Kurt D. Nolte

Sugar Levels Modulate Differential Expression of Maize Sucrose Synthase Genes.

The two genes encoding sucrose synthase in maize (Sh1 and Sus1) show markedly different responses to changes in tissue carbohydrate status. This enzyme is widely regarded as pivotal to sucrose partitioning, import, and/or metabolism by developing plant organs. Excised maize root tips were incubated for varying periods in different sugars and a range of concentrations. The Sh1 mRNA was maximally expressed under conditions of limited carbohydrate supply (~0.2% glucose). In contrast, Sus1 transcript levels were low or nondetectable under sugar-depleted conditions and peaked at 10-fold greater glucose concentrations (2.0%). Responses to other metabolizable sugars were similar, but L-glucose and elevation of osmolarity with mannitol had little effect. Plentiful sugar supplies thus increased expression of Sus1, whereas reduced sugar availability enhanced Sh1. At the protein level, shifts in abundance of subunits encoded by Sh1 and Sus1 were much less pronounced but corresponded to changes in respective mRNA levels. Although total enzyme activity did not show net change, cellular localization of sucrose synthase protein was markedly altered. In intact roots, sucrose synthase was most prevalent in the stele and apex. In contrast, sugar depletion favored accumulation in peripheral cells, whereas high sugar levels resulted in elevated expression in all cell types. The differential response of the two sucrose synthase genes to sugars provides a potential mechanism for altering the pattern of enzyme distribution in response to changing carbohydrate status and also for adjusting the sucrose-metabolizing capacity of importing cells relative to levels of available photosynthetic products.

Companion-Cell Specific Localization of Sucrose Synthase in Zones of Phloem Loading and Unloading

An immunohistochemical approach was used in maize (Zea mays) and citrus (Citrus paradisi) to address the previously noted association between sucrose synthase and vascular bundles and to determine the localization of the low but detectable levels of sucrose synthase that remain in leaves after the import-export transition. Sucrose synthase protein was immunolocalized at the light microscope level using paraffin sections reacted with rabbit sucrose synthase polyclonal antisera and gold-conjugated goat anti-rabbit immunoglobulin G. Immunolabel was specifically observed in phloem companion cells of minor and intermediate veins in mature leaves of both species. Similar localization was apparent in the midrib of mature citrus leaves, with additional labeling in selected files of phloem parenchyma cells. A clear companion-cell specificity was evident in the phloem unloading zone of citrus fruit, where high activity of sucrose synthase has been demonstrated in vascular bundles during periods of rapid import. Sucrose synthase protein was not associated with adjacent cells surrounding the vascular strands in this tissue. The companion-cell specificity of sucrose synthase in phloem of both importing and exporting structures of these diverse species implies that this may be a widespread association and underscores its potential importance to the physiology of vascular bundles.

A new route for synthesis of dimethylsulphoniopropionate in marine algae.

The 3-dimethylsulphoniopropionate (DMSP) produced by marine algae is the main biogenic precursor of atmospheric dimethylsulphide (DMS). This biogenic DMS, formed by bacterial and algal degradation of DMSP,, contributes about 1.5 × 1013 g of sulphur to the atmosphere annually, and plays a major part in the global sulphur cycle, in cloud formation and potentially in climate regulation,. Although DMSP biosynthesis has been partially elucidated in a higher plant,, nothing is known about how algae make DMSP except that the whole molecule is derived from methionine. Here we use in vivo isotope labelling to demonstrate that DMSP synthesis in the green macroalga Enteromorpha intestinalis proceeds by a route entirely distinct from that in higher plants. From methionine, the steps are transamination, reduction and S -methylation to give the novel sulphonium compound 4-dimethylsulphonio-2-hydroxybutyrate (DMSHB), which is oxidatively decarboxylated to DMSP. The key intermediate DMSHB was also identified in three diverse phytoplankton species, indicating that the same pathway operates in other algal classes that are important sources of DMS. The fact that a transamination initiates this pathway could help explain how algal DMSP (and thereby DMS) production is enhanced by nitrogen deficiency.

Purification and Properties of S-Adenosyl-L-methionine: L-Methionine S-Methyltransferase from Wollastonia biflora Leaves

The plant enzyme S-adenosylmethionine:methionine S-methyltransferase (EC 2.1.1.12, MMT) catalyzes the synthesis of S-methylmethionine. MMT was purified 620-fold to apparent homogeneity from leaves of Wollastonia biflora. The four-step purification included fractionation with polyethylene glycol, affinity chromatography on adenosine-agarose, anion exchange chromatography, and gel filtration. Protein yield was about 180 μg/kg of leaves. Estimates of molecular mass from sodium dodecyl sulfate-polyacrylamide gel electrophoresis and native gel filtration chromatography were, respectively, 115 and 450 kDa, suggesting a tetramer of 115-kDa subunits. The 115-kDa subunit was photoaffinity labeled by S-adenosyl[H]methionine. Antibodies raised against W. biflora MMT recognized a 115-kDa polypeptide in partially purified MMT preparations from leaves of lettuce, cabbage, clover, and maize. The pH optimum of W. biflora MMT was 7.2. Kinetic analysis of substrate interaction and product inhibition patterns indicated an Ordered Bi Bi mechanism, with S-adenosylmethionine the first reactant to bind and S-adenosylhomocysteine the last product to be released. The enzyme catalyzed methylation of selenomethionine and ethionine, but not of S-methylcysteine, homocysteine, cysteine, or peptidylmethionine. Tests with other substrate analogs indicated that a free carboxyl group was required for enzyme activity, and that a free amino group was not.

Evidence That the Pathway of Dimethylsulfoniopropionate Biosynthesis Begins in the Cytosol and Ends in the Chloroplast

In the flowering plant Wollastonia biflora (L.) DC. the first step in 3-dimethylsulfoniopropionate (DMSP) synthesis is conversion of methionine to S-methylmethionine (SMM) and the last is oxidation of 3-dimethylsulfoniopropionaldehyde (DMSP-ald) (F. James, L. Paquet, S.A. Sparace, D.A. Gage, A.D. Hanson [1995] Plant Physiol 108: 1439–1448). DMSP-ald was shown to undergo rapid, spontaneous decomposition to dimethylsulfide and acrolein. However, it was stable enough (half-life [greater than or equal to] 1 h) in tertiary amine buffers to use as a substrate for enzyme assays. A dehydrogenase catalyzing DMSP-ald oxidation was detected in extracts of W. biflora mesophyll protoplasts. This enzyme had a high affinity for DMSP-ald (Km = 1.5 [mu]M), was subject to substrate inhibition, preferred NAD to NADP, and was immunologically related to plant betaine aldehyde dehydrogenases. After fractionation of protoplast lysates, [greater than or equal to]90% of DMSP-ald dehydrogenase activity was recovered from the chloroplast stromal fraction, whereas the enzyme that mediates SMM synthesis, S-adenosylmethionine:methionine S-methyltransferase, was found exclusively in the cytosolic fraction. Immunohistochemical analysis confirmed that the S-methyltransferase was cytosolic. Intact W. biflora chloroplasts were able to metabolize supplied [35S]SMM to [35S]DMSP. These findings indicate that SMM is made in the cytosol, imported into the chloroplast, and there converted successively to DMSP-ald and DMSP.

Sucrose Synthase Localization during Initiation of Seed Development and Trichome Differentiation in Cotton Ovules

Sucrose synthase in cotton (Gossypium hirsutum L.) ovules was immunolocalized to clarify the relationship between this enzyme and (a) sucrose import/utilization during initiation of seed development, (b) trichome differentiation, and (c) cell-wall biosynthesis in these rapidly elongating “fibers.” Analyses focused on the period immediately before and after trichome initiation (at pollination). Internal tissues most heavily immunolabeled were the developing nucellus, adjacent integument (inner surface), and the vascular region. Little sucrose synthase was associated with the outermost epidermis on the day preceding pollination. However, 1 d later, immunolabel appeared specifically in those epidermal cells at the earliest visible phase of trichome differentiation. The day following pollination, these cells had elongated 3- to 5-fold and showed a further enhancement of sucrose synthase immunolabel. Levels of sucrose synthase mRNA also increased during this period, regardless of whether pollination per se had occurred. Timing of onset for the cell-specific localization of sucrose synthase in young seeds and trichome initials indicates a close association between this enzyme and sucrose import at a cellular level, as well as a potentially integral role in cell-wall biosynthesis.