Abstract
Na(+) uptake by plant roots has largely been explored using species that accumulate little Na(+) into their shoots. By way of contrast, the halophyte Suaeda maritima accumulates, without injury, concentrations of the order of 400 mM NaCl in its leaves. Here we report that cAMP and Ca(2+) (blockers of nonselective cation channels) and Li(+) (a competitive inhibitor of Na(+) uptake) did not have any significant effect on the uptake of Na(+) by the halophyte S. maritima when plants were in 25 or 150 mM NaCl (150 mM NaCl is near optimal for growth). However, the inhibitors of K(+) channels, TEA(+) (10 mM), Cs(+) (3 mM), and Ba(2+) (5 mM), significantly reduced the net uptake of Na(+) from 150 mM NaCl over 48 h, by 54%, 24%, and 29%, respectively. TEA(+) (10 mM), Cs(+) (3 mM), and Ba(2+) (1 mm) also significantly reduced (22)Na(+) influx (measured over 2 min in 150 mM external NaCl) by 47%, 30%, and 31%, respectively. In contrast to the situation in 150 mm NaCl, neither TEA(+) (1-10 mM) nor Cs(+) (0.5-10 mM) significantly reduced net Na(+) uptake or (22)Na(+) influx in 25 mM NaCl. Ba(2+) (at 5 mm) did significantly decrease net Na(+) uptake (by 47%) and (22)Na(+) influx (by 36% with 1 mM Ba(2+)) in 25 mM NaCl. K(+) (10 or 50 mM) had no effect on (22)Na(+) influx at concentrations below 75 mM NaCl, but the influx of (22)Na(+) was inhibited by 50 mM K(+) when the external concentration of NaCl was above 75 mM. The data suggest that neither nonselective cation channels nor a low-affinity cation transporter are major pathways for Na(+) entry into root cells. We propose that two distinct low-affinity Na(+) uptake pathways exist in S. maritima: Pathway 1 is insensitive to TEA(+) or Cs(+), but sensitive to Ba(2+) and mediates Na(+) uptake under low salinities (25 mM NaCl); pathway 2 is sensitive to TEA(+), Cs(+), and Ba(2+) and mediates Na(+) uptake under higher external salt concentrations (150 mM NaCl). Pathway 1 might be mediated by a high-affinity K transporter-type transporter and pathway 2 by an AKT1-type channel.
Highlights
We propose that two distinct low-affinity Na1 uptake pathways exist in S. maritima: Pathway 1 is insensitive to TEA1 or Cs1, but sensitive to Ba21 and mediates Na1 uptake under low salinities (25 mM NaCl); pathway 2 is sensitive to TEA1, Cs1, and Ba21 and mediates Na1 uptake under higher external salt concentrations (150 mM NaCl)
We have used S. maritima, a species where net uptake of Na1 in 50 mM NaCl is an order of magnitude greater than that of durum wheat (Triticum durum) growing in the same salinity (0.13 mmol/g fresh weight root/min, interpolated from the data in Yeo and Flowers, 1986; net uptake of Na1 by durum wheat in 50 mM NaCl is 0.01–0.02 mmol/g fresh weight root/min; Davenport et al, 2005) to examine Na1 influx and found two distinct low-affinity Na1 uptake pathways: Pathway 1 is insensitive to TEA1 or Cs1, but sensitive to Ba21 and mediates Na1 uptake under low salinities such as 25 mM NaCl; pathway 2 is sensitive to TEA1, Cs1, or Ba21 and mediates Na1 uptake under higher external salt concentrations, such as 150 mM NaCl
We evaluated the effects of cAMP and Ca21, which inhibit Na1 influx through nonselective cation channels (NSCCs) (Maathuis and Sanders, 2001; Demidchik and Tester, 2002), and Li1, which is considered an analog of Na1 and is a competitive inhibitor for Na1 uptake and transport (Serrano et al, 1999; Rubio et al, 2004) on growth and ion accumulation of S. maritima
Summary
The inhibitors of K1 channels, TEA1 (10 mM), Cs1 (3 mM), and Ba21 (5 mM), significantly reduced the net uptake of Na1 from 150 mM NaCl over 48 h, by 54%, 24%, and 29%, respectively. The HKT proteins (high affinity K transporters), which have been shown to transport Na1 (see Rodriguez-Navarro and Rubio, 2006), fall into two subfamilies, one of which (subfamily 2) has only been found in the grasses (Platten et al, 2006). The analysis by Rodriguez-Navarro and Rubio (2006) suggests that HKT transporters mediate high-affinity Na1 uptake and function in low-affinity Na1 transport
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