## 2 Results
### Nitrate Promoted Seed Germination under Salt Stress Was Partially Blocked in ABA Catabolism Mutants  
Both nitrate and NO were proved to induce expression of ABA catabolism gene and thus leads to degradation of ABA. We have generated mutants of ABA catabolism genes, _OsABA8 ox1_ and _OsABABox2_, by CRISPR/Cas9. Germination rates of _ab8 ox1_ and _ab8 ox2_, were delayed compared with WT plant Zhonghua11 (ZH11) (Figure 7a-c), especially the mutant of _ab8 ox1_. Germination rate of both mutants were all significantly enhanced by exogenous gibberellin (Figure S3). In addition, application of nitrate could not accelerate seed germination of _ab8 ox1_ (Figure 7a), but slightly increase germination rate of _ab8 ox2_  
Figure 5: _OsNLP2_ was induced by salt stress to promote expression of _OsNR_ gene families. (**a**) Seed germination of WT plant under water, 100 mM NaCl, 1 mM NaNO3 and NaCl plus NaNO3; (**b,c**) Expression of _OsNLP_ and _OsNR_ gene families in WT seed germinated under water and 100 mM NaCl treatments; (**d**) Expression of _OsNLP2_, _OsNR1_ and _OsNR_2 in WT seed germinated under NaCl and NaCl plus NaNO3 treatments. The data are the means of three independent replications (n = 50) \(\pm\) SD. The asterisk (* and **) indicates a significant difference at the \(p<0.05\) and \(p<0.01\) level, respectively, and ns means not significant, by Student’s \(t\)-test analysis. (Figure 7b). In addition, both _aba8 ox1_ and _aba8 ox2_ were much more sensitive to salt treatment than WT plant (Figure 1a). Moreover, nitrate application has partially promoted seed germination of _abab8 ox2_ under salt stress and was still failed to promote seed germination of _aba8 ox1_ under salt stress (Figure 7c,d). To further confirm this result, the ABA catabolism defective mutants were treated with 100 uM SNP, it was shown that seed germination of _aba8 ox1_ mutant was not promoted by application of SNP (Figure S4a), whereas _aba8 ox2_ was partially accelerated by SNP during seed germination (Figure S4b). Under salt stress, the promoting effect of SNP on seed germination was further enhanced on _aba8 ox2_ (Figure S4d). However, germination of _aba8 ox1_ under salt stress was still unchanged by SNP (Figure S4c), similar with that in CK condition. These results suggest that nitrate promoted seed germination under salt stress was partially blocked in _aba8 ox1_ mutant. It was showed that OsNLP1, OsNLP3 and OsNLP4 can activate the expression of Os_NNR1_ and Os_NNR2_ genes while AtNLP8, a homologous gene of Os_NLP2_, activate the expression of ABA catabolism gene CYP707A2 in Arabidopsis [35; 37; 38; 46]. To further examine the relationship between OsNLP2 and those salt-induced genes, Os_ABABox1_, Os_NR1_ and Os_NR2_ (Figures 4b and 5c), we performed transient trans-activation expression assay for OsNLP2 on both ABA catabolism genes and nitrate reductase encoding genes. We found that expression of Os_ABABox1_ and Os_NNR1_ was significantly activated by OsNLP2 in rice protoplast (Figure 8a,c), and Os_NNR2_ was also slightly activated by OsNLP2 (Figure 8b). In consistent with the result of nitrate application on _aba8 ox2_ mutant, OsNLP2 did not activate the expression of Os_ABABox2_ gene rice protoplast (Figure 8d). These results  
Figure 6: Nitrate-promoted seed germination under salt stress was blocked in triple mutants. (**a,b**) Seed germination of H5 and H10 under treatments of water and 1 mM NaNO3; (**c,d**) Seed germination of H5 and H10 under treatments of NaCl and NaCl plus NaNO3. The data are the means of three independent replications (\(n=50\)) \(\pm\) SD. further confirmed that nitrate-activated OsNLP2 regulated the expression of _OsNR1_ and _OsABA8 ox1_ gene to promote seed germination under salt stress. Figure 8: Transient trans-activation expression assays for NLP2 in rice protoplasts. (**a**,**b**) Transient trans-activation for NLP2 on _OsNR1_ and _OsNR2_ genes; (**c**,**d**) Transient trans-activation for NLP2 on _OsNR1_ and _OsNR2_ genes.