Lemna sulfate uptake

Inoiganic sulfate is the principal form of sulfur acquired by plants. Sulfate uptake has been studied mostly in detached roots, cultured tobacco cells, and in Lemna. It should be noted that in detached roots and root tissues, very little sulfate is incorporated into oiganic sulfur compounds whereas in Lemna, a complete photosynthetic organism, a large proportion of the sulfate taken up is rapidly assimilated. 

Fig. 2. Efifect of growii Lemna at various concentrations of sulfate on the short-term sulfate uptake rate. Uptake was measured at low concentrations of sulfate (2 and 20 fiM) to ascertain the rate of the high-affinity (low K ) uptake system and at a high concentration of sulffite (25 mM) from which the nonsaturating uptake component was calculated. (From Datko and Mudd, 1984a.)...

Most authors continue to report complex uptake kinetics for sulfate (Anderson, 1980 Datko and Mudd, 1984a Deane-Drummond, 1987 Lass and Ullrich-Eberius, 1984 Nissen and Nissen, 1983) although Jones and Smith (1981) reported that the uptake mechanism of cultured tobacco cells exhibits monophasic kinetics with a of 20 nM. However, as discussed below, the measurements made in the latter study might reflect uptake of sulfate into the vacuole and incorporation into organic sulfur compounds rather than influx across the plasma membrane. Lass and Ullrich-Eberius (1984) reported that the uptake kinetics in Lemna can be described by two Michaelis-Menten terms... 

Although the uptake of sulfate in Lemna has been studied extensively (Datko etal., 1978 Datko and Mudd, 1984a,b Thoironeta/., 1981), the distribution of newly acquired sulfur in the various tissues of this aquatic plant has not been investigated. It is therefore not possible to assess whether the uptake and distribution of sulfate occurs in two stages, as appears to be the case in terrestrial plants, or whether only one mechanism is involved. 

The level of sulfotransferase in Lemna and in Phaseolus vulgaris is also subject to strong inhibition by gaseousH2S(Brunoldand Schmidt, 1976,1978 Wyss and Brunold, 1979). However, the extractable acti vity of cysteine synthase is not similarly affected. Removal of H2S firom the gas phase results in rapid restoration of activity which, based on a study of labeling of the enzyme (von Arb and Brunold, 1980), was attributed to synthesis ofthe enzyme de novo. HjS also inhibits the level of APS sulfotransferase in cell suspension cultures of Nicotiana sylvestris in this tissue neither the ATP-sulfiirylase or cysteine synthase activity was affected by H2S or cysteine (Brunold etal., 9Sl). Importantly, the inhibition of APS sulfotransferase by H2S was correlated with an enhanced level of cysteine, suggesting that the H2S inhibition could have been mediated via this reaction product. Uptake of exogenous sulfate was also inhibited by H2S in this system (Brunold et al., 1981). 

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