Higher sensitivity of pad2-1 and vtc2-1 mutants to cadmium is related to lower subcellular glutathione rather than ascorbate contents
Barbara Eva Koffler
Lisa Polanschtz
Bernd Zechmann
Handling Editor: Nstor Carrillo
0
) Institute of Plant Sciences, University of Graz
, Schubertstrasse 51, 8010 Graz,
Austria
Cadmium (Cd) interferes with ascorbate and glutathione metabolism as it induces the production of reactive oxygen species (ROS), binds to glutathione due to its high affinity to thiol groups, and induces the production of phytochelatins (PCs) which use glutathione as a precursor. In this study, changes in the compartment specific distribution of ascorbate and glutathione were monitored over a time period of 14 days in Cd-treated (50 and 100 M) Arabidopsis Col-0 plants, and two mutant lines deficient in glutathione (pad2-1 ) and ascorbate (vtc2-1 ). Both mutants showed higher sensitivity to Cd than Col-0 plants. Strongly reduced compartment specific glutathione, rather than decreased ascorbate contents, could be correlated with the development of symptoms in these mutants suggesting that higher sensitivity to Cd is related to low glutathione contents rather than low ascorbate contents. On the subcellular level it became obvious that long-term treatment of wildtype plants with Cd induced the depletion of glutathione and ascorbate contents in all cell compartments except chloroplasts indicating an important protective role for antioxidants in chloroplasts against Cd. Additionally, we could observe an immediate decrease of glutathione and ascorbate in all cell compartments 12 h after Cd treatment indicating that glutathione and ascorbate are either withdrawn from or not redistributed into other organelles after their production in chloroplasts, cytosol (production centers for glutathione) and mitochondria (production center for ascorbate). The obtained data is discussed in respect to recently proposed stress models involving antioxidants in the protection of plants against environmental stress conditions.
-
Contaminations of soils with heavy metals such as cadmium
(Cd) are a severe problem for plant growth and development as
they accumulate and negatively interfere with many
physiological processes in plants. Environmental pollution by metals
such as Cd became extensive in the late 19th and early 20th
century as mining and industrial activities increased (Benavides
et al. 2005; Gallego et al. 2012). Anthropogenic sources like
zinc smelting, burning of fuel, waste incinerators, urban traffic,
cement factories, and Cd as a by-product of phosphate
fertilizers have led to Cd pollution in the environment (Sanit di
Toppi and Gabbrielli 1999; DalCorso et al. 2008; Jozefczak
et al. 2012). Although Cd is not an essential element, it is
readily absorbed by the roots and transported through xylem
and phloem to different parts of the plants (Mendoza-czatl
et al. 2008; DalCorso et al. 2008). Visible symptoms of Cd
toxicity in plants are leaf roll, chlorosis, browning of root tips
and reduced growth (Baryla et al. 2001; Schtzendbel and
Polle 2002; Chen et al. 2003; Grato et al. 2005; Maughan et al.
2010). Inside the plant Cd leads to an alteration of the
chloroplast ultrastructure, disturbs the synthesis of chlorophyll and
carotenoids, inhibits the enzyme activity of the Calvin cycle
and leads to CO2 deficiency due to stomatal closure, which
cause the inhibition of photosynthesis (Ding et al. 2006;
Ekmekci et al. 2008; He et al. 2008; Grato et al. 2009;
Bouzon et al. 2012). Additionally, Cd causes lipid
peroxidation, disrupts cell transport processes, reduces the uptake of
essential mineral nutrients, reduces the activity of various
enzymes and is responsible for chromosomal aberrations
(Clemens et al. 2001; Schtzendbel and Polle 2002; Ding
et al. 2006; Sanit di Toppi et al. 2008; Grato et al. 2012).
Indirectly, high concentrations of Cd lead to oxidative stress
which activates the accumulation of anti-oxidative defense
enzymes and antioxidants like ascorbate and glutathione
(Sanit di Toppi and Gabbrielli 1999; Hegeds et al. 2001;
Grato et al. 2005; Paradiso et al. 2008; Ekmekci et al. 2008;
Xu et al. 2008; DalCorso et al. 2008; Gill and Tuteja 2010;
Yadav 2010; Monteiro et al. 2011; Gallego et al. 2012; Shan
et al. 2012; Zelinova et al. 2013) which detoxify reactive
oxygen species (ROS) either through reactions catalyzed by
peroxidases or through the ascorbate glutathione cycle (Foyer
and Noctor 2005; Iannone et al. 2010; Foyer and Noctor 2011;
Noctor et al. 2012). In addition, Cd has a high affinity to thiol
groups and forms complexes with reduced glutathione which
are then transported into vacuoles. Glutathione is also able to
protect proteins by reversible binding to their SH groups
which protects them from oxidation by ROS and inhibits the
binding of Cd (Rauser 2001; Maksymiec and Krupa 2006;
Semane et al. 2007; DalCorso et al. 2010; Jozefczak et al.
2012). On top glutathione serves as a direct precursor of
phytochelatins (PCs), which together with metallothioneins
detoxify Cd by complexation, transportation, and deposition
in the vacuole (Akh (...truncated)