The storage and mobilization of nutrients in wood and bark tissues is a typical feature of trees. Sulfur can be stored as sulfate, which is transported from source to sink tissues through the phloem. In the present study two transcripts encoding sulfate transporters (SULTR) were identified in the phloem of grey poplar (Populus tremula x P. alba). Their cell-specific expression was analyzed throughout poplar in source tissues, such as mature leaves, and in sink tissues, such as the wood and bark of the stem, roots and the shoot apex. PtaSULTR1;1 mRNA was detected in companion cells of the transport phloem, in the phloem of high-order leaf veins and in fine roots. PtaSULTR3;3a mRNA was found exclusively in the transport phloem and here in both, companion cells and sieve elements. Both sulfate transporter transcripts were located in xylem parenchyma cells indicating a role for PtaSULTR1;1 and PtaSULTR3;3a in xylem unloading. Changes in mRNA abundance of these and of the sulfate transporters PtaSULTR4;1 and PtaSULTR4;2 were analyzed over an entire growing season. The expression of PtaSULTR3;3a and of the putative vacuolar efflux transporter PtaSULTR4;2 correlated negatively with the sulfate content in the bark. Furthermore, the expression pattern of both PtaSULTR3;3a and PtaSULTR4;2 correlated significantly with temperature and day length. Thus both SULTRs seem to be involved in mobilization of sulfate during spring: PtaSULTR4;2 mediating efflux from the vacuole and PtaSULTR3;3a mediating loading into the transport phloem. In contrast, the abundance of PtaSULTR1;1 and PtaSULTR4;1 transcripts was not affected by environmental changes throughout the whole season. The transcript abundance of all tested sulfate transporters in leaves was independent of weather conditions. However, PtaSULTR1;1 abundance correlated negatively with sulfate content in leaves, supporting its function in phloem loading. Taken together, these findings indicate a transcriptional regulation of sulfate distribution in poplar trees.

We compared three transgenic poplar lines over-expressing the bacterial g-glutamylcysteine synthetase (GSH1) targeted to plastids. Lines Lggs6 and Lggs12 have two copies, while line Lggs20 has three copies of the transgene. The three lines differ in their expression levels of the transgene and in the accumulation of g-glutamylcysteine (g-EC) and glutathione (GSH) in leaves, roots and phloem exudates. The lowest transgene expression level was observed in line Lggs6 which showed an increased growth, an enhanced rate of photosynthesis and a decreased excitation pressure (1-qP). The latter typically represents a lower reduction state of the plastoquinone pool, and thereby facilitates electron flow along the electron transport chain. Line Lggs12 showed the highest transgene expression level, highest g-EC accumulation in leaves and highest GSH enrichment in phloem exudates and roots. This line also exhibited a reduced growth, and after a prolonged growth of 4.5 months, symptoms of leaf injury. Decreased maximum quantum yield (Fv/Fm) indicated down-regulation of photosystem II reaction centre (PSII RC), which correlates with decreased PSII RC protein D1 (PsbA) and diminished light-harvesting complex (Lhcb1). Potential effects of changes in chloroplastic and cytosolic GSH contents on photosynthesis, growth and the whole-plant sulphur nutrition are discussed for each line.

The investigations reported herein were performed to test whether anoxic soil conditions changed the sink strength of the roots for reduced sulphur and therefore transport of sulphur in the phloem, and whether increasing sulphide contents in anoxic soils due to bacteria activity causes changes in sulphate assimilation of the roots. Anoxic conditions caused by flooding of the flooding-tolerant tree species Populus tremula ¥ ¥ ¥ ¥ P. alba led not only to changes in carbon metabolism, but also influenced sulphate assimilation. In the roots, activity and transcript of adenosine 5 ¢ ¢ ¢ ¢ -phosphosulphate (APS) reductase, the key enzyme of the sulphate assimilation pathway, completely disappeared, but the Cys content increased 6.7 times after 15 d of flooding. A higher glutathione content of phloem exudates after 3 and 7 d of flooding indicates an increased transport of reduced sulphur to the roots. However, the export rate of 35 S-sulphur out of mature leaves after flap feeding 35 Ssulphate was not different between flooded and control poplars. Since the allocation to the roots was diminished, enhanced phloem contents of GSH indicate a diminished sink strength of the roots rather than an enhanced sink strength. Therefore, a higher transport of GSH in the phloem could not be responsible for the higher Cys content in the roots. As protein contents were unaffected the higher Cys content in roots could also not originate from protein breakdown. Enhanced O -acetylserine (thiol) lyase (OASTL) activity were measured in the roots demonstrating an uncoupling of sulphate reduction and Cys synthesis. A possible contribution in detoxification of sulphide produced in the anoxic soil is discussed.

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