Senescence‐associated vacuoles with intense proteolytic activity develop in leaves of Arabidopsis and soybean

Abstract

SummaryVacuolar compartments associated with leaf senescence and the subcellular localization of the senescencespecific cysteine-protease SAG12 (senescence-associated gene 12) were studied using specific fluorescent markers, the expression of reporter genes, and the analysis of high-pressure frozen/freeze-substituted samples. Senescence-associated vacuoles (SAVs) with intense proteolytic activity develop in the peripheral cytoplasm of mesophyll and guard cells in Arabidopsis and soybean. The vacuolar identity of these compartments was confirmed by immunolabeling with specific antibody markers. SAVs and the central vacuole differ in their acidity and tonoplast composition: SAVs are more acidic than the central vacuole and, whereas the tonoplast of central vacuoles is highly enriched in c-TIP (tonoplast intrinsic protein), the tonoplast of SAVs lacks this aquaporin. The expression of a SAG12-GFP fusion protein in transgenic Arabidopsis plants shows that SAG12 localizes to SAVs. The analysis of Pro SAG12 :GUS transgenic plants indicates that SAG12 expression in senescing leaves is restricted to SAV-containing cells, for example, mesophyll and guard cells. A homozygous sag12 Arabidopsis mutant develops SAVs and does not show any visually detectable phenotypical alteration during senescence, indicating that SAG12 is not required either for SAV formation or for progression of visual symptoms of senescence. The presence of two types of vacuoles in senescing leaves could provide different lytic compartments for the dismantling of specific cellular components. The possible origin and functions of SAVs during leaf senescence are discussed.

Aging in Legume Symbiosis. A Molecular View on Nodule Senescence in Medicago truncatula

“…Indeed, in situ hybridization analysis indicated that members of this family are specifically transcribed in cells where bacteroids are degraded (W. Van de Velde and S. Goormachtig, unpublished data). As an analogy, SAG12-green fluorescent protein fusions have recently been observed in senescence-associated vacuoles that are only present in chloroplast-containing leaf cells that undergo senescence (Otegui et al, 2005). Whether these SAG12 Cys protease-containing vesicles are involved in degradation of chloroplast proteins is currently unknown.…”

A Mutation in the LPAT1 Gene Suppresses the Sensitivity of fab1 Plants to Low Temperature

“…fab1 plants kept at 2°C for up to 5 weeks are able to recover and set seed after a return to 22°C (Wu et al, 1997). We originally suggested that this chloroplast degradation at low temperature was a form of autophagy; however, recent reports of vacuole formation and chloroplast degradation during natural or dark-induced senescence provide alternative models for the process that occurs following the collapse of photosynthesis in fab1 plants at 2°C (Otegui et al, 2005;Ishida et al, 2008).…”

Senescence in wheat leaves: is a cysteine endopeptidase involved in the degradation of the large subunit of Rubisco?

“…A senescence-associated formation of special vacuoles containing a senescence-specific cysteine protease has been detected in leaves of Arabidopsis and soybean (Otegui et al 2005). Ricinosomes (precursor protease vesicles) containing large amounts of a papain-type cysteine endopeptidase develop at the beginning of programmed cell death in the nucellus of Ricinus communis Greenwood et al 2005).…”

“…High levels of endopeptidases and carboxypeptidases are localized in vacuoles (Brouquisse et al 2001;Huffaker 1990;Otegui et al 2005). Recently, the possibility has been discussed that vacuolar proteases are involved in the degradation of plastidial proteins during senescence of purple nutsedge (Fischer et al 1998).…”