Research progress on diterpenoid active components of endophytic fungi in plants
Endophytic fungi refer to fungi that colonize between or within healthy tissues of host plants throughout their entire or partial life cycle, and typically do not cause significant infection symptoms in the host. They are an important component of plant microecological systems. In the long-term process of coevolution, various endophytic fungi gradually establish a good symbiotic relationship with host plants, especially endophytic fungi that produce many active metabolites, enhancing the growth and competitiveness of host plants. Therefore, many scientists are becoming increasingly interested in studying endophytic fungi as potential producers of novel bioactive compounds in plants. At present, various structural types of compounds such as terpenes, steroids, alkaloids, flavonoids, phenols, etc. have been isolated from endophytic fungal cultures in plants. These compounds not only serve as chemical reserves for anti-tumor, anti-inflammatory, antioxidant, antiviral, and antibacterial activities, but also make significant contributions to solving current and future challenges in agriculture, environment, and medicine.

At present, there are many reports on the secondary metabolites of endophytic fungi in plants, focusing on compounds such as alkaloids, terpenes, and polyketones, while detailed reviews of the structures and biological activities of diterpenoid compounds are relatively scarce. Therefore, this article only reviews the 137 diterpenoid compounds produced by endophytic fungi in plants since 1993 and their biological activities, in order to provide reference for better development and utilization of diterpenoid compounds from endophytic fungi in plants.

According to the literature reviewed since 1993, studies have reported that 37 endophytic fungi isolated from plants can produce diterpenoid compounds, including: taxane type (1), hemiphyllane type (31), harzianone type (5), isoparaffine type (26), guanacastane type (3), clostridin type (4), indole diterpenoid type (45), ginkgolide type (1), phomactin type (4), cycliane type (13), norcyclostane type (3), and abietane type (1). These compounds contain different activities such as anti-tumor, antiviral, anti-inflammatory, antibacterial, and nematode killing. Especially, six types of skeleton compounds such as half day cyclohexane have significant effects in anti-tumor and antibacterial aspects, as summarized in Table 2-3. Therefore, research on diterpenoid compounds of endophytic fungi in plants should focus on this aspect. In addition, due to factors such as climate conditions, sample collection seasons, and geographical locations, there are differences in the diterpenoid compounds metabolized by endophytic fungi of the same plant species, which increases their versatility. Therefore, these compounds have promising applications in medicine, agriculture, environment, and other fields.

However, there are still some problems in the research of diterpenoid compounds from endophytic fungi in plants. The discovery of endophytic fungi has been over 100 years, but the research on active compounds started relatively late, especially in the study of inducing the formation and accumulation of active substances by endophytic fungi, which has only been a focus of attention in recent decades. Therefore, while isolating and identifying, further research should be conducted on the synthesis pathways, biological activity mechanisms, and structure-activity relationships of diterpenoid compounds induced by endophytic fungi in plants, and their active sites should be clarified. This will provide specific basis for the widespread application of diterpenoid compounds. In addition, more research is needed to optimize the growth conditions of endophytic fungi, especially those that are prone to producing metabolites without significant decay. To obtain more active compounds with high pharmacological effects and apply them in the development and research of new drugs. In summary, the combination of optimizing the growth conditions of endophytic fungi with the mechanism and structure-activity of the isolated active compounds has profound significance for the development and utilization of diterpenoid active ingredients in plant endophytic fungi.