3.1. OsMPL423 sequence analysis
The OsMLP423 gene is located on chromosome 4 of rice, with an open reading frame of 474 bp and encoding 157 amino acids. Some homologous genes of OsMLP423 were analyzed, and their amino acid sequences were compared (Fig. 1a). Approximately 1500 bp upstream of the ATG was analyzed by PlantCARE and found to contain multiple elements associated with the stress response (Fig. 1b). These elements include CAAT-box (common cis-acting elements in promoter and enhancer regions), TATA-box response element, Box-4 (cis-acting element involved in light response), ABRE (ABA cis-acting element), CGTCA-motif (Jasmonic acid response cis element), MBS response element, MBS (MYB binding site involved in drought induction), etc. Through the analysis of the RiceGE (Gene Expression Map) website, it is found that OsMLP423 is expressed in all tissues of rice, and its expression level in roots and young stems is significantly increased under drought and salt stresses.
Figure 1. OsMLP423 bioinformatics and expression level analysis. (a) Analysis of homologous amino acid sequence of OsMLP423. (b) Analysis of the relevant elements of the promoter region.
3.2 OsMLP423 localizes to the nucleus and the membrane system
In order to detect the subcellular localization of OsMLP423 in plant cells, OsMLP423 was fused with a GFP reporter gene and transiently expressed in tobacco tissues under the initiation of a strong 35S promoter. The results showed that the GFP signal of the OsMLP423-GFP fusion protein was mainly localized in the nucleus. OsMLP423 was also distributed in the membrane system, and the GFP signal of some OsMLP423-GFP fusion proteins was fused with the membrane localization signal (GFP) (Fig. 2b).
3.3 GUS staining of promoter transgenic plants
To confirm the tissue-specific expression pattern of OsMLP423, we analyzed the activity of β-glucuronidase (GUS) in transgenic plants controlled by the OsMLP423 promoter. Consistent with the RT-PCR results, we found that the OsMLP423 gene was expressed in stems, leaves and spikes (Fig. 2a). After exposing OsMLP423 to different stress treatments, the GUS signal in the treatment group was significantly stronger than that in the control group (Fig. 2c), further indicating that OsMLP423 may be involved in multiple stress responses. The GUS signal was particularly strongly expressed in roots (Fig. 2d), which was consistent with the prediction results of the website, indicating that OsMLP423 may improve osmotic stress by regulating the root water uptake.
Figure 2. Analysis of OsMLP423 expression pattern. (a) OsMLP423pro:GUS transgenic plants showed staining of different tissues. (I: glumes, II: stems, III: mature leaf). (b) Subcellular localization of OsMLP423 in tobacco. Scale bars are 20 mm. (c) GUS staining of OsMLP423pro:GUS transgenic plants under different stress treatments. (Ⅰ: CK; Ⅱ: ABA; Ⅲ: PEG-600; Ⅳ: NaCl ; Ⅴ: 42℃; Ⅵ: 4℃). (d) Detection of GUS activity in roots of OsMLP423 promoter under abiotic stress.
3.4 OsMLP423 is highly induced by various stresses
The relative expression levels of OsMLP423 in overexpression lines were verified by qRT-PCR. Compared with the gene expression data on the RiceGE (gene expression map) website, it was found that both drought and saline-alkali stress treatments induced the expression of OsMLP423. The expression of OsMLP423 under stress treatment was further analyzed by RT-PCR, and it was found that the gene had a higher expression level under salt stress and drought stress compared with control (Fig. 3a, b, c).
Figure 3. Expression analysis of OsMLP423 under different stresses. (a) RT-PCR analysis of expression levels of OsMLP423 over-expressing plants. (b) and (d) RT-PCR analysis of OsMLP423 (b: 50 µM ABA; (c)20% w/v PEG; d: 150 mM NaCl).
3.5 Enhanced tolerance of OsMLP423-overexpressing transgenic plants to drought stress
To verify the importance of OsMLP423 overexpression to drought stress tolerance, wild-type and overexpressed young seedlings were treated with 20% w/v PEG solution, and a few overexpressed plants were randomly selected to measure the plant height after one week of treatment. Plant height of transgenic and wild-type plants was not different under control conditions However, after 7 days of 20% w/v PEG stress, the heights of transgenic lines OE7-4, OE8-3 and OE11-1 were significantly higher than the wild type (Fig. 4a, b). The above-mentioned young seedlings were transferred to sandy soil for two weeks, and then watering was stopped for 5 days (leaves started curling), and then returned to normal watering for 12 days. Compared with wild-type seedlings, OsMLP423-overexpressing plants showed less severe symptoms of drought stress, with delayed and less leaf curling. After restoration of watering, wild-type plant survival was 32%, and overexpression lines had an average survival rate of 60% (Fig. 4c, d, e).
Figure 4. Drought stress treatment of OsMLP423 transgenic lines. (a) OsMLP423 transgenic plants grown at 20% w/v PEG for 7 days. (b) Relative plant height. (c) and (d) Phenotypic differences between OsMLP423 over-expressing and wild-type plants during recovery after drought treatment. (e) Survival rate. Values are mean ± SE (n = 3). Asterisks indicate significant differences between transgenic lines and WT (*P < 0.05, **P < 0.01). n = 20 plants per treatment.
3.6 Enhanced tolerance of OsMLP423-overexpressing transgenic plants to salt stress
To verify the effect of OsMLP423 overexpression on tolerance to salt stress, the overexpressed seedlings were treated with 150 mM sodium chloride solution, and the control group was treated with standard solution without NaCl. After 7 days of treatment, we observed that the height of overexpressing plants was similar to wild type under control conditions. However, after 150 mM NaCl treatment, the plant heights of the overexpression lines OE7-4, OE8-3 and OE11-1 was significantly higher than
that of the wild-type plants (Fig. 5a, b). After NaCl treatment, plants were transferred to the standard solutions without NaCl for 10 days, and average survival rate (63%) of the overexpressing lines was significantly higher than that of the wild type (40%) (Fig. 5c, d, e).
Figure 5. OsMLP423 overexpression in the response to salt stress. (a) OsMLP423 transgenic plants after 150 mM NaCl for 7 days. (b) Relative plant height. (c) and (d) Phenotypic differences in OsMLP423 over-expression and wild-type recovery after 150 mM NaCl treatment. (e) Survival rate. Values are mean ± SE (n = 3). Asterisks indicate significant differences between transgenic lines and WT (*P < 0.05, **P < 0.01). N = 20 plants per treatment.
3.7 OsMLP423-overexpressing lines are sensitive to ABA
To further confirm the involvement of OsMLP423 in ABA-dependent stress responses, the ABA sensitivity of OsMLP423 lines was examined by analyzing seed germination and seedling growth. The germination rate of wild-type plants was 90%, while the germination of seeds of overexpressed lines was delayed by 3 days and the germination rate was significantly lower (Fig. 6a, b, c). Early-stage overexpression seedlings (bud length about 2–3 mm) were treated with 50 µM ABA, and standard nutrient solution without ABA was used in the control group. After 9 days, we randomly selected several overexpressing seedlings to measure plant height. Under control conditions, the height of overexpressing plants was similar to that of wild type. However, after treatment with 50 µm ABA, the plant height of WT was 1.6-2 times that of overexpressing plants (Fig. 6d, e). This result indicates that overexpression of OsMLP423 can enhance the sensitivity of rice to ABA. Exogenous ABA resulted in slower seed germination. The assumed increase of endogenous ABA concentration activated the expression of downstream stress-related genes, and plants enhanced their stress resistance by reducing plant height.
We analyzed the changes in the expression levels of the ABA synthesis and response genes under drought and salt stress treatments by RT-PCR. The results showed that under drought treatment conditions, the expression of ABA synthesis gene OsNCED3; OsNCED4 and ABA response genes OsRAB21 and OsLEA3 was significantly increased in overexpressed plants and was higher than in wild-type plants (Fig. 6f, g, h, i).
Figure 6. ABA treatment of OsMLP423-overexpressing transgenic plants. (a) Germination of OsMLP423 over-expressing plants treated with 50 µM ABA for 7 days. (b) Line graph of germination rate of OsMLP423 over-expressing plants treated with 50 µM ABA for 7 days. (c) OsMLP423 transgenic plants treated with 50 µM ABA for 9 days. (d)and(e) Relative plant height. (f), (g),(h) and (i) The analysis of the expression levels of, respectively, OsNCED3, OsNCED3,OsRAB21 and OsLEA3 under 20% w/v PEG stress.
3.8 OsMLP423 overexpression affects the ROS accumulation and scavenging under different stresses
To detect the activities of enzymes that scavenge reactive oxygen species under stress conditions, we examined the accumulation of reactive oxygen species (ROS) in OsMLP423-overexpressing lines under drought and salt treatments, and assessed the accumulation of superoxide anion (O2−) and hydrogen peroxide (H2O2) by DAB and NBT staining, respectively. Under control conditions, there was no obvious color difference between Nipponbare and transgenic plants. After drought and salt treatments, DAB-stained transgenic leaves had lighter surface browning compared to the control (Fig. 7a), and NBT-stained leaves had fewer surface spots than Nipponbare (Fig. 7b). This further indicated that the overexpressed transgenic plants produced less reactive oxygen species than wild-type plants under drought and salt stress.
Figure 7. Accumulation of reactive oxygen species in OsMLP423 transgenic plants. (a) DAB staining was used to detect superoxide anion (O2−) in the plants overexpressing OsMLP423 before and after drought and salt treatments; (b). Hydrogen peroxide accumulation in OsMLP423 over-expressing plants before and after drought and salt treatments was detected by NBT staining.
In addition, the activities of SOD, POD and CAT under salt and drought stress treatments were determined. The results showed that there was no significant difference in the activities of superoxide dismutase, peroxidase and catalase between the overexpression lines and the wild type under the control conditions, whereas the activities of the three enzymes were significantly increased under the salt and drought stress conditions. The OsMLP423-OE line exhibited significantly higher enzymatic activity than WT (Fig. 8a, b, c). We also found that the content of malondialdehyde in the overexpression line was significantly lower than that in the wild type, indicating that the OsMLP423-OE line had less intracellular oxidative damage under stress conditions (Fig. 8d).
Figure 8. Physiological activity of WT and transgenic plants in response to drought and 150 mM NaCl. (a) SOD activity. (b) POD activity. (c) CAT activity. (d) MDA content. WT: wild type rice; OE7-4, OE8-3 and OE11-1 are three independent transgenic lines.