Introduction

Populus is a genus widely distributed around the world, and many species are highly adaptable to various environmental conditions. As a model species for woody plant research (Ning et al. 2018), Populus has the advantage of having a smaller genome (480 Mb) (Cossu et al. 2012), and that efficient genetic transformation systems are available for different species (Maheshwari and Kovalchuk 2016). A reason why Populus davidiana × P. alba var. pyramidalis Louche (PdPap) is suitable as an urban species is that it has a male vegetative breeding system (Yao et al. 2018). After blooming in spring, no catkins are formed so it can be used as a choice of urban tree species in north China (Yao et al. 2018). Studies have confirmed that the hybrid tolerates low temperatures and drought (Qin et al. 2005; Zhao et al. 2015). However, due to deteriorating environmental conditions in recent years, the hybrid is facing increasingly challenges to growth and development from pathogenic microorganisms (Yu et al. 2019), temperature extremes (Zhao et al. 2015), moisture extremes or deficits (Qu et al. 2014) and ion concentrations (Lu et al. 2004; Chen et al. 2020) that can result in different degrees of damage. Among the many hazards, root rot is one of the most serious and devastating diseases and has attracted widespread attention (Moncrief 2010). Once the disease breaks out, it will cause large-scale root rot and death (Ikeda et al. 2005). Xie et al (1993) was the first to report on the damage status, symptom characteristics, and infection pathways of Fusarium oxysporum in Populus × euamerica (Dode) Guinier that occurred in the Dongting Lake area. Reports note that F. oxysporum has been used for the infection of Populus trichocarpa (Zadworna et al. 2014) and Pdpap (Guo et al. 2020).

Fusarium oxysporum is one of the main pathogenic species of the Fusarium genus, and infects many types of plants, eventually causing root rot and plant wilt (Puhalla 2011). In addition to being parasitic on the host plant, F. oxysporum also survives in soil and in the air, and after 10 years of survival, it may still be strongly pathogenic (Wu et al. 2009). The first physical barrier to be breached by F. oxysporum in plant infection is the plant cell wall (Huang 2001). The degradation of the cell wall is achieved by secreting cellulase, pectinase, and β-glucosidase enzymes (Li and Zhang 2010). The pectin produced after degradation of the cell wall blocks the ducts of the host plant (Florendo et al. 2003), which hinders the absorption of water and eventually causes the plant to wilt and die (Miedes et al. 2013). Fusarium acid (5-butyl-2-picolinic acid) secreted by Fusarium enhances the permeability of the root cell membranes causing serious damage (Wang and Ng 1999). In addition, the acid will also reduce the active oxygen content in mitochondria, ultimately hindering the water absorbing function of plant roots and inhibiting plant growth (Lu et al. 2004). In addition, the presence of pedigree-specific regions makes the F. oxysporum genome highly susceptible to horizontal transfer (Bootsma et al. 2000), resulting in the formation of novel specialized types and physiological races of the pathogen (Köppl and Carr 2003), giving F. oxysporum a wide host range and extremely strong pathogenicity (Ma et al. 2013).

Transcription factors are closely related to the plant stress response (Sakuma et al. 2002). The process by which transcription factors enhance the ability of plants to resist stress is mainly a result of the increased expression of transcription factor genes in response to stress (Ali et al. 2013), and this can then further regulate the expression of downstream functional genes (Stein et al. 2001; Hou et al. 2009). Zeng and Mu (2016) have shown that transcription factors play a vital role in plant stress resistance, and the expression of this stress resistance function is usually coordinated under the common regulation of many transcription factors (Sun et al. 2009). In addition, the adaptation of plants to adverse environmental conditions can be achieved by generating secondary metabolites (Stangarlin et al. 2011). When their growth status is disturbed, secondary metabolites will increase the tolerance of plants to stress (Makkar et al. 2007). Studies have shown that different types of transcription factors can jointly participate in the regulation of plant secondary metabolite biosynthesis through synergistic effects (Wasternack and Strnad 2019), which then respond to an adverse environment.

The largest transcription factor family is the APETALA2/ethylene-responsive factor (AP2/ERF) (Xue and Loveridge 2010), which effectively responds to biotic or abiotic stresses during the process of adaptation, thereby mobilizing various resistance response genes (Zorrilla et al. 2014). Ethylene response factors (ERFs) are a family of transcription factors unique to plants (Qiu et al. 2012), and named because ERF genes can respond to ethylene stress to a certain extent (Mizoi et al. 2012). Different types of ERF transcription factors have different gene expression regulation. For example, ERF transcription factors can simultaneously activate or inhibit expression of genes related to disease control (Fujimoto et al. 2000). In addition, since there is no GCC-box element in the promoter of genes involved in the regulation of maturation and senescence (Jin et al. 2010), ERF transcription factors work together with other types of cis-acting elements (Takagi et al. 2020). Based on the results of published research, there are several ways in which ERF transcription factors function in the process of stress resistance: (1) through direct regulation of PR gene expression; (2) via indirect regulation of key gene expression in hormone synthesis pathways (Mader and Cameron 2006); and, (3) in combination with the promoters of key genes to regulate gene expression and the expression of secondary metabolites (Kagaya et al. 1999). The ERF transcription factor family plays a powerful role in many diverse life activities, including positive responses to biotic/abiotic stresses, growth, and metabolite regulation (Mizoi et al. 2012). ERF member genes respond to stresses, including pathogenic microorganism infection (Shi et al. 2015), temperature extremes (Dossa et al. 2016), ion concentrations, and mechanical damage (Licausi et al. 2013). At the same time, the ERF family, as an important regulatory center of the responses to various types of stress, can comprehensively integrate hormone signals such as jasmonic acid and ethylene (Müller and Bosch 2015). In addition, the heterologous expression of the ERF family member genes can produce different degrees of resistance to various stresses through the activation of resistance pathways (Yi et al. 2004).

Based on the results of previous research on transcriptome data (Diao et al. 2021), the transcription level of Pdpap infected with F. oxysporum is well understood. The transcriptome data was screened with the standard of log2 (Fold Change) > 4 and P adj < 0.05. Based on this, genes were obtained in Pdpap, whose transcription levels continued to increase after infection (Supplementary Table S1). Among these genes, there are three typical transcription factors (ERF109, ERF6, MYB41). Among them, ERF109 is basically at the highest level and therefore an in-depth analysis was carried out on ERF109.

The 809 bp cDNA of the PdPapERF109 gene was isolated from Pdpap. The recombinant vector pBI121-PdPapERF109 for the over-expression of PdPapERF109 was constructed under the control of the CaMV 35S promoter, and it was successfully transformed into Pdpap. In addition, evidence based on the analysis of molecular, morphological, and physiological data showed that the PdPapERF109-overexpressing transformants have enhanced resistance to F. oxysporum CFCC86068. The objectives of this study were to determine the function of the PdPapERF109 gene in resistance to F. oxysporum infection using physiological and biochemical methods to provide a contribution to improving plant disease resistance in hybrid poplars.

Materials and methods

Materials

Wild Pdpap seedlings were cultured on 0.5 × MS (Murashige and Skoog) medium supplemented with 0.01 mg/mL 1-naphthaleneacetic acid (NAA). For stable gene transformation, leaves from 1-month-old plants were transferred to differentiation medium that contained 0.5 mg/mL 6-benzylaminopurine (6-BA), 0.1 mg/mL NAA, and 0.02 mg/mL thidiazuron (TDZ). The tissue cultures were grown in a tissue culture room (Zhang et al. 2009). The seedlings obtained by redifferentiation are transferred to 0.5 × MS medium. In order to simulate growth under natural conditions, the seedlings were transplanted to artificial soil and cultivated in an controlled environment at 22 ± 2 °C with a light intensity of 400 lx/m2/s, a photoperiod of 16 h light and 8 h dark, and a relative humidity of 65–75% (Lu et al. 2011).

The strain of F. oxysporum CFCC86068 was obtained from the China Forestry Culture Collection Center. The pre-activated F. oxysporum was transferred to PDA medium and cultured in dark for 7 days. A 9 mm-diameter borer was used to punch holes at the edge of the hyphae and transferred to a new PDA medium. After culturing at 28 °C for 14 days, spores were washed five times with sterile water, filtered through eight layers of gauze and collected into a 50 mL centrifuge tube. Spore density was calculated using a hemocytometer and stored at room temperature for later use.

RNA extraction and cDNA transformation

Total RNA was extracted from young leaves using a TaKaRa MiniBEST Plant RNA Extraction kit (Takara, Dalian, China), and cDNA was synthesized using PrimeScript™ RT reagent Kit with gDNA Eraser (Takara, Dalian, China).

Evolutionary analysis of the predicted PdPapERF109 protein

To fully understand the evolutionary relationships of the PdPapERF109 protein, 16 gene sequences that encode similar ERF proteins were obtained from the NCBI GenBank database (https://www.ncbi.nlm.nih.gov/) (Stoesser et al. 2014). Using MEGA 5.1 software (Tamura et al. 2011), a neighbor joining (NJ) phylogenetic tree was constructed and the confidence of the branches was estimated using 1,000 iterations of bootstrap resampling and the Poisson model. In addition, to further verify the evolutionary relationships between the ERF proteins, we constructed a second phylogenetic tree using the maximum likelihood (ML) method.

Differential expression of PdPapERF109 gene

To clarify the role of PdpapERF109 gene in the process responds to pathogen infection, 6-month-old wild Pdpap were treated with F. oxysporum at different stages, and the treatment times were at the beginning, then at 6, 12, 24, and 48 h, respectively. The RNA from each culture was subjected to reverse transcription to obtain first-strand cDNAs. The expression of PdPapERF109 was analyzed by reverse transcription quantitative real-time PCR (qRT-PCR) performed on a Stratagene Mx3000P real-time PCR system (Agilent Technologies, Santa Clara, USA) using the 2 × SYBR Green qPCR Master Mix kit (Bimake, Shanghai, China). The amplification curve was generated after analyzing the raw data. Cycle threshold (Ct) value 0.01 was calculated based on the fluorescence threshold. Pdpapactin and PdpapEF1-α were used as reference genes to normalize the gene expression (Guo et al. 2021a). The primer sequences for amplifying the reference genes and PdPapERF109 are in Supplementary Table S2, and the components of the qRT-PCR assays are in Supplementary Table S3. The qRT-PCR amplification conditions were: an initial denaturation of 94 °C for 30 s followed by 44 cycles of 94 °C for 12 s, 58 °C for 30 s, and 72 °C for 45 s, and 79 °C for 1 s. The reaction specificity was determined by performing a melting-curve analysis from 55 ℃ to 99 ℃, with fluorescence readings taken every 0.5 °C for 1 s. The 2−△△Ct method (Livak and Schmittgen 2001) was used to calculate the relative expression levels of the target genes, defined as:

$$ \Delta \Delta {\mathrm{C}}_{{\mathrm{t}}} = \left( {{\mathrm{C}}_{{\mathrm{t-target}}} - {\mathrm{ C}}_{{\mathrm{t-control}}} } \right)_{2} - \, \left( {{\mathrm{C}}_{{\mathrm{t-target}}} - {\mathrm{C}}_{{\mathrm{t-control}}} } \right)_{1} $$
(1)

where “Ct-control” represents the average Ct value of reference genes, “1” represents the untreated group, and “2” represents the treated group. Three biological replicates were performed for each assay.

To determine the spatial expression profiles of the PdPapERF109 gene, six tissue types were sampled: roots, whole stems, leaves, and the base, middle, and top sections of the stem (~ 5 cm in length). RNA was extracted from these tissues from 6-month-old wild seedlings. The RNA from each tissue was subjected to reverse transcription to obtain first-strand cDNAs. The expression pattern of PdpapERF109 gene was analyzed according to the qRT-PCR method described above.

Similarly, the same method was used to analyze the expression level of PdPapERF109 in transformants. After 2 months, the PdPapERF109-overexpressing shoots grown in soil were treated with 20 mL F. oxysporum (1 × 105 cfu/mL) by perfusion into the root. The infection time was at 0 h, and 6, 12, 24, and 48 h; each experimental group had three replicates. The transformants were frozen in liquid nitrogen, and total RNA extracted and reverse transcribed to obtain cDNA. The expression level of PdPapERF109 was determined by qRT-PCR using cDNA as a template from the wild material and transformants for the different pathogen infection times. The primer sequences are given in Supplementary Table S2.

Cloning of the PdPapERF109 gene and vector construction

PCR cloned the PdPapERF109 gene using primer sequences (Supplementary Table S4). The components of the PCR amplifications are shown in Supplementary Table S5. The amplification program for PCR was: 94 °C for 3 min followed by 30 cycles of 94 °C for 30 s, 58 °C for 30 s, and 72 °C for 1.5 min, followed by a final extension step of 72 °C for 20 min.

The DNA fragment and the pBI121 overexpression vector were digested with the restriction endonucleases Xba I and Xma I, and the PdPapERF109 gene was inserted into pBI121 under control of the cauliflower mosaic virus (CaMV) 35S promoter. The pBI121-PdPapERF109 recombinant vector was transferred into Agrobacterium tumefaciens GV3101 by heat shock transformation (Holsters et al. 1978), and then Agrobacterium-mediated transformation was used to introduce the recombinant vector into the Pdpap genome (Guo et al. 2018).

Generating recognized transformant Pdpap plants

Acceptable transformant Pdpap lines were obtained by the leaf disc method (Julia et al. 2017). After cutting the young leaves from 2-month-old Pdpap plants, discs ~ 1 cm diameter were co-cultured with the Agrobacterium strain harboring the pBI121-PdPapERF109 recombinant vector and transferred onto MS differentiation medium containing 50 mg/L kanamycin and 200 mg/L cephalosporin for shoot regeneration. The kanamycin-resistant shoots were rooted on 0.5 × MS medium containing 50 mg/L kanamycin for root regeneration. After selection by kanamycin, acceptable transformants were used for subsequent analysis.

Molecular detection of acceptable transformant Pdpap plants

A DNA extraction kit (Tiangen, Beijing) was used to extract genomic DNA from the acceptable PdPapERF109-overexpressing transformed Pdpap plants. PCR detection was performed on DNA extracted from the plants. The pBI121-PdPapERF109 plasmid as the positive control, wild Pdpap and water as the negative controls. For molecular detection of recombinant vectors, primer pairs pBI121-F were used in the pBI121 vector/PdPapERF109-R in the gene sequence, and PdPapERF109-F in the gene sequence/pBI121-R in the vector. The PCR primer sequences are given in Supplementary Table S4.

Growth and physiological index measurements

To test resistance to F. oxysporum under natural conditions, the wild-type and overexpressing transformant OE4 and OE5 lines were inoculated with F. oxysporum and grown in the artificial chamber for 10 days, after which the growth of the plants was assessed. The experiment had three biological replicates. During the experiment, water was added every 3 days to keep the soil moist to avoid the influence of drought.

The survival rates of wild and transformants inoculated with F. oxysporum at zero, 5, 10, 15, and 20 days were counted according to Sun et al. (2018). Disease levels were classified according to the following criteria: 0 (no symptoms), 1 (1–25% wilted leaves), 2 (26–50% wilted leaves), 3 (51–75% wilted leaves), and 4 (76–100% wilted leaves) (Xu et al. 2014). The experiment had ten samples. Refer to the following equation (Sun et al. 2018) for calculation:

$$ \begin{aligned} & {\mathrm{Survival\,Rate}} = 1 - 100\% \\ & \quad \times [{\mathrm{sum}}\,({\mathrm{number\,of\,plants \times disease\,level}})]/[({\mathrm{total\,number\,of\,plants}}) \\ & \quad \quad \times ({\mathrm{maximal\,disease\,level}})] \\ \end{aligned} $$
(2)

The 2-month-old wild material and PdPapERF109-overexpressing transformant Pdpap plantlets in the same growth state were inoculated with F. oxysporum and sampled at zero, 5, 10, 15, and 20 days as described above. The value of pathogen inoculation of the wild Pdpap plants at day zero was set to 1. The growth status of each infected plantlet was observed, and fresh weight and root lengths measured. The experiment had three replicates.

The 2-month-old PdPapERF109-overexpressing transformants and wild Pdpap plants at similar growth stages were inoculated with F. oxysporum and sampled at 0 h, and at 6, 12, 24, 48 h, and 96 h. The inoculated plants were frozen in liquid nitrogen immediately after sampling. Using the methods of Cheng et al. (2020), the samples were used to determine various physiological indicators, including peroxidase (POD) activity, catalase (CAT) activity (Góth 1991), H2O2 content, malondialdehyde (MDA) content, and the percentage of electrolyte leakage (Nguyen et al. 2016). The experiment had three replicates.

Antioxidant capability test

Nitroblue tetrazolium chloride (NBT) can detect superoxide activity in plant tissues (Kumar et al. 2013). In order to assess the damage to the leaves by ROS due to F. oxysporum infection, the third to sixth leaves of seedlings were sampled 3 days after infection and immediately stained with NBT (Zhou et al. 2020). Dark blue staining indicates that the cells were more damaged by ROS, which means that the antioxidant capacity of the cells was relatively lower.

Statistical analysis

The data were analysed with the Statistical Software Package for Social Science (SPSS) version 17.0. Using the Student’s t-test to compare the data, P < 0.05 was considered significantly different. Significant differences (P < 0.05) are indicated by different lowercase letters.

Results

Phylogenetic analysis

An 809 bp cDNA fragment of the PdPapERF109 gene was cloned from RNA extracted from wild seedlings. ERF protein sequences like PdPapERF109 were downloaded and compared. A NJ phylogenetic tree was constructed to show the evolutionary relationships among the sequences. Bootstrap resampling tested the statistical support of the branches. The NJ analysis is shown in Fig. 1A. To test the reliability of the NJ model in detail, the JTT + G + F model was used to build a ML phylogenetic tree using the same data (Fig. 1B). A comparison of the two phylogenetic trees showed that the two models had good consistency, and that PdPapERF109 shared 100% identify with ERF109 from P. trichocarpa (XM_006384349.2). The PdPapERF109 shared high sequence homology with similar proteins from other species, such as P. trichocarpa (100%, LOC18097788, XM_006384349.2), Lactuca sativa L. (92.2%, LOC111890860, XM_023886942.1), Raphanus sativus L. (91.5%, LOC108816386, XM_018588961.1), Dendrobium catenatum Lind. (91.1%, LOC110108677, XM_020839427.2), Pistacia vera L. (90.4%, LOC116118480, XM_031404463.1), Cynara cardunculus var. Scolymus (90.2%, LOC112529749, XM_025141136.1), Cucurbita pepo subsp. Pepo (86.7%, LOC111810595, XM_023697325.1), Populus alba var. pyramidalis (85.8%, MK955879.1), Populus euphratica (84.8%, LOC105108020, XM_011002160.1), Ricinus communis L. (80.3%, LOC8275304, XM_002518721.3), Manihot esculenta Crantz (76.3%, LOC110624639, XM_021769854.1) and Eucalyptus grandis Hill (76.2%, LOC104455366, XM_010070166.1).

Fig. 1
figure 1

Dendrogram of PdPapERF109 and high homology proteins from other species. A NJ-phylogenetic tree; B ML-phylogenetic tree. Dendrogram was constructed by MEGA5 with neighbor method and Poisson model. PdPapERF109 was marked with red frame. It showed similar evolutionary relationship under two algorithms

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Analysis of differential expression of the PdPapERF109 gene

After Pdpap seedlings were infected by F. oxysporum, the expression of the PdPapERF109 gene was significantly induced (Fig. 2A). With an extension of infection time, the expression level continued to increase and reached the highest at 48 h after infection.

Fig. 2
figure 2

A Analysis of expression patterns of PapapERF109 gene under different stages of F. oxysporum infection; B tissue differential expression of PdPapERF109 in different tissues of Pdpap; C tissue differential expression of PdPapERF109 in different parts of the same tissue. T0–T4: Infection times were 0, 6, 12, 24 and 48 h. Significant differences (P < 0.05) are indicated by lowercase letters. Error bars represented standard deviation of three independent replicates

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To determine the spatial expression profiles of the PdPapERF109 gene, qRT-PCR was used to analyze the expression in different tissues. From the results of this analysis, the expression level is lowest in the leaves and highest in the stems (Fig. 2B). Expression was also compared in different parts of the same tissue, and was lowest in the top part of the stem and highest in the bottom (Fig. 2C).

Molecular detection in putative transformants overexpressing PdPapERF109

After the PdPapERF109 overexpression vector was transformed into wild Pdpap, five putative transformant lines were obtained. DNA was extracted from leaves of the WT and the five putative transformant Pdpap plants, and PCR was performed using primer pairs pBI121-F/PdPapERF109-R and PdPapERF109-F/pBI121-R. After PCR amplification, each line gave DNA fragments amplified by both pairs of primers (Fig. 3A). Bands in lanes 1-1 to 5-2 are the same size as bands in lanes “+1” and “+2”. A single amplified fragment was detected in each lane, while there were no amplified bands in lanes “− −1”, “− −2” and “w−1”, “w−2”. The gel electrophoresis results show that the size of the amplified target bands from the five putative transformant lines are as expected. The recombinant plasmid (positive control) showed that the gene can be amplified only in the transformants, but not in the wild material, indicating that the pBI121-PdPapERF109 vector successfully transformed Pdpap, and the five putative transformants contained the PdPapERF109 gene under the expression of the constitutive CaMV 35S promoter.

Fig. 3
figure 3

A Molecular detection in overexpressing putative transformant Pdpap of PdPapERF109. M: DL 2000 Marker; 1–5: Five overexpressing putative transformant lines were used as amplification templates; +: Positive control using PBI121-PdPapERF109 plasmid as template; −: Negative control with water as template; W: Negative control with wild-type Pdpap as template; −1: PCR detection of putative transformant lines using PBI121-F and PdPapERF109-R as primers; −2: PCR detection of putative transformant lines with PdPapERF109-F and PBI121-R as primers. B: Expression level analysis of PdPapERF109 in transformants. WT: Wild-type Pdpap. OE1–OE5: Transformant lines. Error bars represented standard deviation of three independent replicates. Significant differences (P < 0.05) were indicated by different lowercase letters

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Expression analysis of PdPapERF109 in transformants

As shown in Fig. 4, after inoculation with F. oxysporum, wild Pdpap seedlings had wilted and died, but plants of the PdPapERF109-overexpression transformants (OE4 and OE5) grew well. There was a slight water loss in leaf blades and petioles of the transformants. The results show that overexpression of the PdPapERF109 gene in Pdpap plants results in a strong resistance against F. oxysporum infection compared to wild plants.

Fig. 4
figure 4

The growth status of wild Pdpap and overexpressing transformants OE4 and OE5 lines infected by F. oxysporum and grown for 10 days. CK: Wild Pdpap, OE4, OE5: Transformant lines

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The survival rate showed that, after different infections, the transformants (OE4, OE5) were greater than that of the wild material (Fig. 5A). The fresh weights and root lengths of wild and transformants inoculated at zero (initial step), and at 5, 10, 15, and 20 days were determined, and the values at zero were set to 1. The fresh weights and root lengths are shown in Fig. 5B and C, respectively, and values of the two transformants were consistent with the wild Pdpap plants at D0. In contrast, after F. oxysporum infection, the transformants were superior to the wild plants under the same conditions, indicating that they grow better after infection. These results show that ectopic expression of PdPapERF109 can enhance resistance to F. oxysporum during early growth stages in Pdpap plants.

Fig. 5
figure 5

Morphological characteristics of PdPapERF109 overexpression transformants. A: Survival rate of transformants (OE4 and OE5) and wild Pdpap infected with F. oxysporum. B: Relative fresh weight of wild Pdpap and transformants treated with F. oxysporum; C: Root lengths of wild Pdpap and transformants treated with F. oxysporum. D0–D4: Infection times were 0, 5, 10, 15 and 20 days. The error bars represent standard deviation of three independent replicates. Significant differences (P < 0.05) are indicated by different lowercase letters

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Physiological analysis of transformants infected with F. oxysporum

The results of H2O2 contents are shown in Fig. 6A. Under normal conditions, the levels in the wild plants were 1.0 ± 0.0 times more than the transformants (OE4, OE5). In plants infected with F. oxysporum, the relative differences were 1.3 ± 0.1, 1.3 ± 0.0, 1.3 ± 0.1, 1.2 ± 0.1, and 1.3 ± 0.0 times than those of the transformants after 6, 12, 24, 48, and 96 h. At T0, the H2O2 content of the two transformants was basically the same as in the wild Pdpap. However, after F. oxysporum infection, H2O2 levels in the PdPapERF109-overexpressing lines were lower than in the wild plants under the same treatments.

Fig. 6
figure 6

Physiological characteristic analysis of wild Pdpap and PdPapERF109 overexpression transformants infected with F. oxysporum. Two transformants and WT were used as biological replicates for physiological analysis. A Result of H2O2 content measurement; B Result of POD activity measurement; C result of catalase activity; D Result of malondialdehyde content; E Percentage of electrolyte leakage. T0−T5: Infection times were 0, 6, 12, 24, 48, and 96 h. Error bars represent standard deviation of three independent replicates. Significant differences (P < 0.05) are indicated by different lowercase letters

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The results of POD activity measurements are shown in Fig. 6B. The experiment had three biological replicates. Under normal conditions, POD activity in the transformants was 1.0 ± 0.0 times greater than in the wild materials. The relative differences in transformants were 1.2 ± 0.0, 1.3 ± 0.0, 1.3 ± 0.1, 1.2 ± 0.1, and 1.5 ± 0.0 times that of the wild Pdpap at 6, 12, 24, 48, and 96 h after infection. From these results, it is shown that at T0, POD activity of the two transformants is basically the same as in the wild plants. After infection, POD activity in the overexpression lines was higher than in wild plants at the same infection times.

Catalase activity is shown in Fig. 6C. Under normal conditions, CAT activity in the OE4 and OE5 transformants was 1.0 ± 0.0 times that of the wild material. After infection by F. oxysporum, the relative differences in transformants were 1.1 ± 0.0, 1.1 ± 0.0, 1.2 ± 0.0, 1.1 ± 0.0, and 1.2 ± 0.0 times that in wild Pdpap plants at 6, 12, 24, 48, and 96 h. At T0, CAT activity of the two transformants is similar to that of wild plants. However, after F. oxysporum infection, CAT activity in the two overexpression lines was higher than in wild Pdpap at the same infection times.

Malondialdehyde (MDA) contents of leaves from wild and transformants were measured (Fig. 6D). Under normal conditions, MDA levels of the transformants were 1.0 ± 0.0 times that of the wild plants. Conversely, the wild plants were 1.2 ± 0.0, 1.3 ± 0.1, 1.3 ± 0.0, 1.4 ± 0.1, and 1.3 ± 0.0 times higher than those of the transformants at 6, 12, 24, 48, and 96 h after infection. At T0, MDA contents of the transformants were essentially the same as that of the wild plants. Following infection, the overexpressing lines had lower malondialdehyde levels than the wild Pdpap plants at the same infection times.

The percentage of electrolyte leakage is shown in Fig. 6E. Under normal conditions, electrolyte leakage in wild plants was 1.1 ± 0.0 times that of the transformants. After infection, the wild plants were 1.4 ± 0.1, 1.4 ± 0.0, 1.4 ± 0.1, 1.4 ± 0.0, and 1.2 ± 0.0 times higher than that of overexpressing lines at 6, 12, 24, 48, and 96 h. At T0, the percentage of electrolyte leakage in the two transformants was basically the same as in wild plants. After infection, the percentage of leakage in the PdPapERF109-overexpressing lines was lower than in the wild Pdpap at the same infection times.

Histochemical staining

Infected leaves were stained with nitroblue tetrazolium (NBT) to assess the antioxidant capacity of the transformants (Fig. 7). The staining of the leaves from PdPapERF109-overexpressing transformants after infection (Fig. 7C) is slightly darker than in the uninfected wild controls (Fig. 7A), but the overall appearance is similar. In addition, the differences in color compared with leaves of infected wild plants (Fig. 7B) are obvious. These results indicate that in plants infected with F. oxysporum, the cells in the transformants show less damage, indicating that they have a greater ability to remove reactive oxygen species (ROS), including O2− and H2O2, thereby reducing cell damage and enhancing plant tolerance. This is consistent with POD and CAT activity measurements, and further shows that the overexpression of PdPapERF109 in Pdpap plants reduces the accumulation of intracellular ROS. Overexpression effectively enhances the antioxidant capacity of the plants, thus improving the resistance of Pdpap to F. oxysporum infection.

Fig. 7
figure 7

Results of superoxide staining by NBT. A Leaves of wild PdPap without treatment; B Leaves of wild PdPap with F. oxysporum; C Leaves of overexpressing transformants with F. oxysporum. Staining intensities of WT without treatment and transformants infected by F. oxysporum are similar, but greatly increased in WT treated by F. oxysporum

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Discussion

Transcription factors play an important role in abiotic and biological stress responses (Akhtar et al. 2012). TF families such as WRKY, AP2/ERF, NAC, bZIP, and MYB (Pu et al. 2019) affect stress tolerance by regulating downstream response genes (Joshi et al. 2016). Members of the ERF family play a critical role in the responses to stresses, including pathogenic microorganism infection (Guo et al. 2016), ion concentration damage (Yang et al. 2018), excess water or drought (Labbo et al. 2018), and temperature stress (Licausi et al. 2013). At the same time, in the process of defending against stress, the regulation of jasmonic acid, abscisic acid, and ethylene pathways will be integrated (Müller and Bosch 2015). The results of Yi et al. (2004) showed that ERF family genes produce resistance responses to various pathogens during heterologous expression of plants, which can promote the activation of related resistance pathways, thereby effectively reducing the damage to plants exposed to stress.

Previous studies have shown that the PdPapERF109 gene plays a positive role in various biological and abiotic stresses. There are several reports on the resistance functions of the PdPapERF109 gene in salt stress (Bahieldin et al. 2018), low temperature stress (Wang et al. 2019) and drought resistance (Shi et al. 2015), and these studies have established a foundation for our early prediction and subsequent verification experiments. Research has shown that the JA-responsive ethylene response factor 109 (ERF109) mediates crosstalk between JA signaling and auxin biosynthesis to regulate lateral root formation in Arabidopsis (Cai et al. 2014). This indicates that the PdPapERF109 gene may regulate the formation of lateral roots in Pdpap to achieve an effective response after infection by F. oxysporum.

In this study, differential expression levels for PdPpapERF109 in recognized transformants may be due to different copy numbers and sites of insertion. The overexpression of the gene plays a significant role in the growth and physiological state of plants after pathogen infection. The results indicate that the PdPapERF109 gene plays a role in signaling pathways under stress conditions.

One of the important mechanisms of plant disease resistance is the change and increase in the activities of defense-related enzymes (Gong et al. 2010). By measuring the malondialdehyde content and the percentage of electrolyte leakage, the relative degree of damage to the plant cell membranes can be quantified. The determination of physiological indexes such as H2O2 contents, CAT levels, and POD activity can provide a detailed understanding of the ability of the PdPapERF109-overexpressing transformants to remove ROS from the cells and their antioxidant capacity. Studies have shown that different rapeseed varieties express different degrees of resistance to black shank disease, and these resistance capabilities are reflected in a significant increase in the level of POD activity (Peng et al. 2015). Trichoderma aculeatus resists tomato wilt caused by F. oxysporum by increasing the activity of different defense enzymes, thereby improving the disease resistance of tomatoes (Patel and Saraf 2017). In this study, we measured the physiological indicators of the PdPapERF109-overexpressing transformants, including H2O2, POD, CAT, MDA, and the percentage of electrolyte leakage. The purpose of these experiments was to evaluate the resistance of Pdpap plants through the changes in the activities of defense-related enzymes.

NBT staining showed that the overexpression of PdPapERF109 effectively reduced the accumulation of ROS in plants infected with F. oxysporum.

According to previous research, key transcription factors can positively regulate the expression levels of SOD and POD genes, thereby enhancing their activity for removing ROS (Zhao et al. 2019; Guo et al. 2021b). It is also believed that under stress conditions, key transcription factors promote the production of POD and SOD through physiological or biochemical reactions, thereby promoting the removal of ROS (Cheng et al. 2020). Based on the previous reports, it is speculated that under normal conditions, the PdPapERF109 gene will not directly increase POD and CAT contents in the overexpressing transformants. Similarly, the content of ROS did not change significantly in Pdpap. However, in response to F. oxysporum infection, expression of the PdPapERF109 gene can promote the production of POD and CAT through the action of the PdpapERF109 transcription factor on gene expression, thereby promoting the removal of ROS. Therefore, under normal conditions, there is no significant differences in physiological changes between wild plants and transformants. Our data indicates that the PdPapERF109 gene may play an important role in ROS clearance to protect the plant from external stresses. In addition, the molecular mechanism of gene regulation and the gene network related to PdPapERF109 expression in response to F. oxysporum infection requires further study.

Conclusion

In this study, we cloned a transcription factor gene, PdpapERF109, from Pdpap. Subsequently, overexpressing transgenic lines were obtained in Pdpap. The results of morphological and physiological analyses showed that overexpressing transgenic lines were significantly resistant to F. oxysporum compared with WT. Further studies showed that PdpapERF109 could enhance the ROS scavenging ability of Pdpap under stress. Therefore, we concluded that PdpapERF109 has an important regulatory role during the stress response to F. oxysporum.