuoles are involved in the degradation of chloroplast proteins in tobacco leaves

DANA E. MARTÍNEZ , MARÍA L. COSTA , FACUNDO M. GOMEZ, MARISA S. OTEGUI , JUAN J.

PLANT JOURNAL

Blackwell Synergy

Año: 2008 vol. 56 p. 196 - 206

0960-7412

Massive degradation of photosynthetic proteins is the hallmark of leaf senescence,however the mechanism involved in chloroplast protein breakdown is not completelyunderstood. Since small "senescence-associated vacuoles" (SAVs) with intense proteolyticactivity accumulate in senescing leaves of soybean and Arabidopsis, the main goal of thiswork was to test if SAVs are involved in the degradation of chloroplastic components.SAVs with protease activity were readily detected through confocal microscopy of naturallysenescing leaves of tobacco (Nicotiana tabacum L.). In detached leaves incubated indarkness, acceleration of chloroplast degradation rate by ethylene treatment correlatedwith a two-fold increase of the number of SAVs per cell, compared to untreated leaves. Ina tobacco line expressing GFP targeted to plastids, we detected GFP re-located to SAVsin senescing leaves. SAVs were isolated by sucrose density gradient centrifugation.Isolated SAVs contained chloroplast-targeted GFP, and the chloroplast stromal proteinsRubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) and glutamine synthetase,but they lacked the thylakoid proteins D1 and LHCII of the PSII reaction center and PSIIantenna, respectively. In SAVs incubated at 30ºC there was a steady decrease in Rubiscolevels, which was completely abolished by addition of protease inhibitors. Thus, SAVs arelikely involved in the degradation of the soluble photosynthetic proteins of the chloroplaststroma during senescence of leaves.