Abstract
Salinity imposes a major constraint over the productivity of rice. A set of chromosome segment substitution lines (CSSLs), derived from a cross between the japonica type cultivar (cv.) Nipponbare (salinity sensitive) and the indica type cv. 9311 (moderately tolerant), was scored using a hydroponics system for their salinity tolerance at the seedling stage. Two of the CSSLs, which share a ∼1.2 Mbp stretch of chromosome 4 derived from cv. Nipponbare, were as sensitive to the stress as cv. Nipponbare itself. Fine mapping based on an F2 population bred from a backcross between one of these CSSLs and cv. 9311 narrowed this region to 95 Kbp, within which only one gene (OsHAK1) exhibited a differential (lower) transcript abundance in cv. Nipponbare and the two CSSLs compared to in cv. 9311. The gene was up-regulated by exposure to salinity stress both in the root and the shoot, while a knockout mutant proved to be more salinity sensitive than its wild type with respect to its growth at both the vegetative and reproductive stages. Seedlings over-expressing OsHAK1 were more tolerant than wild type, displaying a superior photosynthetic rate, a higher leaf chlorophyll content, an enhanced accumulation of proline and a reduced level of lipid peroxidation. At the transcriptome level, the over-expression of OsHAK1 stimulated a number of stress-responsive genes as well as four genes known to be involved in Na+ homeostasis and the salinity response (OsHKT1;5, OsSOS1, OsLti6a and OsLti6b). When the stress was applied at booting through to maturity, the OsHAK1 over-expressors out-yielded wild type by 25%, and no negative pleiotropic effects were expressed in plants gown under non-saline conditions. The level of expression of OsHAK1 was correlated with Na+/K+ homeostasis, which implies that the gene should be explored a target for molecular approaches to the improvement of salinity tolerance in rice.
Highlights
Salinity imposes a serious constraint over crop productivity, especially for crops grown under irrigation or in coastal lowlands prone to seawater ingress (Campbell et al, 2017)
The effect of the salinity stress was to reduce the dry matter accumulated by cv. 9311 by 29%, that by chromosome segment substitution lines (CSSLs)-1 by 34%, that by CSSL2 by 31% and that by cv
Nipponbare; it was inducible by salinity stress; changes in its transcript abundance correlated positively with tolerance at both the seedling and tillering stages, as well as with K+ and Na+ homeostasis and correlated negatively with lipid peroxidation; its transcription level affected the abundance of several stress-related genes, and its over-expression improved the productivity of plants challenged with salinity
Summary
Salinity imposes a serious constraint over crop productivity, especially for crops grown under irrigation or in coastal lowlands prone to seawater ingress (Campbell et al, 2017). Its sensitivity depends on the developmental stage of the plant (Fageria, 1985; Khan et al, 1997); it is vulnerable during its early vegetative growth, so that stand density is compromised in salinity-affected fields (Lutts et al, 1995; Gregorio et al, 2002). Some cultivars exhibit their highest sensitivity at the time of tillering and panicle initiation (Zeng et al, 2002).
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