Endophyte Symbiosis: Evolutionary Development, and Impacts of Plant Agriculture

Land plants can absorb soil microbes (bacterial, fungal and algal) into their cells and tissues. Plant endophytes enhance plant growth, stimulate elongation of root hairs, increase branching of roots, allow plants access to more nutrients, and stimulate oxidative stress tolerance. In the rhizophagy cycle, microbes are absorbed from soil directly into plant root cells where nutrients are extracted oxidatively, which provides nutrients to support plant growth. Early land plants lacked true roots, but possessed non-photosynthetic filaments (e.g., caulonemata, rhizoids) that may have cultivated diazotrophic bacteria within their cells as a source of nitrogen, just as bryophyte and pteridophyte rhizoids do today. Extant land plant lineages, such as bryophytes, pteridophytes, gymnosperms, and flowering plants, often produce epidermal structures (e.g., trichomes, papillae, paraphyllia, scales) on their roots, leaves, stems, or thalli; these often contain symbiotic nitrogen-fixing bacteria. Little is understood about how plants interact with soil and plant microbiomes. In this article we present novel endophytic phenomena in diverse lineages of land plants (liverworts, ferns, monocots, and eudicots) and explain how such symbiotic systems might have evolved over hundreds of millions of years. Due to these endophytic and symbiotic systems, land plants have the capability to obtain nutrients from the environment. Cultivation practices used in commercial agriculture can detract from the innate capabilities of plants to use microbes as a source of nutrients and might be harmful to plant health.