**1. Introduction**

Trees, including those outside of forests (i.e., orchards, gardens, trees in urban environments), are terrestrial plants that provide several ecosystem services essential to life and the economy. Trees contribute to carbon sequestration, animal and insect biodiversity maintenance, lignocellulosic biomass production for the industry, food supply (i.e., fruits and seeds), tourism (i.e., forest nature trails), air filtration in urban areas, and wood production for construction. Given their importance and the benefits trees provide, understanding the factors related to their health is pivotal for proper human intervention in forests, woodlands, and orchards.

Tree health depends on numerous factors, and it is now understood that their health, as well as other multicellular organisms, is closely related to the structure of the microbial community of the holobiont (the host and its associated microorganisms) [1]. The theory of plant holobiont seeks to shed more light on the plant as a meta-organism,

in which plant growth, health, and productivity are closely related to the composition and functions of the microbiota inhabiting different niches in the plant [2]. Within this perspective, trees are not autonomous entities, and their health can be considered a function of the plant microbiota (set of all associated microorganisms) or the plant microbiome (set of all associated microbial genomes) [3–4].

Trees establish different symbiotic relationships with a plethora of microorganisms. Therefore, the plant microbiome, also known as a phytobiome, is composed of commensal, mutualistic, and pathogenic microorganisms [5]. The diverse microbial composition contributes to host-microbial homeostasis, meaning the microbial community structure can modulate and maintain tree health over time. It is now known that the colonization of plant tissues by a pathogenic microorganism and disease development may be related to an imbalance of host-microbial homeostasis. In some cases, this imbalance results from atypical environmental conditions or the loss of partner microbes or ancestor microorganisms that are key to tree health.

The microorganisms that are part of the phytobiome include a wide diversity of prokaryotes distributed in different phyla (Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes, etc.), in addition to fungi (including mycorrhizae), protozoa, nematodes, and viruses. In general, the rhizospheric soil and endosphere are the environments most densely populated by microorganisms; nevertheless, the phylloplane is also an important environment for tree-associated microbial communities. Partner microorganisms can contribute directly and indirectly to tree health through nutrient provision (i.e., phosphorus solubilization and nitrogen fixation), plant hormone production (i.e., gibberellins, auxins), pathogen inhibition via competition, and the inhibition of systemic host resistance.
