CaCP15 Gene Negatively Regulates Salt and Osmotic Stress Responses in Capsicum annuum L.

Salt and osmotic stress seriously restrict the growth, development, and productivity of horticultural crops in the greenhouse. The papain-like cysteine proteases (PLCPs) participate in multi-stress responses in plants. We previously demonstrated that salt and osmotic stress affect cysteine protease 15 of pepper (Capsicum annuum L.) (CaCP15); however, the role of CaCP15 in salt and osmotic stress responses is unknown. Here, the function of CaCP15 in regulating pepper salt and osmotic stress resistance was explored. Pepper plants were subjected to abiotic (sodium chloride, mannitol, salicylic acid, ethrel, methyl jasmonate, etc.) and biotic stress (Phytophthora capsici inoculation). The CaCP15 was silenced through the virus-induced gene silencing (VIGS) and transiently overexpressed in pepper plants. The full-length CaCP15 fragment is 1568 bp, with an open reading frame of 1032 bp, encoding a 343 amino acid protein. CaCP15 is a senescence-associated gene 12 (SAG12) subfamily member containing two highly conserved domains, Inhibitor 129 and Peptidase_C1. CaCP15 expression was the highest in the stems of pepper plants. The expression was induced by salicylic acid, ethrel, methyl jasmonate, and was infected by Phytophthora capsici inoculation. Furthermore, CaCP15 was upregulated under salt and osmotic stress, and CaCP15 silencing in pepper enhanced salt and mannitol stress resistance. Conversely, transient overexpression of CaCP15 increased the sensitivity to salt and osmotic stress by reducing the antioxidant enzyme activities and negatively regulating the stress-related genes. This study indicates that CaCP15 negatively regulates salt and osmotic stress resistance in pepper via the ROS-scavenging.


Introduction
The growth, development, and yield of crops are seriously affected by various environmental stresses, such as drought, salt, osmotic, heat, cold, UV radiation, heavy metals, pathogenic bacteria, etc. Drought and salinity are two primary abiotic stresses affecting crop yields globally [1]. They also cause secondary salinization of greenhouse soil, limiting the growth of horticultural crops [2]. The suitable prevention strategy is cultivating high-yield and abiotic stress-resistant crops aided by molecular genetics. Molecular genetics is important in determining pivotal genes and regulatory modules involved in salt and drought tolerance and adaptability of stress-tolerant crop plants [3,4]. Plants activate physiological, morphological, and biochemical processes in response to the changing environment [5,6]. Abiotic stresses enhance the accumulation of reactive oxygen species (ROS) and H 2 O 2 (hydrogen peroxide) in peroxisomes, mitochondria, chloroplasts, and other organelles. ROS production is very common in plants under different stress conditions [7]. ROS causes oxidative stress in plant cells, damaging lipids, metabolites, proteins, and nucleic acids, thus affecting multiple biological processes [8]. Proteases rapidly degrade the damaged proteins through proteolysis, which is necessary to regulate stress-signaling molecules by

RNA Isolation and qRT-PCR Analysis
Total RNA was extracted from tissues and leaves of pepper plants under different stress treatments using Trizol (Invitrogen, Carlsbad, CA, USA) method. Complementary DNA (cDNA) was synthesized using the PrimeScript™Kit (TaKaRa, Tokyo, Japan) reagent. The cDNA concentration was measured using a NanoDrop instrument (UNano 1000F, Hangzhou, China) and normalized to 50 ng/ul. The qRT-PCR tests were performed using SYBR ® Premix Ex Taq™II (TaKaRa) reagents, and capsaicin ubiquitin-coupled protein gene (CaUBI3) (accession number: AY486137.1) was used as the reference gene. The experiment was conducted in triplicate, and the relative expression levels of genes were calculated using the 2 −∆∆Ct comparison threshold method. The primer sequences are shown in Table 1. Table 1. Primer sequences in this study.

VIGS and Transient Overexpression Assay of CaCP15
The fragments of the CaCP15 gene were cloned from pepper line B12 and inserted into the pTRV2 vector, as previously described, and CaPDS (phytoene desaturase in pepper, accession number: LOC107861625) served as the positive control [24]. After four weeks, the transcriptional level of CaCP15 was measured in pTRV2: CaCP15 and pTRV2 plants.
CaCP15-silenced and control plants were treated with NaCl and mannitol (300 mM) for the stress experiment.
Agrobacterium GV3101 cells harboring pSN1301-GUS-CaCP15 or pSN1301-GUS-00 (used as a control) were infiltrated into the leaves of pepper plants at the eight-leaves stage for salt and osmotic assays [22,25].

Physiological Parameters Measurements
Total chlorophyll and malondialdehyde (MDA) contents were measured as previously described [26,27]. The H 2 O 2 and proline contents were determined according to the modified method by Wang et al. [28]. The activities of the antioxidant enzymes (mutase and peroxidase) in pepper leaves were measured as described by Beauchamp et al. and Ranieri et al. [29,30].
2.6. Statistical Analysis SPSS 22.0 software was used to analyze the data (p < 0.05). The histograms were generated using SigmaPlot 14.0. The analyzed data were presented as the means ± standard deviation (SD).

Identification and Characterization of the CaCP15 Gene
CaCP15 (LOC107859299) contained a complete open reading fragment (ORF) of 1032 bp, containing 343 amino acids with a theoretical MW of 37.98 kDa and a calculated pI of 5.44. The CaCP15 was distributed on chromosome 2 (chr2) and predicted to localize in the vacuoles. One intron was found between the nucleotide sites 531-852 ( Figure 1a). CaCP15 had eleven consensus motifs (Figure 1b), and the N terminus of CaCP15 contained a transmembrane helix (position F5-T24). Inhibitor 129 (H38-F95) and peptidase_C1 (V128-T342), highly conserved domains, were found in CaCP15 amino acid sequences (Figure 1c). The secondary structure of CaCP15 mainly contained 34.99% of α helices, 15.16% of strands, 6.41% of β turns, and 43.44% of random coils. Thus, the random coil occupied the largest proportion of secondary structures, followed by α helices and extended strands. Moreover, the tertiary structure of CaCP15 was generated using homologous modeling. The 3D models of CaCP15 were based on template c6u7dA (PDB header: plant protein, Chain:A; PDB Molecule:fbsb; PDBTitle: recombinant stem bromelain precursor) (Figure 1d). The composition and location of the secondary structure of the protein were observed distinctly.

Promoter Analysis of CaCP15
The cis-acting regulatory elements of the CaCP15 promoter were identified to characterize the transcriptional regulation of CaCP15 ( Figure 4). The result revealed the existence of some putative cis-acting regulatory elements modulating stress response and defense-related genes in the promoter region. These elements contained one TC-rich repeats (defense and stress responsiveness), one GT-1 cis-element (salt-stress response), one MYB (drought-stress related), two MYC (drought-stress related), one Myb (regulated anthocyanin pigment), one ABRE (abscisic acid responsiveness), three ARE (auxin responsiveness), and one WUN motif (wound-responsive). Moreover, we identified 19 CAAT-box (promoter and enhancer regions) and some light-responsive elements (One GATA-box, one GA motif, one Box4, and two P-box). In addition, more than half of all putative cis-elements occurred between −1000 to −1 bp within the promoter sequence.

Promoter Analysis of CaCP15
The cis-acting regulatory elements of the CaCP15 promoter were identified to chara terize the transcriptional regulation of CaCP15 ( Figure 4). The result revealed the existen of some putative cis-acting regulatory elements modulating stress response and defens related genes in the promoter region. These elements contained one TC-rich repeats (d fense and stress responsiveness), one GT-1 cis-element (salt-stress response), one MY (drought-stress related), two MYC (drought-stress related), one Myb (regulated anthoc anin pigment), one ABRE (abscisic acid responsiveness), three ARE (auxin responsiv ness), and one WUN motif (wound-responsive). Moreover, we identified 19 CAAT-b (promoter and enhancer regions) and some light-responsive elements (One GATA-bo one GA motif, one Box4, and two P-box). In addition, more than half of all putative c elements occurred between −1000 to −1 bp within the promoter sequence.

Expression Analysis of CaCP15 in Pepper
To explore the potential functions of CaCP15, we analyzed the expression profiles The results showed that CaCP15 was detected in various tissues. Compared with the roo the expression level of CaCP15 in the stems was 8-fold higher, suggesting that CaCP may be involved in stem development (Figure 5a, [22]). The expression of CaCP15 vari in different leaf development stages. Compared with the roots, CaCP15 was highly e pressed in young and mature leaves than that old leaves (Figure 5a). ABA, ETH, SA, an MeJA were respectively sprayed on the leaves of B12 pepper plants at the six-leaf sta leaves. We found that CaCP15 was upregulated to varying degrees under different exo enous hormone treatments. ABA slightly decreased the transcription level of CaCP

Expression Analysis of CaCP15 in Pepper
To explore the potential functions of CaCP15, we analyzed the expression profiles of CaCP15 in various pepper tissues under various stresses by qRT-PCR ( Figures 5 and 6 and [22]). The results showed that CaCP15 was detected in various tissues. Compared with the roots, the expression level of CaCP15 in the stems was 8-fold higher, suggesting that CaCP15 may be involved in stem development (Figure 5a, [22]). The expression of CaCP15 varied in different leaf development stages. Compared with the roots, CaCP15 was highly expressed in young and mature leaves than that old leaves (Figure 5a). ABA, ETH, SA, and MeJA were respectively sprayed on the leaves of B12 pepper plants at the six-leaf stage leaves. We found that CaCP15 was upregulated to varying degrees under different exogenous hormone treatments. ABA slightly decreased the transcription level of CaCP15 7 of 16 within 6 h of the treatment, but the CaCP15 transcripts later increased, reaching the peak at 12 h. Interestingly, CaCP15 was drastically downregulated in the ABA-treated plants at 24 h compared with the control (0 h) (Figure 5b). ETH and SA gradually upregulated CaCP15 within 12 h of the treatment, resulting in 7.5-and 4.8-fold increments in the CaCP15 transcripts, respectively, compared to the control. However, the CaCP15 transcript levels declined rapidly at 48 h (Figure 5c,d). The MeJA treatment slightly downregulated CaCP15 expression at 3 h and 12 h, reaching the lowest point within the first 3 h, after which CaCP15 expression was increased, reaching the peak (5-fold) at 48 h post-treatment (Figure 5e). The results indicated that the CaCP15 gene could be regulated by the four signaling molecules (ABA, ETH, MeJA, and SA).  As shown in Figure 4f, the expression level of CaCP15 was increased in pepper plants inoculated with Phytophthora capsicipc. The P. capsicipc reduced the expression level of CaCP15 within the first 3 h after infection but were gradually upregulated the expression upregulated before 48 h post-treatment, reaching the peak of the expression 3.5-fold higher than the control. After that, there was a sharp reduction at 72 h, reaching the lowest expression level. Interestingly, CaCP15 transcripts were slightly upregulated at 96 h and then reduced to the same expression level at 24 h (Figure 5f). These results suggested that CaCP15 possibly participated in the pepper resistance to pathogens.
To determine the roles of CaCP15 in response to salt, osmotic, drought, cold, heat, and oxidative stresses, we artificially altered the growth environment of pepper plants ( Figure 6, [22]). For the salt and osmotic stress, pepper plants were soaked in NaCl and mannitol solution, respectively. The CaCP15 expression was gradually enhanced by NaCl (300 mM) treatment at 3 h and constant until 6 h, followed by an increment that represented the peak expression (to 3.4-fold) at 12 h (Figure 6a). Similarly, mannitol treatment increased the CaCP15 transcripts, reaching the peak (2.7-fold) at 6 h. However, the expression level of CaCP15 gradually declined until 48 h post-treatment, at which point the transcript levels were lower than those of the control (Figure 6b). Under drought stress, the transcription level of CaCP15 was slightly downregulated in the leaves of pepper plants at 3 h but was upregulated from 3 h to 6 h after uprooting the pepper plants. Interestingly, the transcription level of CaCP15 was rapidly downregulated at 12 h, a same level relative to the control. However, compared to the control (0 h), CaCP15 expression had a 2.0-fold upregulation under drought stress at 24 h, reaching the peak (Figure 6c). To analyze the abundance of CaCP15 transcripts under cold and heat stress, we exposed pepper plants to 4 • C and 40 • C in the illumination incubator. Results showed that the transcription level of CaCP15 showed a downregulation trend at 4°C and 40 • C. As shown in Figure 6d, CaCP15 expression declined drastically within the first 3 h of the 4 • C treatment and remained constant at 6 h. There was a sharp increase in CaCP15 expression at 12 h and a decrease at 24 h. In the 40 • C treatment, CaCP15 expression reduced in the first 1 h and suddenly increased at 3 h. Interestingly, CaCP15 transcript levels were slightly downregulated from 6 h to 24 h (Figure 6e). Pepper plants were also sprayed with 100 mM H 2 O 2 to study whether the CaCP15 gene responded to oxidative stress. Compared with the control (0 h), the transcriptional level of CaCP15 remained stable at 3 h but was dramatically downregulated at 6 h after treatment, reaching the bottom. Thereafter, the CaCP15 expression was sharply enhanced at 12 h and gradually downregulated from 24 h to 48 h (Figure 6f). The results showed that the CaCP15 gene responded positively to these abiotic stresses.

Knockdown of CaCP15 enhnaces Salt and Osmotic Stress Resistance in Pepper
CaCP15 was silenced in pepper by the VIGS technique to verify the function of CaCP15 under salt and osmotic stress [24]. At two weeks after planting, the B12 pepper plants were infiltrated with Agrobacterium cells containing TRV2:00, TRV2:CaPDS, and TRV2:CaCP15 vectors and were subjected to stress treatments after about 45 days of the

Knockdown of CaCP15 Enhnaces Salt and Osmotic Stress Resistance in Pepper
CaCP15 was silenced in pepper by the VIGS technique to verify the function of CaCP15 under salt and osmotic stress [24]. At two weeks after planting, the B12 pepper plants were infiltrated with Agrobacterium cells containing TRV2:00, TRV2:CaPDS, and TRV2:CaCP15 vectors and were subjected to stress treatments after about 45 days of the infiltration. The empty vector TRV2:00 was used as the negative control. Since CaPDS silencing caused leaf photobleaching symptoms, the TRV2: CaPDS plants were used as the positive controls for detecting VIGS efficiency. As shown in Figure 7a, TRV2: CaPDS plants showed obvious leaf photobleaching symptoms, indicating that the VIGS system was successful. Compared with the TRV2:00 plants, TRV2:CaCP15 plants had no morphological changes after 45 days of inoculation (Figure 7a). Therefore, we measured the expression level of CaCP15 in the leaves of TRV2:00 and TRV2:CaCP15 plants by qRT-PCR. The efficiency of CaCP15 silencing was 80% lower in the CaCP15-silenced plants compared with the control, implying that CaCP15 was successfully silenced by the VIGs assays (Figure 7b). To determine the function of CaCP15 under salt or osmotic stress, we exposed the leaf discs (1.0 cm in diameter) from the leaves of TRV2:00 and TRV2:CaCP15 plants to NaCl or mannitol solution (300 mM), with sterile water as the control. After 3 days, the leaf discs of the control plants subjected to salt or osmotic stress exhibited a bleached phenotype compared to those subjected to the control (sterile water), and more obvious than the leaf discs of TRV2: CaCP15 under stress (Figure 7c). Hence, we measured the chlorophyll content of the leaf discs under different treatments. The chlorophyll content in the TRV2:CaCP15 and TRV2:00 plants was reduced after treatment, and the leaf discs of the TRV2:00 plants degraded more than those of CaCP15-silenced plants discs (Figure 7d). Moreover, we also measured the MDA content, which reflected the degree of leaf damage under stress. As shown in Figure 7e, MDA accumulation was gradually increased in TRV2:CaCP15 and TRV2:00 plants after NaCl or mannitol treatment, but the MDA content of the control plants was higher than that of TRV2:CaCP15 plants. These findings proved that CaCP15 silencing could enhance the salt and osmotic stress resistance in pepper.

Transient Overexpression of CaCP15 Reduces Salt and Osmotic Stress Resistance in Pepper
To further investigate the function of CaCP15 in salt and osmotic stress tolerance, we overexpressed CaCP15 in pepper leaves using the 35S:CaCP15 vector, with taking the 35S:00 empty vector as the control. The infiltrated plants were treated with NaCl, mannitol, and water (control). After 12 h post-treatment, the leaves of CaCP15-overexpressing plants had significantly wilted compared with the 35S:00 leaves under salt and osmotic stress (Figure 8a). We further measured the physiological indexes related to the ROS system. The leaves of CaCP15-overexpressing and control plants showed excessive MDA accumulation, which was higher in the CaCP15-overexpressing leaves than in the 35S:  (Figure 8d,e). In contrast, the proline content was reduced in response to salt and osmotic stress in all plants. The degree of reduction in the CaCP15overexpression leaves (14.5% under salt and 26.7% under osmotic stress) was more obvious than in 35S:00 plants (Figure 8f). Furthermore, we also measured the transcriptional level of antioxidant-related genes (CaPOD, CaSOD, and CaCAT) and stress-related genes (CaNHX1, CaP5CS, CaPOX2, and CaSOS1) to analyze the function of CaCP15 in pepper under salt or osmotic stress. As shown in Figure 8g, there was a significant increase 35S:CaCP15 and 35S:00 plants under salt or mannitol stress, and the transcriptional levels of these genes were significantly lower in the CaCP15-overexpression leaves than in the control plants (Figure 8g). Overall, CaCP15 overexpression increased the sensibility to salt and osmotic stresses, suggesting that CaCP15 may play a negative regulatory role in the salt and osmotic stress resistance of pepper. (c) The manifestations of CaCP15-silenced and control leaves discs to salt and osmotic stresses; (d,e) Chlorophyll and MDA contents of the CaCP15-silenced and control leaf discs in response to 300 mM NaCl and mannitol stresses, respectively. The values are the means ± SE (standard error) of three independent replicates. The letters (a-e) represent significant differences according to Tukey's test (p < 0.05).

Transient Overexpression of CaCP15 Reduces Salt and Osmotic Stress Resistance in Pepper
To further investigate the function of CaCP15 in salt and osmotic stress tolerance, we overexpressed CaCP15 in pepper leaves using the 35S:CaCP15 vector, with taking the 35S:00 empty vector as the control. The infiltrated plants were treated with NaCl, mannitol, and water (control). After 12 h post-treatment, the leaves of CaCP15-overexpressing plants had significantly wilted compared with the 35S:00 leaves under salt and osmotic (c) The manifestations of CaCP15silenced and control leaves discs to salt and osmotic stresses; (d,e) Chlorophyll and MDA contents of the CaCP15-silenced and control leaf discs in response to 300 mM NaCl and mannitol stresses, respectively. The values are the means ± SE (standard error) of three independent replicates. The letters (a-e) represent significant differences according to Tukey's test (p < 0.05).

Discussion
PLCPs are a functional proteolytic enzyme family involved in plant growth, development, senescence, immune and stress responses [31]. The PLCPs family has complementary and redundant functions, making it difficult to determine the functional importance of a particular PLCP in plants. In this study, we characterized a multiple stress-induced proteolytic enzyme CaCP15. CaCP15 is a member of the SAG12 subfamily, with two typical conserved domains: "ERFNIN" and "GCNGG" motifs. This is consistent with the other members of the SAG12 subfamily [22]. The CPs sequences of other plants also have "ERFNIN" and "GCNGG" conserved regions, demonstrating that the functions of the two domains are important. The evolutionary tree analyses of the CP proteins showed CaCP15 was homologous to NtCP15 and SlCP15 in tobacco and tomato, respectively. A previous study have proved that SlCP15 is one of the immune proteases in tomatoes [32]. NtCP15 confers resistance to pathogens [33]. Similarly, we verified that CaCP15 expression was increased after P. capsicipc treatment. Besides, MeJA and SA applications increased the CaCP15 transcripts to 5.0-fold compared with the control. MeJA and SA are critical in plant defense against pathogen infection [34,35]. Our results indicated that CaCP15 might be involved in the resistance against pathogenic bacteria through the MeJA-and SA-dependent signaling pathways in pepper. The tissue expression analysis of CaCP15 in pepper showed that the transcription level of CaCP15 was 8-fold higher in the stems than in the roots, suggesting that the gene may play a role in stem development.
In plants, CPs are involved in salt and osmotic stress responses. For example, the expression levels of AtRD21A and AtRD19A in Arabidopsis were increased under salt stress [12]. The transcription level of Cyp15a was increased in pea seedlings treated with NaC1 [13], and the wheat PLCP gene (TaCP) was upregulated by salt stress [36]. SPCP2-overexpressing Arabidopsis thaliana had enhanced salt stress resistance [14]. Salinity stress increased the expressions of CPs genes (LOC_Os01g73980, LOC_Os02g27030, and LOC_Os05g01810) in rice [37]. The two barley CysProt were involved in drought stress response [38]. Furthermore, CaCP11 and CaCP34 participated in salt and mannitol stress resistance of pepper [22,24]. These studies suggest that CPs may play important roles in abiotic stress responses in plants. We identified several cis-elements in putative promoter regions of CaCP15, which could respond to signal molecules and environmental stresses. Interestingly, one GT-1 motif, a cis-acting element involved in response to salt stress [39], and one MYB and two MYC drought-stress-related cis-acting elements were found in the CaCP15 promoter [40]. Hence, we used qRT-PCR analysis to verify the function of CaCP15 under abiotic stress and exogenous plant hormone application. The results revealed that CaCP15 was regulated by salt and osmotic stress, and its transcription level increased by 3.3-fold at 12 h under NaCl treatment and 2.7-fold at 6 h under mannitol treatment compared with at 0 h. In addition, ABA application upregulated CaCP15 expression, and the expression level at 12 h was 2.0-fold higher than that at 0 h. Similarly, ETH treatments also enhanced the CaCP15 expression, and the expression level at 12 h was increased by 7.5-fold compared with at 0 h. Since ABA and ETH signaling pathways are central regulators of abiotic stress responses in plants, we hypothesized that CaCP15 responded to abiotic stress through the ABA or ETH signaling pathway [41][42][43]. We used VIGs and transient overexpression assay to further verify the function of CaCP15 in response to salt and osmotic stress. Chlorophyll content can reflect the damage degree of plants under stress [44]. It was found that deletion or overexpression of CPs, such as AtCEP1 and HvPAP14, could induce changes in the expression of photosynthetic genes in plants [45,46]. Thus, chlorophyll content can be affected by CPs in the cytoplasm [47]. Compared with control plants, the total chlorophyll content in the CaCP15-silenced leaves showed a 36.7% and 64.6% increase after NaCl and mannitol treatments, respectively. ROS-induced lipid peroxidation is an internal indicator of ROS damage, reflected by the MDA content [48]. MDA is generally used to evaluate the degree of ROS-mediated lipid peroxidation in plants under high salt stress [48]. The MDA content in the CaCP15-silenced leaves was lower than control plants after the treatments, and the CaCP15-silenced leaves showed a 31.6% reduction under salt stress and a 33.4% reduction under osmotic stress. However, the transiently overexpressing-CaCP15 leaves showed a 51.1% increase in the MDA content and had 43.3% higher H 2 O 2 contents than the controls control under osmotic stress. Salt stress also slightly increased the MDA and H 2 O 2 contents in the 35S:CaCP15 leaves compared to the 35S:00 plants. H 2 O 2 is a product of ROS [49]. Thus, these results showed that CaCP15 might play a negative role in the abiotic stress response of pepper by clearing ROS accumulation. We also observed the variation in the activities of the major ROS scavenging enzymes (SOD and POD) in the transiently overexpressing-CaCP15 plants experiment with significant differences before and after treatments. The SOD and POD activities of the CaCP15overexpression plants were significantly lower than those of the control. The antioxidant enzyme system and enzyme encoding genes (CaPOD, CaSOD, and CaCAT) were activated under stress conditions to protect pepper from the injuries caused by stress [50]. In our study, the stress treatments reduced the expression of CaPOD, CaSOD, and CaCAT in the CaCP15-overexpression pepper plants. Salt stress significantly improved the activities of the antioxidant enzymes to decompose H 2 O 2, a product of ROS, suggesting that ROS-scavenging plays an important role in salt tolerance mechanism [51]. These results showed that CaCP15 overexpression reduced the stress resistance of pepper by reducing the ROS scavenging enzymes activities.
Proline protects against osmotic stress, and NtP5CS1 is involved in proline biosynthesis under salt stress [52]. The proline content and the expression of CaP5CS were lower in the CaCP15-overexpressing leaves than in control under stress. CaCP15 increased the sensitivity of plants to salt and osmotic stress. Moreover, the stress response genes, such as SOS, NHX1, P5CS, etc., could be activated under stress [53][54][55][56]. The transcription levels of NtSOS1 and NtNHX1 were significantly increased in AlSRG1 transgenic tobacco under salt or osmotic stress, increasing their abiotic stress resistance [57]. ZmMKK4 regulated osmotic stress response in transgenic tobacco by ROS-scavenging, and NtPOX1 was upregulated in the ZmMKK4-overexpressing plants [58]. In our study, the expression of CaSOS1, CaPOX2, and CaNHX1 in CaCP15-overexpression pepper leaves was reduced under stress compared to the control leaves, showing that CaCP15 overexpression enhanced the sensitivity of pepper to salt and osmotic stress.

Conclusions
In conclusion, CaCP15 is a SAG12 protein containing two highly conserved domains. The expression profile revealed that CaCP15 was associated with the development of pepper stems and was involved in abiotic and biotic stress responses. CaCP15 silencing in pepper enhanced salt and osmotic stress resistance. Contrarily, transient overexpression of CaCP15 reduced salt and osmotic stress resistance by decreasing the antioxidant enzyme activities and negatively regulating the stress-related genes. In summary, CaCP15 may negatively regulate salt and osmotic stress resistance in pepper. This study demonstrates the molecular and physiological responses of CaCP15 to salt and osmotic restress in plant. Our future studies will focus on determining the factors the interacting with CaCP15 under salt and osmotic stress to understand the regulatory pathways and mechanisms related to abiotic stress for breeding stress-resistant pepper varieties.