An Ecological Insight into the Multifaceted World of Plant-Endophyte Association

Under natural conditions plants are not individual entities; they are associated with diverse microbiota to form the plant holobiont. The concept of plant holobiont is being actively explored to address the issues related to plant's health. Endophytes are a class of plant-associated microbes, which reside within the internal tissues of plants. They have been ubiquitously reported in all plants investigated so far. The plant-endophyte interactions may exhibit different modes of symbiotic association, ranging from beneficial (mutualism), neutral (commensal), to even pathogenic. Although we have a fair idea of the factors affecting plant-microbe interactions, the intricacies involved in fine-tuning their association are just beginning to unfold. Some of the pertinent questions surrounding the plant-endophyte symbiosis include: how are endophytes different from other beneficial microbes like rhizobia, mycorrhizae, and rhizobacteria? What mechanisms ensure that endophytes gain an unsurpassed entry and colonization into plants without eliciting a strong defense reaction? Why do different strains of the same microbial species enter into diverse modes of symbiotic association with plants? What factors cause the switch in the lifestyle of endophytes? In the present review, these questions have been addressed in the light of recent data and finally, concluded with gaps in endophyte research, which could be deliberated in future endeavors.