Effects of nitric oxide on nitrogen metabolism and the salt resistance of rice (Oryza sativa L.) seedlings with different salt tolerances.

Salt stress inhibits rice productivity seriously. Nitric oxide (NO) is an endogenous signaling molecule in plants that can improve the resistance of rice to abiotic stresses. Previous studies also showed that nitrogen metabolism is essential for rice stress-tolerance. However, the physiological and molecular mechanisms by how NO affects the nitrogen metabolisms of rice seedlings remain unclear. A hydroponic experiment with two rice varieties, Jinyuan85 (salt tolerant) and Liaojing763 (salt sensitive), was carried out to explore whether NO could alleviate the negative effects of salt stress on nitrogen metabolism and increase salt resistance of rice seedlings. The results showed that (1) the application of NO alleviated the inhibitory effects of salt stress on plant height and biomass accumulation, and increased the nitrogen content of rice leaf. (2) the accumulation of the sucrose and proline was markedly increased in salt stress after application of NO, and peroxidase activities was increased by 107% and 67.7% for Jinyuan85 and Liaojing763, respectively. (3) NO significantly increased the activities of glutamate dehydrogenase, sucrose synthase and sucrose phosphate synthase in both rice varieties under salt stress. (4) Additionally, NO regulated the expression levels of AMT, NIA and SUT genes, but these regulation effects are different with rice varieties and treatments. The results suggested that NO mainly increased the glutamate dehydrogenase and peroxidase activities and sucrose accumulation to enhance the nitrogen metabolism and antioxidative capacity, and alleviated the negative effects of salt stress on rice performance.

iTRAQ-Based Protein Profiling and Biochemical Analysis of Two Contrasting Rice Genotypes Revealed Their Differential Responses to Salt Stress.

Salt stress is one of the key abiotic stresses causing huge productivity losses in rice. In addition, the differential sensitivity to salinity of different rice genotypes during different growth stages is a major issue in mitigating salt stress in rice. Further, information on quantitative proteomics in rice addressing such an issue is scarce. In the present study, an isobaric tags for relative and absolute quantitation (iTRAQ)-based comparative protein quantification was carried out to investigate the salinity-responsive proteins and related biochemical features of two contrasting rice genotypes-Nipponbare (NPBA, japonica) and Liangyoupeijiu (LYP9, indica), at the maximum tillering stage. The rice genotypes were exposed to four levels of salinity: 0 (control; CK), 1.5 (low salt stress; LS), 4.5 (moderate salt stress; MS), and 7.5 g of NaCl/kg dry soil (high salt stress, HS). The iTRAQ protein profiling under different salinity conditions identified a total of 5340 proteins with 1% FDR in both rice genotypes. In LYP9, comparisons of LS, MS, and HS compared with CK revealed the up-regulation of 28, 368, and 491 proteins, respectively. On the other hand, in NPBA, 239 and 337 proteins were differentially upregulated in LS and MS compared with CK, respectively. Functional characterization by KEGG and COG, along with the GO enrichment results, suggests that the differentially expressed proteins are mainly involved in regulation of salt stress responses, oxidation-reduction responses, photosynthesis, and carbohydrate metabolism. Biochemical analysis of the rice genotypes revealed that the Na⁺ and Cl- uptake from soil to the leaves via the roots was increased with increasing salt stress levels in both rice genotypes. Further, increasing the salinity levels resulted in increased cell membrane injury in both rice cultivars, however more severely in NPBA. Moreover, the rice root activity was found to be higher in LYP9 roots compared with NPBA under salt stress conditions, suggesting the positive role of rice root activity in mitigating salinity. Overall, the results from the study add further insights into the differential proteome dynamics in two contrasting rice genotypes with respect to salt tolerance, and imply the candidature of LYP9 to be a greater salt tolerant genotype over NPBA.