Endogenous abscisic acid levels are not linked to methyl jasmonate-promoted senescence of detached rice leaves

Both abscisic acid (ABA) and jasmonates are known to promote leaf senescence. Since ABA and jasmonates have both chemical and physiological similarities, we are interested to know whether senescence of detached rice leaves induced by methyl jasmonate (MJ) is mediated through an increase in endogenous ABA levels. In darkness, the endogenous level of ABA in detached rice leaves remained unchanged in the first day of incubation in water and increased about 5 times its initial value in the second day. However, the pattern of senescence, as judged by protein loss, was rapid during the first day. MJ significantly promoted senescence of detached rice leaves. Contrary to our expectation, endogenous ABA levels decreased in MJ-treated detached rice leaves. Similar to the effect of MJ, endogenous ABA levels decreased in detached rice leaves which were induced to senesce by treatment with NH₄Cl. These results suggest that endogenous ABA levels are not linked to MJ-induced senescence of detached rice leaves.     

Ammonium, calcium, and leaf senescence in rice

The possible involvement of calcium in the regulation of ammonium-promoted senescence of detached rice leaves was investigated. Calcium effectively reduced ammonium-promoted senescence of detached rice leaves. The effect of ammonium on the senescence was also significantly reduced by the calcium ionophore A23187. Ammonium-promoted senescence of detached rice leaves may be mediated through blocking the entrance of calcium ions into the cytosol.     

Water stress, ammonium, and leaf senescence in detached rice leaves

Ammonium accumulation in relation to water stress-promoted senescence of detached rice leaves was investigated. The effect of water stress on the senescence of detached rice leaves is associated with the accumulation of ammonium. The accumulation of ammonium in detached rice leaves by water stress is attributed to a decrease in glutamine synthetase activity. Ammonium accumulation in detached rice leaves, induced by water stress, was accompanied by an increase in tissue sensitivity to ethylene which, in turn, accelerated leaf senescence.     

Comparison of phosphorus mobilization during monocarpic senescence in rice cultivars with sequential and non-sequential leaf senescence

The senescence pattern of the three uppermost leaves of four rice (Oryza sativa L.) cultivars viz. Ratna, Jaya, Masuri and Kalojira was analysed in terms of decline of chlorophyll and by measuring [³²P]-phosphate retention and export from leaf to grains during the reproductive development. With the advancement of reproductive development, the cultivars Masuri and Kalojira showed a sequential mode of senescence, but the cultivars Ratna and Jaya showed a non-sequential mode of leaf senescence where the flag leaf senesced earlier than the older second leaf. Foliar spraying with benzyladenine (0.5 mM) significantly delayed, and abscisic acid (0.1 mM) accelerated, leaf senescence. In untreated control plants, the second leaf had the highest export of labelled phosphate among the leaves at the grain formation stage (0–7 days) in Masuri and Kalojira. This was compensated by the flag leaf at the grain development stage (7–14 days), whereas export of [³²P]-phosphate was highest from the flag leaf of Ratna and Jaya at the grain development stage. Compared with the control, benzyladenine treatment caused higher retention of [³²P]-phosphate in the leaves and also export to the grains, but abscisic acid treatment gave lower retention and export of [³²P]-phosphate to the grains. The amount of [³²P]-phosphate export from a mother to a daughter shoot developed in the axil of the second leaf of plants with the panicle removed, was less than that to panicles remaining on control plants of all cultivars. When the panicle had been excised, the greatest export of [³²P]-phosphate took place from the second leaf to the daughter shoot in all cultivars. Excision of the panicle delayed leaf senescence as compared with intact controls and maintained an age-related leaf senescence pattern in all the four cultivars. The results presented here demonstrate that mobilization of phosphorus from leaf to grains, regardless of cultivar or age and position of the leaf, correlates well with the senescence of that leaf.     

Degradation of ribulose-1,5-bisphosphate carboxylase and chlorophyll in senescing barley leaf segments triggered by jasmonic acid methylester, and counteraction by cytokinin

Barley ( Hordeum vulgare L. cv. Salome) primary leaf segments responded to the application of a putative plant growth regulator, ± jasmonic acid methylester (JA-Me). with accelerated senescence, as indicated by the loss of chlorophyll and the rapid decrease in activity and immunoreactive protein content of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase, EC 4.1.1.39). The senescence-promoting action of JA-Me differed in light and in darkness; e.g. the initial rates of chlorophyll and RuBP carboxylase breakdown were markedly higher in light than in darkness in the presence of 4.10⁻⁵ M JA-Me. Cytokinin (benzyladenine, 4.10⁻⁵ M ) stopped the loss of chlorophyll and RuBP carboxylase during senescence; however, the rapid drop induced by JA-Me in the early phase of leaf segment senescence could not be prevented by concomitant or previous addition of BA. On the other hand, BA added 24 h after JA-Me application resulted in a recovery of chlorophyll and RuBP carboxylase at the later stages, indicating a possible rapid inactivation of JA-Me in the tissues. The activities of a number of other chloroplastic and cytosolic enzymes were not significantly altered in JA-Me-treated leaf segments compared with controls floated on water. Time-dependent chlorophyll decrease in isolated chloroplasts did not change upon JA-Me addition to the isolated organelles. It is suggested that JA-Me acts on chloroplast senescence by promoting cytoplasic events which eventually bring about the degradation of chloroplast constituents.     

Effects of benzyladenine and abscisic acid on the disassembly process of photosystems in anArabidopsis delayed-senescence mutant,ore9

Using wild-type (WT) leaves and those from anore9 delayed-senescenceArabidopsis mutant, we investigated the delaying and accelerating effects of benzyladenine (BA) and abscisic acid (ABA), respectively, on the degradation process of the photosynthetic apparatus during dark-induced senescence (DIS). In the mutant, delays were seen for both the breakdown of chlorophyll (Chl) and the decrease in photochemical efficiency of photosystem II (Fv/Fm). Moreover, each step was prolonged in the disassembly process of the Chl-protein complexes. In the presence of BA, Chl degradation was retarded to a similar extent for both the mutant and the WT, but the decrease in Fv/Fm was not. However, in the presence of ABA, the two processes were accelerated in both genotypes. Therefore, although theore9 mutation causes this functional delayed-senescence, it may not be related to the non-functional delay that happens afterwards. In contrast, BA seems to affect both processes.     

大穗型水稻生育后期叶片和根系生理的特性

Ｇ Ｅ Ｒ１ 是从玉米 Ｄ Ｎ Ａ 导入水稻的后代变异材料中筛选出的大穗型高产变异系⒚对 Ｇ Ｅ Ｒ１ 生育后期叶片和根系生理特性的研究表明：与受体湘早籼 ８ 号和三系杂交稻汕优 ６３相比, Ｇ Ｅ Ｒ１ 的顶三叶叶绿素含量、蛋白质含量、光合速率、过氧化物酶和超氧化物歧化酶活性降幅小,丙二醛含量增幅小,叶片衰老得到延缓,生育后期根量和根系伤流量较大、发根力和根系 α萘胺氧化力较强⒚生育后期叶片和根系的延缓衰老使得叶片功能期长、光合产物供应充足,是实现大穗多穗条件下水稻高结实率和充实度好的主要内在原因⒚     

A comparative study of the effects of abscisic acid and methyl jasmonate on seedling growth of rice

The effects of abscisic acid (ABA) and methyl jasmonate (MJ) on growth of rice seedlings were compared. The lowest tested concentration of ABA and MJ that inhibited seedling growth was found to be 4.5 and 0.9 µM, respectively. Growth inhibition by ABA is reversible, whereas that by MJ is irreversible. GA₃ was found to be more effective in reversing inhibition of shoot growth by ABA than by MJ. KCl partially relieved MJ-inhibited, but not ABA-inhibited, growth of rice seedlings. The beneficial effect of K⁺ on growth of rice seedlings in MJ medium could not be replaced by Li⁺, Na⁺ or Cs⁺. MJ treatment caused a marked release of K⁺ into the medium. In order to understand whether cell wall-bound peroxidase activity was inversely related to rice seedling growth, effects of ABA and MJ on cell wall-bound peroxidase activity were also examined. Results indicated that both ABA and MJ increased cell wall-bound peroxidase activity in roots and shoots of rice seedlings. Although MJ (4.5 µM) was less effective in inhibiting root growth than ABA (9 µM), MJ was found to increase more cell wall-bound peroxidase activity in roots than ABA.     

Reversal of the inhibitory action of abscisic acid by benzyladenine in excised watermelon cotyledons

Cotyledons of watermelon ( Citrullus vulgaris Schrad. cv. Fairfax) were excised from the embryo after 24 h of imbibition and cultured for several days on filter paper with water or abscisic acid (ABA) solution. In some experiments the cotyledons were pretreated with benzyladenine (BA) for times ranging from 5 min to 2 h before transfer to ABA.
A treatment with 10⁻⁵ M ABA blocked all developmental parameters examined (growth and increase in appropriate markers for glyoxysome, peroxisome and plastid development). This blocking can be prevented by an initial treatment with 10⁻⁴ M BA for 2 h. This pretreatment with BA overrides the action of ABA: the final developmental responses are not just restored to the level of the water control, but they are almost as high as those obtained by treating the cotyledons with BA only. If BA is administered for three days together with ABA the reversal of inhibition is much less efficient.     

Ammonium in relation to proline accumulation in detached rice leaves

Ammonium accumulation in relation to prolineaccumulation in detached rice leaves under stressconditions was investigated. Ammonium accumulation indark-treated detached rice leaves preceded prolineaccumulation. Ammonium accumulation caused by waterstress coincided closely with proline accumulation indetached rice leaves. Exogenous NH₄Cl andmethionine sulfoximine (MSO), which caused anaccumulation of ammonium in detached rice leaves,increased proline content. It was found that prolinein NH₄Cl- or MSO-treated rice leaves is lessutilized than in water-treated rice leaves (controls). These results are in agreement with the observationthat a decrease in proline utilization contributes tothe accumulation of proline in dark-treated and waterstressed rice leaves. Although ammonium contentincreased in Cd- and Cu-treated rice leaves, theincrease in ammonium content was only observed afterthe increase in proline content.     

The toxicity induced by phosphinothricin and methionine sulfoximine in rice leaves is mediated through ethylene but not abscisic acid

The possible role of ethylene and abscisic acid (ABA) in regulating thetoxicity of detached rice leaves induced by phosphinothricin (PPT) andmethionine sulfoximine (MSO), both known to be glutamine synthetase (GS)inhibitors, was studied. During 12 h of incubation, PPT and MSOinhibited GS activity, accumulated NH₄ ⁺ and inducedtoxicity of detached rice leaves in the light but not in darkness. PPT and MSOtreatments also resulted in an increase of ethylene production and ABA contentin a light dependent way. Addition of fluridone, an inhibitor of ABAbiosynthesis, reduced ABA content in rice leave but did not preventNH₄ ⁺ toxicity of rice leaves induced by PPT and MSO.Cobalt ion, an inhibitor of ethylene biosynthesis, affected PPT- andMSO-inducedtoxicity of detached rice leaves but had no effect on PPT- and MSO-inducedNH₄ ⁺ accumulation. Results suggest that ethylene but notABA may be responsible for PPT- and MSO-induced toxicity of detached riceleaves.     

Proteomic Characterization of Tissue Expansion of Rice Scutellum Stimulated by Abscisic Acid

We found that appropriate treatment with a highly potent and long-lasting abscisic acid analog enhanced the tissue expansion of scutellum during early seedling development of rice, accompanied by increases of protein and starch accumulation in the tissue. A comparative display of the protein expression patterns in the abscisic acid analog-treated and non-treated tissues on two dimensional gel electrophoretogram indicated that approximately 30% of the scutellar proteins were induced by abscisic acid. The abscisic acid-induced proteins included sucrose metabolizing, glycolytic, and ATP-producing enzymes. Most of these enzyme proteins also increased during the seedling growth. In addition, the expression of some isoforms of UDP-glucose pyrophosphorylase, 3-phosphoglycerate kinase, and mitochondrial ATP synthase beta chain was stimulated in the scutellum, with suppressed expression of α-amylase. We concluded that abscisic acid directly and indirectly stimulates the expression of numerous proteins, including carbohydrate metabolic enzymes, in scutellar tissues.     

Physiology of rice tungro virus disease: Proline accumulation due to infection

Tungro virus infection stimulates proline accumulation in leaves of rice ( Oryza sativa L.), especially in a sensitive cultivar, Taichung Native 1. Disease-induced proline accumulation increases with the severity of the disease. Proline also accumulates in senescing, detached healthy rice leaves. The magnitude of proline accumulation in these leaves was further accentuated by ABA and retarded by kinetin. In the absence of drought stress, virus infection induces severe symptoms (stunting) in a drought tolerant cultivar (Lalnakanda 41) when compared to cultivars with intermediate (MW 10) and high sensitivity (Cauvery) to drought. Thus tungro virus mimics water stress in inducing proline accumulation in rice leaves. In both cases a common factor, ABA, may mediate proline accumulation. In drought stress, proline accumulation is associated with tolerance, while in virus stress, proline accumulation is connected with sensitivity. It is, therefore, clear that proline cannot always act to relieve physiological stress.     

Changes in abscisic acid levels during dormancy release in freesia corms

A high level of free-abscisic acid (ABA) was detected when corms were still in deep dormancy. The level of free-ABA decreased as the corm dormancy disappeared and increased temporarily after complete release from dormancy. A gradual slight increase of bound-ABA was observed during dormancy release.Treatment of dormant corms with benzyladenine (BA) increased sprouting but the sprouts did not show normal growth. Ethylene treatment induced complete sprouting and subsequent normal growth. Changes in ABA levels and ethylene production are discussed in relation to dormancy release in freesia corms.     

Role of abscisic acid in chilling tolerance of rice (Oryza sativa L.) seedlings. I. Endogenous abscisic acid levels

Changes of ABA levels in chilled rice (Oryza sativa L.) seedlings of two varieties were determined. On exposure to chilling, ABA concentration rapidly increased in the chilling-tolerant cultivar (cv. Tainung 67, TNG.67) but not in the chilling-sensitive cultivar (cv. Taichung Native 1, TN.1). Both detached shoots and roots of TNG.67 seedlings showed a significant ABA increase after exposure to chilling. TN.1 seedlings could not accumulate ABA under low temperature but well-watered status. Exogenous application of the ABA biosynthetic inhibitor, fluridone, reduced ABA accumulation, as well as survival ratio of chilled TNG.67 seedlings. Electrolyte leakage and leaf conductance were also increased by the inhibitor and the effects could be reversed by exogenously applied ABA. ABA concentrations in xylem sap of TNG.67 seedlings increased within 4 h after chilling, and this was temporally coincident with the reduction of leaf conductance. The roles of endogenous ABA in the tolerance of rice seedlings to chilling on a whole plant basis are discussed and suggested.