Download Abstract - Plant Sulfur Network

INFLUENCE OF SHORT TERM SULFUR STARVATION ON
PHOTOSYNTHESIS-RELATED COMPOUNDS AND PROCESSES
IN TOBACCO1
Małgorzata Lewandowska1, Agnieszka Bajda2, Ewa Swiezewska2 and Agnieszka
Sirko1
1
Department of Plant Biochemistry, Institute of Biochemistry and Biophysics PAS,
Pawinskiego 5A, 02-106 Warsaw, Poland; 2Department of Lipid Biochemistry, Institute
of Biochemistry and Biophysics PAS, Pawinskiego 5A, 02-106 Warsaw, Poland
Abstract
Sulfur metabolism is an important component of the general metabolism of plants. The
reductive assimilation pathway of sulfate is connected with availability of
photosynthetic electrons and carbohydrates, which fluctuates diurnally (Kopriva and
Koprivova 2003). On the other hand photosynthesis is affected by S starvation. Evident
proof for mutual dependence is chlorosis occurring in the leaves of S-starved plants,
producing insufficient amount of chlorophyll and lipids, what in turn yields in reduction
of photosynthetic activity. S-deprivation results in decreased amount of SAM, necessary
for methylation during chlorophyll biosynthesis (Nikiforova et al. 2003, 2005, 2006).
Chloroplasts are characterized by the presence of high amount of galactolipids, reaching
about 80% of total lipids in thylakoids. Another lipid component of thylakoid
membranes are phospholipids and sulfolipids (Hölzl and Dörmann 2007). Anionic
thylakoid lipids, phosphatidylglycerol (PG) and sulfoquinovosyldiacylglycerol (SQDG),
are limiting for chloroplast structure and function. SQDG is not limiting under optimal
growth conditions in bacteria and Arabidopsis, but starts to be essential under phosphate
starvation, because one of the functions of SQDG is substitution of PG to maintain the
proper balance of anionic charge in the thylakoid membrane (Yu and Benning 2003). In
Chlamydomonas reinhardtii SQDG serves as an internal source of S for protein
synthesis during early phase of response to S deprivation. At prolonged S-starvation the
Sulfur Metabolism in Higher Plants, pp. xx-xx
Edited by A. Sirko et al.
 2009 Backhuys Publishers, Leiden, The Netherlands
next S source seems to be Rubisco (Sugimoto et al. 2007). Decrease of SQDG is
compensated by increased amounts of PG (Sugimoto et al. 2008). Additional connection
between photosynthesis and sulfur metabolism is the common presence of iron-sulfur
clusters in many photosynthesis-associated proteins.
Photosynthetic pigments belong to a vast family of isoprenoid derivatives.
Isoprenoids are present in majority of living organisms, in plants apart from
photosynthesis they participate in a variety of other biological functions such as
respiration, growth, cell cycle control, plant defense, and adaptation to environmental
conditions. Isoprenoids are produced from isopentenyl diphosphate (IPP) either by the
cytosolic mevalonate (MVA) or the plastidial deoxyxylulose/methylerythritol (MEP)
pathway. There are evidences for interaction between both pathways, presumable
undergoing regulation by various physiological and environmental factors such as
circadian clock, age of tissue or environmental stresses. Cellular pool of IPP is tightly
regulated. Therefore, increased amounts of individual isoprenoids without decreasing
levels of other isoprenoids require higher production of IPP (Rohmer 1999; Estévez et
al. 2001; Sharkey et al. 2008; Swiezewska and Danikiewicz 2005).
Analysis of transcriptional changes in tobacco plants grown for 2 days in sulfurdeficient conditions revealed changes in expression of multiple genes encoding proteins
related to photosynthesis (Wawrzyńska et al. 2005; M. Lewandowska, unpublished
data). This result is not surprising since optimal energy distribution in photosynthesis
should be adjusted to plant nutritional status and it is consistent with the results obtained
for Arabidopsis using macro- and microarrays (Hirai et al. 2003; Maruyama-Nakashita
et al. 2003; Nikiforova et al. 2003).
Tobacco plants grown for 2 days in S-deficient conditions had no physiological stress
symptoms like chlorosis or lower biomass, nevertheless we decided to measure the
amount of chlorophyll a, b and carotenoids using method of Lichtenthaler and Wellburn
(1983). Plants grown in S-deficient condition had strongly reduced amounts of
carotenoids and a slight decrease of both chlorophylls as compared to the plants grown
in the optimal conditions (Fig. 1A, B). Further analysis revealed that in mature leaves
the amount of plastoquinone was also slightly increased by short-term sulfur starvation
(Fig. 1C). Plastoquinones play important roles in the light-dependent reaction of
photosynthesis as an electron carries, and in synthesis of carotenoids. The antioxidant
role of plastoquinol (reduced plastoquinone) has also been earlier reported by Hundal et
al. (1995). Another function of electron transport components in the plastoquinone
region of the chain, reported by Vener et al. (1998) and Pfannschmidt et al. (2001), is
participation in control of gene expression within chloroplast and in nucleus.
Additionally, an increased amount of solanesol (trans-polyprenol) during S deficit
and no change in cis-polyprenols were observed (Fig. 1D). Solanesol is a main
polyprenol in tobacco, and has gained the attention because of its role as a donor of a
isoprenoid moiety for the synthesis of metabolically active quinones and vitamin K
analogs (Tang et al. 2008). Despite several studies there is still not much information
concerning its function. Polyprenyl phosphates probably together with dolichyl
phosphates take part in synthesis of glycoproteins. Additionally, polyprenyl and/or
dolichyl phosphates are suggested to act as donors of isoprenoid groups during protein
prenylation. The biological role of free polyisoprenoid alcohols and their carboxylic
esters is still not known, but they are considered as membrane constituents modulating
the properties of the biological membranes. In animals, protection by dolichols of
cellular membranes against peroxidation is speculated, whereas in plants protective
function of isoprene formed in plant photosynthetic tissue in generally accepted
(Swiezewska and Danikiewicz 2005).
Fig. 1. Level of chlorophylls (A), carotenoids (B), plastoquinone (C) and solanesol and
polyprenols (D) in mature leaves of tobacco grown in either optimal or S-deficient conditions.
The decreased amount of glutathione (GSH) as a consequence of S limitation leads to
the higher sensitivity of plants to stress. To substitute the scavenging reactive oxygen
species function of GSH-ascorbate cycle plants induce synthesis of aromatic secondary
compounds, as observed in S-deprived Arabidopsis (Nikiforova et al. 2003, 2006). Our
earlier analysis showed reduction of total non-protein thiols in all parts of the plant and
an elevated level of dehydroascorbate (Wawrzyńska et al. 2005). It could be possibly
speculated that an increased amount of solanesol in tobacco plants grown in S-deficient
conditions might play protective role in the cells although its effect on the physical
properties of the membranes cannot be ruled out. Alternatively, increased pool of
solanesol might be reused, after phosphorylation, for the increased synthesis of
plastoquinone. Perhaps that might be specific for Solanaceae plants mechanism of
increasing resistance to environmental stresses.
Results from photosynthetic activity measurements (not shown) indicated that plants
grown for 2 days in S-deficient conditions did not have significant damages in
photosynthetic apparatus. Probably plants at early step of S deprivation are able to
overcome the stress yielding in chlorophyll, carotenoids or glutathione decreases
through modifications of chloroplast metabolism, such as increase of the pools of
plastoquinone and solanesol and changes in gene expression.
Acknowledgements: This work was supported by grant PBZ-KBN-110/P04/2004 from
MNiSW.
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