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Article

Promoted Growth of Sweet Potato Root by Bacillus amyloliquefaciens HN11 and Enhanced Uptake of Fosthiazate

by
Sukun Lin
,
Xin Hu
,
Xulang Ye
,
Zhixiang Zhang
* and
Hanhong Xu
*
State Key Laboratory of Green Pesticides, South China Agricultural University, Guangzhou 510642, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(5), 1098; https://doi.org/10.3390/agronomy15051098
Submission received: 27 March 2025 / Revised: 23 April 2025 / Accepted: 27 April 2025 / Published: 30 April 2025
(This article belongs to the Special Issue Role of Plant Growth-Promoting Microbes in Agriculture—2nd Edition)
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Abstract

:
Many microbial agents, such as Bacillus amyloliquefaciens, have been reported to promote the growth of plant roots, which may enhance the uptake of systemic pesticides by plant roots. Through experimental methods, such as microscopic observation and HPLC (High Performance Liquid Chromatography) detection, the colonization behavior of B. amyloliquefaciens HN11 in sweet potato rhizosphere and its effects on sweet potato growth and fosthiazate uptake were studied. The results show that B. amyloliquefaciens HN11 could effectively colonize the rhizosphere of sweet potatoes and significantly promote the growth of sweet potato roots, leading to increase the yield of sweet potatoes. Moreover, the colonization of B. amyloliquefaciens HN11 promoted the absorption of fosthiazate by sweet potato roots under drip irrigation. The control efficiency against root knot nematodes of sweet potato also improved under this management approach. In summary, drip application of fosthiazate to sweet potato roots inoculated with B. amyloliquefaciens HN11 revealed a new approach to insecticide application. This method could improve the effective utilization rate of fosthiazate and the control efficiency of root knot nematodes, help farmers increase production and income, be environmentally friendly and meet the requirements of sustainable development. This study provides new references for the application direction of microbial agents.

1. Introduction

Sweet potato (Ipomoea batatas Lam) belongs to the Convolvulaceae family and is not only an important food crop, but is also widely used as animal feed and industrial raw materials [1]. It is native to tropical South America and mainly produced in tropical and subtropical regions around the world [2]. The edible parts of sweet potato include leaves, stems, and tubers, its tubers are rich in nutrients and contain a variety of amino acids, vitamins, minerals, tocopherols, and beta carotene, making them increasingly popular among people [3]. Although sweet potato is considered more resistant to pests and diseases than most other leafy crops, it is still sensitive to over 20 viruses, 40 diseases, and 40 pests [4]. Plant parasitic nematodes, such as root knot nematodes (Meloidogyne spp.), pose a major pathogenic threat to sweet potato cultivation in China, Japan, South Korea, and South Africa [5]. A previous report showed that global plant parasitic nematodes caused a 10.2–11.4% decrease in sweet potato yield and significantly reduced the quality of sweet potato [6]. In recent years, with the continuous cropping of sweet potatoes and soil degradation, the harm of plant parasitic nematodes has become more serious.
For the control of plant parasitic nematodes, the application of nematicides is an important management tool [7]. In China, currently, the commonly used nematicides include fosthiazate (CAS number: 98886-44-3). The molecular weight of fosthiazate is 283.35, with a water solubility (20 °C) of 9.85 g/L and a boiling point (66.6 Pa) of 198 °C, fosthiazate is soluble in most organic solvents and toxic to mammals, its LD50 in rats is 57 mg/kg [8]. Due to its efficient nematicidal activity, the use of fosthiazate in China is increasing year by year [9]. Previous studies revealed the residual behavior of fosthiazate in soil and crops, such as cucumbers and tomatoes, and may pose potential risks [10,11,12]. With the improvement in the quality of human life, humans have put forward higher requirements for the application of pesticides [13]. Therefore, it is necessary to explore how to improve the effective utilization rate of fosthiazate or reduce the dosage of fosthiazate while ensuring the effectiveness of nematode control. This is of great significance for the environment, food safety, and helping farmers reduce production costs and increase yields.
In recent years, Bacillus amyloliquefaciens has been increasingly widely used in agricultural production due to its significant biocontrol and growth promoting effects, safety, and environmental friendliness [14,15]. B. amyloliquefaciens is a type of plant growth-promoting rhizobacteria (PGPR) that is easy to survive and colonize in plant rhizosphere, can effectively promote plant growth, and has the ability to prevent and control harmful organisms [16]. Our research team previously isolated a strain of B. amyloliquefaciens from the rhizosphere soil of Azadirachta indica (A. Juss., 1830) and named it B. amyloliquefaciens HN11. Through preliminary experiments, we found that B. amyloliquefaciens HN11 has great potential in antagonizing plant pathogens and promoting plant growth [17]. Evaluating the colonization ability of B. amyloliquefaciens in crop rhizosphere and its effects on crop growth is a fundamental task and a prerequisite for ensuring the application of B. amyloliquefaciens [18]. Thus, in this study, the colonization pattern of B. amyloliquefaciens HN11 in the rhizosphere of sweet potatoes under greenhouse and field conditions was studied, and the growth promoting effect of B. amyloliquefaciens HN11 on sweet potatoes was evaluated. In addition, the effect of B. amyloliquefaciens HN11 on the uptake of fosthiazate from rhizosphere soil by sweet potato roots and its effect on the control of root knot nematodes were studied under field condition. This study aims to provide a theoretical basis for the improvement of effective utilization rate of fosthiazate, and the application of microbial agent B. amyloliquefaciens HN11.

2. Materials and Methods

2.1. Test Bacterial Strains

B. amyloliquefaciens HN11 was isolated and identified from the rhizosphere soil of A. indica by the State Key Laboratory of Green Pesticides and was preserved in the China General Microbiological Culture Collection Center, with the strain preservation number CGMCC No. 7948. B. amyloliquefaciens HN11-GFP was constructed by inserting green fluorescent protein (GFP) into the genome of B. amyloliquefaciens HN11 through CRISPR-Cas9 in the State Key Laboratory of Green Pesticides. Microbial agent (Zhuorun, Hangzhou, China), mainly composed of B. amyloliquefaciens QST713, with an effective live bacterial count of ≥3 × 108 CFU/mL, provided by Bayer Crop Science China Limited Company (Hangzhou, China).
In the ultra-clean workbench, individual colonies were selected from the activated B. amyloliquefaciens HN11 and HN11-GFP agar strains and transferred to 10 mL of Luria-Bertani (LB) liquid culture media, respectively, then cultivated in a shaker for 12 h to obtain seed liquid. The shaking parameters were 37 °C and 150 r/min. The seed liquid was inoculated into the LB liquid culture medium at a 1% inoculation rate and cultured for 12 h in a shaker. The shaking parameters were the same as above. The number of colonies was calculated using the agar plate counting method and diluted with sterile water gradient to obtain 108, 107, 106, and 105 CFU/mL of B. amyloliquefaciens HN11 and HN11-GFP bacterial liquid for later use.

2.2. Chemicals and Reagents

Yeast extract, tryptone, agar powder, and sodium chloride were purchased from Thermo Fisher Scientific China Limited Company (Shanghai, China). Fosthiazate standard (purity: 98%) was purchased from Intebel Biotechnology Limited Company (Guangzhou, China). A 5% fosthiazate soluble concentrate was provided by Zhenge Biotechnology Limited Company (Zhaoqing, China). Analytical grade acetonitrile (purity > 99.5%), HPLC (High Performance Liquid Chromatography) grade acetonitrile (purity > 99.9%), and HPLC grade methanol (purity > 99.9%) were purchased from Fuchen Chemical Reagent Factory (Tianjin, China). N-Propyl ethylene diamine (PSA) was purchased from Tengda Technology Limited Company (Tianjin, China). All the solutions for experiment were stored at −18 ± 1 °C.

2.3. Observation of Colonization of B. amyloliquefaciens HN11-GFP in Sweet Potato Roots

Under greenhouse condition, sweet potato seedlings were potted and treated with B. amyloliquefaciens HN11-GFP bacterial solution at a concentration of 107 CFU/g soil and treated with an equal amount of sterile water as a control. After 2 days of cultivation at 30 °C, relative humidity of 80%, and light exposure time of 12 h, sweet potato seedlings were sampled to detect the colonization behavior of B. amyloliquefaciens HN11-GFP in sweet potato roots. When taking samples, the sweet potato seedlings were gently removed, and the surface was carefully rinsed with sterile water. The residual water on the surface of the root system was absorbed by clean absorbent paper, and then the root system was cut into small sections of about 0.8 cm using a clean surgical knife. The automatic slicer was used for slicing, and the slice thickness was 50 μm. The prepared root and slice samples were placed on the glass slide, covered with a cover glass, and observed upside down under a confocal laser scanning microscope (CLSM). The CLSM parameters were excitation wavelength of 488 nm and signal collection range of 500 to 600 nm. The images were analyzed using Leica LAS X software (v 3.7.4).

2.4. Colonization Dynamics of B. amyloliquefaciens HN11-GFP in Sweet Potato Roots and Rhizosphere Soil

Under greenhouse and field conditions, samples of sweet potato roots and rhizosphere soil were taken at 2, 4, 6, 8, and 14 days after inoculation with B. amyloliquefaciens HN11-GFP. The inoculation concentration in greenhouse was 107 CFU/g soil. The field inoculation concentration was 109 CFU/mL, the dosage was 6 L/mµ, and the water consumption is 600 L/mµ. The recovered B. amyloliquefaciens HN11-GFP was counted by dilution coating plate method and dual wavelength fluorescent protein excitation light source irradiation (Figure S1). After sampling, the samples were weighed and ground in a mortar with sterile water. A total of 1 g of sample and 9 mL of sterile water were added to a centrifuge tube containing glass beads, and shaken for 10 min. The suspension was further diluted in a gradient, and 0.1 mL was taken and coated on an LB plate. The plate was incubated at 37 °C for 12 h, and the number of colonies was recorded after growth. The experiment was repeated 3 times, the average was taken, and the data are presented in log (CFU/g). The counting formula for dilution coating plate method is [19]:
The number of bacteria (CFU/g) = (A × C)/B
where A represents the number of bacterial colonies grown in the plate, B represents the volume of diluent added to the plate, C represents the dilution factor.

2.5. Effect of B. amyloliquefaciens HN11 on the Growth of Sweet Potatoes Under Greenhouse Condition

Under greenhouse condition of 30 °C, relative humidity of 80%, and 12 h of light exposure, sweet potato seedlings with consistent growth status were treated with different concentrations of B. amyloliquefaciens HN11 (108, 107, 106, and 105 CFU/g soil). LB liquid medium treatment and sterile water treatment were used as controls. Each treatment group contained 5 sweet potato seedlings, and the experiment was repeated 3 times. After 7 days of treatment, the growth of sweet potato roots was observed, and the total root length and average root diameter were scanned and calculated using a root scanner. Based on the analysis of total root length and average root diameter, the optimal concentration of B. amyloliquefaciens HN11 for promoting root growth of sweet potato was obtained.
After completing the above experiment, sweet potato seedlings with consistent growth status were treated with the optimal concentration of B. amyloliquefaciens HN11 and treated with an equal amount of sterile water as a control. Each treatment group contained 5 sweet potato seedlings, and the experiment was repeated 3 times. The growth of sweet potatoes was observed 14 days after treatment, and various growth indicators (total root length, root tip number, average root diameter, total root surface area, total root volume, and fresh weight of plant) were recorded using a root scanner and electronic scale. Based on the analysis of the above growth indicators, the effect of B. amyloliquefaciens HN11 on the growth of sweet potatoes was evaluated in detail.

2.6. Effect of B. amyloliquefaciens HN11 on the Growth of Sweet Potatoes Under Field Condition

Under field condition, the effect of B. amyloliquefaciens HN11 on sweet potato growth was further validated from 2023 to 2024. The experimental sites were located in Zhanjiang, Guangdong, China (21°22′ N, 109°89′ E) and Shanwei, Guangdong, China (22°94′ N, 115°64′ E). The planting varieties were Pushu 32 and Zherenshu 2007001, respectively. The soil type of the experimental plot was sandy soil. Three groups of treatments were set up for the experiment, with each treatment covering an area of 0.16 acre (1 mµ), and the experiment was repeated 3 times. Treatment-1 (Tr-1) was drip irrigation application of B. amyloliquefaciens HN11 on the basis of conventional management. The effective dosage of B. amyloliquefaciens HN11 (109 CFU/mL) was 6 L/mµ. After 5 and 33 days of transplantation, it was diluted 100 times with water and applied, respectively. Treatment-2 (Tr-2) was used as a positive control, and microbial agent (Zhuorun) was applied by drip irrigation on the basis of conventional management, with an effective dosage of 600 mL/mµ. It was diluted 1000 times with water and applied, respectively, after 5 and 33 days of transplantation. Treatment-3 (CK) was used as a negative control, and 600 L/mµ of water were applied after 5 and 33 days of transplantation, respectively. After processing, the growth status of sweet potatoes was regularly monitored. Finally, the number and weight of sweet potatoes were measured using a five-point sampling method, with 20 seedlings collected at each point, for a total of 100 seedlings, to determine their number and weight of sweet potatoes.

2.7. Effect of B. amyloliquefaciens HN11 on Fosthiazate Uptake in Sweet Potato Roots

The experiment time and location were consistent with 2.6 and was designed according to the ‘Guideline for the testing of pesticide residues in Crops (NY/T 788-2018)’ issued by the Ministry of agriculture and rural affairs of the people’s republic of China [20]. Two sets of treatments were set up in the experiment, when applying 5% fosthiazate through drip irrigation, the treatment group applied B. amyloliquefaciens HN11 (109 CFU/mL) at effective dosage of 6 L/mµ simultaneously, and the dosage of 5% fosthiazate was 1500 mL/mµ (recommended dose). The water consumption was 600 L/mµ, and the application time was 5 days after transplanting. The control group had the same conditions as the treatment group except for not using B. amyloliquefaciens HN11.
Sweet potato root samples were collected 1, 3, 5, 7, 14, 21, and 28 days after application of fosthiazate. The sweet potato root sample were chopped up and 5.0 g was taken out and placed in a 100 mL plastic centrifuge tube. A total of 25.0 mL of acetonitrile (analytical grade) was added, and the samples were shaken on a vortex shaker for 2 min to ensure sufficient interaction between the solvent and the entire samples. After oscillation, ultrasonic extraction was performed for 30 min and then filtered into a 100 mL plastic centrifuge tube containing 3.0 g NaCl. After further oscillation for 2 min, high-speed centrifugation was performed for 5 min at a speed of 3500 rpm. A total of 5 mL of acetonitrile phase was transferred into a round bottom flask and evaporated to dryness under vacuum in a 40 °C water bath. Finally, the flask was washed with 5.0 mL of methanol (HPLC grade), and 1.5 mL of washing solution was transferred to a 4.0 mL centrifuge tube containing 150 mg PSA. After vigorous shaking for 1 min, it was centrifuged at high speed for 5 min at 3500 rpm. The supernatant was filtered through a 0.22 μm organic solvent filter membrane into a chromatographic analysis bottle and stored at −18 ± 1 °C for subsequent HPLC detection.
Fosthiazate was determined using HPLC, the model was Shimadzu LC-20A, equipped with UV detector, purchased from Shimadzu company of Kyoto, Japan. HPLC was performed with an Agilent Eclipse XDB-C18 column (250 mm × 4.6 μm × 5 μm), by using a mobile phase of acetonitrile-water (40:60), with a flow rate of 1 mL/min, column temperature of 35 °C, detection wavelength of 220 nm, and injection volume of 10 μL.
Fosthiazate standard was continuously diluted with methanol (HPLC grade) to 5, 1, 0.5, 0.1, and 0.05 mg/L working solutions for the determination of the standard curve. The linearity of the detection method was evaluated by the correlation index (R2) of the standard curve. The accuracy and precision of the detection method was evaluated by addition and recovery experiment. Fosthiazate at concentrations of 6, 3, and 1 mg/kg was added to the blank samples of sweet potato roots, respectively, and the samples were prepared according to the above method. Subsequently, the recovery rates of different levels of addition and the relative standard deviation (RSD) of the repeated samples were measured.

2.8. Effect of B. amyloliquefaciens HN11 on the Control Efficiency to Root Knot Nematode of Sweet Potato

The sweet potato fields with severe damage of root knot nematode (Meloidogyne enterolobii) were selected for the experiment in the two experimental sites (Zhanjiang and Shanwei). Four sets of treatments were set up in the experiment. When applying 5% fosthiazate through drip irrigation, treatment 1 (Tr-1) applied B. amyloliquefaciens HN11 simultaneously, and treatment 2 (Tr-2) did not apply B. amyloliquefaciens HN11, and treatment 3 (Tr-3) only applied B. amyloliquefaciens HN11. Drip irrigation with an equal amount of water was used as a control (CK). The water consumption was 600 L/mµ. The dosage of 5% fosthiazate was 1500 mL/mµ, and the effective dosage of B. amyloliquefaciens HN11 (109 CFU/mL) was 6 L/mµ. The application time was 5 days after transplanting sweet potato seedlings. Each treatment area was 1 mµ, and the experiment was repeated 3 times. The damage caused by root knot nematode of sweet potato was investigated 28 days after application of fosthiazate using a five-point sampling method, 20 seedlings were collected from each point, for a total of 100 seedlings. The control efficiency was calculated based on the survey results and the methods of Tian et al., the calculation formula is as follows, and ‘RKI’ represents the root knot index [21].
Control efficiency = (D − E)/D
where D represents the RKI of the control group, E represents the RKI of the treatment group.

3. Results

3.1. Colonization of B. amyloliquefaciens HN11-GFP in Sweet Potato Rhizosphere

The colonization of B. amyloliquefaciens HN11-GFP in sweet potato root after 2 days of inoculation under soil culture condition was observed using CLSM. As shown in Figure 1a, B. amyloliquefaciens HN11-GFP can emit green fluorescence, Figure 1b showed CLSM observation of sweet potato root tip surface, it can be seen that a large amount of B. amyloliquefaciens HN11-GFP cells colonized the surface of sweet potato roots. From the perspective of colonization sites, B. amyloliquefaciens HN11-GFP mainly colonize the meristem region of the root tip. Further observation results showed that no B. amyloliquefaciens HN11-GFP was observed in sweet potato roots (Figure 1c). The results indicate that after 2 days of inoculation under soil culture condition, B. amyloliquefaciens HN11-GFP could successfully colonize on the surface of sweet potato roots, and the colonization situation varied in different parts.
The colonization dynamics of B. amyloliquefaciens HN11-GFP in sweet potato root and rhizosphere soil under greenhouse and field conditions are shown in Figure 2. The results show that after 14 days of inoculation, B. amyloliquefaciens HN11-GFP could be detected in sweet potato root and rhizosphere soil under both experimental conditions. However, there were differences in the colonization results under the two experimental conditions. After 14 days of inoculation, the density of B. amyloliquefaciens HN11-GFP in sweet potato roots and rhizosphere soil was still 104–105 CFU/g root and 105–106 CFU/g rhizosphere soil. The results indicate that B. amyloliquefaciens HN11-GFP could stably colonize in sweet potato root and rhizosphere soil 14 days after application.

3.2. Effect of B. amyloliquefaciens HN11 on the Growth of Sweet Potatoes

Under greenhouse condition, different concentrations of B. amyloliquefaciens HN11 bacterial solution treatment had a certain promoting effect on the growth of sweet potato roots (Figure 3a). Through data analysis, it was found that the application concentration of B. amyloliquefaciens HN11 at 108, 107, and 106 CFU/g soil had significant growth promoting effect, with 107 CFU/g soil having the best growth promoting effect. The application concentration of B. amyloliquefaciens HN11 at 105 CFU/g soil had a certain promoting effect, but not significant. And LB liquid medium had no significant effect on the growth of sweet potato roots (Figure 3b). Furthermore, the growth promoting effect of B. amyloliquefaciens HN11 was mainly reflected in promoting an increase in the total length of sweet potato roots but had no significant effect on the average diameter of sweet potato roots (Figure 3b).
Based on the above experimental results, the optimal concentration of 107 CFU/g soil was selected to continue the experiment. Under greenhouse condition, after 14 days of treatment with B. amyloliquefaciens HN11 at a concentration of 107 CFU/g soil, sweet potato root growth was significantly more vigorous (Figure 4a). Through data analysis, it was found that the optimal concentration of B. amyloliquefaciens HN11 did not have a significant effect on the average diameter of sweet potato roots after 14 days of treatment. However, it could significantly promote the increase in the total length of sweet potato roots. The increase in the total length of sweet potato roots was not only reflected in the increase in individual root length, but also in the increase in the number of roots (i.e., root tips). The increase in total root length gave it a larger surface area and volume, which in turn facilitated the absorption of water and nutrients by the sweet potato root system and the growth of the aboveground part (Figure 4b).
Under field condition, the growth promoting effect of B. amyloliquefaciens HN11 on sweet potatoes was verified, and the results are shown in Figure 5. The results show that after treatment with B. amyloliquefaciens HN11, both the number (Figure 5a) and weight (Figure 5b) of sweet potatoes in Zhanjiang and Shanwei were significantly higher than those in the negative control group. The yield increasing effect of B. amyloliquefaciens HN11 was mainly achieved by promoting an increase in the number of sweet potatoes, which may be related to the promotion of sweet potato rooting by B. amyloliquefaciens HN11. Because sweet potato tubers develop from roots. Compared with the positive control product (B. amyloliquefaciens QST713), B. amyloliquefaciens HN11 had the same or even better yield increasing effect. There were certain differences in the results between Zhanjiang and Shanwei, which may be related to different planting varieties.

3.3. Effect of B. amyloliquefaciens HN11 on Fosthiazate Uptake in Sweet Potato Roots

The blank sample of sweet potato root did not interfere with the retention time of fosthiazate (Figure S2a,b). The R2 of the standard curve of fosthiazate was 0.9999, indicating a good linear relationship (Figure S2c). The average recovery rates of fosthiazate in sweet potato roots was 91.29% ± 4.06% to 99.39% ± 0.76% (Figure S2d), all within the acceptable range, and the RSDs were less than 10%, indicated that this analysis method could be used to detect the content of fosthiazate in sweet potato roots.
The effect of B. amyloliquefaciens HN11 on fosthiazate uptake in sweet potato roots was shown in Figure 6. The concentrations of fosthiazate in sweet potato roots first increased, peaked on day 7, and then gradually decreased after day 7 until day 28. At Zhanjiang experimental site, on day 1, there was no significant difference in the concentrations of fosthiazate between the two groups; however, from day 3 to day 28, the concentrations of fosthiazate in sweet potato roots in the treatment group were significantly higher than those in the control group, which indicated that the colonization of B. amyloliquefaciens HN11 promoted the uptake of fosthiazate from rhizosphere soil by sweet potato roots. The results of Shanwei experimental site were basically consistent with those of Zhanjiang experimental site, except for day 1, the concentrations of fosthiazate in the sweet potato roots in the treatment group were significantly higher than those of the control group.

3.4. Effect of B. amyloliquefaciens HN11 on the Control Efficiency to Root Knot Nematode of Sweet Potato

After 28 days of treatment, the control efficiency to root knot nematode of Tr-2 were 61.31% ± 6.21% in Zhanjiang and 58.40% ± 2.10% in Shanwei, respectively, and the control efficiency to root knot nematode of Tr-1 reached 82.74% ± 3.90% in Zhanjiang and 83.20% ± 4.20% in Shanwei, respectively. The field research results in two experimental sites (Zhanjiang and Shanwei) showed that the control efficiency of Tr-1 were significantly higher than those of Tr-2. The above results indicate that drip application of fosthiazate to sweet potato roots inoculated with B. amyloliquefaciens HN11 could improve the control efficiency of root knot nematodes.

4. Discussion

B. amyloliquefaciens play an important role in the growth and health of plants, providing various beneficial functions for plants, such as promoting plant growth and enhancing plant tolerance to abiotic stress, as well as inducing systemic resistance and directly antagonizing soil borne pathogens to protect host plants [22]. In recent years, B. amyloliquefaciens have shown great potential as microbial agents or biofertilizers in agricultural production [23]. Rhizosphere colonization is one of the most important characteristics of B. amyloliquefaciens that determines their survival and reproduction, which is a necessary condition for B. amyloliquefaciens to exert beneficial functions on host plants [24].
Our research results found that B. amyloliquefaciens HN11 can successfully colonize on the surface of sweet potato roots and mainly colonize in the meristem region in the form of microcolonia (Figure 1), and there was less colonization in other regions, which may be related to root exudates. Zhang et al. also reported similar results [19]. Numerous studies show that the colonization of microorganisms in plant rhizosphere is closely related to root exudates. Root exudates are distributed in various parts of the root tip, mainly in the front half of the tip, especially in the meristem region and root cap. The cell division in the meristem region is most active, and the secreted substances contribute to cell division and the production of new cells. Root cap cells mainly secrete sticky substances to help protect the root tip and lubricate the soil [25,26]. Therefore, the colonization differences of B. amyloliquefaciens HN11-GFP in different regions of sweet potato root tips may be related to differences in root exudates in different regions. Under both greenhouse and field conditions, B. amyloliquefaciens HN11 was able to colonize stably in the rhizosphere of sweet potatoes for a long time (Figure 2). These results indicate that B. amyloliquefaciens HN11 has the foundation for application on sweet potatoes. And B. amyloliquefaciens HN11 is an aerobic bacterium [27], while sweet potatoes are often grown in sandy soil [28], which undoubtedly increases the application advantage of B. amyloliquefaciens HN11 on sweet potatoes.
Moreover, our study demonstrated for the first time that the colonization of B. amyloliquefaciens can promote the uptake of fosthiazate by sweet potato roots. Compared with the same fosthiazate application rates without B. amyloliquefaciens application, the content of fosthiazate in sweet potato roots was significantly higher (Figure 6), which means that applying B. amyloliquefaciens HN11 with fosthiazate simultaneously can improve the effective utilization rate of fosthiazate. This method of nematode management did not pursue increasing the amount of fosthiazate used, was more environmentally friendly and safe. The enhanced uptake of fosthiazate from soil could be mainly attributed to the following factor. B. amyloliquefaciens HN11 facilitated the uptake of fosthiazate from soil by increasing the uptake surface area of sweet potato roots. As shown in Figure 4, the colonization of B. amyloliquefaciens HN11 increased the number and length of sweet potato roots, resulting in a larger root surface area for effective capture of fosthiazate. Furthermore, the colonization of B. amyloliquefaciens HN11 also increased the field yield of sweet potatoes (Figure 5). For farmers, this nematode control method not only saved management costs but also increased profits.
As shown in Figure 7, compared with the same fosthiazate application rates without B. amyloliquefaciens application, the control efficiency to root knot nematodes was significantly better, which may be mainly attributed to this factor. It was because B. amyloliquefaciens HN11 increased the content of fosthiazate in sweet potato roots, resulting in better control efficiency to root knot nematodes. Previous studies show that the inoculation of arbuscular mycorrhizal fungi (AMF) can promote the growth of plant roots [29,30], and the colonization of AMF in cotton roots promoted the absorption of flonicamid by cotton roots [31], which was similar to our research findings. Their research advantage was that AMF has multiple mechanisms that promote pesticide absorption. However, our research advantage lies in the potential beneficial effects of B. amyloliquefaciens HN11 on plant resistance to biotic stress. Previous studies show that some of Bacillus have strong inhibitory effects on parasitic nematodes, and the secondary metabolites or volatile substances they produced play important roles [32,33]. Moreover, some of B. amyloliquefaciens have biocontrol activity against various plant diseases [34,35]. Therefore, B. amyloliquefaciens HN11 may have significant biocontrol activity against certain sweet potato diseases, but further experiments are needed to verify it.

5. Conclusions

In conclusion, this study for the first time documented that B. amyloliquefaciens HN11 could effectively colonize the sweet potato rhizosphere and significantly promote the growth of sweet potato roots. It also increased the uptake of fosthiazate from rhizosphere soil and improved the effective utilization rate of fosthiazate applied through drip irrigation. With the increasing use of pesticides in drip application in many countries [36,37,38], these results provide a new perspective for improving pesticide application practices through drip irrigation and provide reference for the current application direction of microbial agents. Our research results indicate that the combined application of B. amyloliquefaciens HN11 and fosthiazate can improve the utilization efficiency of fosthiazate, promote the increase in sweet potato yield and provide new reference for green prevention and control of sweet potato nematodes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15051098/s1, Figure S1: Detection method of B. amyloliquefaciens HN11-GFP colonization dynamics in rhizosphere soil and sweet potato root under greenhouse and field conditions. Figure S2: Liquid chromatogram of fosthiazate standard at 0.5 mg/kg (a) and sweet potato root blank sample (b), fosthiazate standard curve (c), and recovery rate of fosthiazate added to sweet potato roots (d).

Author Contributions

Conceptualization, Z.Z.; methodology, S.L. and X.H.; investigation, S.L. and X.Y.; resources, H.X.; data curation, X.H.; writing—original draft preparation, S.L.; writing—review and editing, S.L. and Z.Z.; visualization, X.Y.; supervision, H.X.; project administration, Z.Z. and H.X.; and funding acquisition, Z.Z. and H.X. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by grants from the National key research and development program of China (No.2023YFD1701103).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. CLSM observation of B. amyloliquefaciens HN11-GFP (a), and CLSM observation of B. amyloliquefaciens HN11-GFP located on the root surface (b) and interior (c) of sweet potato after 2 days of inoculation. Note: ‘Tr’ represents the sweet potato root treated with B. amyloliquefaciens HN11-GFP (107 CFU/g soil) for 2 days, while ‘CK’ represents the sweet potato root treated with equal amount of sterile water.
Figure 1. CLSM observation of B. amyloliquefaciens HN11-GFP (a), and CLSM observation of B. amyloliquefaciens HN11-GFP located on the root surface (b) and interior (c) of sweet potato after 2 days of inoculation. Note: ‘Tr’ represents the sweet potato root treated with B. amyloliquefaciens HN11-GFP (107 CFU/g soil) for 2 days, while ‘CK’ represents the sweet potato root treated with equal amount of sterile water.
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Figure 2. Detection results of B. amyloliquefaciens HN11-GFP colonization dynamics in rhizosphere soil and sweet potato root under greenhouse and field conditions. Note: the data are presented as mean ± standard error (SE), the widths of wave represent the values of SE (n = 3).
Figure 2. Detection results of B. amyloliquefaciens HN11-GFP colonization dynamics in rhizosphere soil and sweet potato root under greenhouse and field conditions. Note: the data are presented as mean ± standard error (SE), the widths of wave represent the values of SE (n = 3).
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Figure 3. Growth of sweet potato root treated with different concentrations of B. amyloliquefaciens HN11 for 7 days (a), as well as total length and average diameter (b). Note: Tr-1, Tr-2, Tr-3, Tr-4, Tr-5, and CK represent different concentrations of B. amyloliquefaciens HN11 (108, 107, 106, and 105 CFU/g soil, as well as LB liquid medium and sterile water). In (b), ‘-’ represents the mean value, the box range is ±1 times the SE, the whisker range is ±1.5 times the SE, and the different letters above box indicate significant differences in root growth indicators (total length and average diameter) due to concentration effects at p < 0.05 level based on Duncan’s multiple range test (n = 3).
Figure 3. Growth of sweet potato root treated with different concentrations of B. amyloliquefaciens HN11 for 7 days (a), as well as total length and average diameter (b). Note: Tr-1, Tr-2, Tr-3, Tr-4, Tr-5, and CK represent different concentrations of B. amyloliquefaciens HN11 (108, 107, 106, and 105 CFU/g soil, as well as LB liquid medium and sterile water). In (b), ‘-’ represents the mean value, the box range is ±1 times the SE, the whisker range is ±1.5 times the SE, and the different letters above box indicate significant differences in root growth indicators (total length and average diameter) due to concentration effects at p < 0.05 level based on Duncan’s multiple range test (n = 3).
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Figure 4. Growth (a) and various growth indicators (b) of sweet potato after 14 days of treatment with B. amyloliquefaciens HN11 at optimal concentration (107 CFU/g soil) under greenhouse condition. Note: ‘Tr’ represents the treatment of B. amyloliquefaciens HN11 at optimal concentration, while ‘CK’ represents the treatment of sterile water. In (b), ‘-’ represents the mean value, the box range is the mean ±1 times the SE, the whisker range is from the minimum value to maximum value, ‘**’ represents a highly significant difference between the two treatments at the p < 0.01 level, according to the Tukey’s HSD test, and ’ns’ represents no significant difference between the two treatments (n = 3).
Figure 4. Growth (a) and various growth indicators (b) of sweet potato after 14 days of treatment with B. amyloliquefaciens HN11 at optimal concentration (107 CFU/g soil) under greenhouse condition. Note: ‘Tr’ represents the treatment of B. amyloliquefaciens HN11 at optimal concentration, while ‘CK’ represents the treatment of sterile water. In (b), ‘-’ represents the mean value, the box range is the mean ±1 times the SE, the whisker range is from the minimum value to maximum value, ‘**’ represents a highly significant difference between the two treatments at the p < 0.01 level, according to the Tukey’s HSD test, and ’ns’ represents no significant difference between the two treatments (n = 3).
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Figure 5. The effect of B. amyloliquefaciens HN11 on the number (a) and weight (b) of sweet potatoes per hundred plants at two experimental sites (Zhanjiang and Shanwei). Note: ‘Tr-1’ represents the treatment of B. amyloliquefaciens HN11, ‘Tr-2’ represents the treatment of B. amyloliquefaciens QST713 (positive control), and ‘CK’ represents the water treatment. ‘-’ represents the mean value, the box range is the mean ±1 times the SE, the whisker range is from the minimum value to maximum value, and the different letters above box indicate significant differences in measurement indicators (the number and weight of sweet potatoes per hundred plants) due to different treatment at p < 0.05 level based on Duncan’s multiple range test (n = 3).
Figure 5. The effect of B. amyloliquefaciens HN11 on the number (a) and weight (b) of sweet potatoes per hundred plants at two experimental sites (Zhanjiang and Shanwei). Note: ‘Tr-1’ represents the treatment of B. amyloliquefaciens HN11, ‘Tr-2’ represents the treatment of B. amyloliquefaciens QST713 (positive control), and ‘CK’ represents the water treatment. ‘-’ represents the mean value, the box range is the mean ±1 times the SE, the whisker range is from the minimum value to maximum value, and the different letters above box indicate significant differences in measurement indicators (the number and weight of sweet potatoes per hundred plants) due to different treatment at p < 0.05 level based on Duncan’s multiple range test (n = 3).
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Figure 6. Fosthiazate concentration in sweet potato roots at two experimental sites (Zhanjiang and Shanwei). Note: ‘Tr’ represents the treatment of B. amyloliquefaciens HN11, and ‘CK’ represents the water treatment. Data represent mean values ± SE, ‘**’ represents a highly significant difference between the two treatments at the p < 0.01 level, according to the Tukey’s HSD test, and ’ns’ represents no significant difference between the two treatments (n = 3).
Figure 6. Fosthiazate concentration in sweet potato roots at two experimental sites (Zhanjiang and Shanwei). Note: ‘Tr’ represents the treatment of B. amyloliquefaciens HN11, and ‘CK’ represents the water treatment. Data represent mean values ± SE, ‘**’ represents a highly significant difference between the two treatments at the p < 0.01 level, according to the Tukey’s HSD test, and ’ns’ represents no significant difference between the two treatments (n = 3).
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Figure 7. The control efficiency to root knot nematode of sweet potato after 28 days of treatment at two experimental sites (Zhanjiang and Shanwei). Note: ‘Tr-1’ represents the treatment of B. amyloliquefaciens HN11 and fosthiazate, ‘Tr-2’ represents the treatment of fosthiazate, and ‘Tr-3’ represents the treatment of B. amyloliquefaciens HN11. Data represent mean values ± SE, ‘-’ represents the mean value, the box range is the mean ±1 times the SE, the whisker range is from the minimum value to maximum value, and the different letters above box indicate significant differences in measurement indicators (the control efficiency to root knot nematode) due to different treatment at p < 0.05 level based on Duncan’s multiple range test (n = 3).
Figure 7. The control efficiency to root knot nematode of sweet potato after 28 days of treatment at two experimental sites (Zhanjiang and Shanwei). Note: ‘Tr-1’ represents the treatment of B. amyloliquefaciens HN11 and fosthiazate, ‘Tr-2’ represents the treatment of fosthiazate, and ‘Tr-3’ represents the treatment of B. amyloliquefaciens HN11. Data represent mean values ± SE, ‘-’ represents the mean value, the box range is the mean ±1 times the SE, the whisker range is from the minimum value to maximum value, and the different letters above box indicate significant differences in measurement indicators (the control efficiency to root knot nematode) due to different treatment at p < 0.05 level based on Duncan’s multiple range test (n = 3).
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Lin, S.; Hu, X.; Ye, X.; Zhang, Z.; Xu, H. Promoted Growth of Sweet Potato Root by Bacillus amyloliquefaciens HN11 and Enhanced Uptake of Fosthiazate. Agronomy 2025, 15, 1098. https://doi.org/10.3390/agronomy15051098

AMA Style

Lin S, Hu X, Ye X, Zhang Z, Xu H. Promoted Growth of Sweet Potato Root by Bacillus amyloliquefaciens HN11 and Enhanced Uptake of Fosthiazate. Agronomy. 2025; 15(5):1098. https://doi.org/10.3390/agronomy15051098

Chicago/Turabian Style

Lin, Sukun, Xin Hu, Xulang Ye, Zhixiang Zhang, and Hanhong Xu. 2025. "Promoted Growth of Sweet Potato Root by Bacillus amyloliquefaciens HN11 and Enhanced Uptake of Fosthiazate" Agronomy 15, no. 5: 1098. https://doi.org/10.3390/agronomy15051098

APA Style

Lin, S., Hu, X., Ye, X., Zhang, Z., & Xu, H. (2025). Promoted Growth of Sweet Potato Root by Bacillus amyloliquefaciens HN11 and Enhanced Uptake of Fosthiazate. Agronomy, 15(5), 1098. https://doi.org/10.3390/agronomy15051098

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