Mercuric Chloride Effects on Root Water Transport in Aspen Seedlings

Abstract

HgCl 2 (0.1 mM) reduced pressure-induced water flux and root hydraulic conductivity in the roots of 1-year-old aspen (Populus tremuloides Michx.) seedlings by about 50%. The inhibition was reversed with 50 mM mercaptoethanol. Mercurial treatment reduced the activation energy of water transport in the roots from 10.82 ؎ 0.700 kcal mol ؊1 to 6.67 ؎ 0.193 kcal mol ؊1 when measured over the 4°C to 25°C temperature range. An increase in rhodamine B concentration in the xylem sap of mercury-treated roots suggested a decrease in the symplastic transport of water. However, the apoplastic pathway in both control and mercurytreated roots constituted only a small fraction of the total root water transport. Electrical conductivity and osmotic potentials of the expressed xylem sap suggested that 0.1 mM HgCl 2 and temperature changes over the 4°C to 25°C range did not induce cell membrane leakage. The 0.1 mM HgCl 2 solution applied as a root drench severely reduced stomatal conductance in intact plants, and this reduction was partly reversed by 50 mM mercaptoethanol. In excised shoots, 0.1 mM HgCl 2 did not affect stomatal conductance, suggesting that the signal that triggered stomatal closure originated in the roots. We suggest that mercury-sensitive processes in aspen roots play a significant role in regulating plant water balance by their effects on root hydraulic conductivity.Several criteria have been used to infer the presence of water-transporting channels in cell membranes. These include a high ratio of osmotic to diffusional water permeability (P f /P d Ͼ1), low Arrhenius activation energy (E a Ͻ 6 kcal mol Ϫ1 ) for water transport, and its reversible inhibition by mercury sulfhydryl reagents (for reviews, see Chrispeels and Agre, 1994;Verkman et al., 1996;Maurel, 1997). The transport of water through the lipid bilayer has a high E a , usually above 10 kcal mol Ϫ1 (Macey, 1984). Water transport can also be via water channel proteins (aquaporins), which have been found in the tonoplasts (Maurel et al., 1993) and plasma membranes (Kammerloher et al., 1994) of plants. It is generally acknowledged that the transport of water via channels is less temperature dependent and has a lower E a (Ͻ 6 kcal mol Ϫ1 ) than transport via the lipid pathway (Finkelstein, 1987;Chrispeels and Agre, 1994). Water transport via aquaporins is characteristically inhibited by mercurial reagents, which react with sulfhydryl groups in the channel proteins and result in closure of the channels. This closure inhibits water transport and increases E a to the level of that for transport through the lipid pathway (Macey, 1984). An inhibition of water transport by mercury was reported in cell membranes isolated from higher plants Niemietz and Tyerman, 1997) and in whole root systems (Maggio and Joly, 1995;Carvajal et al., 1996). However, the effects of mercury reagents on E a have not been investigated in intact higher plants.Based on the composite transport model (Steudle and Frensch, 1996), water transport is via three parallel pathways, apoplastic, ...

Hydraulic Conductance and Mercury-Sensitive Water Transport for Roots of Opuntia acanthocarpa in Relation to Soil Drying and Rewetting

“…For both the distal root region and the dissected stele of O. acanthocarpa, inhibition by 50 m HgCl 2 was fully reversed by subsequent treatment with 10 mm 2-mercaptoethanol, suggesting that a toxic reaction was unlikely, as is also observed for roots of Allium cepa (Barrowclough et al, 2000) and A. deserti (North and Nobel, 2000). The 32% reduction of L P by 50 m HgCl 2 for roots of O. acanthocarpa under wet conditions is smaller than inhibitions using that concentration for A. cepa (57%-84% inhibition; Barrowclough et al, 2000), A. deserti (60%; North and Nobel, 2000), Capsicum annuum (66%; Carvajal et al, 1999), Cucumis melo (80%; Carvajal et al, 2000), barley (Hordeum vulgare; 90%; Tazawa et al, 1997), P. tremuloides (47%; Wan and Zwiazek, 1999), or bread wheat (66%; Carvajal et al, 1996). In any case, not all aquaporins are sensitive to mercury (Daniels et al, 1994;Otto and Kaldenhoff, 2000), and the penetration of HgCl 2 may by blocked by suberized cell layers (Barrowclough et al, 2000), such as the endodermis or periderm.…”

“…Such studies on whole root systems or root regions indicate that aquaporins can account for 60% to 80% of the root L P . HgCl 2 (100 m) decreases L P for the root system of Populus tremuloides in 1 h without reducing the respiration rate, suggesting that the inhibition of root water uptake is not due to metabolic inhibition (Wan and Zwiazek, 1999). In contrast, HgCl 2 rapidly depolarizes the plasma membrane (half-maximal depolarization at 8 m) for cortical root cells of bread wheat (Triticum aestivum) and has other effects in addition to the direct inhibition of water channel activity (Zhang and Tyerman, 1999).…”

“…Sulfhydryl reagents, such as HgCl 2 , inhibit water flow via most (Maurel, 1997) but not all aquaporins (Daniels et al, 1994); subsequent use of 2-mercaptoethanol to reverse this inhibition facilitates the study of aquaporin-mediated water uptake by whole roots (Maggio and Joly, 1995;Wan and Zwiazek, 1999;Barrowclough et al, 2000). Such studies on whole root systems or root regions indicate that aquaporins can account for 60% to 80% of the root L P .…”

Overexpression of PIP2;5 Aquaporin Alleviates Effects of Low Root Temperature on Cell Hydraulic Conductivity and Growth in Arabidopsis

“…Low soil temperature is often a major factor restricting the growth and yield of plants even if the soil is not frozen (Bonan, 1992;Wan and Zwiazek, 1999;Wan et al, 2001). In plants that are sensitive to cold soils, growth reduction is accompanied by the reduction of water uptake, which usually starts within a few minutes after the temperature decrease (Bigot and Boucaud, 1996;Lee et al, 2005a).…”

Aquaporin structure–function relationships: Water flow through plant living cells

“…Due to mercury-induced conformational changes and identification of conserved surface loops in plasma membrane aquaporins from higher plants, mercury is thought to bind to sulphydryl groups of the aquaporin proteins, physically blocking the channels and reducing their hydraulic conductivity [9]. Partial recovery of the water flow rate following the application of mercuric chloride was also observed in tomato and aspen root systems, implying the presence of aquaporins as the regulators of plant water status [84,85]. However, the inhibition of water flow with mercurial reagents is neither completely understood nor a general characteristic of aquaporins [28].…”