As sulfur constitutes one of the macronutrients necessary for the plant lif
e cycle, sulfur uptake and assimilation in higher plants is one of the cruc
ial factors determining plant growth and vigour, crop yield and even resist
ance to pests and stresses. Inorganic sulfate is mostly taken up as sulfate
from the soil through the root system or to a lesser extent as volatile su
lfur compounds from the air. In a cascade of enzymatic steps inorganic sulf
ur is converted to the nutritionally important sulfur-containing amino acid
s cysteine and methionine (Hell, 1997; Hell and Rennenberg, 1998; Saito, 19
99). Sulfate uptake and allocation between plant organs or within the cell
is mediated by specific transporters localised in plant membranes. Several
functionally different sulfate transporters have to be postulated and have
been already cloned from a number of plant species (Clarkson et al., 1993;
Hawkesford and Smith, 1997; Takahashi et al., 1997; Yamaguchi, 1997). Follo
wing import into the plant and transport to the final site of reduction, th
e plastid, the chemically relatively inert sulfate molecule is activated th
rough binding to ATP forming adenosine-5 ' -phosphosulfate (APS). This enzy
matic step is controlled through the enzyme ATP-sulfurylase (ATP-S). APS ca
n be further phosphorylated to form 3 ' -phosphoadenosine-5 ' -phosphosulfa
te (PAPS) which serves as sulfate donor for the formation of sulfate esters
such as the biosynthesis of sulfolipids (Schmidt and Jager, 1992). However
, most of the APS is reduced to sulfide through the enzymes APS-reductase (
APR) and sulfite reductase (SIR). The carbon backbone of cysteine is provid
ed through serine, thus directly coupling photosynthetic processes and nitr
ogen metabolism to sulfur assimilation. L-serine is activated by serine ace
tyltransferase (SAT) through the transfer to an acetyl-group from acetyl co
enzyme A to form O-acetyl-L-serine (OAS) which is then sulhydrylated using
sulfide through the enzyme O-acetyl-L-serine thiol lyase (OAS-TL) forming c
ysteine. Cysteine is the central precursor of all organic molecules contain
ing reduced sulfur ranging from the amino acid methionine to peptides as gl
utathione or phytochelatines, proteines, vitamines, cofactors as SAM and ho
rmones. Cysteine and derived metabolites display essential roles within pla
nt metabolism such as protein stabilisation through disulfide bridges, stre
ss tolerance to active oxygen species and metals, cofactors for enzymatic r
eactions as e.g. SAM as major methylgroup donor and plant development and s
ignalling through the volatile hormone ethylene. Cysteine and other metabol
ites carrying free sulfhydryl groups are commonly termed thioles (confer Pi
g. 1). The physiological control of the sulfate reduction pathway in higher
plants is still not completely understood in all details. The objective of
this paper is to summarise the available data on the molecular analysis an
d control of cysteine biosynthesis in plants, and to discuss potentials for
manipulating the pathway using transgenic approaches.