Diversity of Seed Endophytes: Causes and Implications

The immense importance of microbial symbioses with plants, animals, and other eukaryotes is meeting with ever increasing awareness and interest. Heritable symbionts—those transmitted directly from hosting parents to hosting progeny—are particularly intimate associations with profound ecological, evolutionary, and applied consequences. However, heritable symbioses also tend to be inconspicuous and are often understudied. Heritable fungal symbionts of plants, which we call seed endophytes, have been discovered and rediscovered in a few grass species (family Poaceae) starting well over a century ago, but have been intensively researched only in the last 45 years since their ability to produce antimammalian alkaloids was revealed to cause major toxicoses to livestock. The characterization of those fungal Epichloë species has been followed gradually by documentation of other seed endophytes with bioactive alkaloids, such as those found in locoweeds (family Fabaceae) and morning glories (family Convolvulaceae). As the known species diversity of seed endophytes and their hosts has expanded, so too has our knowledge of their alkaloid diversity, defenses against invertebrates, positive and negative effects on host plants, effects on pathogens and beneficial symbionts (e.g., mycorrhizal fungi), protection from abiotic stresses such as drought, and cascading population, community, and ecosystem consequences. Recent studies have even revealed endophyte contributions to plant diversity, including an Epichloë gene apparently transferred to a host grass in which it confers disease resistance. Here we review the current knowledge of seed-endophyte symbioses with emphasis on their phylogenetic, genetic, and functional diversity.