The role of exogenous melatonin in modulating root morphology and physiological response to drought in soybean seedlings

doi:10.21203/rs.3.rs-4774239/v1

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The role of exogenous melatonin in modulating root morphology and physiological response to drought in soybean seedlings

https://doi.org/10.21203/rs.3.rs-4774239/v1

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This study investigated how melatonin helps soybean seedlings grow in drought conditions by studying root morphology and physiological characteristics in the 'Suinong 26' variety. The experiment started at the V₂ stage with water control and melatonin spraying. Three treatments were established: drought stress (D), drought stress with melatonin spraying (D + M), and normal water supply as control. The study compared the effects of melatonin spraying on the growth of soybean seedling roots and shoots. Compared to control, D treatment inhibited soybean seedling growth significantly. D + M treatment increased root length, volume, and dry weight on days 10 and 20. Root activity and antioxidant enzyme activities, as well as sugar and protein contents, also increased on days 3, 6, and 9. Root morphological and physiological indicators were significantly improved, correlating positively with shoot photosynthesis and dry matter accumulation. Melatonin spraying enhances soybean seedling growth under drought stress by regulating root characteristics and key physiological factors, leading to improved photosynthesis, shoot development, and dry matter accumulation.

Antioxidant enzyme

Drought tolerance

Melatonin

Root growth

Soybean seedling

Soy is a major source of high-quality plant protein and oil for humans (Hartman et al., 2011), playing an essential role in livestock production. Additionally, due to its rich content of bioactive substances such as isoflavones, peptides, lecithin, saponins, and oligosaccharides (Mark, 2016), soybean is widely used in the healthcare industry. Moreover, soybean significantly promotes sustainable agriculture worldwide thanks to its excellent nitrogen-fixing capabilities (Peoples, 1996) and strong environmental adaptability (Foyer et al., 2016). Given the foreseeable future, the demand for soybeans will continue to rise. However, due to limitations in arable land and cropping patterns, the area under soybean cultivation cannot be substantially increased. Thus, improving yield per unit area is imperative, requiring breakthroughs in both varieties and cultivation techniques. Despite this necessity, innovations in soybean breeding and cultivation have stagnated, and soybean production faces numerous stressors, including drought (Sabagh et al., 2019). Drought stress, with its high frequency and extensive impact (Dai, 2013), is the most significant abiotic constraint on global food production. In the foreseeable future, the misalignment between rainfall distribution and crop water storage needs will intensify (Trenberth et al., 2015), severely hindering further increases in soybean yield.

Numerous studies confirm that melatonin application can significantly improve crop growth under drought stress and elucidate the corresponding physiological mechanisms. On one hand, melatonin participates in ABA metabolism to regulate stomatal closure (Li et al., 2021), enhances root morphological traits such as root length, lateral root number, and root surface area, thereby increasing water uptake capacity and improving water retention ability under drought conditions (Cui et al., 2017; Silalert & Pattanagul, 2021; Wang et al., 2016), which is crucial for maintaining necessary physiological activities. On the other hand, melatonin exhibits robust ROS scavenging abilities under drought stress. Specifically, (1) melatonin directly reacts with reactive oxygen species like superoxide anion (O₂⁻), hydroxyl radical (OH·), and singlet oxygen (¹O₂), reducing their concentrations (Arnao, 2014); (2) it activates ROS-scavenging enzymes such as ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione peroxidase (GPX), and glutathione transferase (GST) (Cui et al., 2017); (3) it promotes the accumulation of non-enzymatic antioxidants like ascorbic acid (AsA), glutathione (GSH), and oxidized glutathione (GSSG), thereby enhancing the plant's antioxidant capacity (Altaf et al., 2022). By regulating water status and ROS levels, melatonin inevitably improves photosynthetic capacity and nitrogen nutritional status under drought stress. This includes enhancing light energy absorption and promoting electron transport in PSII (Dong et al., 2018), increasing photosynthetic pigment content (Ahmad et al., 2021), improving gas exchange related to photosynthesis (Wang et al., 2021), and boosting the activity of key enzymes involved in nitrogen metabolism and nitrogen accumulation in soybean under drought stress (Cao et al., 2022).

However, most studies elucidate the mechanisms by which melatonin promotes crop growth under drought stress from the perspective of adaptive adjustments at the leaf physiological and molecular levels. Though some research addresses root morphology and physiological indices, there is a lack of comprehensive analysis on whether melatonin mitigates the adverse effects of drought stress through regulation of root morphology and physiology, thereby promoting soybean seedling growth. Therefore, this study uses 'Suinong 26' as the experimental variety, employing pot experiments to simulate drought stress and melatonin application. It aims to comprehensively analyze the relationship between root morphology and physiological indices and key growth indices of the aerial parts, enriching the understanding of the physiological mechanisms by which melatonin enhances drought resistance in soybeans through the synergy between roots and shoots.

Experimental Materials

The experiment was conducted at the National Coarse Grain Engineering Technology Research Center in the High-Tech Zone of Daqing City, Heilongjiang Province. The soybean variety 'Suinong 26' was used as the test material. One day before sowing, seeds of uniform shape, size, and free of damage or disease were selected and disinfected with 5% sodium hypochlorite. Pot cultivation combined with water control was used to simulate drought stress. The cultivation pots were 15.0 cm high and 10.0 cm in diameter, with six 0.5 cm diameter holes at the bottom. The cultivation substrate was black soil taken from the top 20 cm layer of a cornfield. Before filling the pots with substrate, two layers of window screen were placed at the bottom to prevent substrate leakage and restrict root growth within the pot. Subsequently, 1 kg of black soil, sieved to remove plant debris and stones, was added. Part of the black soil was used to measure soil moisture content, and the field capacity was determined using the ring knife method. Before sowing, each pot was thoroughly watered with tap water. Six seeds were evenly placed on the wet soil surface and covered with 2 cm of black soil to complete sowing. When the seedlings reached the V1 stage, thinning was performed, leaving three uniformly growing seedlings per pot. The pots were placed in a movable rain shelter to prevent rainwater interference.

Experimental Design

From sowing to the V2 stage, soil moisture content in all pots was maintained at 75%-80% of field capacity using the weighing method and supplemental watering (weighing soil moisture content at 8:00 and 16:00 daily and adding tap water as needed). The day the plants reached the V2 stage was designated as day 0. The uniformly growing seedlings were divided into three groups: the first group had soil moisture maintained at 75%-80% of field capacity and was sprayed with distilled water on day 0 at 20:30 until the leaves were covered with droplets but not dripping, serving as the control (CK); the second group underwent water control starting on day 0, reaching drought stress levels (45%-50% of field capacity) by day 3 and sprayed with distilled water like CK, serving as the drought stress treatment (D); the third group followed the same water control as D but was sprayed with 100 µmol/L melatonin solution on day 0 at 20:30, with the same spray amount as CK, serving as the drought stress plus melatonin treatment (D + M). The experimental soil had an organic matter content of 3.22 g kg^− 1, a bulk density of 1.22 g cm^− 3, and a pH of 6.83. The soil also contained 162.63 mg kg^− 1 of available nitrogen, 32.54 mg kg^− 1 of available phosphorus, and 183.69 mg kg^− 1 of available potassium.

Sampling Time

Samples were collected on days 0, 10, and 20. Representative plants were selected for measuring photosynthetic gas exchange parameters. After morphological index measurements, the seedlings were divided into roots, stems, and leaves, placed in envelopes, killed at 105°C for 30 minutes, and then dried at 80°C to a constant weight. Photosynthetic pigment content in mature leaves was measured on days 3, 6, and 9. Mature leaves and roots were quickly frozen in liquid nitrogen and stored at -80°C for physiological index measurements.

Measurement Indices and Methods

Morphological Indices

Plant height and root length were measured with a ruler; stem diameter was measured with a vernier caliper at the middle of the first internode (the cotyledon scar was designated as the 0th internode); leaf area was measured using a Yaxin-1241 leaf area meter (Beijing Yaxin Liyi Technology Co., Ltd.); dry matter was weighed on a balance, and growth rates from day 0 to day 10 and day 0 to day 20 were calculated using the following formula:

$$\:GR=\frac{\text{D}\text{M}\text{A}2-\text{D}\text{M}\text{A}1}{\text{D}\text{a}\text{y}\text{s}}$$

DMA1 represents the dry matter accumulation of soybean plants on day 0 (g·plant⁻¹), DMA2 represents the dry matter accumulation of soybean plants on days 10 and 20 (g·plant⁻¹), Days refers to the number of days elapsed from day 0 to days 10 or 20, and GR stands for the growth rate (g·day⁻¹).

Photosynthetic Indices

Photosynthetic pigment content was determined by immersing 0.1 g of soybean trifoliate functional leaves in 10 mL of absolute ethanol, kept in the dark at room temperature for 24 hours, following the ethanol colorimetric method of Lichtenthaler⁽Lichtenthaler 1987). The calculation formulas are as follows:

Chlorophyll a (Chla) = 13.95 OD₆₆₅ − 6.88 OD₆₄₉

Chlorophyll b (Chlb) = 24.96 OD₆₄₉ − 7.32 OD₆₆₅

Total chlorophyll content = Chla + Chlb

Carotenoids (Car) = (1000 OD₄₇₀ − 2.05 Chla − 114.8 Chlb) ÷ 245

Photosynthetic pigment content (mg·g^− 1FW) = (ΔOD × total extract volume × dilution factor) ÷(sample weight)

Net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), and intercellular CO₂ concentration (Ci) of functional leaves were measured using an LI-6400 portable photosynthesis system (LI-COR Biosciences, USA). The artificial red-blue light source was set to an intensity of 1000 µmol·m^− 2·s^− 1, airflow rate to 500 µmol·s^− 1, and a buffer bottle was used to maintain CO₂ concentration at 400 µmol·mol-1. Each treatment was replicated five times.

Physiological Indices

Soluble sugar content was determined following the method of Hu et al.( Hu et al. 2009). Soluble protein content was measured according to Bradford(Bradford 1976). Sucrose and fructose content were determined using the method of Cao et al.( Cao et al. 2019). Superoxide dismutase (SOD) activity was measured following Ries ( Ries 1977). Peroxidase (POD) activity was measured according to Choudhary et al.( Choudhary 2011). Catalase (CAT) activity was measured following Chance(Chance & Maehly 1954) and Fu(Fu & Huang 2001). Ascorbate peroxidase (APX) activity was determined using the method of Cakmak et al.( Cakmak & Marschner 1992).

Data Processing and Plotting

Microsoft Excel 2020 was used for data entry and organization, and RStudio was used for data analysis and plotting.

Growth of Soybean Seedlings under Drought Stress

As shown in Fig. 1, drought stress inhibited the growth characteristics of soybean seedlings, while the application of melatonin under drought stress improved seedling growth. Compared to the control (CK), the drought stress treatment (D) resulted in reductions of 75.35%, 29.99%, 66.16%, 46.34%, 38.35%, 64.04%, 63.42%, 57.03%, and 59.86% in growth rate, root length, root volume, plant height, stem diameter, leaf area, leaf dry weight, stem dry weight, and root dry weight, respectively (Fig. 1A, B, C, D). Compared to the D treatment, the application of melatonin under drought stress (D + M) further increased growth rate, root length, root volume, plant height, stem diameter, leaf area, leaf dry weight, stem dry weight, and root dry weight, with increases of 36.59%, 18.79%, 38.15%, 22.82%, 9.95%, 23.05%, 6.71%, 15.98%, and 18.68% on days 3, 6, and 9, respectively (Fig. 1A, B, C, D).

Root activity, antioxidant enzyme activities, and osmotic regulator content

As shown in Fig. 2, drought stress inhibited the physiological characteristics of soybean roots, while the application of melatonin under drought stress improved these physiological indicators. Compared to the control (CK), the drought stress treatment (D) increased root activity, SOD, POD, CAT, and APX activities by 43.75%-70.00%, 12.02%-29.63%, 15.07%-18.51%, 65.47%-175.90%, and 30.52%-31.19%, respectively (Fig. 2A, B). The contents of soluble sugars and soluble proteins in roots increased by 23.97%-51.33% and 30.85%-43.71%, respectively (Fig. 2C). Fructose content in roots increased by 24.48%, 29.29%, and 25.86%, while sucrose content decreased by 5.09%, 10.05%, and 18.11% (Fig. 2D). Compared to the D treatment, the application of melatonin under drought stress (D + M) further increased root activity, SOD, POD, CAT, APX, soluble sugars, soluble proteins, fructose, and sucrose contents, with increases of 33.33%-133.33%, 5.70%-9.74%, 2.66%-5.73%, 12.83%-31.55%, 11.81%-16.29%, 10.01%-14.07%, 11.41%-13.03%, 7.71%-11.99%, and 12.66%-21.24% on days 3, 6, and 9, respectively (Fig. 2A, B, C, D).

Photosynthetic Pigment Content and Gas Exchange Parameters

As shown in Fig. 3, drought stress inhibited the chlorophyll content and photosynthetic gas exchange parameters of soybean leaves, while the application of melatonin under drought stress improved these related indicators. Compared to the control (CK), the drought stress treatment (D) reduced chlorophyll a, chlorophyll b, total chlorophyll (a + b), carotenoids, Pn, Tr, Gs, and Ci by 26.83%-62.90%, 4.77%-44.16%, 31.36%-51.60%, 6.24%-53.68%, 42.80%-76.76%, 52.27%-85.24%, 14.77%-25.61%, and 9.32%-16.57%, respectively (Fig. 3A, B). Compared to the D treatment, the application of melatonin under drought stress (D + M) increased chlorophyll a, chlorophyll b, total chlorophyll (a + b), carotenoids, Pn, Tr, Gs, and Ci by 16.11%-24.40%, 4.22%-18.34%, 14.88%-23.78%, 5.12%-30.53%, 22.12%-53.61%, 38.75%-99.83%, 9.99%-10.36%, and 5.45%-8.08% on days 3, 6, and 9, respectively (Fig. 3A, B).

Correlation of Root Morphological and Physiological Indices with Key Growth Indicators of the Aboveground Parts

As shown in Fig. 4, the relationships among root morphological and physiological indices and their correlations with soybean plant morphology, dry matter accumulation, photosynthetic gas exchange parameters, leaf photosynthetic pigments, and MDA content were analyzed. The results showed that, except for a weak correlation between root sucrose content (Suc) and other root morphological and physiological indices, root activity (RVig) had a strong positive correlation with root length (RLen), root dry matter (RDM), and root volume (RVol), and a strong negative correlation with antioxidant enzyme activities, fructose content, soluble protein, and soluble sugars (Fig. 4A). Antioxidant enzyme activities had a strong positive correlation with fructose content, soluble protein, and soluble sugars (Fig. 4A, B). Mantel test results showed significant or highly significant positive correlations between all root morphological and physiological indices and plant morphology, dry matter accumulation, photosynthetic gas exchange parameters, and leaf photosynthetic pigment content, except for the nonsignificant correlation between root sucrose content and plant morphology (DY) and dry matter accumulation (DM), indicating that root morphology and physiology are crucial for the growth of soybean seedlings' aboveground parts. Further biplot analysis revealed that root morphological and physiological indices, except for sucrose content, were associated with most aboveground morphological traits, dry matter accumulation, photosynthetic gas exchange parameters, and leaf photosynthetic pigment content, with root dry matter accumulation being the most associated indicator. Additionally, root activity, root volume, root length, and root dry matter accumulation showed positive correlations with most aboveground morphological and physiological indices (Fig. 4B). Random forest analysis identified root activity, root length, and root volume as the top three root indices most important for the growth rate of soybean seedlings (Fig. 4C).

Upon sensing drought stress, plants activate their drought resistance mechanisms, making adaptive adjustments at molecular, physiological, and morphological levels. At the physiological level, levels of stress-related hormones as abscis acid (ABA) rapidly increase, while levels of growth-promoting hormones likeins and cytokinins decrease (Xu et al., 2016). Stomatal closure occurs, leading to restricted photosynthesis (Huang et al., 2019). The activities of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) are elevated to enhance reactive oxygen species (ROS) scavenging capabilities (Hasanuzzaman, 2020). Additionally, the contents of osmolytes like soluble sugars and proteins increase to improve osmotic adjustment capacity [36]. Morphologically, plant height, stem diameter, and leaf area all reduce, reflecting both a limitation in plant growth and an adaptive strategy to minimize aerial volume and water loss (Aroca et al., 2012). Concurrently, root growth patterns alter, with increases in root length and lateral root number, accompanied by a reduction in root diameter, to expand the root absorption zone and enhance water and nutrient uptake (Comas, 2013).

Roots, being primary organs for water absorption and early drought perception (Huang & Gao, 2000), are inherently involved in the plant’s response to drought stress. Lozano et al. (2020) highlighted that changes in root morphological characteristics represent adaptive strategies to drought stress, with these changes demonstrating systematic traits. Our study data reveal that after 20 days of drought treatment, soybean root length, root volume, and root dry matter accumulation were significantly lower than those in the well-watered control group (Fig. 1B), which contradicts some reports suggesting that drought stress promotes root development (Glynn, 2007; Fenta, 2014; Comas, 2013). This discrepancy may be due to the prolonged duration of drought stress severely limiting soybean growth. Furthermore, melatonin application was found to alleviate the inhibitory effects of prolonged drought on soybean root growth to some extent (Fig. 2A), promoting increases in root length, lateral root number, root volume, and root surface area, thereby enhancing water absorption capability under long-term drought stress.

Under drought conditions, plants require more efficient water uptake by roots to meet the demands of the aboveground parts, rendering root vitality crucial (Brunner et al., 2015). Studies have indicated that plants with higher root vitality exhibit stronger resilience to drought conditions (De Vries et al., 2016). Our results show that drought stress reduces root vitality, but melatonin application mitigates this inhibition (Fig. 2A), undoubtedly aiding soybeans in maintaining higher levels of water and nutrient absorption under drought stress (Comas et al., 2013), thus facilitating better conditions for aboveground growth.

Drought stress inevitably leads to the accumulation of reactive oxygen species () in various plant organs, including soybean roots. The buildup of ROS can cause multifaceted damage to root cells, disrupting their normal function and inhibiting growth and development, thereby exacerbating water uptake challenges for the plant (Jaleel et al., 2009). To counteract ROS accumulation, plants typically increase the activity of antioxidant enzymes and the levels of osmolytes. Our study shows that after 3, 6, and 9 days of drought stress, soybean seedling roots exhibited significantly elevated activities of antioxidant enzymes such as SOD, POD, APX, and CAT, alongside increased contents of soluble proteins, soluble sugars, and fructose. Exogenous melatonin application further enhanced these antioxidant enzyme activities and osmolyte levels (Fig. 2B, C, D). These findings align with Zhao et al. (2019), who also reported that melatonin application boosts both antioxidant enzyme activity and osmotic adjustment capacity in soybean roots under drought stress, mitigating the adverse effects of ROS accumulation.

Correlation analysis revealed significant positive relationships between root vitality and root morphology (root length, root volume), as well as dry matter accumulation. Conversely, these indices showed negative correlation with antioxidant enzyme activities and osmolyte contents, except for major carbohydrates like sucrose (Fig. 4A). This suggests a mutual promotive relationship among root vitality, morphology, and dry matter accumulation, where increases under drought conditions might exert a “dilution” effect on enzyme activities and substance contents, warranting further investigation. Moreover, Mantel analysis indicated significant positive correlations between root vitality, root morphology, and dry matter with shoot morphology, dry matter accumulation, and photosynthesis (Fig. 4A). Subsequent dendrogram (Fig. 4B) and random forest analysis (Fig. 4C) also demonstrated close associations between these root traits and other key growth indicators and growth rates of soybean seedlings. Aside from sucrose, root antioxidant enzyme activities, osmolyte levels, and key carbohydrate contents also correlated positively to varying degrees with shoot morphology, dry matter accumulation, and photosynthesis, suggesting that improving these root parameters benefits aerial parts of the plant. This is consistent with numerous studies proposing thatmelatonin enhances crop growth under drought stress by regulating root growth (Dai et al., 2020; Zhu et al., 2023; Zhang et al., 2023).

In conclusion, adjustments in root morphology and physiological characteristics are critical indicators of a plant’s drought resistance. Our findings suggest that melatonin application can further enhance these root adjustments, thereby improving drought tolerance. However, the molecular mechanisms through which melatonin regulates root morphology and physiological traits of soybean under drought stress, promoting overall plant growth, require further exploration.

Melatonin application enhances various root morphological and physiological indices in soybean under long-term drought stress, including root activity, root length, root volume, root dry matter, as well as the activities of key antioxidant enzymes and the contents of osmotic regulators in roots. These improvements in root indices, in turn, enhance photosynthesis, morphology, and dry matter accumulation in the aboveground parts, thereby promoting the growth of soybean seedlings.

Author contribution: Jiayu Zhu and Tianyi Wu: Conceptualisation, Investigation, Funding acquisition, Writing and Reviewing and Editing;Xiaohan Shang, Hongchang Jia, Dezhi Han and Yuxian Zhang: Investigation, Project administration, Validation, Formal analysis, Resources;Xijun Jin: Supervision

Institutional Review Board Statement: Not applicable

Informed Consent Statement: Not applicable

Data Availability Statement: All data analysed during the current study are included in this published article. The detailed data can be provided on reasonable request.

Acknowledgements:This study was supported by Heilongjiang Province’s “Revealing the List and Commanding the Leaders” Scientific and Technological Research Project (2021ZXJ05B011), Natural Science Foundation of Heilongjiang Province(LH2022C063), China Agriculture Research System of MOF and MARA (CARS-04-PS18) and the Heilongjiang Provincial research Institute research expenses project (CZKYF2023-1-B016).

Conflicts of interest: The authors declare no conflicts of interest in the current study.

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The role of exogenous melatonin in modulating root morphology and physiological response to drought in soybean seedlings

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Version 1

Abstract

Figures

Introduction

Materials and Methods

Experimental Materials

Experimental Design

Sampling Time

Measurement Indices and Methods

Morphological Indices

Photosynthetic Indices

Physiological Indices

Data Processing and Plotting

Results

Growth of Soybean Seedlings under Drought Stress

Root activity, antioxidant enzyme activities, and osmotic regulator content

Photosynthetic Pigment Content and Gas Exchange Parameters

Correlation of Root Morphological and Physiological Indices with Key Growth Indicators of the Aboveground Parts

Discussion

Conclusion

Declarations

References

Status:

Version 1