Although it might sound like we’re veering into science fiction territory, the endosphere is actually part of a plant’s microbiome, like the rhizosphere and the phyllosphere. It is the community of microbes which live inside the plant itself – that is, between and in its cells. It’s only in the last few decades that research on the endosphere has accelerated – this has found that in fact a wide variety of microbes including bacteria and fungi live inside plants for at least a part of their lifecycle.^ref They are known as endophytes – and some of these are symbiotic whilst others can be pathogens.

Endophytes are found throughout the plant, in leaves, roots and stems, in spaces between cells as well as within cells themselves; the greatest number are found in roots, then leaves, then fruit/flowers. The types of microbes in residence depends on the microenvironment in each part of the plant, the specific physical and chemical characteristics in each environment attract different microbes.^ref

To enter the plant in the first place, microbes come from outside, through the root tips and hairs, through stomata and trichome pores in leaves, fruit & flowers, through holes in the stem made by insects, or by producing enzymes which break down plant cell walls to create an opening. Often these microbes are present in the rhizosphere or phyllosphere, and they migrate into the plant for all or part of their lifecycle.^ref Usually they live between cells, but some examples of bacteria and fungi entering plants cells have also been found. Endophytes can be transmitted vertically (from mother plant to seed), and horizontally (from the outside environment).^ref

Of all the spheres, the endosphere is the hardest to study, so there isn’t a huge amount of research which demonstrates what endophytes actually do when they are inside plants and how the host plant might benefit. Some findings are that endophytes are able to detect Reactive Oxygen Species (“ROS”) and may be able to help plants fight high ROS levels (eg. acting as an anti-oxidant).^ref Others have found endophytic fungi which produce the plant growth regulators gibberellic acid and indole acetic acid (auxin), and that this contributes to greater root & shoot mass.^{ref1 ref2} One study found an endophyte which conferred resistance to Dutch Elm Disease in vitro^ref. Finally a large number of endophytes associated with trees have been found to produce Taxol^ref, the best-selling cancer drug ever manufactured^ref and this promises to be a way for greater volumes of the drug to be created.^ref So like bacteria & fungi across the microbiome, these microbes appear to be pop-up pharmacies within the tree.

The endosphere probably doesn’t need to be your prime concern from a bonsai perspective. Like the other components of the tree’s microbiome, you want to foster a healthy one, which benefits the tree, and not an unhealthy one. Doing this mainly involves not killing them off!