**2. The ecological community of microorganisms**

Symbiotic relationships are common to all organisms. Trees establish three main symbiotic relationships with microorganisms: commensalism, mutualism, and parasitism (pathogenic microorganisms). Commensals represent a wide diversity of microorganisms that internally and externally inhabit plant tissues; they benefit from the resources made available by the host tree (contact surface, space, moisture, mucilage, cellular debris, etc.) and do not cause any harm to the host. In contrast, the benefits of commensal microorganisms are considered "neutral" to their host [6]. Although the classical definition of commensalism defines the absence of benefits to the host, this may not be truly applicable to the networks of microbial interactions that develop in plants. The development of a community of commensal microorganisms results in the occupation of a niche that, in their absence, would be held by other groups of microorganisms, including pathogens. Hence, these microorganisms' survival strategies, including cooperation with other commensal microorganisms, pose an obstacle to the pathogen colonization and development in these niches. From this perspective, the benefit of a single commensal species may be neutral, although the gains from the presence of a commensal microbial community are positive to the host, despite not being straightforward to establish the actual gains from the commensal relationship between microorganisms and plants.

While the community of commensal microorganisms can act as an obstacle to establishing primary pathogens, this community can harbor opportunistic pathogens. Factors related to this phenomenon include unusual environmental conditions (i.e., long drought periods or excess water), physical damage to plant tissues (i.e.,

#### *Perspective Chapter: Microorganisms and Their Relationship with Tree Health DOI: http://dx.doi.org/10.5772/intechopen.110461*

mechanical damage to the roots, stems, leaves, and fruits), extreme temperatures, and other factors that can affect key components of plants' "innate immunity" [7]. For instance, acute oak decline (AOD) has been a recurring problem in European forests and is associated with opportunistic pathogens, as in the case of the fungus *Armillaria gallica* Marxm. & Romagn (Agaricales: Physalacriaceae). This fungus is an important saprophytic species in wood decay in forests, and it can develop a wide network of hyphae in the subterranean soil and is unable to colonize vigorously growing hosts [8]. Nevertheless, *A. gallica* invades oak trees weakened by insect defoliation or drought, colonizing the root system and causing root rot [9]. The transition from saprophytic to pathogenic lifestyle indicates that a microorganism can alter its relationship with the host depending on environmental conditions and the plant's immune status [9].

The symbiotic mutualistic relationship is a key survival strategy between plants and microorganisms. It is now recognized that mycorrhizal fungi were undoubtedly the most important microorganisms for the successful terrestrial colonization by plants. The evolution of symbiosis with mycorrhizal fungi occurred simultaneously with the establishment of plants on land 450 million years ago [10]. Arbuscular mycorrhizal fungi (AMF—phylum Glomeromycota) were the first fungi to establish a mutualistic symbiosis with plants, and, currently, this group is associated with the roots of over 85% of all plant species [11]. However, the establishment of plants on land also occurred concomitantly with the diversification of other mutualistic symbioses. In forest environments, symbiosis with ectomycorrhizae (ECM—phyla Basidiomycota and Ascomycota) is found in various gymnosperm and angiosperm lineages [12]. Both mycorrhizae groups contribute to the nutrition of their hosts by making scarce nutrients available (i.e., phosphorus and nitrogen), as well as increasing the tolerance of their hosts to abiotic (salinity and water stress) and biotic (pathogen attack) stress. The biogeographical distribution of these symbionts is influenced by climatic factors, such as temperature and rainfall regime. In tropical forests, mutualistic symbiotic association with AMF is more common, whereas ECM fungi are more diverse in temperate and boreal ecosystems [13, 14].

Another important association that may have contributed to the plants' successful terrestrial colonization is symbiosis with nitrogen-fixing bacteria. Association with nitrogen-fixing, nodule-forming bacteria, termed root nodule symbiosis (RNS), is restricted to four angiosperm orders: Fabales, Fagales, Cucurbitales, and Rosales [15]. Most leguminous tree species in tropical forests are capable of forming root nodules and fixing atmospheric nitrogen [16]. In some situations, trees can establish a mutualistic symbiotic association with rhizobia in conjunction with mycorrhizal fungi. The synergistic interaction between these two types of mutualism can improve plant performance, especially in soils nutritionally poor in phosphorus and nitrogen or saline soils. For instance, co-inoculation of the AMF *Rhizophagus fasciculatus* (Thaxt.) Gerd. & Trappe (Glomerales: Glomeraceae) and the actinobacterium Frankia (root nodule symbiont) improved the tolerance to salt stress of the tree species *Casuarina equisetifolia* and *C. obesai* (Fagales) [17].

Unlike RNS, nitrogen fixation through plant-cyanobacteria association is widely distributed in terrestrial plants [18]. Association with nitrogen-fixing cyanobacteria can occur in below-ground (rhizosphere) and above-ground (phyllosphere) environments. These microorganisms can live with mosses growing on the soil, be associated with bryophytes and arboreal epiphytes, or grow in the leaf cavities of plants [18–20]. In fact, evidence has shown that nitrogen fixation by cyanobacteria is associated with "feather moss" (i.e., *Hylocomium splendens*, *Ptilium crista-castrensis*, *Pleurozium* 

*schreberi*, and *Sphagnum caprifolium*), an important source of nitrogen input into boreal forest ecosystems [21–22].

The phytobiome also harbors pathogenic microorganisms (symbiotic parasitism relationship). Forest pathogens include fungi, oomycetes, bacteria, phytoplasmas, parasitic higher plants, viruses, and nematodes [23]. Despite a plethora of causative agents, the diseases of forest trees are primarily caused by fungal and oomycete pathogens [24]. The development of some tree diseases has been correlated with changes in the phytobiome. Thus, it is believed that the population growth of a particular pathogen may be related to the decline in the population of beneficial microorganisms. For example, one study reported that pine plants with pine wilt disease (PWD) caused by the nematode *Bursaphelenchus xylophilus* had a lower diversity of beneficial fungi and bacteria in the rhizosphere compared to the rhizosphere of healthy pines [25]. Recent studies have focused on analyzing the microbiome of diseased trees in an attempt to map unknown pathogen groups. Research on the microbiome of oak trees with AOD has reported that this concerning disease is caused by a polymicrobial complex, that is, the onset of AOD is related to the interaction of different bacterial species (*Brenneria goodwinii*, *Gibbsiella quercinecans*, and *Rahnella victoriana*) that work synergistically to develop the disease [26–28]. Some studies suggest that the physiological state of the host can influence the abundance and diversity of pathogenic microorganisms. For example, a recent study evaluated the diversity of foliar endophytes in *Platycladus orientalis* and *Styphnolobium japonicum* trees with different ages (individuals ranging from 10 to 5000 years old for *P. orientalis* and 10 to 1700 years old for *S. japonicum*) [29]. The authors demonstrated that the abundance of latent pathogens (fungi) increased as the trees aged. Thus, the abundance of pathogens *Collectotrichum gloeosporioides* and *Botryosphaeria dothidea* in *S. japonicum* and *Pestalotiopsis funerea* and *Amyloporia subxantha* in *P. orientalis* increased linearly with tree age, indicating that tree age is also an important structuring factor for host communities [29].

Although forest pathology is a recent science, it has been growing rapidly in recent decades mainly due to recent tree death events in forest ecosystems throughout Europe and North America. Forest decline diseases have concerned scientists and government bodies and are becoming increasingly problematic for tree health worldwide [28, 30]. Hence, shedding more light on the role of the tree microbiome will be crucial for properly managing forest environments.
