**4. Functions of the host-associated microbiota**

Plant-associated microorganisms can assist the health of the host in different ways. Many endophytic microorganisms can make previously scarce nutrients available in the soil or increase host tolerance to stressors. Mycorrhizae play an important role in maintaining tree health among endophytic microorganisms. The role of these microorganisms is related not only to making phosphorus and nitrogen available to the host but also to increasing plant tolerance to water stress and pathogen attack. In one experiment, the authors evaluated the effects of inoculation of an AMF (*Glomus etunicatum*) on pistachio (*Pistacia vera*) seedlings subjected to water stress. They observed that plants inoculated with the fungus had an increased tolerance to water stress compared to the control (seedlings subjected to water stress and not inoculated) [45]. Among the mechanisms related to increased stress tolerance and possibly induced by the mycorrhizal association, the authors highlighted a greater accumulation of osmotic adjustment compounds (i.e., soluble sugar content), increased activity of antioxidant enzymes (i.e., catalase and peroxidase), secondary metabolite production (i.e., flavonoids), and nutrient accumulation (nitrogen and calcium) [45].

Further findings have also suggested that association with mycorrhizal fungi improves host resistance to pathogens [46]. For instance, a recent study used amplicon sequencing to determine the presence of a wide taxonomic range in the rhizosphere of apple (*Malus domestica* Borkh) rootstocks [47]. The authors observed that roots of the G.890 rootstock (which is tolerant to apple replant disease—ARD) harbored a significantly higher percentage of AMF species, indicating a possible active role of endophytic fungal communities in apple tree tolerance to soil pathogens (including *Rhizoctonia* spp., *Phytophthora* spp., and *Pratylenchus penetrans*) that cause ARD [47]. In another study [25], the authors evaluated the effects of inoculating ECM species (*Suillus bovinus* and *Amanita vaginata*) and dark septate endophytes (DSE, *Gaeumannomyces cylindrosporus* and *Paraphoma chrysanthemicola*) on the tolerance of pines (*Pinus tabulaeformis*) to PWD caused by the pine nematode *Bursaphelenchus xylophilus*. The authors demonstrated that inoculating pines with ECM/DSE reduced

disease severity caused by *B. xylophilus* and increased the recruitment of beneficial bacterial and fungal groups in the rhizosphere of pines [25]. In addition to improving nutrition and stimulating defense mechanisms in their hosts, ectomycorrhizal fungi can assist their host against pathogen attacks by forming a thick fungal mantle that acts as a mechanical barrier against the penetration of soil pathogens [48].

Ectomycorrhizae are known to benefit their host trees in different ways. This mutualistic symbiosis is especially useful for forest plantations intended for the production of wood, cellulose, or the recovery of degraded areas. However, mutualism between trees and ECM species can stimulate another highly profitable economic activity: truffle farming. Truffles are the reproductive structures of hypogean ectomycorrhizal fungi and are appreciated in haute cuisine [49]. A recent study evaluated the effect of mycorrhization of pecan trees under subtropical conditions in Brazil [50]. The authors demonstrated that inoculation of pecan seedlings with the ectomycorrhizal species *Tuber aestivum* and *T. brumale* increased plant growth and, in addition, ECM produced high-value edible structures (truffles) [50]. These results indicate the possibility of developing a highly profitable economic activity associated with pecan orchards in subtropical regions.

Recruitment of plant growth-promoting bacteria (PGPB) is another important strategy trees adopt to increase their resistance to pathogen attacks; PGPB is known to promote plant growth through different mechanisms, including producing or stimulating plant hormones (i.e., gibberellins, auxins, and cytokinins), nitrogen fixation, phosphate solubilization, among others. In addition, PGPB may exhibit biocontrol activity against plant pathogens; numerous mechanisms have been described as being responsible for the biocontrol ability of PGPB, such as direct competition for space, emission of volatile compounds, siderophore production, lytic enzymes (i.e., proteases, chitinases, and lipases), antibiotics (i.e., amphisin, 2,4-diacetylphloroglucinol, and oomycin A), and induction of systemic resistance of the host plant [51–53].

The ability of the plant to recruit partner microorganisms indicates that the rhizosphere is a reservoir of beneficial microorganisms amenable to selecting and applying disease biocontrol programs. For example, an endophytic PGPB (*Bacillus velezensis* OEE1) was isolated from olive (*Olea europaea*) roots, and its disease biocontrol ability was tested against fungal (*Fusarium solani*, *Botrytis cinera*, etc.) and oomycete (*Phytophthora* spp.) pathogens [54]. The authors observed that the biocontrol ability of fungal pathogens and oomycetes by the isolate *B. velezensis* OEE1 was related to a wide range of competitive characteristics, including phosphate solubilization and producing siderophores, extracellular hydrolytic enzymes (amylases, cellulases, and pectinases), biosurfactant (surfactins), and secondary metabolites [54]. In another study, bacteria were isolated from the rhizosphere of avocado trees that survived root rot infestations caused by the oomycete *Phytophthora cinnamomi* [55]. The authors evaluated the antagonistic activity of the isolates against *P. cinnamomi* and selected a potential PGPB (*Bacillus acidiceler*) that could inhibit the growth of the oomycete by producing volatile compounds, indicating the potential use of this PGPB in biocontrol programs for pathogenic oomycetes [55].
