Salt intolerance in Arabidopsis: shoot and root sodium toxicity, and inhibition by sodium-plus-potassium overaccumulation

Abstract

Arabidopsis plants in NaCl suffering half growth inhibition do not suffer osmotic stress and seldom shoot Na (+) toxicity; overaccumulation of Na (+) plus K (+) might trigger the inhibition. It is widely assumed that salinity inhibits plant growth by osmotic stress and shoot Na(+) toxicity. This study aims to examine the growth inhibition of Arabidopsis thaliana by NaCl concentrations that allow the completion of the life cycle. Unaffected Col-0 wild-type plants were used to define nontoxic Na(+) contents; Na(+) toxicities in shoots and roots were analyzed in hkt1 and sos1 mutants, respectively. The growth inhibition of Col-0 plants at 40 mM Na(+) was mild and equivalent to that produced by 8 and 4 mM Na(+) in hkt1 and sos1 plants, respectively. Therefore, these mutants allowed to study the toxicity of Na(+) in the absence of an osmotic challenge. Col-0 and Ts-1 accessions showed very different Na(+) contents but similar growth inhibitions; Ts-1 plants showed very high leaf Na(+) contents but no symptoms of Na(+) toxicity. Ak-1, C24, and Fei-0 plants were highly affected by NaCl showing evident symptoms of shoot Na(+) toxicity. Increasing K(+) in isotonic NaCl/KCl combinations dramatically decreased the Na(+) content in all Arabidopsis accessions and eliminated the signs of Na(+) toxicity in most of them but did not relieve growth inhibition. This suggested that the dominant inhibition in these conditions was either osmotic or of an ionic nature unspecific for Na(+) or K(+). Col-0 and Ts-1 plants growing in sorbitol showed a clear osmotic stress characterized by a notable decrease of their water content, but this response did not occur in NaCl. Overaccumulation of Na(+) plus K(+) might trigger growth reduction in NaCl-treated plants.

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

“…Although shoot Na + exclusion is widely acclaimed to be one of the most important salt tolerance mechanisms, both root and shoot Na + accumulation have been shown to affect plant growth under salt stress (Álvarez-Aragón et al, 2016). This study also revealed that under salt stress in Arabidopsis, not only Na + accumulation is the cause for growth impairment, but also K + accumulation.…”

Coordinated Transport of Nitrate, Potassium, and Sodium

“…Sodium and potassium interact at two main levels: the interference of Na + with K + nutrition, and the substitution of Na + for K + as highly dynamic and mobile cellular osmolyte in conditions of K + shortage (Haro et al, 2010;Kronzucker and Britto, 2011;Kronzucker et al, 2013;Alvarez-Aragon et al, 2016). Soil salinity is often associated with elevated levels of Na + (Munns and Tester, 2008).…”

“…Soil salinity is often associated with elevated levels of Na + (Munns and Tester, 2008). Although it is not clear what cytosolic levels of Na + are harmful to the plant cell (Kronzucker and Britto, 2011;Alvarez-Aragon et al, 2016), this cation is usually excluded from the cytosol. Due to their physicochemical similarity, Na + and K + can compete for binding to amino acids of protein surfaces, pockets of allosteric regulation or selectivity filters of ion channels (Benito et al, 2014).…”

The combination of K+ deficiency with other environmental stresses: What is the outcome?

“…A study on barley showed that cytosolic K + /Na + ratios did not explain growth inhibition because of salt stress (Kronzucker et al ). Another study on Arabidopsis showed that growth inhibition because of NaCl treatments was caused by the osmotic component of the treatment rather than the toxic component (Alvarez‐Aragon et al ). In that sense, isotonic combinations of KCl/NaCl were able to inhibit plant growth without giving rise to low K + /Na + ratios.…”