Base to Tip and Long-Distance Transport of Sodium in the Root of Common Reed [Phragmites australis (Cav.) Trin. ex Steud.] at Steady State Under Constant High-Salt Conditions

Abstract

We analyzed the directions and rates of translocation of sodium ions (Na(+)) within tissues of a salt-tolerant plant, common reed [Phragmites australis (Cav.) Trin. ex Steud.], and a salt-sensitive plant, rice (Oryza sativa L.), under constant high-salt conditions using radioactive (22)Na tracer and a positron-emitting tracer imaging system (PETIS). First, the test plants were incubated in a nutrient solution containing 50 mM NaCl and a trace level of (22)Na for 24 h (feeding step). Then the original solution was replaced with a fresh solution containing 50 mM NaCl but no (22)Na, in which the test plants remained for >48 h (chase step). Non-invasive dynamic visualization of (22)Na distribution in the test plants was conducted during feeding and chase steps with PETIS. Our results revealed that (22)Na was absorbed in the roots of common reed, but not transported to the upper shoot beyond the shoot base. During the chase step, a basal to distal movement of (22)Na was detected within the root tissue over >5 cm with a velocity of approximately 0.5 cm h(-1). On the other hand, (22)Na that was absorbed in the roots of rice was continuously translocated to and accumulated in the whole shoot. We concluded that the basal roots and the shoot base of common reed have constitutive functions of Na(+) exclusion only in the direction of root tips, even under constant high-salt conditions. This function apparently may contribute to the low Na(+) concentration in the upper shoot and high salt tolerance of common reed.

The Role of Na+ and K+ Transporters in Salt Stress Adaptation in Glycophytes

“…As will be seen later, SOS1 activity is very important for K + uptake, especially under low-K + conditions. Recently, it has been consistently shown that the difference between tolerant reed and sensitive rice is due to the difference in ability to reduce transport of Na + to the shoot; only the reed plants restricted Na + transport to the shoot base while the rice plants lacked this capacity (Fujimaki et al, 2015). This restriction was shown to involve Na + retrieval from the xylem and recirculation to the soil through the phloem.…”

“…One such action was shown involving AtHKT1;1-mediated Na + recirculation from the leaf to the root via the phloem (Berthomieu et al, 2003; Horie et al, 2005). Although this means of Na + transport was later found to be either negligible or non-existent (Horie et al, 2007), there is increasing evidence of its occurrence, at least in the root (Tian et al, 2010; Fujimaki et al, 2015). Fujimaki et al (2015) used the positron-emitting tracer imaging system to analyse the direction of movement of radioactive 22 Na + in the roots of reed and rice plants.…”

“…Although this means of Na + transport was later found to be either negligible or non-existent (Horie et al, 2007), there is increasing evidence of its occurrence, at least in the root (Tian et al, 2010; Fujimaki et al, 2015). Fujimaki et al (2015) used the positron-emitting tracer imaging system to analyse the direction of movement of radioactive 22 Na + in the roots of reed and rice plants. They found that whereas rice plants continuously transported and accumulated Na + in the shoot, reed plants' roots absorbed and moved Na + only to the base of the shoot, where it was retrieved and recirculated via the phloem back to the root medium.…”

Recent Advances in Radioisotope Imaging Technology for Plant Science Research in Japan

“…In rice, sodium in the root continued to migrate to the upper leaves, whereas it moved towards the root tip in a reed. These results suggested that the salt tolerance mechanism of reed maintained a low concentration of sodium in the ground part by returning the sodium absorbed from the root [32]. Positron imaging of zinc has been performed using 62 Zn (t 1/2 = 9.2 hours) [33].…”

The elemental composition of halophytes correlates with key morphological adaptations and taxonomic groups

“…These species are not specialized in using Na for osmoregulation, and excessive Na can disturb their elemental composition homeostasis (Kudo et al, 2010). Some eury-hygro-halophytes species, like Phragmites australis, can transport Na absorbed by upward stream back to the roots, keeping low Na concentrations in the shoots and high Na in the root vacuoles (Fujimaki et al, 2015;Matsushita and Matoh, 1992;Takahashi et al, 2007;Vasquez et al, 2006).…”