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BackgroundRice growth is frequently affected by salinity. When exposed to high salinity, rice seed germination and seedling establishment are significantly inhibited. With the promotion of direct-seeding in Asia, improving rice seed germination under salt stress is crucial for breeding.ResultsIn this study, anindicalandrace Wujiaozhan (WJZ) was identified with high germinability under salt stress. A BC1F2population derived from the crossing WJZ/Nip (japonica, Nipponbare)//Nip, was used to quantitative trait loci (QTL) mapping for the seed germination rate (GR) and germination index (GI) under H2O and 300 mM NaCl conditions. A total of 13 QTLs were identified, i.e. ten QTLs under H2O conditions and nine QTLs under salt conditions. Six QTLs,qGR6.1,qGR8.1,qGR8.2,qGR10.1,qGR10.2andqGI10.1were simultaneously identified under two conditions. Under salt conditions, three QTLs,qGR6.2,qGR10.1andqGR10.2for GR were identified at different time points during seed germination, which shared the same chromosomal region withqGI6.2,qGI10.1andqGI10.2for GI respectively. TheqGR6.2accounted for more than 20% of phenotypic variation under salt stress, as the major effective QTL. Furthermore,qGR6.2was verified via the BC2F2population and narrowed to a 65.9-kb region with eleven candidate genes predicted. Based on the microarray database, five candidate genes were found with high transcript abundances at the seed germination stage, of whichLOC_Os06g10650andLOC_Os06g10710were differentially expressed after seed imbibition. RT-qPCR results showed the expression ofLOC_Os06g10650was significantly up-regulated in two parents with higher levels in WJZ than Nip during seed germination under salt conditions. Taken together, it suggests thatLOC_Os06g10650, encoding tyrosine phosphatase family protein, might be the causal candidate gene forqGR6.2.ConclusionsIn this study, we identified 13 QTLs from a landrace WJZ that confer seed germination traits under H2O and salt conditions. A major salt-tolerance-specific QTLqGR6.2was fine mapped to a 65.9-kb region. Our results provide information on the genetic basis of improving rice seed germination under salt stress by marker-assisted selection (MAS).

Background Rice growth is frequently affected by salinity. When rice plants are exposed to high salinity, seed germination and seedling establishment are significantly inhibited. In particular, with the promotion of rice direct-seeding in Asia, improving rice salt tolerance during seed germination is of strong importance for rice breeding. Results In this study, we found that the indica rice landrace Wujiaozhan (WJZ) showed a high capability of seed germination under both water (H 2 O) and salt (NaCl) conditions, particularly under high salt stress. The BC 1 F 2 population produced by crossing WJZ with japonica Nipponbare (Nip) was used to evaluate the germination traits under water (H 2 O) and salt (300 mM NaCl) conditions using germination rate (GR) and germination index (GI). A total of 13 quantitative trait loci (QTLs) were identified, including eight QTLs of GR, two QTLs of GI under H 2 O conditions, six QTLs of GR, and three QTLs of GI under 300 mM NaCl conditions. Six QTLs ( qGR6.1 , qGR8.1 , qGR8.2 , qGR10.1 , qGR10.2 and qGI10.1 ) contributed to GR under both H 2 O and 300 mM NaCl conditions. Three QTLs ( qGR6.2 , qGR10.1 and qGR10.2 ) under 300 mM NaCl conditions were identified at different time points of seed germination and shared the same region with qGI6 , qGI10.1 and qGI10.2 for GI. These QTLs could be used to improve seed germination ability via marker-assisted selection (MAS). One major effective salt-tolerance-specific QTL, qGR6.2, on chromosome 6 was further confirmed via the BC 2 F 2 population, which explained more than 20% of the phenotypic variation. Fine mapping results showed that qGR6.2 was narrowed to a 65.9-kb region between the Z654 and Z619 molecular markers, with eleven candidate genes being predicted. Based on the microarray database, there were high transcript abundances of six genes ( LOC_Os06g10650 , LOC_Os06g10660 , LOC_Os06g10690 , LOC_Os06g10710 , LOC_Os06g10730 and LOC_Os06g10750 ) at all developmental stages, and only LOC_Os06g10750 was differentially expressed after salt incubation. RT-qPCR showed that two genes ( LOC_Os06g10650 and LOC_Os06g10750 ) were significantly differentially expressed at 300 mM NaCl during seed germination. This result suggested that LOC_Os06g10650 and LOC_Os06g10750 might be the causal candidate genes for the major effective salt-tolerance-specific QTL qGR6.2 identified in WJZ, which may facilitate map-based cloning and help to elucidate the molecular mechanism underlying salt tolerance during seed germination. Conclusions In our study, we identified 13 QTLs from indica landrace WJZ that confer seed germination traits under water and salt conditions. A major salt-tolerance-specific QTL qGR6.2 was confirmed and fine mapped to a 65.9-kb region flanked by the Z654 and Z619 markers. Our results provide information on the genetic basis of improving salt tolerance during seed germination by MAS.

Supplementary Data set v2.pdf for a paper entitled "Identification and fine mapping of qGR6.2, a novel locus controlling rice seed germination under salt stress"

The Stress Associated RNA-binding protein 1 (SRP1) repressed by ABA, salt and cold encodes a C2C2-type zinc finger protein in Arabidopsis. The knock-out mutation in srp1 reduced the sensitivity of seed to ABA and salt stress during germination and post-germinative growth stages. In contrast, SRP1-overexpressing seedlings were more sensitive to ABA and salt compared to wild type plants. In the presence of ABA, the transcript levels of ABA signaling and germination-related genes including ABI3, ABI5, EM1 and EM6 were less induced in srp1 compared to WT. Interestingly, expression of ABI2 encoding a protein phosphatase 2C protein were significantly up-regulated in srp1 mutants. By in vitro analysis, SRP1 was identified as a novel RNA-binding protein directly binding to 3’UTR of ABI2 mRNA. Moreover, transient expression assay proved the function of SRP1 in reducing the activity of luciferase whose coding sequence was fused with the ABI2 3’UTR. Together, it is suggested that SRP1 is involved in the ABA signaling by post-transcriptionally repressing ABI2 expression in Arabidopsis.