Storage and remobilisation of sulphur in beech trees (Fagus sylvatica)

Three-year-old beech trees were fed ³⁵S-sulphate in August 1993 via a flap in a mature leaf of an upper branch. Harvest of beech trees was performed 24 h after feeding ³⁵S-sulphate, before leaf senescence, after leaf abscission, in early winter (January 1994). in late winter (March 1994). before bud break and after bud break. Twenty-four h after feeding ³⁵S-sulphate, 0.7 ± 0.5% of the ³⁵S-radioactivity taken up was exported out of the fed leaf. When trees were analysed 2 months later, i.e., before leaf senescence, this value had increased to 22 ± 7%. The exported ³⁵S-radioactivity was located in the branch containing the fed leaf (2.8 ± 13%). in basipetal parts of the trunk (41 ± 77%) and in the main rool (21 ± 6%). Leaves and apical parts of the trunk were no sink organs for the exported sulphur. Along the tree axis the main proportion of the radiolabel was located in the wood, predominantly in the acid soluble fraction. In the bark the greater portion of the radiolabel was found in the acid insoluble fraction. In both tissues the bulk of the ³⁵S of the soluble fraction was sulphate together with small amounts of glutathione. This pattern did not change until bud break. After bud break, basipetal parts of the trunk lost part of its ³⁵S-radioactivity. Of the ³⁵S-radioactivity which had been exported out of the fed leaf during the previous autumn, 16 ± 2% remained in the trunk, whereas 47 ± 7% of the ³⁵S was found in branches, mainly in the newly developed leaves. The present results show that sulphur, mainly in the form of sulphate, is stored along the tree axis in both bark and wood of beech trees and is re-mobilised during leaf development in spring.     

Net Uptake of Sulfate and its Transport to the Shoot in Spinach Plants Fumigated with H2S or SO2: Does Atmospheric Sulfur Affect the “Inter-Organ” Regulation of Sulfur Nutrition?*

Spinach plants (Spinacea oleracea L. cv. Estivato) were grown on nutrient solutions under deficient, normal and excess sulfate supply. In both young and mature plants net uptake of sulfate and its transport to the shoot increased with increasing sulfate supply, but both processes proceeded at a higher rate in young as compared to mature plants. The relative sulfate transport, i.e. the relative amount of the sulfate taken up that is transported to the shoot, decreased with increasing sulfate supply. Apparently, net uptake of sulfate is not strictly controlled by the sulfur demand of the shoot, but xylem loading appears to counteract excess transport of sulfate to the shoot. Fumigation with H₂S or SO₂ reduced net uptake of sulfate by the roots in sulfur-deficient plants and absolute as well as relative sulfate transport to the shoot independent of the three sulfate levels supplied to the plant. At the same time thiol contents of the shoot and the root were enhanced by fumigation with H₂S and SO₂. These findings are consistent with the idea that thiols produced in the leaves can mediate demand-driven control of sulfate uptake by the roots and its transport to the shoot.     

Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

Overexpression of {gamma}-Glutamylcysteine Synthetase, but not of Glutathione Synthetase, Elevates Glutathione Allocation in the Phloem of Transgenic Poplar Trees

Phloem exudates were collected along the tree axis at differentheights of wild type and transgenic poplar. Over-expressionof bacterial     

Sulphate uptake and xylem loading of young pea (Pisum sativum L.) seedlings

Sulphate uptake and xylem loading was analysed in young pea (Pisum sativum) seedlings. The rate of sulphate uptake into intact 8-days-old pea seedlings (determined by a 1 h exposure to radiolabelled sulphate in the nutrient solution) was 585 nmol sulphate g^–1 root fresh weight h^–1. When the cotyledons were removed on day 6 the 8-days-old seedlings took up only 7% of the controls. Interruption of the phloem transport by steam girdling of the stem or the root (1 h before incubation with radiolabelled sulphate) diminished sulphate uptake by approximately 50%. The addition of sucrose to the nutrient solution during incubation did not restore sulphate uptake rates indicating that the decrease was not due to a lack of energy. Apparently, a signal from the shoot and/or the cotyledons is necessary to stimulate sulphate uptake into the roots of pea seedlings. Glutathione fed to the roots for 3 h prior to incubation with radiolabelled sulphate diminished sulphate uptake by approximately 50%. The relative proportion of the sulphate taken up that was loaded into the xylem remained unchanged (between 7 and 9% of total uptake), even when the stem was girdled above the cotyledons or when the seedlings were pre-exposed to glutathione. Only removal of the cotyledons or girdling of the root below the cotyledons increased the proportion of sulphate loaded into the xylem to 13–15% of total uptake upon exposure to glutathione. Apparently, a signal from the cotyledons represses xylem loading to some extent.     

S-methylmethionine plays a major role in phloem sulfur transport and is synthesized by a novel type of methyltransferase.

All flowering plants produce S-methylmethionine (SMM) from Met and have a separate mechanism to convert SMM back to Met. The functions of SMM and the reasons for its interconversion with Met are not known. In this study, by using the aphid stylet collection method together with mass spectral and radiolabeling analyses, we established that l-SMM is a major constituent of the phloem sap moving to wheat ears. The SMM level in the phloem ( approximately 2% of free amino acids) was 1.5-fold that of glutathione, indicating that SMM could contribute approximately half the sulfur needed for grain protein synthesis. Similarly, l-SMM was a prominently labeled product in phloem exudates obtained by EDTA treatment of detached leaves from plants of the Poaceae, Fabaceae, Asteraceae, Brassicaceae, and Cucurbitaceae that were given l-(35)S-Met. cDNA clones for the enzyme that catalyzes SMM synthesis (S-adenosylMet:Met S-methyltransferase; EC 2.1.1.12) were isolated from Wollastonia biflora, maize, and Arabidopsis. The deduced amino acid sequences revealed the expected methyltransferase domain ( approximately 300 residues at the N terminus), plus an 800-residue C-terminal region sharing significant similarity with aminotransferases and other pyridoxal 5'-phosphate-dependent enzymes. These results indicate that SMM has a previously unrecognized but often major role in sulfur transport in flowering plants and that evolution of SMM synthesis in this group involved a gene fusion event. The resulting bipartite enzyme is unlike any other known methyltransferase.     

Oxidation and reduction of sulfite contribute to susceptibility and detoxification of SO2 in Populus × canescens leaves

Key Message

The critical level for SO ₂ susceptibility of Populus × canescens is approximately 1.2 μL L ^?1 SO ₂ . Both sulfite oxidation and sulfite reduction and assimilation contribute to SO ₂ detoxification.

Abstract

In the present study, uptake, susceptibility and metabolism of SO₂ were analyzed in the deciduous tree species poplar (Populus × canescens). A particular focus was on the significance of sulfite oxidase (SO) for sulfite detoxification, as SO has been characterized as a safety valve for SO₂ detoxification in herbaceous plants. For this purpose, poplar plants were exposed to different levels of SO₂ (0.65, 0.8, 1.0, 1.2 μL L^?1) and were characterized by visible injuries and at the physiological level. Gas exchange parameters (stomatal conductance for water vapor, CO₂ assimilation, SO₂ uptake) of the shoots were compared with metabolite levels (sulfate, thiols) and enzyme activities [SO, adenosine 5′-phosphosulfate reductase (APR)] in expanding leaves (80–90 % expanded). The critical dosage of SO₂ that confers injury to the leaves was 1.2 μL L^?1 SO₂. The observed increase in sulfur containing compounds (sulfate and thiols) in the expanding leaves strongly correlated with total SO₂ uptake of the plant shoot, whereas SO₂ uptake rate was strongly correlated with stomatal conductance for water vapor. Furthermore, exposure to high concentration of SO₂ revealed channeling of sulfite through assimilatory sulfate reduction that contributes in addition to SO-mediated sulfite oxidation to sulfite detoxification in expanding leaves of this woody plant species.     

Long-distance transport of 35S-sulphur in 3-year-old beech trees (Fagus sylvatica)

³⁵S-L-cysteine was fed to a mature leaf of 3-year-old beech trees via a flap. After 1 to 4 h the distribution of ³⁵S-radioactivity was analysed in the leaves as well as the bark and wood of the trunk and the main root. Transport of ³⁵S out of the fed leaf amounted to 0.3–1.2% of the total ³⁵S taken up. The branches of the trees did not act as sink organs for the exported radioactivity. The main portion of the ³⁵S-radioactivity transported out of the fed leaf was found in basipetal parts of the trunk. Only a small portion of ³⁵S-radioactivity was transported in acropetal direction. The distribution of the ³⁵S-radioactivity within the trunk showed a higher portion of ³⁵S in the bark than in the wood. In both tissues, bark (70 to 80%) and wood (60 to 70%), the ³⁵S was predominantly found in the HCl soluble fraction. However, ³⁵S-cysteine, the compound fed to the leaves was not exported out of the fed leaf. Along the trunk ³⁵S-cysteine was neither determined in bark nor in wood sections. The only low molecular mass S-compounds found was ³⁵S-glutathione (GSH). The ³⁵S-sulphate detected in bark and wood origined from cysteine oxidation in the leaf tissue and from contamination of the ³⁵S-cysteine feeding solution. The ratio of GSH to sulphate decreased with increasing distance from the fed leaf. Apparently, ³⁵S-radioactivity was transported as sulphate and GSH in the phloem in basipetal direction, but GSH was removed preferentially out of the phloem along the transport path. ³⁵S-radioactivity exported out of the phloem and transported into the wood of the trunk was not retranslocated in the xylem. It may therefore be assumed that part of the ³⁵S translocated was stored in ray cells, medullary sheath cells and/or pith parenchyma cells. Girdling experiments in which the bark of the trunk was peeled off basipetal to the branch containing the fed leaf support these assumptions.