The Role of Ethylene in the Senescence of Oat Leaves

Abstract

The evolution of ethylene, both from the endogenous source and from added 1-aminocyclopropane-1-carboxylic acid (ACC), has been foUlowed in close relationship with the senescent loss of chlorophyll from seedling oat leaves. In white light, where chlorophyll loss is slow, the ethylene evolution increases slowly at first, but when the loss of chlorophyll becomes more rapid, ethylene evolution accelerates. CoCI2 inhibits this increase and correspondingly maintains the chlorophyll content, with an optimum concentration of 10 micromolar. The rapid rate of chlorophyll loss in the dark is slightly decreased by 3-aminoethoxyvinyl glycine (AVG), by cobalt, and slightly stimulated by ACC. The slower chlorophyll loss in white light, however, is almost completely inhibited by silver ions, greatly decreased by cobalt and by AVG, and strongly increased by ACC. Since the chlorophyll loss is accompanied by proteolysis, it represents true senescence. Chlorophyll loss in light is also strongly antagonized by C02, 1% CO2 giving almost 50% chlorophyll maintenance in controls, while in the presence of added ACC or ethylene gas, the chlorophyll loss is 50% reversed by about 3% CO2. The ethylene system in leaves is thus more sensitive to CO2 than that in fruits. Indoleacetic acid also clearly decreases the effect of ACC. It is shown that kinetin, C02, Ag+, and indoleacetic acid, all of which oppose the effect of ethylene, nevertheless increase the evolution of ethylene by the leaves, and it is suggested that ethylene evolution may, in many instances, mean that its hormonal metabolism is being prevented.Abscisic acid somewhat increases ethylene evolution also, but its action in promoting senescence in light is antagonized only partially by Ag+, C02+, or AVG. For this and a number of other reasons it is concluded that ethylene and abscisic acid both independently control leaf senescence in the light.The study of leaf senescence continually brings to light more and more interactions. The earliest researches tended to concentrate on the nucleic acids (1 1, 24, 43, 44), then proteolysis came to the fore, together with the balance between proteolysis and protein synthesis, and the specific roles of individual amino acids (29 and see 35). Comparison between the behavior of isolated leaves and those attached to the plant then brought out the important part played by transport phenomena (22,32,38 (e.g. 18), it was natural to look for a function of this hormone in senescence, and indeed it was reported early to increase leaf senescence (6,12, 27). Our own experiments (14) '

Effects of Exogenous 1,3-Diaminopropane and Spermidine on Senescence of Oat Leaves

“…Inhibition of ethylene production by PAs has also been reported for senescing petals of Tradescantia (27), leaf tissue of beans, and apple protoplasts (4). Figure IB shows that Chi content decreased only little during the first 24 h of dark-incubation, but rapidly between 24 and 48 h, as was reported earlier (11,13). Dap and Spd inhibit Chl loss and, as with ethylene production, inhibition of Chl loss is antagonized by Ca2+.…”

Bicarbonate/CO2-Facilitated Conversion of 1-Amino-cyclopropane-1-carboxylic Acid to Ethylene in Model Systems and Intact Tissues

“…Increased production of ethylene in response to enhanced levels of CO2 has been demonstrated for detached leaves and leaf discs of cocklebur (15), intact sunflower plants (6,12), tobacco leaf discs (1,2,18), and detached senescing oat leaves (14). The effect is obtained when CO2 is supplied either directly as a gas or indirectly as bicarbonate, and no increase in ethylene production ' is obtained when CO2 is removed with a KOH trap (15,18).…”

“…White light also affects ethylene production by leaf tissue, presumably through modulation of endogenous CO2 levels. For example, exposure of leaf tissue to white light has been found to inhibit ethylene production relative to controls maintained in darkness ( 11,14,15,18). Moreover, light inhibition of ethylene production can be overcome by the addition of NaHCO3 or CO2 (15,18).…”

Large‐scale identification of leaf senescence‐associated genes

“…Many studies in this context have used detached leaves¯oating or immersed in hormone solutions. Exogenous application of ethylene or its immediate precursor 1-amino-cyclopropane-1-carboxylic acid (ACC; 1 mM) induced premature leaf yellowing and the evolution of endogenous ethylene positively correlated with the progression of this process (Gepstein and Thimann, 1981). It is expected that the effects of ethylene on physiological and biochemical processes of leaf senescence will also be re¯ected by differential gene expression during this process.…”