Soil salinization seriously affects crop yield. The loss of crops in irrigated areas due to soil salinization in the world is up to 11 billion US dollars (Zhou et al., 2012). Salinization increases the soil osmotic pressure, decreases the water permeability and permeability, reduces the effective utilization rate of nutrients, affects the normal growth of plants, and significantly reduces crop yield (Li et al., 2018; Yin, 2011).

Potato is the fourth largest food crop in the world (Barrell et al., 2013), and the degree of soil salinization is increasing year by year, which seriously affects the development of potato industry. Maintaining the balance of Na⁺ and water is a prerequisite for ensuring the normal growth of plants. Salt stress can inhibit plant growth, accelerate leaf senescence and reduce yield. Gong (1996) showed that there were significant differences in salt tolerance among different potato varieties. Wang (1998) analyzed the salt tolerance of potato test tube seedlings and found that under salt stress, the biological yield of test tube seedlings decreased, and the leaf area growth and dry matter accumulation were inhibited.

Plants respond differently to salt stress. Zhang et al. (2015) analyse the growth and changes of endogenous hormone content in leaves of Corylus heterophylla under saline alkali stress showed that the biomass and endogenous hormone level of Corylus heterophylla under saline alkali stress were significantly changed, specifically, the net increase of plant height, new shoot length, leaf number, leaf water content, aboveground biomass and total biomass were reduced, the root shoot ratio was significantly increased, and the endogenous zeatin riboside (ZR), indoleacetic acid (IAA) The content of abscisic acid (ABA) and gibberellin (GAs) changed significantly, and the ratio of (GA+IAA+ZR)/ABA decreased significantly. High salt stress significantly increased the content of endogenous hormones ABA and ethylene in maize plants, while the content of IAA, GAs and cytokinins (CKs) decreased significantly (Tuna et al., 2008). Studies have shown that when kidney beans are under salt stress, endogenous ABA content increases significantly, which can reduce leaf expansion speed and stomatal conductance, thereby reducing leaf water transpiration and reducing salt ion damage (Cachorro et al., 1995). When the effect of salt stress is slight, the content of GAs increases, significantly reducing leaf stomatal resistance, accelerating transpiration, and enhancing effective water use (Zhang et al., 2006).

In recent years, there have been many studies on the effects of NaCl stress on the physiological characteristics and photosynthesis of potato, but the mechanism of the response of endogenous hormone levels in potato plantlet to NaCl stress is still unclear. Therefore, in this study, the plantlet of the common potato cultivar "Atlantic" were used as experimental materials, and salt stress treatment was carried out after 7 days of culture under normal conditions. According to the growth status of the plantlet, samples were taken on the 1st, 9th, 15th and 22nd days after stress, and the response characteristics of the plantlet to salt stress were explored by measuring and analyzing the changes of endogenous hormone levels in the plantlet, which provides a reference for the study of salt stress response mechanism, salt tolerance characteristics and salt resistance mechanism of potato.

1 Results and Analysis

1.1 Effect of salt stress on cytokinin (CKs) content in roots and leaves of potato plantlet

Zeatin (Z) and zeatin riboside (ZR) are plant growth regulators of cytokinins, which can promote plant cell division, prevent chlorophyll and protein degradation, maintain cell vitality and delay plant senescence. Whether under salt stress or not, the content of Z and ZR in the leaves of potato plantlet gradually decreased with the extension of culture days, but the content of ZR was significantly higher than that of Z in the same period. For example, on the 22nd day of stress, the content of Z and ZR in fresh leaves of stress treatment reached the minimum, reaching 31.07 ng/g and 80.73 ng/g respectively (Figure 1A; Figure 1B). At the same time, the content of Z+ZR in leaves of plantlet under salt stress was also significantly lower than that of the control (Figure 1C); The content of ZR in leaves of plantlet under NaCl stress is about 1.4~2.6 times that of Z, while the content of ZR in the control group is about 2.6~4.5 times that of Z. It can be seen that under salt stress, the content of ZR in leaves of plantlet decreases faster than that of Z, thus increasing the Z/ZR ratio (Figure 1D).

Different from leaves, the Z content of roots of potato plantlet in treatment and control gradually increased with the increase of culture days, and the Z content in salt stress treatment was significantly higher than that in control (Figure 1E); However, the change trend of ZR content in roots of the same treatment and control is similar to that of leaves, and the treatment is significantly lower than that of the control. The difference of Z between the control and treatment is lower than that of ZR (Figure 1F), so the Z+ZR content in roots of plantlet under stress treatment is lower than that of the control, and gradually decreases with the extension of stress time (Figure 1G), but its Z/ZR ratio gradually increases (Figure 1H). Further comparison showed that ZR content in leaves and roots of potato plantlet was higher than Z content, but Z and ZR content in leaves were higher than that in roots. It can be seen that the content of cytokinin in potato plantlet was reduced to varying degrees under salt stress.

Figure 1 The effect of salt stress on CKs content of potato plantlet

1.2 Effect of salt stress on IAA content in roots and leaves of potato plantlet

Indoleacetic acid (IAA) is the first discovered plant hormone, which has a significant promoting effect on plant growth and development. When plants are under stress, the auxin synthesis process is inhibited. The IAA content in the leaves of potato plantlet under salt stress was significantly lower than that of the control, and it increased first and then decreased with the increase of culture time. At the 22nd day of culture (the 15th day of stress treatment), the IAA content in the control group was about 2.78 times that of the treatment group (Figure 2A). Different from the leaves, the IAA content in the roots of potato plantlet under salt stress was significantly higher than that in the control, and the IAA content decreased first and then increased with the extension of culture time, and reached the minimum value at the 22nd day of culture (the 15th day of stress treatment) (Figure 2C). The content of Z+ZR in the roots of plantlet decreased with the prolongation of culture days, and the content of Z+ZR under salt stress was lower than that of the control (Figure 1F), so the IAA/(Z+ZR) ratio in the roots increased - decreased - increased with the prolongation of time, and reached the maximum value at the 22nd day of stress, but the ratio of the control group was smaller than that of the treatment group (Figure 2D). It can be seen that under salt stress, IAA content and IAA/(Z+ZR) in leaves of potato plantlet decreased, but IAA content and IAA/(Z+ZR) in roots increased significantly.

Figure 2 The effect of salt stress on the content of IAA and IAA/(Z+ZR) ratio of potato plantlet

1.3 Effect of salt stress on ABA and ACC contents in roots and leaves of potato plantlet

ABA is a kind of stress hormone, which is extremely important in regulating the adaptation of plants to adversity, mainly through regulating the opening and closing of stomata, maintaining the water balance in tissues, enhancing the water absorption of roots, and improving the water conductivity to enhance plant resistance. Under salt stress, the content of ABA in leaves and roots of potato plantlet increased significantly, and the longer the stress time was, the higher the content of ABA was, but the change of ABA content in the control group in the same period was small (Figure 3A; Figure 3C); On the 22nd day of salt stress, the endogenous ABA content in leaves and roots of salt stressed plantlet reached the maximum. At the same time, it can be seen from the figure that ABA content in leaves of potato plantlet is much higher than that in roots, which may be related to the function of ABA under stress. ACC is the direct precursor of ethylene synthesis. Under stress conditions, the content of ACC and ethylene in plants increases. Under salt stress, the content of ACC in leaves and roots of potato plantlet increases gradually with the prolongation of culture days. The difference between treatment and control gradually increases, and reaches the maximum on the 22nd day of salt stress. At this time, the content of ACC in leaves of the control group is significantly lower than that of the stress treatment group, In the root, the stress treatment group was significantly higher than the control group (Figure 3B; Figure 3D). It can be seen that salt stress significantly increased ABA content in leaves and roots and ACC content in roots of potato plantlet, but had no significant effect on ACC content in leaves.

Figure 3 The effect of salt stress on the ABA and ACC content of potato plantlet

1.4 Effect of salt stress on hormone ratio of roots and leaves of potato plantlet

The level of endogenous hormones in plants is closely related to the normal growth of potatoes (Roumeliotis et al., 2012). Therefore, the changes of hormone contents in leaves and roots of plantlet can be used to explain the growth rate and biomass distribution of plantlet under salt stress.

First, compared with the control, the content of Z in the leaves of potato plantlet under salt stress decreased, while the content of Z in the roots increased, so its Z_Root/Z_Leaf ratio increased significantly with the number of days of culture, and was significantly higher than the control (Figure 4A); Under the same culture conditions, the change trend of ZR_Root/ZR_Leaf ratio of the plantlet under salt stress was similar to that of the control, showing that it decreased first and then increased, and reached the minimum value at the 9th day of stress treatment, but the ratio was always lower than that of the control (Figure 4B); As a result of these changes, the (Z+ZR)_Root/(Z+ZR)_Leaf ratio of the plantlet gradually increased significantly with the increase of culture days, and reached the maximum value at 22 days of stress. At this time, the (Z+ZR)_Root/(Z+ZR)_Leaf ratio under salt stress was significantly higher than the control (Figure 4C). Secondly, different from other hormone ratios, the IAA_Root/IAA_Leaf ratio of the plantlet forced by salt stress decreased rapidly at first, and then increased linearly after 15 days of stress, finally reaching the maximum value, but the IAA_Root/IAA_Leaf ratio of the control group did not change much in the whole culture process (Figure 4D). Thirdly, similar to Z_Root/Z_Leaf, the ratio of ABA_Root/ABA_Leaf and ACC_Root/ACL_leaf of the plantlet under salt stress gradually increased with the prolongation of culture days, and the ratio of ABA_Root/ABA_Leaf in the control group also showed a similar trend, but the ratio of ACC_Root/ACL_leaf in the control group gradually decreased (Figure 4E; Figure 4F). In addition, when the ratio of endogenous hormones between root and leaf of the plantlet was≥1, it indicated that the hormone content in root was higher than that in leaf, or equivalent to that in leaf; When the ratio<1, the hormone content in leaves was higher than that in roots. Therefore, under salt stress, ZR and ABA contents in leaves of the plantlet were significantly higher than those in roots, while Z, IAA and ACC contents were lower than those in roots. The results showed that under salt stress, the distribution of endogenous hormones in roots and leaves of the plantlet was readjusted, and they actively responded to the dynamic balance and coordinated with each other, so that they could adapt to the external stress more effectively.

Figure 4 The effect of salt stress on the ratio of potato plantlet root and leaf

2 Discussion

Salt stress can cause various physiological and biochemical responses of plants, such as the reduction of photosynthetic rate and cell water potential. The most intuitive expression is that growth and development are inhibited, and biomass is one of the most direct indicators to measure plant salt resistance (Yao et al., 2015).

As an important plant growth regulator, hormone is a kind of micro organic substance synthesized in plants to regulate plant growth and development. In recent years, important progress has been made in the research on the regulation of plant hormones in response to salt stress (Liu, 2018), for example, ethylene is involved in regulating the inhibition of salt stress on root growth (Li et al., 2015). First of all, under salt stress, the synthesis and transportation of CTK in roots of plants are blocked, and the level of CTK in plants is reduced, leading to plant growth inhibition and premature senescence (McSteen and Zhao, 2008). This study showed that under salt stress, the Z and ZR content in leaves decreased, while the Z content in roots increased while the ZR content decreased, but the increase in Z was less than the decrease in ZR. Therefore, under salt stress, the synthesis of plant like growth regulators Z and ZR is blocked, which reduces the Z+ZR content in the leaves and roots of plantlet. By inhibiting the excessive growth of plants, they consume their own nutrients and water to ensure normal life activities of plants. Secondly, IAA is involved in multiple physiological processes of plants. Wang et al. (2009) responded to salt stress by regulating IAA content in Arabidopsis thaliana and controlling lateral root formation and elongation. ABA is a key factor for plants to respond to stress and a signal molecule in response to abiotic stress (Zhang et al, 2006; Chaves et al., 2009). ABA is a large amount synthesized by plants under stress, which is mainly used to regulate stomatal movement, prevent water loss of plants, and ensure normal life activities of plants (Jia and Luch, 2003; Liu et al., 2008; Zhou et al., 2010; Yan et al., 2011; Sun et al., 2013).

The results of this study show that ABA content in roots and leaves of potato plantlet increases under salt stress. However, there are few studies on the relationship between IAA and salt stress. In this study, NaCl (100 mmol/L) stress reduced IAA content in roots and leaves of potato plantlet, and the fresh weight of roots also decreased. Thirdly, ethylene is a gaseous plant hormone, which can regulate many plant growth and physiological processes. In recent years, there have been many studies on the ethylene synthesis precursor ACC. During seed germination, the amount of ethylene synthesized by the ethylene synthesis precursor ACC forced by salt stress significantly decreased (Zapata et al., 2004; Chang et al., 2010; Meng et al., 2014). In this study, under salt stress, the ACC content in the leaves of the plantlet was significantly lower than that in the control, but the ACC content in the roots increased instead, which may be the result of interaction with other hormones. In addition, under salt stress, the distribution of endogenous hormones in plants will be readjusted due to stress, so the hormone ratio in roots and leaves will change. The results of this study showed that the ratios of Z_Root/Z_Leaf, (Z+ZR) _Root/(Z+ZR) _Leaf, IAA_Root/IAA_Leaf, ABA_Root/ABA_Leaf and ACC_Root/ACCL_eaf of the plantlet under salt stress were significantly increased, while the ratios of ZR_Root/ZR_Leaf were significantly decreased. It can be seen that when the plantlet are under salt stress, the internal hormones actively respond to the dynamic balance and coordinate with each other, so as to adapt to the external stress.

To sum up, under salt stress, the contents of endogenous hormones Z, ZR, IAA, ABA and ACC in roots and leaves of potato plantlet changed to varying degrees, which positively responded to salt stress and provided a theoretical basis for the study of salt resistance mechanism of potatoes and the cultivation of new salt resistant varieties.

3 Materials and Methods

3.1 Material cultivation and treatment

The plantlet of common potato cultivar "Atlantic" were provided by Gansu Provincial Key Laboratory of Crop Genetic Improvement and Germplasm Innovation. The stronger single node stem segments of potato plantlet were inoculated into the liquid MS (Murashige and Skoog, 1962) medium (MS+sucrose 3%, without agar, pH 5.8), and each bottle was cut into 5 stem segments, a total of 80 bottles. The incubation temperature was (23±1)℃, the illumination time was 16 h/d, the illumination intensity was 2000 lx, and the incubation time was 7 days. Four expanded leaves were grown. Take the liquid MS medium containing 100 mmol/L NaCl as the salt stress medium, and the liquid MS medium without NaCl as the control, respectively replace the previous liquid MS medium for salt stress treatment (40 bottles for the control and 40 bottles for the treatment). Culture under the same conditions, take samples respectively after 1 day, 9 days, 15 days and 22 days of culture, take 10 bottles for the control and treatment each time, and measure the relevant indicators respectively.

3.2 Determination of endogenous hormone content

The extraction methods of zeatin (Z), zeatin Riboside (ZR), auxin (IAA) and abscisic acid (ABA) were improved according to Dobrev and Kamínek (2002). Weigh about 3 g of fresh materials (roots or leaves), grind them into powder with liquid nitrogen, extract them overnight at - 20 ℃ with 5 mL of methanol/water/formic acid (15:4:1, v:v:v, pH 2.5) mixture precooled at -20 ℃, centrifugate at - 20 ℃ for 15 min, take the supernatant, add 2 mL of the extract under the same conditions, extract for 30 min, centrifugate and combine the supernatant, pass the pre activated C18 column, and obtain the mixed sample containing all hormones. According to Dobrev and Kamínek (2009) methods, cytokinins (Z and ZR) were separated from mixed samples. The separation methods of IAA and ABA were similar. The Z, ZR, IAA and ABA separated were quantified by high performance liquid chromatography (HPLC).

The content of 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of ethylene synthesis, was determined according to the method of Lizada and Yang (1979) and improved. Weigh 2 g of material, add 5 mL of 0.2 mol/L TCA (trichloroacetic acid), grind it into slurry in ice bath, extract it for 12 h at 4 ℃, centrifuge it for 20 min at 12 000 g and 4℃, take the supernatant for storage at 4 ℃, add 1.5 mL of supernatant into penicillin vial, inject 0.5 mL of mixed solution (5% sodium hypochlorite: saturated sodium hydroxide=2:1), keep it still at 4 ℃ for 2 h, shake it for 1 min, and then keep it still for 5 min, extract a certain amount of gas, and use gas chromatography (GC) to quantify it, Determine the content of ACC by detecting the synthetic amount of ethylene. Analyze and draw the standard curve with their respective standard samples.

HPLC separation conditions: improved according to the method of Svačinová et al. (2012). The determination conditions of Z and ZR are the same. The mobile phase: ethyl acetate: methanol: acetonitrile=5:590 (v:v:v); The chromatographic column is C18 column (250 mm×4.6 mm, 5 μm); Detection wavelength: 450 nm; Flow rate: 1 mL/min; Sample volume; 20 μL; Column temperature: 25 ℃. IAA and ABA determination conditions are similar, and the chromatographic column used is Eclipse plus C18 (250 mm×4.6 mm, 5 μm) The mobile phase is 20% acetonitrile and 0.5% acetic acid, the detection wavelength of IAA is 254 nm, the detection wavelength of ABA is 260 nm, the flow rate is 1 mL/min, the injection amount is 20 µL, and the column temperature is 25 ℃. ACC determination conditions: PorapakQ chromatographic column, column length 2 m, inner diameter 3 mm, detection port temperature 150 ℃, injection port temperature 130 ℃, column temperature 60 ℃, gas hydrogen flow 25 mL/min, air flow 300 mL/min.

3.3 Data statistics and analysis

The test was repeated for 5 times, and all data were expressed as mean ± standard error. The data were statistically analyzed by using SPSS 19.0 software. After the analysis of variance (ANOVA) of the data among the treatments, Duncan method was used to detect the significance of the difference, the level of significance of the difference α=0.05.

Authors’ contributions

WDX was the executor of the experimental design and research, and completed the revision of the thesis; CD participated in data analysis; WJM participated in the experimental design, and LZR was the designer and director of the project. All authors read and approved the final manuscript.

Acknowledgements

This research was supported by Youth Program of Natural Science Foundation of Qinghai Province (2020-ZJ-972Q).

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