Abstract
Rice (Oryza sativa) is a staple food that feeds more than half the world population. As rice is highly sensitive to soil salinity, current trends in soil salinization threaten global food security. To better understand the mechanistic basis of salinity tolerance in rice, three contrasting rice cultivars—Reiziq (tolerant), Doongara (moderately tolerant), and Koshihikari (sensitive)—were examined and the differences in operation of key ion transporters mediating ionic homeostasis in these genotypes were evaluated. Tolerant varieties had reduced Na+ translocation from roots to shoots. Electrophysiological and quantitative reverse transcription PCR experiments showed that tolerant genotypes possessed 2-fold higher net Na+ efflux capacity in the root elongation zone. Interestingly, this efflux was only partially mediated by the plasma membrane Na+/H+ antiporter (OsSOS1), suggesting involvement of some other exclusion mechanisms. No significant difference in Na+ exclusion from the mature root zones was found between cultivars, and the transcriptional changes in the salt overly sensitive signaling pathway genes in the elongation zone were not correlated with the genetic variability in salinity tolerance amongst genotypes. The most important hallmark of differential salinity tolerance was in the ability of the plant to retain K+ in both root zones. This trait was conferred by at least three complementary mechanisms: (1) its superior ability to activate H+-ATPase pump operation, both at transcriptional and functional levels; (2) reduced sensitivity of K+ efflux channels to reactive oxygen species; and (3) smaller upregulation in OsGORK and higher upregulation of OsAKT1 in tolerant cultivars in response to salt stress. These traits should be targeted in breeding programs aimed to improve salinity tolerance in commercial rice cultivars.
Highlights
Soil salinization is a major abiotic constraint for crop productivity worldwide (Munns and Gilliham 2015; Yang and Guo 2018)
Salinity stress resulted in a significant reduction in plant height, root length, tiller number, Fresh weight (FW) and DW of shoot and root, and Water content (WC) in all cultivars (Table 1, Figure 1)
Compared to Koshihikari, Reiziq and Doongara showed lower reduction in shoot DW under salt stress and performed at a moderate concentration of salinity (50 mM); under the more severe treatment (100 mM NaCl), variety Reiziq’s performance was the best (Figures 1B, C). Consistent with these findings were measurements of chlorophyll content and efficiency of operation of PSII in salt-grown plants, with Soil–Plant Analyses Development (SPAD) and Fv/Fm parameters being more reduced in Koshihikari compared with other cultivars (Figures 1D, E)
Summary
Soil salinization is a major abiotic constraint for crop productivity worldwide (Munns and Gilliham 2015; Yang and Guo 2018). As a result, depending on their habitat and severity of stress, plants have evolved various strategies to minimize the damage associated with salt These mechanisms include osmotic adjustment, Na+ exclusion and sequestration, and K+ retention in the cytosol, control of xylem ions loading, and oxidative stress tolerance (Ashraf et al, 2008; Adem et al, 2014; Bose et al, 2014a; Chakraborty et al, 2016). All of these mechanisms should be considered in the genetic design of a salt-tolerant rice plant
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