The control of protein breakdown and synthesis in the senescence of oat leaves

Abstract

During the senescence of detached first leaves of oat (Avena sativa L. cv. Victory) seedlings (grown in continuous light) the protein is hydrolyzed and the proteases increase, but the expected simple relation between these two factors is not always realized. The present experiments examine the timing, the influence of light and darkness and the action of the protein synthesis inhibitors cycloheximide (CHI) and cordycepin. Transfer from dark to light delays the breakdown of both chlorophyll (Chl) and protein, but some residual proteolysis is ascribed to the enzyme initially present. Transfer to CHI resembles transfer to light, while the action of cordyceptin is similar but much weaker. Repeated determinations of the acid protease, which is the most active one and the first to appear, show that this enzyme is formed in the light about as rapidly as in the dark, though with different kinetics. In spite of this there is little proteolysis in light in the first 5 days. One possible explanation of that could be that protein is rapidly resynthesized in light, but treatment with [14C]‐leucine shows that such resynthesis is no faster in light than in darkness. It is therefore concluded that the protease initially does not have access to its substrates and, as a corollary, that the senescence process must be controlled by the gradual impairment of the vacuolar membrane, allowing protease to enter the cytosol and attack the proteins there and in the organelles. This concept is supported by many observations on the timing and on the known changes in membrane permeability during senescence.

Leaf Senescence in a Nonyellowing Mutant of Festuca pratensis

“…It has often been proposed, most recently by Veierskov and Thimann (29), that the senescence process in chloroplasts may be controlled by gradual degeneration of the tonoplast, allowing proteinases to enter the cytosol and attack the proteins there and in the plastid. Veierskov and Thimann (29) postulated that thylakoid membranes in Bf993 are abnormally stable because the chloroplast membrane may be resistant to proteinase entry.…”

“…It has often been proposed, most recently by Veierskov and Thimann (29), that the senescence process in chloroplasts may be controlled by gradual degeneration of the tonoplast, allowing proteinases to enter the cytosol and attack the proteins there and in the plastid. Veierskov and Thimann (29) postulated that thylakoid membranes in Bf993 are abnormally stable because the chloroplast membrane may be resistant to proteinase entry. This interpretation has not been favored by Thomas (24) and does not explain why the extrinsic proteins of the thylakoids and those of the stroma are degraded in a manner comparable to the equivalent proteins of the wild type, whereas it is only the highly hydrophobic intrinsic polypeptides of thylakoid membranes of mutant leaf tissue that are abnormally stable.…”

Effects of O2and CO2partial pressure on senescence of oat leaves and broccoli miniflorets

“…Special care was taken to restore the atmospheric conditions immediately after sampling. Three leaf segments were used for chlorophyll and 10 for protein determination as described by Veierskov & Thimann (1988). Stomatal resistance was measured with a steady state porometer (L1-1600 from Lambda Instruments Co., United States), leaf area on a portable area meter (L1-3000 also from Lambda Instruments Co., United States).…”

“…The loss of protein during senescence has been attributed to a concurrent increase in proteolytic activity in the vacuole (Wittenbach et al 1982), and in oat chloroplasts aminopeptidases have been identified, although their role during senescence has not been detetmined (Thayer et al 1988). Recent experiments, however, have revealed that the rate of proteolysis may not be related to the level of proteases (van Loon et al 1987;Veierskov & Thimann 1988).…”

Increased Proteolysis of Senescing Rice Leaves in the Presence of NaCl and KCl

“…However, their activity is not always correlated with changes in proteins in situ (7,11,19). Also, the interpretation ofresults from some studies on proteolytic enzymes has been complicated by variations in assay methods (7,19) and in experimental systems, such as the detachment of leaves (13,24,25) and light/dark conditions (25).Compartmentation is an appealing hypothesis in that the major substrate protein is localized in chloroplasts while the majority of the proteolytic activity is in the vacuoles (21,23,24,26,28) and, during senescence, there is a change in membrane permeability, resulting in increased leakage of solutes (4,22). However, chloroplastic (1,8,12,17) and vacuolar membranes (23) remain relatively unchanged until the later stages of senescence.…”