Regulation of the uptake and distribution of Na⁺ in shoots of rice (Oryza sativa) variety Pokkali: role of Ca²⁺ in salt tolerance response

Abstract

Soil salinity is a major factor affecting crop productivity worldwide. This study explores mechanisms that contribute to salt tolerance in rice (Oryza sativa L.). Hydroponically grown, 2‐week‐old salt tolerant and sensitive indica rice varieties, Pokkali and Jaya, respectively, were exposed to a 48‐h stress period with NaCl (0–250 mM). When exposed to 200 mM NaCl, micromolar levels of external Ca2+ elevated survival of both varieties. The Ca2+ levels required were lower for Pokkali than for Jaya, but resulted in significantly higher survival. Estimates of Na+ and K+ in root and shoot compartments were made by flame photometry, while X‐ray microanalysis was used to localize Na+ in the extracellular matrix of the shoot. Transpirational bypass flow was estimated using the apoplastic tracer, 8‐hydroxypyrene‐1,3,6‐trisulphonic acid, trisodium salt. Our data demonstrate a Ca2+‐dependent reduction in Na+ transport to shoots, which correlated with a decline in bypass flow and of Na+ in the transpirational stream. In addition, the Na+ that enters the shoot is partitioned among several distinct compartments. Survival is inversely correlated with Na+ levels in the shoot apoplastic fluid, which surrounds the cell and influences cytosolic composition. Pokkali maintained lower Na+ in its apoplast compared with the salt sensitive Jaya at the same total shoot Na+. Na+ in the apoplast appears to be regulated by sequestration into intracellular compartments. This sink supplements the primary response of reducing Na+ influx into the shoot and effectively buffers the apoplastic fluid in Pokkali. All of these mechanisms are operational in Jaya as well but are deployed less effectively.

A Plant Ca2+ Pump, ACA2, Relieves Salt Hypersensitivity in Yeast

“…Adaptive responses to maintain low [Na ϩ ] cyt , especially in photosynthesizing tissues, are complex and occur at both the whole plant and cellular levels (3,41). When exposed to salinity, Arabidopsis uses a pathway comprising a Ca 2ϩ -binding protein SOS3, which activates the kinase SOS2 that, in turn, phosphorylates the plasma membrane Na ϩ /H ϩ antiporter SOS1, thereby activating it.…”

“…Cells were either taken as such or stressed with 400 mM NaCl, and 2-ml aliquots were collected over a range of time points extending up to 18 h. Cells were collected by centrifugation, washed four times in ice-cold Buffer B (10 mM Tris-HCl, pH 6.0, 2 mM MgCl 2 , 1% glucose, 0.6 M sorbitol), and dried at 50°C for 4 days. Dried cells were acid-digested in a 4:1 diacid mixture of perchlorate, and nitrate and Na ϩ levels were estimated by flame photometry using a Systronics Flame Photometer 128 (Ahmedabad, India) (41). In a few experiments, growth was carried out at Ca 2ϩ concentrations of 100 M and 5 mM.…”

Casparian strip development and its potential function in salt tolerance

“…The endodermal and hypodermal cell walls of roots also contained varying amounts uptake in rice increased after exposing the roots to organic acids and sulphide. 16,17 Since bypass flow is reported to be the major pathway of Na + entry into the shoots in rice, [18][19][20] characterization of the apoplastic barriers in rice roots is of additional importance. Using the enhanced suberin1 (esb1) mutant, Baxter et al correlated suberin in the root with both water movement through the plant and solute accumulation in the shoot in Arabidopsis thaliana.…”

“…6,19 Accumulating evidence indicates that plants have developed different strategies to minimise the access of salt to the stelar tissues by sequestering/excreting Na + into the intracellular and extracellular compartments away from the cytosol. 19,22,23 In parallel, plants react to different environmental factors (such as salinity and drought), reinforcing the level of their apoplastic barriers in roots.…”

“…6,19 Accumulating evidence indicates that plants have developed different strategies to minimise the access of salt to the stelar tissues by sequestering/excreting Na + into the intracellular and extracellular compartments away from the cytosol. 19,22,23 In parallel, plants react to different environmental factors (such as salinity and drought), reinforcing the level of their apoplastic barriers in roots. 15,24 In terms of the relationship between the development of endoand exodermal Casparian strips and the growth of roots under stress, studies have demonstrated that the distance of Casparian strip formation from the root tip decreases significantly in maize roots under osmotic stress and in cotton roots under salt stress.…”