Potyvirus is the largest genus of plant viruses causing significant losses in a wide range of crops in tropical and subtropical regions of the world [
32]. Growing evidences showed that diverse potyvirus species could infect
P. edulis plants in many parts of the world [
33]. Telosma mosaic virus (TeMV), a potyvirus, was reported to infect
Telosma cordata plants in Vietnam, subsequently patchouli plants in Indonesia and recently passion fruit in Thailand and Haikou and Fujian province of China [
13‐
17]. However, up until now, the infection of TeMV and its effects on phytochemical contents of
P. edulis plant especially fruits remain largely unknown.
The identification of a virus typically requires the application of a number of methods including physical, biological, serological and molecular methods. In this present study, firstly, TeMV infection associated with mosaic and distorted leaves and mosaic skin on green fruit (Fig.
1a and
b) was identified with traditional electron microscope and polymerase chain reaction (PCR) (Fig.
1c and
d, Table
1). However, the two detection methods depend on prior knowledge of morphological characteristics of virus particles or specific sequence of the potential virus. Recently, next generation high-throughput parallel sequencing platforms have proved to be highly efficient in identification of diverse plant and animal viruses [
34‐
39]. Thus, sRNA-seq was used to identify TeMV infection in
P. edulis plants. We analyzed and characterized some common features of the virus-derived small interfering RNA (vsiRNAs) specifically from TeMV, including the amount (0.44%), the length distribution (mainly 21-nt and 22-nt), the bias of first nucleotide (mainly G and C), the polarity distribution (equally from positive and negative strands) and frequency distribution (hot and cold spots) of vsiRNAs along the TeMV genome, the coverage (73.2%) and the average depth (9.5) (Fig.
2). Taken together, these methods mentioned above confirmed the infection of TeMV in
P. edulis plants.
TeMV infection adversely affected the developments of
P. edulis fruits (Table
2). However, the effects of viral infection on the phytochemical components remain unknown. The total proteins and total sugars levels increased significantly in TeMV-infected
P. edulis fruit compared with TeMV-free
P. edulis fruit (Fig.
3a and
b). Similar observations were also reported for begomovirus-infected pumpkin/bitter gourd and for potyvirus-infected plum, where increase in the level of total proteins and total sugars was caused by virus infection, respectively [
18,
19,
40]. Host nutrition can play a key role for the outcome of pathogen infections in host, since it is critical for immune-defense and resistance to pathogens. Poor nutrition, in particular protein or sugar depletion, is a major factor in high incidence and host mortality due to infectious diseases. Thus, the increased total proteins and total sugars may be implicated in pathogen defense. In contrast, total fat and total acid was decreased due to TeMV infection (Fig.
3c and
d). Thus, virus infection could differently and selectively modulate the nutrition components (three primary metabolites) of
P. edulis fruit. Recently, more evidence showed that changes of secondary metabolites were involved in host plant resistance in response to invading pathogens. Phenols, a secondary metabolite, played important roles in host-pathogen interaction, disease development and defence reaction of infected plants [
41]. Our results showed that the level of total phenols was obviously higher in TeMV-infected than TeMV-free
P. edulis fruit (Fig.
3f). Therefore, the increased quantity of total phenols in virus-infected
P. edulis fruit presumably appears to contribute towards the resistance against viral infection. Virus pathogens are known to cause oxidative damage such as tissue necrosis to plants by triggering excess production of reactive oxygen species (ROS), which in turn could defend against invading pathogens at moderate level [
42,
43]. Plant cells are protected against the oxidative damage caused by ROS through a complex antioxidant system, comprising antioxidants like ascorbic acid (Vitamin C, Vc) and antioxidant enzymes like superoxide dismutases (SOD) and catalases (CAT).Very few reports are available for antioxidative enzymes activity in plants subjected to biotic stresses especially, viral infection. SOD is the key antioxidative enzyme and catalyzes dismutation of superoxide free radical (O
2−) into H
2O
2 and O
2. In turn, CAT break down H
2O
2 in the living. In this study, a significant decrease of Vc (30%) (Fig.
3e) but an obvious increase of SOD (63%) and CAT (27%) activities were observed in the virus-infected
P. edulis fruit (Fig.
4). Our observations are in agreement with report for geminivirus-infected bitter gourd and tomato leaf curl palampur virus-infected pumpkin [
18,
40].