DNA Repair Gene ZmRAD51A Improves Rice and Arabidopsis Resistance to Disease

RAD51 (DNA repair gene) family genes play ubiquitous roles in immune response among species from plants to mammals. In this study, we cloned the ZmRAD51A gene (a member of RAD51) in maize and generated ZmRAD51A overexpression (ZmRAD51A-OE) in rice, tobacco, and Arabidopsis. The expression level of ZmRAD51A was remarkably induced by salicylic acid (SA) application in maize, and the transient overexpression of ZmRAD51A in tobacco induced a hypersensitive response. The disease resistance was significantly enhanced in ZmRAD51A- OE (overexpressing) plants, triggering an increased expression of defense-related genes. High-performance liquid chromatography (HPLC) analysis showed that, compared to control lines, ZmRAD51A-OE in rice plants resulted in higher SA levels, and conferred rice plants resistance to Magnaporthe oryzae. Moreover, the ZmRAD51A-OE Arabidopsis plants displayed increased resistance to Pseudomonas syringae pv. tomato DC3000 when compared to wild types. Together, our results provide the evidence that, for the first time, the maize DNA repair gene ZmRAD51A plays an important role in in disease resistance.


Introduction
In plant immune systems, PAMP (pathogen-associated molecular pattern)-triggered immunity (PTI) through pattern recognition is the first line of defense, which keeps most potential invaders in check [1]. Effector-triggered immunity (ETI) is a second line of defense by recognition of attacker-specific effector molecules [1]. The local activation of a PTI or ETI response triggers systemic acquired resistance (SAR) [2]. SAR is accompanied by the increased levels of SAR signal salicylic acid (SA), which then causes the accumulation, nuclear translocation, and turnover of the transcription cofactor NPR1 (non-expressor of PR1 genes), leading to the activation of pathogeneses-related (PR) genes [3][4][5]. SAR provides long-lasting, broad spectrum resistance to secondary infection [6]. Studies have shown that the DNA repaired protein RAD51D and SNI1 (suppressor of NPR1 inducible 1) coregulated NPR1-independent PR gene expression [7]. DNA damage repair proteins SSN2 (Suppressor of sni1 2) and RAD51D replace the transcription repressor SNI1 at PR gene promoters, and their coordinated action ensures plant immune gene expression during plant defense [8].
The RAD51 recombination protein was first discovered in Saccharomyces cerevisiae [9]. It is homologous to the Escherichia coli RecA protein, which plays a vital role in homologous recombination, a well-known repair process of DNA double strand breaks (DSBs) [9]. During the lifetime of plant, DNA damage always occurs, including DSBs, caused by environmental stresses or intercellular

Cloning and Characterization of ZmRAD51A
The full-length of ZmRAD51A (NP_001104918.2) coding sequence was obtained from the cDNA of maize B73 leaves by using PCR with specific primers. The ZmRAD51A transcript has 9 exons, encoding 340 amino acids ( Figure S1). The predicted 3D model of ZmRAD51A showed similar structures with that of human RAD51 ( Figure 1A). An unrooted phylogenetic tree generated by known RAD51 proteins exhibited a high association between ZmRAD51A and OsRAD51A1, and the two proteins share 92.46% sequence identity ( Figure 1B). Amino acid sequence alignment of ZmRAD51A and other RAD51 proteins from Zea mays, Arabidopsis thaliana, Lycopersicon esculentum, Triticum aestivum and Oryza sativa Japonica suggested conservation of RAD51 across the species ( Figure 1C). The spatial expression pattern analysis revealed that ZmRAD51A is highly expressed in maize ears, tassels, filaments, roots, and stems, but is relatively lower in leaves ( Figure 2A). By employing previously reported transcriptome database, we further investigated the expression profiles of ZmRAD51A gene in tissues at different developmental stages. ZmRAD51A showed high expression levels in seeds, endosperm, embryos, SAM (shoot apical meristem), and roots ( Figure 2B). ZmRAD51A was lower expressed in leaves, supported by the qRT-PCR (Quantitative real time polymerase chain reaction) result ( Figure 2B). To test whether ZmRAD51A expression in maize leaves responds to biotic stress in SA signaling pathways, we applied SA to maize leaves at the three-leaf stage. The transcript level of ZmRAD51A increased from 1 to 24 h after SA treatment, where the highest transcript level (~13-fold up-regulation) was observed in the first hour after SA treatment ( Figure 2C). The spatial expression pattern analysis revealed that ZmRAD51A is highly expressed in maize ears, tassels, filaments, roots, and stems, but is relatively lower in leaves ( Figure 2A). By employing previously reported transcriptome database, we further investigated the expression profiles of ZmRAD51A gene in tissues at different developmental stages. ZmRAD51A showed high expression levels in seeds, endosperm, embryos, SAM (shoot apical meristem), and roots ( Figure 2B). ZmRAD51A was lower expressed in leaves, supported by the qRT-PCR (Quantitative real time polymerase chain reaction) result ( Figure 2B). To test whether ZmRAD51A expression in maize leaves responds to biotic stress in SA signaling pathways, we applied SA to maize leaves at the three-leaf stage. The transcript level of ZmRAD51A increased from 1 to 24 h after SA treatment, where the highest transcript level (~13-fold up-regulation) was observed in the first hour after SA treatment ( Figure 2C). The expression scale of high, medium, and low are represented as red, yellow, and blue colors at right, respectively. V = vegetative growth stage, R = reproductive growth stage, H = hours, DAS = days after sowing, and DAP = days after pollination, GH = greenhouse, V = vegetative, VT = vegetative tasseling, R = reproductive. (C) Gene expression levels of ZmRAD51A in leaf samples response to salicylic acid (SA) application. Three-leaf stage maize seedlings received SA and leaves were harvested at 0 h, 1 h, 3 h, 12 h and 24 h after SA treatment. ZmActin [28] was used for normalization. The expression levels of 0 h was used as the control and assigned value of 1. Data represent means SD. Three biological replicates were performed.

Transient Overexpression of ZmRAD51A in Nicotiana Benthamiana Leaves Induced a Hypersensitive Response
The 35S:ZmRAD51A construct, driven by CaMV35S promoter, was generated. The A. tumefaciens GV3101 containing 35S:ZmRAD51A vector and pCAMBIA1301 (control) infiltrated N. benthamiana leaves to verify hypersensitive response cell death. DAB (Diaminobenzidine) staining showed a large amount of H2O2 was accumulated in transformed N. benthamiana leaf overexpressing ZmRAD51A after 48 h ( Figure 3). Trypan blue staining analysis correlated with the visual damage index [29]. ZmRAD51A transient overexpression leaf showed darker trypan blue staining than that of pCAMBIA1301 ( Figure 3). These results suggest that transient overexpression of ZmRAD51A in tobacco leaves induced a hypersensitive response and H2O2 accumulation in response to stress. The expression scale of high, medium, and low are represented as red, yellow, and blue colors at right, respectively. V = vegetative growth stage, R = reproductive growth stage, H = hours, DAS = days after sowing, and DAP = days after pollination, GH = greenhouse, V = vegetative, VT = vegetative tasseling, R = reproductive. (C) Gene expression levels of ZmRAD51A in leaf samples response to salicylic acid (SA) application. Three-leaf stage maize seedlings received SA and leaves were harvested at 0 h, 1 h, 3 h, 12 h and 24 h after SA treatment. ZmActin [28] was used for normalization. The expression levels of 0 h was used as the control and assigned value of 1. Data represent means SD. Three biological replicates were performed.

Transient Overexpression of ZmRAD51A in Nicotiana Benthamiana Leaves Induced a Hypersensitive Response
The 35S:ZmRAD51A construct, driven by CaMV35S promoter, was generated. The A. tumefaciens GV3101 containing 35S:ZmRAD51A vector and pCAMBIA1301 (control) infiltrated N. benthamiana leaves to verify hypersensitive response cell death. DAB (Diaminobenzidine) staining showed a large amount of H 2 O 2 was accumulated in transformed N. benthamiana leaf overexpressing ZmRAD51A after 48 h ( Figure 3). Trypan blue staining analysis correlated with the visual damage index [29]. ZmRAD51A transient overexpression leaf showed darker trypan blue staining than that of pCAMBIA1301 ( Figure 3). These results suggest that transient overexpression of ZmRAD51A in tobacco leaves induced a hypersensitive response and H 2 O 2 accumulation in response to stress.

Transgenic Overexpression of ZmRAD51A Increased SA Synthesis and Conferred Rice Resistance to M. oryzae
To test whether ZmRAD51A overexpression confers disease resistance in rice, 70 pieces of rice calli were induced and 19 transgenic lines expressing ZmRAD51A (ZmRAD51A-OE) were generated. Three transgenic lines renamed OE-1, 2, and 3, were randomly selected for further analysis. Transgenic expression analysis showed the three transgenic lines had different expression levels ( Figure S2). The growth of ZmRAD51A-OE transgenic rice at the seedling stage was similar to the empty vector (EV) transgenic lines (Figure 4). Demonstrated by high-performance liquid chromatography (HPLC), we found that SA accumulation drastically increased in ZmRAD51A-OE plants (peak area 400.596 g -1 ) compared with control plants (peak area 74.749 g -1 ) without any pathogen inoculation ( Figure 4). ZmRAD51A-OE transgenic plants at 4-5-leaf stage were challenged with M. oryzae, which can cause rice blast. Interestingly, overexpression of ZmRAD51A reduced macroscopic blast symptoms, with fewer and smaller blast lesions than the control at 7 dpi of M. oryzae ( Figure 5A). ZmRAD51A-OE plants showed the lesion area ranging from 11.3 (OE-1) to 19.0% (OE-3) at 7 dpi, while that of the control plants was 51.4% ( Figure 5B). In addition, the expression levels of rice blast resistant genes, OsPAL06, and OsPIKM [30], were respectively increased 2.3-fold and 5.1-fold in ZmRAD51A-OE transgenic lines at 7 dpi of M. oryzae ( Figure 5C). These results together suggest that the overexpression of ZmRAD51A provides an enhanced resistance to rice blast in ZmRAD51A-OE lines.
To investigate whether ZmRAD51A gene affect rice yield, we measured rice yield-related traits, including the seed size and 1000-grain weight, among EV control and ZmRAD51A-OE lines. The seeds of ZmRAD51A-OE rice showed no significant size (in terms of length and width) and 1000-grain weight when compared to EV control rice ( Figure S3).     OsActin [31] was used for normalization. The expression levels of inoculated EV was used as the control and assigned value of 1. Data represent means SD. Three biological replicates were performed. OsActin [31] was used for normalization. The expression levels of inoculated EV was used as the control and assigned value of 1. Data represent means SD. Three biological replicates were performed.

ZmRAD51A Overexpression in Arabidopsis Enhanced Resistance to Pst DC3000 Triggering by Increased SA-Related Genes Expression
To investigate whether ZmRAD51A confers disease resistance to other species, we generated ZmRAD51A overexpression Arabidopsis plants. We firstly examined the impact of ZmRAD51A overexpression in Arabidopsis upon the infection of Pst DC3000 (Pseudomonas syringae pathovar tomato DC3000). The ZmRAD51A-OE lines showed significant resistance to Pst DC3000 at 7 dpi ( Figure 6A,B). Col-0 plants had more lesions than ZmRAD51A-OE plants after Pst DC3000 infection ( Figure 6A). The disease severity (DI, disease index) of ZmRAD51A-OE lines was 23.11% (OE-3) and 20.97% (OE-7) on average, but the Col-0 presented a DI of 50.78% at 7 dpi ( Figure 6B). We further examined the expression levels of the genes that were involved in SA-dependent defense signaling pathway and pathogen resistance at 7 dpi, including AtEDS1 (Enhanced Disease Susceptibility 1) [32], AtNDR1 (Nonrace Specific Disease Resistance 1) [33], AtRPM1 (Resistance to P. syringae 1) [34], and AtTAO1 (Target of AvrB Operation) [35]. All the four genes that participated either in the SA signal pathway or in the resistance to Pst DC3000 pathogens were expressed significantly higher in the ZmRAD51A-OE plants than in the Col-0 plants ( Figure 6C). the expression levels of the genes that were involved in SA-dependent defense signaling pathway and pathogen resistance at 7 dpi, including AtEDS1 (Enhanced Disease Susceptibility 1) [32], AtNDR1 (Nonrace Specific Disease Resistance 1) [33], AtRPM1 (Resistance to P. syringae 1) [34], and AtTAO1 (Target of AvrB Operation) [35]. All the four genes that participated either in the SA signal pathway or in the resistance to Pst DC3000 pathogens were expressed significantly higher in the ZmRAD51A-OE plants than in the Col-0 plants ( Figure 6C).

ZmRAD51A Is a Conserved DNA Repair Protein in Maize
The structure analyses on amino acid sequences and 3D model suggests that ZmRAD51A is conserved across plants to mammals. Notably, ZmRAD51A showed a close evolutionary relationship with OsRAD51A1, which can bind single and double stranded DNA, and promote homology dependent renaturation as well as strand exchange reactions [16], suggesting a similar role of ZmRAD51A in maize. Although the ZmRAD51A has been demonstrated to play important roles in meiotic recombination and DNA repair in maize, little is known about its function in disease resistance. In this study, for the first time, we isolated ZmRAD51A and functionally characterized its roles on disease resistance function in model plants.

ZmRAD51A Is Involved in SA-Signal Defense Responses
The expression patterns of ZmRAD51A showed ZmRAD51A is enriched in ears, roots, tassels, and filaments (Figure 2A), where mitosis and meiosis are more active, supporting that ZmRAD51A plays roles in the repair of DSBs. Although ZmRAD51A is usually less expressed in maize leaf, interestingly, it was significantly upregulated (up to 13-fold) within 1 h upon SA application ( Figure 2B). Such a quick response was also observed in other resistance genes, including the bacterial blight disease resistance gene Xa1 in rice [37] and the Ralstonia solanacearum resistance gene AhRRS5 in peanuts [38]. SA is a well-known phytohormone signaling molecule involved in controlling the defense gene expression against disease [39]. The exogenous application of SA to a plant induces systemic acquired resistance (SAR), increases the expression of pathogeneses-related (PR) genes and enhances resistance to a broad range of pathogens [40][41][42]. Similar to those SA-dependent defense-related genes [31,43,44], our results suggested that ZmRAD51A may also involve in SA signaling pathways against pathogen infection. In addition, the transient overexpression of ZmRAD51A in N. benthamiana showed that it can induce a hypersensitive response, causing cell death and also the accumulation of H 2 O 2 in hypersensitive responses (Figure 3), indicating that ZmRAD51A may be involved in reactive oxygen species (ROS) signaling against disease. These results together suggested that ZmRAD51A may be involved in SA-dependent cell death and disease resistance during pathogen infection.

ZmRAD51A Confers Disease Resistance in Transgenic Plants
Phenylalanine ammonia lyase (PAL) is a SA pathway-associated gene and a key enzyme that controls the biosynthesis of SA [45]. OsPAL06 knockout mutant showed increased susceptibility to M. oryzae and developed typical leaf blast symptoms, accompanied by a reduction of the SA level [45]. In this study, OsPAL06 increased significantly in ZmRAD51A-OE transgenic lines at 7 days after being inoculated with M. oryzae ( Figure 5C), indicating ZmRAD51A conferred resistance to rice blast, accompanied by increasing of the SA level. Moreover, ZmRAD51A also enhanced Arabidopsis resistance to Pst DC3000, by triggering the expression of SA-dependent signal genes and defense-related genes, AtEDS1 (Enhanced Disease Susceptibility 1), AtNDR1 (Nonrace Specific Disease Resistance 1), AtRPM1 (Resistance to P. syringae 1), and AtTAO1 (Target of AvrB Operation) ( Figure 6C). AtEDS1 operates in the upstream of SA-mediated defenses and requires resistance to be mediated by several R genes in Arabidopsis [32]. The increased expression level of AtEDS1 in ZmRAD51A-OE Arabidopsis indicated that ZmRAD51A is involved in SA-mediated defense pathway. It has been reported that AtNDR1 was required for disease resistance signaling mediated by members of disease resistance proteins in Arabidopsis in response to infection by P. syringae [46], while AtRPM1 and AtTAO1 were conferring disease resistance in response to Pst DC3000 [34,35]. The up-regulation of AtNDR1, AtRPM1, and AtTAO1 in ZmRAD51A-OE plants suggested that ZmRAD51A plays a positive role in resistance against Pst DC3000. Given that ZmRAD51A plays an important role in DNA recombination in maize [18], the dual roles of ZmRAD51A in disease resistant gene and DNA recombination suggest an interesting mechanistic link between defense response and DNA recombination, which is also supported by the previous reports of microbial pathogens causing DNA damage [47]. The abundance of double strand breaks is reduced by plant defense responses, suggesting that the mechanisms for activating DNA repair processes may share some similarity with the induction of PR genes [48,49]. More recently, DNA damage has also been found to be associated with SA signaling, where the increased expression of PR genes and the growth suppression of Fusarium solani were found in peas [50].

Plant Materials and Treatments
Maize B73 plants were grown in a greenhouse at 28 • C under a 16 h light/8 h dark photoperiod. Healthy maize seedlings at the three-leaf stage were sprayed with 1 × 10 -3 M salicylic acid (SA). After 0h, 1 h, 3 h, 12 h, and 24 h, leaves were harvested and stored at -80 • C for subsequent RNA extraction.
Other plants were further cultured and harvested roots, leaves, stems, tassels, filaments, husks, and ears were stored at -80 • C for subsequent RNA extraction.

Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) Analysis
Total RNA from maize, rice, and Arabidopsis tissues was extracted by TRIzol (Thermo Fisher Scientific, Waltham, MA, USA). The reverse transcription kit (Roche Molecular Systems, Inc., Pleasanton, CA, USA) was used for cDNA synthesize. The SYBR green PCR master mix (Roche Molecular Systems, Inc., Pleasanton, CA, USA) was used for qRT-PCR reaction. The qRT-PCR was performed by Applied Biosystems 7300 (Applied Biosystems, Foster City, CA, USA) according to the manufacturer's instruction. The qRT-PCR primers used in this study are listed in Table S1. The relative expression levels of each gene were calculated by formula 2 −∆∆Ct [38].

Full-Length cDNA Cloning and Vector Construction
The full-length cDNA of ZmRAD51A was isolated from maize leaves cDNA by using PCR with specific primers (Forward ATGGCAGAAGCTGTGGTGTT and reverse CTATATGCGCAACTCCAGACC) and PrimeSTAR Max DNA Polymerase (TaKaRa Bio Inc., Kusatsu, Shiga, Japan). Then, the products were cloned and sequenced. The full-length cDNA of ZmRAD51A was constructed into the pCAMBI1301vector to obtain a p35S:ZmRAD51A fusion gene.

Cell Death Assays in Nicotiana Benthamiana
Agrobacterium tumefaciens strain GV3101 containing 35S:ZmRAD51A and pCAMBI1301 (control) constructs were respectively injected into N. benthamiana leaves by syringe, which contained the volume of about 100 µL as described by Stella et al. [57]. Photos of N. benthamiana leaves phenotype were taken 48 h after Agrobacterium infection. DAB and trypan blue staining were performed as previously described [38,58]. N. benthamiana leaves were treated with DAB (3 -Diaminobenzidine, Sigma, St. Louis, MO, USA) solution (1 mg mL −1 ) overnight and then cleared with 95% ethanol. For trypan blue staining, the leaves were boiled in lactophenol-trypan blue solution (1 mg mL −1 ) for 5 min and destained overnight in chloral hydrate (2.5 g mL −1 ). The DAB and trypan blue staining leaves were observed under a microscope (Leica DM5000 B, Leica Microsystems Ltd., Heerbrugg, Switzerland).

Rice Transformation
The seeds of rice Zhonghua 11 were used to induce embryogenic calli. Well growth embryogenic calli were harvested for infection by A. tumefaciens GV3101 harboring the 35S:ZmRAD51A constructs and pCAMBI1301 (control) as previously described [37]. After 2 days co-cultivated with A. tumefaciens GV3101 in N6 medium containing 200 µM acetosyringone, embryogenic calli were thoroughly washed with ddH 2 O, and then transferred to a new N6 medium, containing 0.25 g mL −1 cefotaxime and 0.05 g mL −1 hygromycin. After several selections, the embryogenic calli propagated plants in regeneration medium.

Measurement of SA
SA extraction and quantification were performed according to the method described in the previous study [59,60]. The leaves of rice transformed of pCAMBI1301 and ZmRAD51A were excised and frozen in liquid nitrogen (N 2 ). The SA was extracted from 0.3 g leaf powder by sequentially subjecting them to 70% and 90% methanol. The 2-methoxybenzoic acid (Sigma-Aldrich, St. Louis, MO, USA) was used as an internal SA standard. The reverse-phase high-performance liquid chromatography (RP-HPLC) (Agilent 1200 series with a C18 column [150 mm × 4.6 mm, 5 µm], Agilent Technologies, Santa Clara, CA, USA) was used for SA measurement.

Rice Pathogens Inoculation
ZmRAD51A overexpression rice and control of rice plants with the fourth leaf fully expanded were selected for disease analysis. Rice seedlings were inoculated with Magnaporthe oryzae Guy 11 using the spraying method to assess the resistance of transgenic rice to blast [61]. Disease was scored by measuring the percent lesion area (lesion length/leaf length) at 7 days after inoculation [31].

Arabidopsis Transformation
Arabidopsis plants Col-0 were transformed by A. tumefaciens GV3101 carrying the 35S:ZmRAD51A using the floral dip method [62]. Transformed Arabidopsis seeds were screened on Murashige and Skoog plates containing 20 mg mL −1 hygromycin. DNA was extracted from the Arabidopsis plant using the CTAB (cetyl trimethyl ammonium bromide) method [63], which was used as a template for PCR to further determine positive transgene integration. The 2× Taq Master Mix (Dye Plus; Vazyme Biotech Co. Ltd., Nanjing, China) was used for the PCR reaction. After several screens, the homozygous T3 generation was used for experiments.

Arabidopsis Pathogen Inoculation
Pseudomonas syringae pv. (pathovar) tomato DC3000 (Pst DC3000) was cultured at 28 • C on King's B (KB) medium. After 2 days, a single Pst DC3000 colony was inoculated in 5 mL KB medium for 1 day at 28 • C. One milliliter of the culture was inoculated in 100 mL KB medium for 1 day. The bacterial was harvested and suspended in a solution which included 0.01% Silwet L-77 and 10 mM MgSO 4 with final OD 600 = 1.0. Two transgenic Arabidopsis lines (OE-3 and OE-7) and Col-0 were sprayed with Pst DC3000 suspension. Whole Arabidopsis plants were sprayed by the bacterial suspension and then covered with plastic film for 3 days. Disease index was evaluated after 7 days post inoculation (dpi) as Niu et al. described [64]. Three biological replicates were set.

Statistical Analyses
The data were subjected to excel software 2016 with analysis of student's test to evaluate differences between control and treatment samples. Statistical significance was set at * p < 0.05, ** p < 0.01 and *** p < 0.001.