Research progress on the function of plant endophytes in enhancing plant resistance to biological stresses

Plant growth and development can be threatened by a variety of adversities, which cause problems such as loss of nutrients and significant decrease of yied. Using traditional agrochemicals to regulate the plant resistence to stresses can result in the resistance in insect pests and diseases not only in traditional plant varieties but also in transgenic plants. These agrochemicals cannot be degraded by biological means, and cause environmental pollution. Also, prevalence of these chemicals can even cause severe health issues to the farmers, livestock, and consumers. Therefore, application of naturally available microbes is a safe and alternative complementary way to tackle the pests and phytopathogens. Endophytes living in almost every plant are natural components of plant microecosystems and may have more positive and direct effects on plants because of their special ecological niches. However, the mechanism of endophytic bacteria in improving host biostress resistance is still poorly understood. In this review, we describe the origin, diversity and resistance to biotic stress of endophytes. Firstly, we provide an overview of the transmission routes that endophyte can take to colonize plants, including vertically via seeds and pollen, and horizontally via soil, atmosphere, and insects; secondly, summarize and analyze the diversity of endophyte species and distribution diversity in plants; finally, the basic characteristics and action mechanisms of endophytes in enhancing the tolerance of plants to biotic stress (anti-pathogenic bacteria and insect resistence) are described in detail. Endophytes use niche competition or nutrient competition to promote plant induced resistance to inhibit pathogen infection, or use synthetic antibiotics, alkaloids, chitin and other secondary metabolites to inhibit the growth of pathogenic bacteria or nematodes, so as to prevent and control insect pests and diseases. In addition, based on the research status of endophyte enhancing plant biotic stress resistance, the future development direction is prospected, and this article provides reference for the development and utilization of more environmentally friendly biological control agents.