Studies On Na～+ Uptake Pathway Of Suaeda Maritima

Salinity stress is one of the most serious environmental factors limiting the growth and productivity of agricultural crops worldwide. High concentrations of salts cause ion imbalance and hyperosmotic stress in plants. As a consequence of these primary effects, secondary stresses such as oxidative damage often occur. Studying Na⁺ uptake pathways is an important means to the understanding of plant ion homeostasis and osmo-balance, and diminish Na⁺ influx to cope with such stressful environments.The data on which current hypotheses rest had largely come from experiments on glycophytes (including the model plant Arabidopsis), which accumulate little Na⁺ into their leaves when compared with most halophytes. We used the succulent halophyte Suaeda maritima, a member of the Chenopodiaceae to examine its Na⁺ influx pathway. Na⁺ absorption rate of Suaeda maritima with different time when plants were in both 25 and 150 mmol/L NaCl had no significant difference.Here we report that cAMP and Ca²⁺ (blockers of non-selective cation channels, NSCCs) and Li⁺ (a competitive inhibitor of Na⁺ uptake) did not have any significant effect on the uptake of Na⁺ by the halophyte Suaeda maritima when plants were in 25 or 150 mmol/L NaCl (near-optimal for growth). However, TEA⁺, Cs⁺ and Ba²⁺ (inhibitors of K⁺ channels) significantly reduced the net uptake of Na⁺ from 150 mmol/L NaCl over 48 h, by 54%, 24% and 29%, respectively. In contrast to the situation in 150 mmol/L NaCl, neither TEA⁺ nor Cs⁺ significantly reduced net Na⁺ uptake in 25 mmol/L NaCl. Ba²⁺ did significantly decrease net Na⁺ uptake (by 47%) in 25 mmol/L NaCl. The data suggest that neither NSCCs nor LCT are major pathways for Na⁺ entry into root cells. We propose that two distinct low-affinity Na⁺ uptake pathways exist in Suaeda maritima: Pathway 1 is insensitive to TEA⁺ or Cs⁺, but sensitive to Ba²⁺ and mediates Na⁺ uptake under low salinities (25 mmol/L NaCl);Pathway 2 is sensitive to TEA⁺, Cs⁺ and Ba²⁺ and mediates Na⁺ uptake under higher external salt concentrations (150 mmol/L NaCl).