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

Abstract

NaCI and KCI enhanced the degradation of chlorophylls and proteins in detached rice (Oryza sativa) leaves in a concentrationdependent manner. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) accounted for 73 to 80% of the protein lost by day 4 in the light. NaCI at 50 millimolar increased proteolysis by 21% over the control in 4 days, but the addition of cycloheximide reduced the increase to about one-half. Cycloheximide alone had no effect on proteolytic activity during this period. Leaf segments taken from 10-day-old seedlings contained the highest proteolytic activity. Both NaCI and KCI increased the activity of Rubiscodegrading endoproteinases (the amount of ninhydrin-positive compounds measured from HCI-hydrolyzates of trichloroacetic acid-soluble supematant), but decreased the activity of hemoglobin-and Rubisco-degrading exoproteinases (the amount of ninhydrin-positive compounds measured directly from trichloroacetic acid-soluble supematant). Efflux of amino acids from senescing leaf segments into the incubation media increased 7-and 12-fold in the presence of KCI and NaCI, respectively. The increased efflux resulted in a negative correlation between salt concentration and amino acid content of leaf segments at the later stage of senescence. It is concluded that, in addition to the induction of new proteinase synthesis, the increased efflux of protein hydrolyzates may play a significant role in increasing proteolysis of salt-treated leaves, especially at the later stages of senescence.Leaf senescence and proteolysis have been studied in cereal crops. Rubisco2 is the most abundant substrate protein degraded during senescence (5,8,10,17,18,27). The major regulatory mechanisms involved in proteolysis of senescing leaves include: the synthesis of proteolytic enzymes and the changes in their activity during senescence (2, 5-7, 11, 13, 17, 18, 24, 25, 27) and the physical integrity of cellular organelles and the spatial separation between substrate proteins and enzymes (4,(23)(24)(25).The synthesis or existence of endoproteinases with acid pH optima has been reported for several plant species (2, 6). ' 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. Also, recent evidence indicates that chloroplasts contain a variety of proteolytic enzymes (3,16,20,26).In this report, we present evidenc...

Expression of asparagine synthetase genes in sunflower (Helianthus annuus) under various environmental stresses

“…C: control; Cu: copper-treated plants. [5,6,11,12,24]. Under such circumstances, the synthesis of free amino acids, and especially the amides glutamine and asparagine, is increased to prevent ammonium toxicity and to reassimilate the nitrogen released [16,22].…”

“…Unfavourable growth conditions, such as high temperature, exposure to heavy metals, soil salinity, or nutrient deficiency [5,6,11,12], can reduce the photosynthetic capacity of the plant, resulting in a decreased carbon availability and an enhanced protein degradation which lead to internal ammonium accumulation [11]. The synthesis of amides, especially asparagine, is stimulated by some environmental stresses [1,16,22].…”

Heat-inducible rice hsp82 and hsp70 are not always co-regulated

“…Cadmium which is another protein-denaturing agent, has been used by other researchers to induce the gene in other plants (Edelman et al 1988). We were also able to induce hsp70 with high temperature, or with high salinity, two stresses that have been shown to stimulate protein turnover, either in wheat (Ferguson et al 1990) or in rice (Kang and Titus 1989). Prolonged darkness was an unanticipated inducer of hsp70 in our experiments, although previous studies of hsp70 in Chlamydomonas (von Gromoff et al 1989) and of hsp83 in Pharbitis (Felsheim and Das 1992) have shown there is a transient increase in hs gene expression when dark-grown cells are suddenly exposed to light.…”

Identification of Selection Signatures in Commercial Asian Rice Subspecies Deciphered Selection Footprints for Four Overrepresented Biological Processes

“…Furthermore, it can be helpful to understand the recent selection patterns in these subspecies. We found the four overrepresented biological processes, including GO:0006508, GO:0016310, GO:0030163 and GO:0055085 which can contribute in some important traits such as immunity (phosphorylation) [56], senescing leaves (proteolysis and protein catabolic) [49], and root membrane transporting (transmembrane transport) which can be highly important for absorption, and transporting of ions in rice [67]. Additionally, there were overlapped biological processes underlying selection pressures among several scenarios which can have effective roles in economical traits of Asian rice subspecies; For example, developmental process was identi ed as an overrepresented biological process among three scenarios (Tropical japonica vs. Temperate japonica, Tropical japonica vs. Indica, and Tropical japonica vs. Indica) which can have a fundamental role in growth and development of rice seeds [68].…”

“…The Os04g0677100 involves in proteolysis (GO:0006508) and protein catabolic process (GO:0030163). These GOs can play important roles, such as senescing leaves [49] and releasing trans-membrane proteins [50].…”