Seed Endophytes and Their Roles in Host Plant Stress Resistance

Seed endophytes are microorganisms that exist in the internal tissues of seeds and do not cause any obvious tissue damage or adverse effects on the host. Most of them are passed down from generation to generation by vertical transmission and become the first microorganisms of the plant and then as the basis to establish the plant endophyte community. Various kinds of plant species have been investigated for seed endophytes, and the results showed that the diversity of seed endophytes is lower than that of leaves, stems, and roots. However, it was found that some core endophytes are kept in the seed. They can promote host plant growth by phytohormone production, nitrogen fixation, potassium and phosphorus dissolving, etc., or through improving other nutrient absorption. Particularly, they can enhance host plant stress resistance by producing active metabolites or/and regulating a host plant's antioxidant system and related resistance genes. There are so many kinds of seed endophytes that have been reported, and among them, the most common endophytic bacteria are Firmicutes, Proteobacteria, Actinomycetes, and Bacteroidetes, while the most common endophytic fungi are Ascomycetes and Basidiomycetes. The present paper reviews the diversity of seed endophytes and their role in host plant stress resistance, and some problems in their applications in agricultural production are discussed.