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.     

Galactose inhibition of auxin-induced cell elongation in oat coleoptile segments

Auxin-induced cell elongation in oat coleoptile segments was inhibited by galactose; removal of galactose restored growth. Galactose did not appear to affect the following factors which modify cell elongation: auxin uptake, auxin metabolism, osmotic concentration of cell sap, uptake of tritium-labeled water, auxin-induced wall loosening as measured by a decrease in the minimum stress-relaxation time and auxininduced glucan degradation. Galactose markedly prevented incorporation of [¹⁴C]-glucose into cellulosic and non-cellulosic fractions of the cell wall. It was concluded that galactose inhibited auxin-induced long-term elongation of oat coleoptile segments by interfering with cell wall synthesis.     

Variation in stomatal aperture in leaves of Avena fatua L. observed by low-temperature scanning electron microscopy

Abstract. A novel technique to record the variability of stomatal aperture over the leaf surface is described. This combines observations of leaf surfaces using low-temperature scanning electron microscopy (LTSEM), with digital image analysis to produce the most accurate aperture measurements obtained to date. Leaf samples are rapidly immobilized by cryo-fixation in liquid nitrogen and stored in a purpose-built cryo-storage system. Specimens can be collected in the field, remote from the cryopreparation system, and stored for up to several weeks before being examined on the LTSEM. The advantages of this method are that the time frame of the measurements is accurately known, and is identical for all stomatal apertures in a sample, and the precision of the measurements is not limited by the resolving power of the microscope. Measurements of stomatal aperture were obtained from leaves of field grown Avena fatua using the above procedure. Leaf surface conductance (g_sur) was determined by porometry immediately before cryo-fixation of the same region of the leaf. Measurements of aperture size showed a high degree of variability within each specimen, with coefficients of variation similar to those found in previous studies. Stomatal conductance (g_s) was calculated from stomatal dimensions using formulae derived elsewhere. A linear regression between the computed values of g_s and porometric estimates of g_sur showed good agreement with the regression line passing through the origin with a slope of 1.0 (R²=0.96). Applications of the experimental system are discussed.     

Identification of C-banded chromosomes in meiosis and the analysis of nucleolar activity in Avena byzantina C. Koch cv ‘Kanota’

Summary The Giemsa C-banding technique was used to identify individual meiotic and somatic chromosomes in 21 monosomic lines of Avena byzantina C. Koch cv Kanota (genome designation AACCDD). The hexaploid complement is composed of three sets of seven chromosome pairs. The heterochromatin in the putative diploid progenitors is located at the telomeres (genome A), at the centromeric and interstitial regions (genome C), or more evenly spread throughout the set (genome D). Comparisons based on C-banding between A. byzantina and its diploid progenitor species allowed us to allocate individual chromosomes into specific genomes. The C-banding technique may be useful for interspecific chromosome pairing analyses. Nucleolar activity and competition were studied using a silver-staining procedure. Only three chromosome pairs showed nucleolar organizer regions, thus indicating that nucleolar competition occurs naturally in hexaploid oats.     

Comparative evaluation of iron solubilizing substances (phytosiderophores) released by oats and corn: Iron-efficient and iron-inefficient plants

Release of phytosiderophores from barley (Hordeum vulgare L.) in response to Fe-deficiency stress prompted further testing of other graminaceous (grass) species for phytosiderophore release and results have prompted characterization of these plants into a Strategy II designation. This classification denotes an enhanced release of phytosiderophore in response to Fe-deficiency stress with a concomitant uptake of Fe by the plant. The objective of this study was to determine if Fe-inefficient and Fe-efficient corn (Zea mays L.) differ in their release of Fe solubilizing substances in response to Fe-deficiency stress. We have not identified the specific structure of these substances but refer to them as phytosiderophores to further characterize their behavior. By our indirect method, there was no measurable release of Fe solubilizing substances (phytosiderophores) from either the Fe-efficient WF9 or the Fe-inefficient ys₁ corn despite WF9 being greener and apparently more Fe efficient than ys₁. Fe-efficient Coker 227 oats (Avena byzantina C. Koch.) has been found to release a phytosiderophore whereas the Fe-inefficient TAM 0-312 does not. Iron-stressed Coker 227 oats released Fe solubilizing substances when grown in the same solution with WF9 corn which resulted in greening and Fe uptake by WF9 corn. Iron efficiency in these two graminaceous species appears to be controlled by different mechanisms.     

Binding of 124-kilodalton oat phytochrome to liposomes and chloroplasts

Binding of the radio-iodinated 124-kDa oat ( Avena sativa L. cv. Garry) phytochrome to liposomes and chloroplasts was investigated as a model system in order to understand the molecular affinity of phytochrome toward cellular organelles in plants. The binding of intact (124 kDa) phytochrome to liposomes and chloroplasts is hydrophobic in nature, as in the case of the degraded (118/114 kDa) phytochrome, but electrostatic interactions play a greater role in the intact phytochrome. The physiologically active P_fr form of the intact phytochrome showed a binding preference over the inactive P_r form with neutral liposomes and chloroplasts. However, the P_fr form of intact phytochrome exhibits smaller binding preference than the P_fr form of degraded phytochrome over their respective P_r forms (see Kim, I.-S. and Song, P.-S. 1981, Biochemistry 20: 5482–5489, for degraded phytochrome binding). These results indicate that the 6/10 kDa N-terminus segment, which is lost in the degraded phytochrome, plays an important role in determining the protein surface properties of the intact phytochrome. A competitive binding study on phytochrome also suggested that the P_fr form had a greater binding affinity for chloroplasts than the P_r form. However, the physiological activity of the P_fr form may not be explained simply by the observed difference in binding affinity between the two forms of phytochrome.     

Photoreversible association of phytochrome with membranes. I. Distinguishing between two light-induced binding responses

Abstract Two types of association between phytochrome and crude membrane fractions from oat (Avena sativa L.) are distinguished and compared, and that which comprises only a small fraction of the total phytochrome in extracts prepared in the absence of added divalent cations (Watson & Smith. 1982b) has been studied in detail. Extraction in the presence of phenylmethylsulphonyl fluoride shows that proteolysis of P_r (the red-light absorbing form) probably does not account for the lower levels of membrane-associated phytochrome measured after far-red light than after red light. Difference spectra of soluble and membrane-associated phytochrome indicate that the latter is much less susceptible to spectral degradation in vitro than is the soluble pool. The stoichiometry of association with the membranes is such that for each phytochrome molecule associated after far-red light there are three associated after red light and it is argued that this stoichiometry is maintained independent of the extraction pH. The characteristics of this photo-reversible association of phytochrome with membranes are compared to the characteristics of the widely studied light-induced enhancement of phytochrome pelletabilily that is dependent on electrostatic interaction of phytochrome and membranes.