Eli Zamski

Salt stress-induced responses in cucumber plants

Summary A previous study of the formation and removal of reactive oxygen species (ROS) in plants in relation to salt stress gave heterogeneous results that depended upon the mode of plant treatment, the age of the plant tissue and organs, and the experimental setup. This report demonstrates the relationship between ROS scavenging activities and the degree of damage sustained by cucumber plants under salt stress, in a standardized assay system. Cucumber seedlings were grown in a hydroponic solution for 1 to 2 weeks prior to salt treatment. NaCl or KCl was added to the solution at a final concentration of 100 mmol/L, with root exposure to the salt being direct and homogeneous. Plant treatment with NaCl for 4 days resulted in reduced growth, increased leakage of solutes from the leaf tissue and modification of chloroplast structure. NaCl treatment increased the activities of the antioxidative enzymes catalase and glutathione reductase, and the content of the antioxidants ascorbic acid and reduced glutathione, but did not affect the activity of superoxide dismutase (SOD). The distribution of Na + and K + ions in the plant suggests that the salt-derived injuries and the effects on antioxidative systems reflect a response to osmotic stress.

Salicylic acid stimulates secretion of the normally symplastic enzyme mannitol dehydrogenase: a possible defense against mannitol-secreting fungal pathogens

The sugar alcohol mannitol is an important carbohydrate with well-documented roles in both metabolism and osmoprotection in many plants and fungi. In addition to these traditionally recognized roles, mannitol is reported to be an antioxidant and as such may play a role in host–pathogen interactions. Current research suggests that pathogenic fungi can secrete mannitol into the apoplast to suppress reactive oxygen-mediated host defenses. Immunoelectron microscopy, immunoblot, and biochemical data reported here show that the normally symplastic plant enzyme, mannitol dehydrogenase (MTD), is secreted into the apoplast after treatment with the endogenous inducer of plant defense responses salicylic acid (SA). In contrast, a cytoplasmic marker protein, hexokinase, remained cytoplasmic after SA-treatment. Secreted MTD retained activity after export to the apoplast. Given that MTD converts mannitol to the sugar mannose, MTD secretion may be an important component of plant defense against mannitol-secreting fungal pathogens such as Alternaria. After SA treatment, MTD was not detected in the Golgi apparatus, and its SA-induced secretion was resistant to brefeldin A, an inhibitor of Golgi-mediated protein transport. Together with the absence of a known extracellular targeting sequence on the MTD protein, these data suggest that a plant’s response to pathogen challenge may include secretion of selected defensive proteins by as yet uncharacterized, non-Golgi mechanisms.

Immunolocalization of Mannitol Dehydrogenase in Celery Plants and Cells

Immunolocalization of mannitol dehydrogenase (MTD) in celery (Apium graveolens L.) suspension cells and plants showed that MTD is a cytoplasmic enzyme. MTD was found in the meristems of celery root apices, in young expanding leaves, in the vascular cambium, and in the phloem, including sieve-element/companion cell complexes, parenchyma, and in the exuding phloem sap of cut petioles. Suspension cells that were grown in medium with mannitol as the sole carbon source showed a high anti-MTD cross-reaction in the cytoplasm, whereas cells that were grown in sucrose-containing medium showed little or no cross-reaction. Gel-blot analysis of proteins from vascular and nonvascular tissues of mature celery petioles showed a strong anti-MTD sera cross-reactive band, corresponding to the 40-kD molecular mass of MTD in vascular extracts, but no cross-reactive bands in nonvascular extracts. The distribution pattern of MTD within celery plants and in cell cultures that were grown on different carbon sources is consistent w ith the hypothesis that the Mtd gene may be regulated by sugar repression. Additionally, a developmental component may regulate the distribution of MTD within celery plants.

Light, dark and growth regulator involvement in groundnut (Arachis hypogaea L.) pod development

Gynophore elongation and pod formation were studied in peanut plants (Arachis hypogaea L.) under light and dark conditions in vivo. The gynophores elongated until pod formation was initiated. Pod (3–20 mm length) development could be totally controlled by alternating dark (switched on) and light (switched off) conditions, repeatedly. Gynophore elongation responded conversely to light/dark conditions, compared to pods. In this study we aimed to correlate the light/dark effects with endogenous growth substances. The levels of endogenous growth substances were determined in the different stags of pod development. Gynophores shortly after penetration into the soil, ‘white’ gynophores, released twice the amount of ethylene as compared to the aerial green ones, or to gynophores bearing pods. Ethylene inhibitors had no effect on the percent of gynophores that developed pods, but affected pod size which were smaller compared to the control. A similar level of IAA was extracted from gynophore tips of green gynophores, ‘white’ gynophores and pods. ABA levels differed between the three stages and were highest in the green gynophores and lowest in the pods.

Subcellular localization of celery mannitol dehydrogenase. A cytosolic metabolic enzyme in nuclei.

Mannitol dehydrogenase (MTD) is the first enzyme in mannitol catabolism in celery (Apium graveolens L. var dulce [Mill] Pers. Cv Florida 638). Mannitol is an important photoassimilate, as well as providing plants with resistance to salt and osmotic stress. Previous work has shown that expression of the celery Mtd gene is regulated by many factors, such as hexose sugars, salt and osmotic stress, and salicylic acid. Furthermore, MTD is present in cells of sink organs, phloem cells, and mannitol-grown suspension cultures. Immunogold localization and biochemical analyses presented here demonstrate that celery MTD is localized in the cytosol and nuclei. Although the cellular density of MTD varies among different cell types, densities of nuclear and cytosolic MTD in a given cell are approximately equal. Biochemical analyses of nuclear extracts from mannitol-grown cultured cells confirmed that the nuclear-localized MTD is enzymatically active. The function(s) of nuclear-localized MTD is unknown.

Characterization of sucrose phosphate synthase, sucrose synthase and invertase in roots of two nearly isogenic carrot lines that differ in their capacity to accumulate sucrose

Summary Activities of sucrose phosphate synthase (SPS), sucrose synthase (SuSy) and acid invertase (Inv) were assayed in two carrot lines: one with high-sucrose, low-reducing-sugar contents (HS) and the other with low-sucrose, high-reducing-sugar contents (LS). In general, SPS and SuSy exhibited lower activities in the LS vs. HS lines, except for the high SuSy activity in the LS xylem. SPS and SuSy activities were much higher in the phloem than in the xylem of the HS line. The kinetics of SuSy activity, in the degrading direction, were similar in HS and LS lines. K m values were 4–5 mmol/L sucrose and V max values were 23–26 μmol fructose · g FW −1 · h −1 . In the synthesizing direction, K m and V max of SuSy activity were higher in the LS than in the HS line at pH 6.5, but identical at pH 8.0. In the presence of exogenous UDP-glucose, sucrose synthesis by SuSy rose 1.5-fold in the HS line and 4.25-fold in the LS line. Roots of both lines showed similar acid Inv activities. Our results suggest that each carrot line has specific Inv isozymes because they responded differently to increased sucrose concentration and pH. Inv activity of the HS line was induced by increasing substrate concentrations, whereas it decreased in the LS line. Inv activity in the LS line did not change between pHs 4 and 8, whereas in the HS line activity decreased at pHs higher than 6.5.

The role of ooze exudation in the migration of Erwinia amylovora cells in pear trees infected by fire blight

The bacterium Erwinia amylovora causes a severe disease called fire blight in pear trees. In pear trees that were artificially inoculated with E. amylovora cells the bacteria were found neither in the conducting elements of the xylem nor in the phloem (vessels and sieve tubes). They were found only in the intercellular spaces of the parenchyma cells of the bark. Therefore, the bacteria were not migrating through the conducting systems. Exopolysaccharides secreted by the bacteria in the intercellular spaces of the parenchyma cells decreased the water potential in the spaces, resulting in water and solute leakage from the living cells into the spaces. This led to cell plasmolysis and disintegration. We suggest that when ooze (the bacteria, materials secreted by the bacteria, water, and solutes leaked from the host cells) is accumulated in the intercellular spaces, it drives the air outside, blocks the oxygen supply, and inflicts additional stress on the dying cells. The outcome of the accumulated ooze in th...

The Relationship between Pyrophosphatase and Branching Enzyme Activity with Amyloplast Size in Maize Endosperm

Summary This study aimed to clarify the relationship between amyloplast size (age) and activities of alkaline pyrophosphatase (PPase) and branching enzyme (BE) within developing Zea mays L. endosperm cells. PPase and BE activities per starch granule were increased when the organelle grew in size, although decreased when calculated per granule surface area. The amyloplast specific PPase and the stromal marker enzyme BE had identical distribution (percent of total activity) in the stroma. The amount of free Pi increased in the stroma with the increased granule size up to 8 μ in diameter (about one third of the final size) and then remained steady. The maximum Pi content at this granule size was 5.2 mmol per 10 6 amyloplasts (about 5.5 fmol per granule) and PPase activity released about 1 nmol min -1 per 10 6 amyloplasts. Attempts to detect endogenous PPi in amyloplasts were unsuccessful.