Introduction

Endophytes widely exist in plants and are an important part of plant microecosystems. In the long evolutionary process, they have established a special symbiotic relationship with parasitic plants, which can promote the growth of host plants, enhance their adaptability, enhance their tolerance to abiotic and biological threats, and promote the accumulation of secondary metabolites [30]. According to relevant reports [23, 24, 31] and observations on the incidence of sclerotiniosis at the mulberry fruit base of the Chongqing Academy of Agricultural Sciences, TWC is an immune cultivar against sclerotiniosis, and JZS, DBE, NYSJ, and YK display good disease resistance, indicating that a higher microbial community diversity index means higher resistance, which is in line with the results of Allison [32] and Wu et al. [17].

Plant endophytic fungi mainly include Ascomycota, Basidiomycota, and Zygomycota [33]. In this study, the endophytic fungi in the mulberry stems included 5 phyla in addition to Fungi_unclassified, namely, Ascomycota, Basidiomycota, Zygomycota, and a very small amount of Rozellomycota and Chytridomycota. Ascomycota is the dominant phylum, which is similar to the case of mulberry fruit [34]. The study indicates that there are significant differences among the community structures of different cultivars. Ascomycota and Basidiomycota are common phyla, and Zygomycota was found only in four typical cultivars, namely, NYSJ, JZS, DBE, and TWC. A small number of Chytridiomycota were found in A7 (0.03%) and T6 (0.13%), the two cultivars collected in **njiang. Rozellomycota was found in YK. It is speculated that these differences may be closely related to the mulberry cultivar or resistance. As shown in the heatmap graph, specific genera or dominant genera with significantly high abundances exist in several typical mulberry cultivars. The most noticeable genus was Ampelomyces, of which the abundance in TWC was 16.47–8975.69 times that in the other cultivars. It is considered to be a biocontrol bacterium of epiparasitism. The secretion of antibacterial quinone compounds [35] directly affects the health of plants during the interaction between microorganisms and plants, thus improving plant disease resistance [36]. Some strains were later registered as commercial fungicide products after being used as biocontrol bacteria for the prevention and control of powdery mildew in important crops such as grapes and cucumbers [37]. Importantly, they are not just the host strains of powdery mildew. They are also speculated to be parasitic on other pathogenic fungi, including Botrytis cinerea, Alternaria solani, Colletotrichum coccodes, and Cladosporium cucumerinum [38]. Such findings provide a new approach for the biological control of mulberry fruit sclerotiniosis. In addition, a large number of Ascomycota_unclassified existing in NYSJ are likely to fall within a new species that is speculated to be capable of playing an important role in biological control.

Although sequencing technology allows researchers to obtain large microorganism resources, many of the microorganism cultivars may play only a minor role in ecological functions [39]. In this study, relevant functional information for 77.00%–98.74% of OTUs was not obtained, which is speculated to be because access to functional prediction information is limited owing to the incomplete information in the fungal function prediction database [22] or because the current investigations into mulberry endophytic fungi are still too scarce, which means that microbial resources and ecological functions need to be further developed. The study indicates that there are certain differences in the nutritional types and ecological flora among the cultivars. Overall, saprotroph–symbiotroph, pathotroph, and saprotroph were the main types. According to the clustering results, the typical species are TWC, NYSJ, BYW, DBE, and YK, which is in line with the results of the endophytic diversity analysis. Therefore, combining the results of endophytic fungal diversity and ecological function prediction, it can be inferred that the endophytic fungal diversity and richness of the cultivars with strong disease resistance are higher, and the ecological function is significantly different from that of the susceptible cultivars. The following points need to be emphasized: the abundance of plant pathogens in TWC was the lowest, and other fungi were highly abundant. Ampelomyces was the absolutely dominant genus (98.69%) among the other fungi. According to the analysis above, the high resistance of TWC to sclerotiniosis may be correlated with Ampelomyces. BYW has a low abundance of plant pathogens and an absolute predominance of wood saprotrophs. As the dominant genus that grows in BYW only, Ceriporia (91.47%) has many functions in agriculture, and it can improve the utilization efficiency of soil nutrients by crops [40, 41]. Notably, Ceriporia can also synthesize a series of secondary metabolites and has gained attention as a microbial source of biocatalysts for the biotransformation of natural terpenoid products [42]. The in vitro antifungal activities of terpenoids against a variety of plant pathogenic fungi were evaluated by the mycelial growth rate method, and the results showed that terpenoids exhibited broad-spectrum antifungal activity [43]; thus, the fungi can be considered potential biocontrol fungi [44]. Although the relative abundance of pathotrophs in LS is high, the dominant fungus (Hirsutella) is a parasitic fungus that has a good biological control effect on plant pathogenic nematodes such as mites, stem nematodes, root-knot nematodes, and some lepidopteran insects [45].

Conclusion

In this study, high-throughput sequencing technology was applied to obtain information on the endophyte communities in the stems of different mulberry cultivars. By comparative analyses, we found that there are certain differences in terms of the diverse structures of endophyte communities of different cultivars and that the higher the microbial diversity is, the stronger the plant resistance is. It was also found that the resistant endophytic fungi demonstrate particularity in quantity or strains, which may be of certain value in investigations into plant resistance. Furthermore, cluster analysis also showed differences between mulberries with high resistance and other species in terms of ecological function. The research results can provide theoretical references for the construction of mulberry microbial information systems, the screening of antagonistic strains against sclerotinia, the ecological utilization of mulberry microorganisms and the interactions between mulberry and microbes.