Senescence Of Oat Research Articles

Changes in lipoxygenase properties and activity related to postgerminative growth and senescence in oat (Avena sativa L cv. Victory 1 )

Lipoxygenase (LOX), one of the main oxidative catalysts in plants, is involved in the regulation of growth and senescence. We investigated changes in LOX activity or its properties as they related to the development of oat plants at four stages (germination, growth, natural senescence, and dark-incubated senescence). LOX activity was high during early growth and at senescence. At pH 4.5, activity showed an abrupt surge compared with a normal enzyme reaction pattern at pH 6.5. The optimum reaction temperature was 25°C; LOX and peroxidase exhibited similar activity patterns. Polyacrylamide gel electrophoresis revealed that the purified LOX consisted of three isoenzymes in germinating seeds, two in growing seedlings, and three during both natural and dark-induced senescence. As determined by isoelectric focusing, the isoelectric points (pl) of LOX ranged from 3.6 to 6.5 throughout the four developmental stages; for natural or dark-induced senescence, the pl was 9.0.

Effects of methyl jasmonate (MeJA) on the dark-induced senescence in oat (Avena sativa L.) leaf segments

To investigate the relationship between methyl jasmonate (MeJA) and ethylene in leaf senescence, we studied the effects of MeJA on ethylene production and ethylene biosynthetic enzyme activities in oat(Avena sativa L.) leaf segments incubated in darkness. MeJA promoted dark-induced senescence judged from the contents of chlorophyll and protein, and increased ethylene production 6 times of the control. MeJA also increased the activities of ethylene biosynthetic enzymes, 1-aminocyclopropane carboxylic acid (ACC) synthase and ACC oxidase as compared to control. In MeJA-treated leaf segments, ACC synthase activity reached its maximum level at 24 h of incubation and ACC oxidase activity peaked at 6 h of incubation. Aminoethoxyvinylglycine (AVG) and Co2+, inhibitors of ACC synthase and ACC oxidase respectively, reduced MeJA-induced ethylene production. They also delayed leaf senescence that was promoted by the treatment of MeJA. From these results, we can suggest that MeJA increased the activities of ACC synthase and ACC oxidase, these increased activities lead to increase in ethylene production and this increased ethylene production might promote dark-induced leaf senescence.

Inhibitory effect of lepidimoide on senescence in Avena leaf segments

Summary The effect of lepidimoide (Na 2- O -rhamnopyranosyl-4-deoxy-α-L- threo -hex-4-enopyranosiduronate) and its free carboxylic acid (hereafter referred as lepidimoic acid), on senescence in oat ( Avena sativa L. cv. Victory I) leaf segments in the dark was studied. Lepidimoic acid inhibited loss of total chlorophyll in excised oat leaf segments during the initial phase of senescence. The loss of total chlorophyll was also retarded in the presence of 10 μmol/L kinetin. Lepidimoic acid preserved chlorophyll content synergistically with kinetin. Lepidimoide (Na salt) preserved chlorophyll content in the absence or presence of kinetin to a similar extent as did lepidimoic acid. These facts suggest that lepidimoide causes, especially in cooperation with cytokinin, a retardation of leaf senescence as characterized by the loss of total chlorophyll in the leaves.

Mechanisms of polyamine action during senescence responses induced by osmotic stress

The peeled Avena sativa L. leaf-system has been used as a simple and rapid model, which allows the study of the mechanisms of polyamine action during senescence responses induced by osmotic stress. The use of guazatine, an inhibitor of polyamine oxidase activity, has demonstrated the existence of a positive correlation between high levels of spermidine and spermine and delay of senescence in oat leaves and mesophyll protoplasts. The availability of antibodies specific for key polypeptides of thylakoid membranes in combination with ultrastructural studies have led to the discovery that spermine stabilizes the molecular composition and preserves the integrity of thylakoid membranes. The analyses of malondialdehyde and lipoxygenase levels has revealed that polyamines and guazatine treatments may prevent membrane destabilization, at least in part, by inhibiting lipid peroxidation. Finally, the availability of a cDNA coding for oat arginine decarboxylase (ADC) and the generation of ‘site-directed’ antibodies specific for ADC have led to the establishment of the basis for a model of regulation of ADC gene expression in oat leaves.

Effects of Oxidative Stress Caused by Oxygen and Hydrogen Peroxide on Energy Metabolism and Senescence in Oat Leaves

Effects of Oxidative Stress Caused by Oxygen and Hydrogen Peroxide on Energy Metabolism and Senescence in Oat Leaves

The interaction of senescence and wounding in oat leaves. II. Chlorophyll breakdown caused by wounding in light

The influence of wounding on the senescence of oat ( Avena sativa L.) leaf segments held in continuous white light was studied. It was noticed that leaf segments held in white light, which in general delays senescence, actually showed increased loss of chlorophyll within a few millimeters of the cut ends. When the whole segment was subjected to different types of wounding, chlorophyll loss was promoted in all cases and the loss increased with increase in the intensity of wounding. While 50% of the initial chlorophyll content was retained in the control segments after 6 days, wounding reduced that figure to as little as 24%. The increase in chlorophyll loss was already measurable by the second day. The presence of calcium and magnesium promoted the loss of chlorophyll (Chl). The action of wounding on proteolysis, however, was the opposite, for wounding synergized the effect of light in delaying proteolysis. A number of previously reported senescence-modifying substances were tested and even in presence of these substances wounding still promoted chlorophyll loss. However, these compounds differed in their effect on the protein content, some promoting proteolysis and others inhibiting it. Chl loss was not antagonized by reducing agents. The effect of wounding in light was reversed as soon as the segments were transferred from the light to darkness. Possible mechanisms for these peculiar effects are discussed.

Senescence in oat leaves: Changes in translatable mRNAs

Changes in translatable mRNA populations during the senescence of oat (Avena sativa L. cv. Victory) leaves were examined by analyzing the in vitro translation products of isolated RNA. Total RNA was isolated from oat leaves of 7‐day‐old seedlings, and also after these leaves were aged for different lengths of time under various conditions. Polypeptides from in vitro translations were separated by two‐dimensional gel electrophoresis to estimate any changes in translatable mRNA populations associated with senescence. Corresponding leaf samples were monitored for loss of chlorophyll as a measure of the extent of senescence. The aging of excised leaves in the light for 4 days resulted in the disappearance or substantial quantitative decrease of a number of mRNA species, while only five new translatable mRNA species were produced. Three of these mRNAs were unique to aging of leaves under light. Two of these mRNA species were also produced during the early stages of senescence in attached leaves of seedlings grown under light. The translatable mRNA populations of leaves aged for 4 days either on intact seedlings or detached and kept in the light in the presence of kinetin were very similar. Aging of excised leaves in the dark on water for 24 h resulted in very extensive changes in translatable mRNA populations. Over thirty polypeptides disappeared or were substantially reduced in quantity, while about an equal number appeared de novo or were substantially increased in quantity. Aging of these leaves for an additional 24 or 48 h resulted in only a few additional changes in translatable mRNAs. The presence of kinetin during aging of excised leaves in the dark inhibited few of the numerous changes in mRNAs that occured during the first 24 h, but did inhibit most of the changes that occured after 48 or 72 h of aging in the dark. When leaves were first aged in the dark and then returned to light, most of the initial changes in translatable mRNAs expression were reversed. Such changes in mRNAs thus appear to be light‐regulated and not necessarily associated with senescence.

Isolation of vacuoles from developing oat leaves

Procedures for the isolation of vacuoles from the first leaf of oats at different stages of development were compared as to yield, size and purity of the vacuoles isolated. Mechanical disruption or polybase-induced lysis of isolated protoplasts did not lead to clean vacuole preparations. In contrast, relatively pure vacuoles were obtained by phosphate-dependent osmotic lysis. Liberation of intact vacuoles required a minimum concentration of K 2HPO 4 in the lysis medium, lower concentrations leading to vacuole fragmentation. The maximum concentration by which complete lysis occurred decreased progressively when leaves were fully grown and started to senesce. Only vacuoplasts (vacuoles with adhering cytoplasm) could be obtained from leaves older than 17 days. The implications for the control of senescence in oat leaves are discussed.

Differential Changes in the Amount of Protein Complexes in the Chloroplast Membrane during Senescence of Oat and Bean Leaves

Antibodies against the individual subunits of protein complexes in the chloroplast membranes were used to follow the amounts of these polypeptides during foliar senescence. No change was found in the amount of polypeptides of photosystem I reaction center and the chloroplast coupling factor during senescence of oat (Avena sativa L.) and bean (Phaseolus vulgaris L.) leaves. A significant decrease in the amount of the different components of the cytochrome b(6)-f complex was detected. This change may account for the decrease in the rate of electron transport, which might be the rate limiting step of photosynthesis in senescing leaves.

Relation between Respiration and Senescence in Oat Leaves

The respiration of excised oat (Avena sativa cv Victory) leaves and their sensitivity to inhibitors was followed during senescence under varied conditions. The respiration rate, which in controls reaches its peak on the third day in darkness, is lowered at the time of fastest loss of chlorophyll (as reported earlier) by seven unrelated reagents that all delay dark senescence. When senescence is delayed by white light or by cytokinins, the respiratory rise is correspondingly delayed. Kinetin and l-serine, which act as antagonists on senescence, also act as antagonists on the respiratory rate. However, an exception to this close correspondence between senescence and the respiratory rise is offered by the lower aliphatic alcohols, which delay dark senescence and yet accelerate the onset of the respiratory rise.The respiration of freshly cut leaves is insensitive to KCN up to 8 millimolar, but sensitive to benzhydroxamate (BAM), 1 to 2 millimolar BAM causing 25% promotion and higher concentrations inhibiting. At the respiratory peak, however, part of the respiration becomes KCN-sensitive. Low concentrations of alcohols in darkness, or 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron, in light, also render part of the respiration KCN-sensitive, but this sensitivity soon disappears again. Some 10 to 15% of the respiration is insensitive to both inhibitors. Thus, cyanide sensitivity comes and goes, while BAM sensitivity is always present. The current concept of the cyanide-resistant pathway as an overflow, therefore, does not fit well with behavior of these leaves. The respiratory rise in leaf senescence is similar to, but not identical with, the climacteric in ripening fruits and the aging phenomenon in tuber slices.