Comparative studies on tolerance of rice genotypes differing in their tolerance to moderate salt stress

Abstract

Background: Moderate salt stress, which often occurs in most saline agriculture land, suppresses crop growth and reduces crop yield. Rice, as an important food crop, is sensitive to salt stress and rice genotypes differ in their tolerance to salt stress. Despite extensive studies on salt tolerance of rice, a few studies have specifically investigated the mechanism by which rice plants respond and tolerate to moderate salt stress. Two rice genotypes differing in their tolerance to saline-alkaline stress, Dongdao-4 and Jigeng-88, were used to explore physiological and molecular mechanisms underlying tolerance to moderate salt stress.

Results: Dongdao-4 plants displayed higher biomass, chlorophyll contents, and photosynthetic rates than Jigeng-88 under conditions of salt stress. No differences in K⁺ concentrations, Na⁺ concentrations and Na⁺/K⁺ ratio in shoots between Dongdao-4 and Jigeng-88 plants were detected when challenged by salt stress, suggesting that Na⁺ toxicity may not underpin the greater tolerance of Dongdao-4 to salt stress than that of Jigeng-88. We further demonstrated that Dongdao-4 plants had greater capacity to accumulate soluble sugars and proline (Pro) than Jigeng-88, thus conferring greater tolerance of Dongdao-4 to osmotic stress than Jigeng-88. Moreover, Dongdao-4 suffered from less oxidative stress than Jigeng-88 under salt stress due to higher activities of catalase (CAT) in Dongdao-4 seedlings. Finally, RNA-seq revealed that Dongdao-4 and Jigeng-88 differed in their gene expression in response to salt stress, such that salt stress changed expression of 456 and 740 genes in Dongdao-4 and Jigeng-88, respectively.

Conclusion: Our results revealed that Dongdao-4 plants were capable of tolerating to salt stress by enhanced accumulation of Pro and soluble sugars to tolerate osmotic stress, increasing the activities of CAT to minimize oxidative stress, while Na⁺ toxicity is not involved in the greater tolerance of Dongdao-4 to moderate salt stress.

Keywords: Dongdao-4; Jigeng-88; Moderate salt stress; Oryza sativa L.; Osmotic regulation; ROS detoxifying mechanism; Rice.

Fig. 1

Effect of salt stress on contents of soluble sugar (a) and Proline (b) in shoot of Dongdao-4 and Jigeng-88 seedlings grown at normal and salt stress conditions. Expression levels of putative proline synthase genes (Os05g0455500 and Os01g0848200; c and d), transporter genes (Os03g0644400 and Os07g0100800; e and f) in the two rice genotypes were analysed. Total RNA was extracted from rice seedlings grown under control and salt stress conditions for one week. Transcript levels were measured by real-time PCR. Actin was used as an internal control. Error bars are calculated based on three biological replicates. Data are means ±SE (n ≥ 4). Means with different letters are significantly different (P < 0.05) within the same treatments. Asterisks (*) indicate significant differences between control and salt stress of the same genotype which were determined by Student’s t-test (** 0.001 < P < 0.01, *** P < 0.001)

Fig. 2

Effects of osmotic stress on growth performance of Dongdao-4 and Jigeng-88 plants after transferred to solution containing40 mM mannitol for 1 day, and 80 mM mannitol for 1 day, and then exposed to 120 mM mannitol for 5 days, finally recovered for another 3 and 15 days. Bars, 10 cm

Fig. 3

Effect of salt stress on accumulation of reactive oxygen species (ROS) and contents of oxidants and antioxidant enzymes in shoot of Dongdao-4 and Jigeng-88 seedlings. ROS accumulation in Dongdao-4 and Jigeng-88 was detected at 5 day with 3,3′-diaminobenzidine (DAB) (a) and nitroblue tetrazolium (NBT) staining (b). Bar, 0.1 cm. c malondialdehyde (MDA), d catalase (CAT), e peroxidase (POD), f superoxide dismutase (SOD). Data are means ±SE (n ≥ 4). Means with different letters are significantly different (P < 0.05) within the same treatments. Asterisks (*) indicate significant differences between control and salt stress of the same genotype which were determined by Student’s t-test (*0.01 < P < 0.05, **0.001 < P < 0.01, *** P < 0.001)

Fig. 4

Summary of the numbers of total and shared genes differentially expressed upon treatment by salt stress in shoots of two rice genotypes. a The number of genes up- or -down-regulated by salt stress. b A venn diagram showing the genes up-regulated by salt stress. The numbers of genes shared and distinct to each genotype are shown. c A Venn diagram showing the genes down-regulated by salt stress. The numbers of genes shared and distinct to each genotype are shown

Fig. 5

Singnificantly enriched GO terms at biological process ontology level for specifically up- (a) and down-regulated (b) DEGs in shoots of Dongdao-4 and Jigeng-88. GO terms were defined as significant enriched if false discovery rate (FDR) was ≤0.05. Green and red bars represent significant enriched GO terms in Dongdao-4 and Jigeng-88, respectively. Blue and grey bars represent GO terms which were not significantly enriched in Dongdao-4 and Jigeng-88, respectively