Lichen as Multipartner Symbiotic Relationships

Abstract

Lichens have long been considered as composite organisms composed of algae and/or cyanobacteria hosted by a fungus in a mutualistic relationship. Other organisms have been gradually discovered within the lichen thalli, such as multiple algal species, yeasts, or even viruses. Of pivotal relevance is the existence of the lichen microbiome, which is a community of microorganisms that can be found living together on the lichen surface. This community performs a growing number of functions. In this entry, we explore the journey of lichens being considered from a dual partnership to a multi-species symbiotic relationship.

Full text

Citation: Morillas, L.; Roales, J.; Cruz,
C.; Munzi, S. Lichen as Multipartner
Symbiotic Relationships. Encyclopedia
2022,2, 1421–1431. https://doi.org/
10.3390/encyclopedia2030096
Academic Editors: Milva Pepi and
Raffaele Barretta
Received: 5 May 2022
Accepted: 25 July 2022
Published: 3 August 2022
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Entry
Lichen as Multipartner Symbiotic Relationships
Lourdes Morillas 1, 2, * , Javier Roales 1,3 , Cristina Cruz 1and Silvana Munzi 1,4
1Center for Ecology, Evolution and Environmental Changes & CHANGE—Global Change and
Sustainability Institute, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Bloco C2,
1749-016 Lisbon, Portugal; jroabat@upo.es (J.R.); ccruz@fc.ul.pt (C.C.); ssmunzi@fc.ul.pt (S.M.)
2Departamento de Biología Vegetal y Ecología, Universidad de Sevilla, Av. Reina Mercedes 10,
41080 Seville, Spain
3
Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Ctra. Utrera Km 1,
41013 Seville, Spain
4Centro Interuniversitário de História das Ciências e da Tecnologia, Faculdade de Ciências,
Universidade de Lisboa, Campo Grande, 1749-016 Lisbon, Portugal
*Correspondence: lmorillas@fc.ul.pt
Definition:
Lichens have long been considered as composite organisms composed of algae and/or
cyanobacteria hosted by a fungus in a mutualistic relationship. Other organisms have been gradually
discovered within the lichen thalli, such as multiple algal species, yeasts, or even viruses. Of pivotal
relevance is the existence of the lichen microbiome, which is a community of microorganisms that
can be found living together on the lichen surface. This community performs a growing number of
functions. In this entry, we explore the journey of lichens being considered from a dual partnership
to a multi-species symbiotic relationship.
Keywords: symbiosis; microbiome; partnership; mycobiont; photobiont; holobiont; bacterial layer
1. General Context of Lichens
Lichens are just one of many symbiotic relationships that can be established between
heterotrophic fungi and photoautotrophic partners, such as plants, mosses, cyanobacte-
ria, and algae. Partnerships between fungi and vascular plants are highly diverse and
ecologically relevant. Some of these partnerships, such as those of ectomycorrhizas [
1
],
endomycorrhizas, or the unique orchid mycorrhizal associations [2], are well known. The
relationships between fungi and cyanobacteria or algae are also well known and very
diverse, including the relation between algicolous fungi and bacteria or algae [
3
] and
lichens [
4
]. Algicolous fungi can parasitize algae or cyanobacteria [
5
,
6
] or alternatively can
establish a mutualistic relationship, as in the case of mycophycobioses [
7
]. Lichen-forming
fungi may establish symbiotic relationships with algae or cyanobacteria and form a unique
identity, i.e., the lichen. However, although unique, lichens are just one example of the
highly diverse partnerships between fungi and photosynthetic organisms.
More than 18,000 fungal species, comprising a highly diverse group and representing
around 20% of those currently identified, participate in lichen partnerships. They occur
in all terrestrial ecosystems, ranging from polar to tropical areas and from coastal to high
mountain ecosystems. Lichens form vegetative structures called thalli and can grow on
a large variety of substrates such as minerals, rocks, bare soil, and the wood or leaves of
plants, even in streams and marine zones [8], as well as on synthetic material surfaces.
The symbiotic condition of lichens remained unknown for a long time and, until
1869 [
9
], they were thought to be individual organisms. The German mycologist Anton de
Bary introduced the term “symbiosis” to describe the condition of dissimilar organisms
living together [
10
] supporting Schwendener’s theory that lichen is formed by two separate
organisms, a fungus and an alga. From then on, lichens were considered to be an obligate
partnership between a fungus (mycobiont) and either cyanobacteria and/or green algae,
Encyclopedia 2022,2, 1421–1431. https://doi.org/10.3390/encyclopedia2030096 https://www.mdpi.com/journal/encyclopedia

Encyclopedia 2022,21422
that acted as a photoautotrophic partner (photobiont) [
8
,
11
]. Figure 1shows the location of
the mycobiont and the photobiont within heteromerous lichens, in which the algae and
fungal components are arranged in definite layers. The stability of this symbiotic associa-
tion depends on the mutualistic–antagonistic relationships of a multitude of interlinked
organisms, also known as the “holobiont” [
12
,
13
]. This complex relationship is determined
by the symbionts’ interactive intimacy, stability of environmental conditions, and partner
availability [14,15].
Encyclopedia 2022, 1, FOR PEER REVIEW 2
that acted as a photoautotrophic partner (photobiont) [8,11]. Figure 1 shows the location
of the mycobiont and the photobiont within heteromerous lichens, in which the algae and
fungal components are arranged in definite layers. The stability of this symbiotic
association depends on the mutualistic–antagonistic relationships of a multitude of
interlinked organisms, also known as the “holobiont” [12,13]. This complex relationship
is determined by the symbionts’ interactive intimacy, stability of environmental
conditions, and partner availability [14,15].
Figure 1. (a) Correspondence between the lichen structure and (b) their individual components in a
heteromerous thallus. Depiction of tripartite lichen (c) and homoiomerous lichen (d).
Thalli are specialized structures unique to lichen-forming fungi and are not observed
when the mycobiont grows in isolation. The thalli hold the photosynthetic partners and
are one of the most complex structures in the entire fungal kingdom. Thallus structures
are mainly determined by the fungal partner and can be grouped into three main growth
forms: crustose, foliose, or fruticose. Crustose lichens, lacking the lower cortex, are
completely attached to the substrate, while foliose and fruticose lichens are only partially
attached through anchor like structures such as rhizines and hapters [16]. The internal
space of the thallus can be made up of either an internal stratification with a fungal upper
layer and underlying algal layer (heteromerous, Figure 1a), or an even distribution of the
Figure 1.
(
a
) Correspondence between the lichen structure and (
b
) their individual components in a
heteromerous thallus. Depiction of tripartite lichen (c) and homoiomerous lichen (d).
Thalli are specialized structures unique to lichen-forming fungi and are not observed
when the mycobiont grows in isolation. The thalli hold the photosynthetic partners and are
one of the most complex structures in the entire fungal kingdom. Thallus structures are
mainly determined by the fungal partner and can be grouped into three main growth forms:
crustose, foliose, or fruticose. Crustose lichens, lacking the lower cortex, are completely
attached to the substrate, while foliose and fruticose lichens are only partially attached
through anchor like structures such as rhizines and hapters [16]. The internal space of the
thallus can be made up of either an internal stratification with a fungal upper layer and
underlying algal layer (heteromerous, Figure 1a), or an even distribution of the mycobiont
and photobiont (homoiomerous, Figure 1d). Lichens do not have a waxy cuticle to isolate

Encyclopedia 2022,21423
the thalli from their surroundings and therefore everything in the lichen’s environment is
absorbed into its structure, including water and nutrients coming from air and rain. They
also lack vascular tissues (such as xylem and phloem in plants) to distribute nutrients and
water around their thalli.
The general structure of a heteromerous lichen is shown in Figure 1and it is mainly
composed of layers of fungus and alga. The upper cortex is the outer layer of the lichen
thallus, and it is formed by the mycobiont. The cells in this layer are tightly packed to
provide certain physical and chemical protection from the environment. The algal layer
contains the photobiont, which frequently is a green alga. Cyanobacteria, if present, can be
located in small vacuoles called cephalopodiums, which exist on top of the upper cortex or
within the tissues of the lichen when there is a green algal layer already present (secondary
photobiont, Figure 1c), or in a layer under the upper cortex (primary photobiont). Part
of the lichen thallus is composed of filamentous fungal cells that form the medulla. This
layer is loosely packed and has a threadlike structure. The lower cortex, with a similar
structure to the upper cortex, protects the medulla and give support to the rhizines or
other basal attachments that allow lichens to get linked to their substrate. Rhizines are
fungal multicellular structures originated on the lower surface with no vascular purposes:
no water or nutrients can be absorbed by them, and their unique function is to support
the lichen attachment. Although this is the most frequent structural organization found
in a thallus, some lichens present no distinguishable layers of mycobiont and photobiont.
In these cases, the components are distributed in one big uniform layer, resulting in a
gelatinous growth form (Figure 1d).
Most lichen-forming fungi belong to the phylum Ascomycota, while only 0.3% of
lichenized fungi are known to be derived from Basidiomycota [
8
,
17
]. The majority of
lichen-forming algae belong to the green algae (85%), and 10% have a cyanobacterium as
primary photosynthetic partner, but also brown or yellow-green algae have been identified
in these relationships [
18
,
19
]. Sexual reproduction of the fungal partner involves the growth
of fruiting bodies from the thallus which produce ascospores to be dispersed. In addition to
this sexual mechanism, lichens have also developed other processes of asexual reproduction
to disperse both partners together in varied and specific joint propagules [16].
2. From a Dual Partnership to a Multi-Species Symbiotic Relationship
In this intimate and long-term partnership, the fungal partner provides water, mineral
nutrients, and sheltering structures for the photobiont, which contributes photosyntheti-
cally fixed carbon as the energy source for the system. Although the fungus can be very
specific when selecting its photobiont [
20
,
21
], generalist fungi have also been frequently
described [
22
–
25
]. Yahr et al. [
26
] established three categories according to the range of
photobionts with which they are able to lichenize: photobiont specialists, intermediates and
generalists. Following this scheme, photobiont specialists partner with single algal lineages,
while photobiont generalists accept a high variety of algal partners and can stablish associ-
ations with a number of strains according to environmental conditions. Intermediates form
symbiotic relationships with a reduced number of algal partners. Photobiont generalists are
frequently associated to lichens with wide both geographical range and ecological niches,
which can associate with locally adapted photobionts in different climatic regions [
25
,
27
,
28
].
The exact factors that determine the photobiont selection are well known and appear to
be related to phylogenetic specialization, fungus reproductive strategy, photobiont cell
availability and ecological factors such as climate or substrate [
29
,
30
]. Surprisingly, multiple
algal species have been observed in association with the same thallus in a large number
of studies [
24
,
31
,
32
]. For example, Backor et al. [
33
] confirmed the presence of multiple
algal genopypes in a single lichen thallus, Casano et al. [
34
] found that Ramalina farinacea
thalli represent a specific and selective form of symbiotic association involving the same
two Trebouxia phycobionts, and Del Campo et al. [
35
] concluded that ecological diversifi-
cation and speciation of lichen symbionts in different habitats could include a transient
phase consisting of associations with more than one photobiont in individual thalli. This

Encyclopedia 2022,21424
pattern of algal coexistence is probably promoted by their different and complementary
ecophysiological responses which facilitate the proliferation of the lichen in a wide range
of habitats and geographic areas [34].
In the last decade, it has been shown that lichens are far from being a simple asso-
ciation between two unrelated organismal groups, and instead involve a bacterial (and
other fungi including yeast) component which is a key contributor to the biology of the
holobiont. The inclusion of bacteria within the lichen partnership was first observed around
the 1930’s
[36–38]
(Figure 2), and it has been described as a discontinuous monolayer on the
thallus surface (Figure 1). At that time, rudimentary methods only facilitated associating
these bacteria with a possible and unspecific role in nitrogen fixation. At the beginning of
the 21st century, the first molecular analyses started using bacterial isolates (e.g., Gonza-
lez et al. [
39
]). However, as culture-dependent methods can only account for 0.001–15%
of the bacterial diversity [
40
], most microorganisms had remained unrevealed [
41
]. More
recent studies allowed the observation that the structure of the nitrogen-fixing bacteria
present in the cyanolichens is different from that of chlorolichens and that chlorolichens
have a higher diversity of nitrogen-fixing bacteria than cyanolichens [
42
]. Also, in addition
to the main cyanobiont, other cyanobacteria have been found within the microbiota of
lichen thalli and substrates [
43
,
44
], which can also contribute with part of the nitrogen
input to the symbiosis.
Once new methods were developed to complement the previously applied techniques,
the limitations of the bacterial isolation were overcome, which allowed researchers to
holistically explore the bacterial community. Fingerprinting techniques [
45
] and molecular
cloning methods (e.g., Hodkinson and Lutzoni, [
46
]) allowed the production of microbial
community profiles of lichen-associated microbiota. Bates et al. [
47
] revealed the microbial
community associated with lichens based on next generation pyrosequencing for the first
time. Thus, multi-omics approaches, that allow the integration of multiple omics research
and datasets explaining the mechanisms underlying biological processes and molecular
functions [
48
], along with bioinformatic tools have put the spotlight on the host-specific
bacterial microbiome [
49
–
52
]. Recent findings about bacterial associations with lichens
support their relationship as a multi-species symbiosis in which different roles are played
by an increasingly recognized diversity of organisms associated with the thalli (see [
49
]
and references therein; Figure 2).
3. Potential Roles of Recently Discovered Partners
Bacterial and secondary fungal communities inhabiting lichens have been found
through next-generation sequencing techniques (e.g., Grube et al. [
45
], Tuovinen et al. [
53
]).
Key roles as functional components in structuring the thalli and modulating the response
to environmental factors could be performed by these overlooked organisms [
49
,
50
,
54
].
Since the first half of the 20th century, individual strains of bacteria have been isolated
from lichens, with Alpha proteobacteria making up the dominant group, first observed by
Cardinale et al. [
55
]. Studies of the diversity of lichen-associated bacteria suggest that dif-
ferent parts of the thallus, providing different chemical and physiological micro-niches, can
influence microbial colonization ([
45
,
46
] and references therein). Another main factor driv-
ing the bacterial composition is the fungal partner. Accordingly, Aschenbrenner et al. [56]
demonstrated that the lichen Lobaria pulmonaria presented a core and shared fraction of its
bacterial biome, as well as a transient fraction. They also demonstrated that bacteria were
present in the lichen’s vegetative propagules which allowed them to vertically transmit
through asexual reproduction. Traditionally, it has been accepted that the mycobiont builds
up the thallus as a result of the specific interaction with a suitable algal partner, and then
numerous associated and potentially interacting bacterial and other partners colonize the
lichen more or less specifically. Recently, it has been hypothesized that the microbiome
contributes to the lichenization process [57].
Similar questions to those posed regarding the role and specificity of bacteria within
lichens have also been raised in relation to secondary fungal communities inhabiting lichens.

Encyclopedia 2022,21425
Spribille et al. [
58
] found that a specific group of basidiomycetous yeasts played a part in
the microbiome of two epiphytic lichens (Figure 2), and that the abundance of the yeast
within the lichen was correlated with concentrations of vulpinic acid, which is a secondary
metabolite associated with lichen defenses. More recently, Cernajova and Skaloud [
59
]
discovered previously unknown cystobasidiomycete symbionts in a number of Cladonia
species in the northern hemisphere.
The consistent presence of basidiomycetous yeast within lichens in these studies sug-
gested these were the third partner within the lichen complex. However, Millanes et al. [
60
]
considered Cyphobasidium spp. to be a lichen-related fungi that can form galls on the
thalli instead of as a previously unseen third mutualistic partner. The relevance of these
lichen-related yeasts was also discussed by Oberwinkler [
61
], who pointed out that “it is
obvious that basidiomycetous yeasts in lichen thalli are not a third component of symbiosis,
but rather the vegetative propagules of mycoparasites”. Supporting this view, Lende-
mer et al. [
51
] failed to detect basidiomycete yeasts in over 97% out of 339 lichen species
from the Appalachian Mountains in North America in a metagenomic study. Although
their metagenomic approach is likely less sensitive than PCR assays with specific primers,
these findings raise questions about the ubiquity and specificity of yeasts in lichens.
Encyclopedia 2022, 1, FOR PEER REVIEW 5
interacting bacterial and other partners colonize the lichen more or less specifically.
Recently, it has been hypothesized that the microbiome contributes to the lichenization
process [57].
Similar questions to those posed regarding the role and specificity of bacteria within
lichens have also been raised in relation to secondary fungal communities inhabiting
lichens. Spribille et al. [58] found that a specific group of basidiomycetous yeasts played
a part in the microbiome of two epiphytic lichens (Figure 2), and that the abundance of
the yeast within the lichen was correlated with concentrations of vulpinic acid, which is a
secondary metabolite associated with lichen defenses. More recently, Cernajova and
Skaloud [59] discovered previously unknown cystobasidiomycete symbionts in a number
of Cladonia species in the northern hemisphere.
The consistent presence of basidiomycetous yeast within lichens in these studies
suggested these were the third partner within the lichen complex. However, Millanes et
al. [60] considered Cyphobasidium spp. to be a lichen-related fungi that can form galls on
the thalli instead of as a previously unseen third mutualistic partner. The relevance of
these lichen-related yeasts was also discussed by Oberwinkler [61], who pointed out that
“it is obvious that basidiomycetous yeasts in lichen thalli are not a third component of
symbiosis, but rather the vegetative propagules of mycoparasites.” Supporting this view,
Lendemer et al. [51] failed to detect basidiomycete yeasts in over 97% out of 339 lichen
species from the Appalachian Mountains in North America in a metagenomic study.
Although their metagenomic approach is likely less sensitive than PCR assays with
specific primers, these findings raise questions about the ubiquity and specificity of yeasts
in lichens.
Figure 2. Temporal line indicating the timing at which the different components of the lichen
partnership were discovered. This figure reflects the journey from considering lichens as a single
organism to the complex multipartner symbiotic relationship known nowadays
[9,10,34,36,58,62,63].
Figure 2.
Temporal line indicating the timing at which the different components of the lichen
partnership were discovered. This figure reflects the journey from considering lichens as a single
organism to the complex multipartner symbiotic relationship known nowadays [
9
,
10
,
34
,
36
,
58
,
62
,
63
].
4. The Increasing Complexity Surrounding the Concept of Lichen
The discovery of potential new partners increased the complexity of species interac-
tions within these supraorganisms. However, the lack of experimental evidence regarding
the lichen microbiome hindered our ability to reveal the network of interactions within this
holobiont. As opposed to what was traditionally believed, the photobiont should not be

Encyclopedia 2022,21426
limited to a single strain of algae [
34
] and protists and even viruses can form symbiotic
associations with lichens [62,63] (Figure 2).
Conceptualizing a lichen means accounting for a vast array of related microorganisms,
providing the ideal example of a holobiont, composed of a dominant mycobiont and diverse
microbiome [
64
]. This evolved network of biotic connections whose morphology is shaped
by the mycobiont, is at the service of the fitness of the entire superorganism. Hawksworth
and Grube [
65
] re-defined the lichen symbiosis as: ‘a self-sustaining ecosystem formed
by the interaction of an exhabitant fungus, an extracellular arrangement of one or more
photosynthetic partners and an indeterminate number of other microscopic organisms’ [
66
].
While a detailed analysis of the scientific scrutiny focused on lichens is beyond the
purpose of this text, a rough indication of the increasing interest in the lichen microbiome
can be obtained by looking at the publications of the last 20 years (Figure 3). Although
a linear relationship is missing, since 2014, the concept of a lichen symbiotic association,
including more than just the mycobiont and the photobiont was well established. The
development of “omic” technologies is a key element that has allowed the proliferation of
studies in this research field [
67
]. The number of citations of a specific paper can increase
with the time from its publication, which can explain the low number of citations for the
most recent works; however, since 2015, the annual number of citations has exceeded 50,
testifying to an increasing interest in this specialized topic.
Encyclopedia 2022, 1, FOR PEER REVIEW 6
4. The Increasing Complexity Surrounding the Concept of Lichen
The discovery of potential new partners increased the complexity of species
interactions within these supraorganisms. However, the lack of experimental evidence
regarding the lichen microbiome hindered our ability to reveal the network of interactions
within this holobiont. As opposed to what was traditionally believed, the photobiont
should not be limited to a single strain of algae [34] and protists and even viruses can form
symbiotic associations with lichens [62,63] (Figure 2).
Conceptualizing a lichen means accounting for a vast array of related
microorganisms, providing the ideal example of a holobiont, composed of a dominant
mycobiont and diverse microbiome [64]. This evolved network of biotic connections
whose morphology is shaped by the mycobiont, is at the service of the fitness of the entire
superorganism. Hawksworth and Grube [65] re-defined the lichen symbiosis as: ‘a self-
sustaining ecosystem formed by the interaction of an exhabitant fungus, an extracellular
arrangement of one or more photosynthetic partners and an indeterminate number of
other microscopic organisms’ [66].
While a detailed analysis of the scientific scrutiny focused on lichens is beyond the
purpose of this text, a rough indication of the increasing interest in the lichen microbiome
can be obtained by looking at the publications of the last 20 years (Figure 3). Although a
linear relationship is missing, since 2014, the concept of a lichen symbiotic association,
including more than just the mycobiont and the photobiont was well established. The
development of “omic” technologies is a key element that has allowed the proliferation of
studies in this research field [67]. The number of citations of a specific paper can increase
with the time from its publication, which can explain the low number of citations for the
most recent works; however, since 2015, the annual number of citations has exceeded 50,
testifying to an increasing interest in this specialized topic.
Figure 3. Number of papers related to the lichen microbiome and their citations. Data obtained using
the search terms “lichen microbiome” or “lichen microbiota” or “lichen bacterial community” or
“lichen microbial” or “lichen bacteria” or “lichen-associated bacteria” in the Web of Science
(https://www.webofscience.com/wos/woscc/basic-search). Accessed on 13 February 2022. All
results were checked to ensure that they were referred to the presence of microorganisms in the
lichen symbiosis.
5. Mutualism or Parasitism?
Although lichens are usually considered mutualistic symbioses, many lichen
characteristics identify them as controlled parasitic interactions [68,69]. Initial studies in
Figure 3.
Number of papers related to the lichen microbiome and their citations. Data obtained
using the search terms “lichen microbiome” or “lichen microbiota” or “lichen bacterial community”
or “lichen microbial” or “lichen bacteria” or “lichen-associated bacteria” in the Web of Science
(https://www.webofscience.com/wos/woscc/basic-search) accessed on 13 February 2022. All
results were checked to ensure that they were referred to the presence of microorganisms in the
lichen symbiosis.
5. Mutualism or Parasitism?
Although lichens are usually considered mutualistic symbioses, many lichen char-
acteristics identify them as controlled parasitic interactions [
68
,
69
]. Initial studies in this
field deemed lichens as algae parasitized by fungi because they found algal cells in a
lichen thallus which were dead or penetrated by fungal haustoria. However, other au-
thors considered lichens as mutualists based on the seemingly healthy and long-lasting

Encyclopedia 2022,21427
relationship among its partners. Two different experimental approaches have been de-
veloped to study selectivity among the lichen partners. The first one is based on
in vitro
resynthesis of the independently cultured mycobiont with different photobiont species.
An alternative approach involves the assessment of the specimens collected in different
geographical areas and the identification of the partners present in the lichen thalli [
70
].
In vitro
studies give researchers the opportunity to observe initial stages of the lichenization
process. Several developmental stages of lichenization have been described depending on
the interacting alga, and timing of these events is variable depending on the species, media,
and incubation conditions. The first developmental stage which has been described is the
“pre-contact” stage, crucial for the establishment of symbiont recognition mechanisms and
biont specificity [
71
,
72
], where the partners are in close proximity to share extracellular
secretions but not physical contact. In a second phase, the “contact” stage, the two bionts
start making physical contact by fungal appressoria (which are flattened hyphal tips that
bind to the host cell surface and start a penetration peg), whereas, in a third phase termed
the “growth together” stage, the two partners grow together in a network to form cellular
masses containing both bionts [73–76].
Attempts of resynthesis in the laboratory starting from the isolated partners have
been made with various and inconsistent outcomes [
52
,
68
,
69
,
77
–
86
], making the study of
lichenization mechanisms hard due to the lack of consistently repeatable results.
In vitro
re-synthesis experiments showed that the interaction of a mycobiont with its compatible
algal partner triggers specific morphological differentiation that is not seen when in contact
with incompatible algal partners [
68
,
71
,
87
]. Conversely, parasitic behaviors of the fungus
can be observed in interaction with nonlichenized algae or lichenized algae from different
lichen species [
68
,
71
,
87
,
88
]. Switching to parasitism and saprotrophic nutrition is also
known in some lichens, like Ochrolechia frigida when growing without its alga partner. In
algal-free stages, this species seems to be capable of saprotrophic nutrition on mosses,
phanerogams and other lichens [89].
In agreement with an optional parasitic or saprotrophic lifestyle of the mycobiont,
Munzi et al. recorded for the first time in lichens a high activity of extracellular enzymes
able to digest organic matter of different types and that are usually found in mycorrhizal
fungi, which also alternate between different lifestyles (unpublished). A common charac-
teristic of axenic reconstitutions is either that they do not progress beyond the soredia or
squamule stages or, if they do, the resulting thalli do not resemble closely the corresponding
natural lichens in shape, size, and full differentiation. Gene expression studies [
72
] and
specific exudation patterns of lichen photobionts [
90
] also point to extracellular commu-
nication between lichen symbionts without cellular contact [
57
]. For example, ribitol was
capable of overcoming fungal growth arrest [
90
], fungal lectins induced chemotropism
of compatible Nostoc cells in cyanolichens [
91
], and chitinase, a defense enzyme in plants
against pathogens, was downregulated in the photobiont during resynthesis stages [
87
].
Interestingly, the results of proteomic analyses in the thalli of the lichen Xanthoria pari-
etina [
92
] included expression of proteins linked to the signaling compound pathways
mentioned above.
Our knowledge about the cytological and biochemical interactions between the sym-
bionts in lichens is still scarce [
88
]. However, increasing evidence indicates conservation of
signaling pathways involved in the establishment of other major symbioses between plants
and mutualistic microbes. Both common effectors and genes have been found to be essential
for the establishment of rhizobial, Frankia, mycorrhizal and fungal endophytic symbioses,
including plant-produced strigolactones, microbial partner-produced chitooligosaccharides
(COs) and lipo-chitooligosaccharides (LCOs) [
93
–
99
], genes encoding Vapyrin [
93
,
100
,
101
]
and several transcription factors [
102
,
103
]. Three of these transcription factors (CYCLOPS,
NSP1, and NSP2) are well conserved between actinorhizal, legume, non-legume, and
mycorrhizal symbioses [
96
]. It is therefore possible that some or all these factors are also
present and play essential roles in lichenization, and their absence or differential presence
may relate to re-synthesis failures or establishment of associations with parasitic outcomes.

Encyclopedia 2022,21428
6. Conclusions and Prospects
A large scientific effort is still needed to achieve the goal of revealing the physio-
logical mechanisms operating in convoluted lichen symbioses and the roles of the various
organisms involved in this complex holobiont. As the latest scientific evidence has shown,
this research challenge must be addressed by considering lichens as self-sustained and
adaptable systems of partnerships, just like us!
Author Contributions:
Conceptualization, S.M. and L.M.; methodology, S.M. and L.M.; software,
J.R.; validation, S.M., L.M., J.R. and C.C.; formal analysis, J.R.; investigation, L.M.; resources, C.C.,
L.M. and S.M.; data curation, L.M. and J.R.; writing—original draft preparation, L.M., J.R. and S.M.;
writing—review and editing, S.M., L.M., J.R. and C.C.; visualization, L.M. and J.R.; supervision, C.C.
and S.M.; project administration, L.M.; funding acquisition, L.M. All authors have read and agreed to
the published version of the manuscript.
Funding:
This project has received funding from the European Union’s Horizon 2020 Research
and Innovation programme under the Marie Skłodowska–Curie grant agreement #793965 (Med-
N-Change).
Acknowledgments: We are grateful to Lucy Sheppard for language revision and useful insights.
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
References
1. Agerer, R. Characterization of Ectomycorrhiza. Methods Microbiol. 1991,23, 25–73.
2. Rasmussen, H.N. Recent developments in the study of orchid mycorrhiza. Plant Soil 2002,244, 149–163. [CrossRef]
3. Hawksworth, D.L. Observations on three algicolous microfungi. Notes R. Bot. Gard. Edinburgh 1987,44, 549–560.ill.
4.
Hawksworth, D.L. The variety of fungal-algal symbioses, their evolutionary significance, and the nature of lichens. Bot. J. Linn.
Soc. 1988,96, 3–20. [CrossRef]
5. Kohlmeyer, J.; Demoulin, D. Parasitic and Symbiotic Fungi on Marine Algae. Bot. Mar. 1981,24, 9–18. [CrossRef]
6.
Sønstebø, J.H.; Rohrlack, T. Possible implications of Chytrid parasitism for population subdivision in freshwater cyanobacteria of
the genus Planktothrix. Appl. Environ. Microbiol. 2011,77, 1344–1351. [CrossRef]
7. Kohlmeyer, J.; Kohlmeyer, E. Is Ascophyllum nodosum Lichenized? Bot. Mar. 1972,15, 109–112. [CrossRef]
8. Nash, T.H.I. Lichen Biology, 2nd ed.; Cambridge University Press: Cambridge, UK, 2008; ISBN 9780521871624.
9. Schwendener, S. Die Algentypen der Flechtengonidien; Universitætsbuchdruckerei von C. Schultze: Basel, Switzerland, 1869.
10.
Oulhen, N.; Schulz, B.J.; Carrier, T.J. English translation of Heinrich Anton de Bary’s 1878 speech, ‘Die Erscheinung der Symbiose’
(‘De la symbiose’). Symbiosis 2016,69, 131–139. [CrossRef]
11. Rikkinen, J. Molecular studies on cyanobacterial diversity in lichen symbioses. MycoKeys 2013,6, 3–32. [CrossRef]
12.
Margulis, L.; Fester, R. Symbiosis as a Source of Evolutionary Innovation: Speciation and Morphogenesis; Margulis, L., Fester, R., Eds.;
MIT Press: Cambridge, MA, USA, 1991; ISBN 0262132699.
13.
Douglas, A.E.; Werren, J.H. Holes in the hologenome: Why host-microbe symbioses are not holobionts. mBio
2016
,7, e02099-15.
[CrossRef]
14. Rafferty, N.E.; Caradonna, P.J.; Bronstein, J.L. Phenological shifts and the fate of mutualisms. Oikos 2015,124, 14–21. [CrossRef]
15.
Chomicki, G.; Renner, S.S. Partner abundance controls mutualism stability and the pace of morphological change over geologic
time. Proc. Natl. Acad. Sci. USA 2017,114, 3951–3956. [CrossRef]
16.
Büdel, B.; Scheidegger, C. Thallus morphology and anatomy. In Lichen Biology; Nash, T.H.I., Ed.; Cambridge University Press:
Cambridge, UK, 2008; pp. 40–68, ISBN 9780521871624.
17.
Lücking, R.; Dal-Forno, M.; Sikaroodi, M.; Gillevet, P.M.; Bungartz, F.; Moncada, B.; Yánez-Ayabaca, A.; Chaves, J.L.; Coca,
L.F.; Lawrey, J.D. A single macrolichen constitutes hundreds of unrecognized species. Proc. Natl. Acad. Sci. USA
2014
,111,
11091–11096. [CrossRef] [PubMed]
18.
Honegger, R. Lichen-Forming Fungi and Their Photobionts. In Plant Relationships; Deising, H.B., Ed.; Springer: Berlin/Heidelberg,
Germany, 2009; pp. 307–333, ISBN 978-3-540-87407-2.
19.
Henskens, F.L.; Green, T.G.A.; Wilkins, A. Cyanolichens can have both cyanobacteria and green algae in a common layer as major
contributors to photosynthesis. Ann. Bot. 2012,110, 555–563. [CrossRef]
20. Piercey-Normore, M.D.; DePriest, P.T. Algal switching among lichen symbioses. Am. J. Bot. 2001,88, 1490–1498. [CrossRef]
21.
Magain, N.; Miadlikowska, J.; Goffinet, B.; Serusiaux, E.; Lutzoni, F. Macroevolution of specificity in cyanolichens of the genus
Peltigera section Polydactylon (Lecanoromycetes, Ascomycota). Syst. Biol. 2017,66, 74–99. [CrossRef]

Encyclopedia 2022,21429
22.
Wirtz, N.; Lumbsch, H.T.; Green, T.G.A.; Türk, R.; Pintado, A.; Sancho, L.; Schroeter, B. Lichen fungi have low cyanobiont
selectivity in maritime Antarctica. New Phytol. 2003,160, 177–183. [CrossRef] [PubMed]
23.
Guzow-Krzemi´nska, B. Photobiont flexibility in the lichen Protoparmeliopsis muralis as revealed by ITS rDNA analyses. Lichenol.
2006,38, 469–476. [CrossRef]
24.
Muggia, L.; Vancurova, L.; Škaloud, P.; Peksa, O.; Wedin, M.; Grube, M. The symbiotic playground of lichen thalli—A highly
flexible photobiont association in rock-inhabiting lichens. FEMS Microbiol. Ecol. 2013,85, 313–323. [CrossRef]
25.
Blaha, J.; Baloch, E.; Grube, M. High photobiont diversity associated with the euryoecious lichen-forming ascomycete Lecanora
rupicola (Lecanoraceae, Ascomycota). Biol. J. Linn. Soc. 2006,88, 283–293. [CrossRef]
26.
Yahr, R.; Vilgalys, R.; DePriest, P.T. Geographic variation in algal partners of Cladonia subtenuis (Cladoniaceae) highlights the
dynamic nature of a lichen symbiosis. New Phytol. 2006,171, 847–860. [CrossRef]
27.
Fernández-Mendoza, F.; Domaschke, S.; García, M.A.; Jordan, P.; Martín, M.P.; Printzen, C. Population structure of mycobionts
and photobionts of the widespread lichen Cetraria aculeata. Mol. Ecol. 2011,20, 1208–1232. [CrossRef]
28.
Vargas Castillo, R.; Beck, A. Photobiont selectivity and specificity in Caloplaca species in a fog-induced community in the Atacama
Desert, northern Chile. Fungal Biol. 2012,116, 665–676. [CrossRef]
29.
Scheidegger, C. Systematische Studien zur Krustenflechte Anzina carneonivea (Trapeliaceae, Lecanorales). Nov. Hedwigia
1985
,
41, 191–218.
30.
Hoz, C.J.P.-D.L.; Magain, N.; Lutzoni, F.; Goward, T.; Restrepo, S.; Miadlikowska, J. Contrasting Symbiotic Patterns in Two Closely
Related Lineages of Trimembered Lichens of the Genus Peltigera.Front. Microbiol. 2018,9, 2770. [CrossRef]
31.
del Campo, E.M.; Casano, L.M.; Gasulla, F.; Barreno, E. Suitability of chloroplast LSU rDNA and its diverse group I introns
for species recognition and phylogenetic analyses of lichen-forming Trebouxia algae. Mol. Phylogenet. Evol.
2010
,54, 437–444.
[CrossRef] [PubMed]
32.
Moya, P.; Molins, A.; Chiva, S.; Bastida, J.; Barreno, E. Symbiotic microalgal diversity within lichenicolous lichens and crustose
hosts on Iberian Peninsula gypsum biocrusts. Sci. Rep. 2020,10, 14060. [CrossRef]
33.
Baˇckor, M.; Peksa, O.; Škaloud, P.; Baˇckorová, M. Photobiont diversity in lichens from metal-rich substrata based on ITS rDNA
sequences. Ecotoxicol. Environ. Saf. 2010,73, 603–612. [CrossRef]
34.
Casano, L.M.; Del Campo, E.M.; García-Breijo, F.J.; Reig-Armiñana, J.; Gasulla, F.; Del Hoyo, A.; Guéra, A.; Barreno, E. Two
Trebouxia algae with different physiological performances are ever-present in lichen thalli of Ramalina farinacea. Coexistence
versus Competition? Environ. Microbiol. 2011,13, 806–818. [CrossRef]
35.
del Campo, E.M.; Catalá, S.; Gimeno, J.; del Hoyo, A.; Martínez-Alberola, F.; Casano, L.M.; Grube, M.; Barreno, E. The genetic
structure of the cosmopolitan three-partner lichen Ramalina farinacea evidences the concerted diversification of symbionts. FEMS
Microbiol. Ecol. 2013,83, 310–323. [CrossRef]
36. Uphof, J.C.T. Purple bacteria as symbionts of a lichen. Science 1925,61, 67. [CrossRef] [PubMed]
37. Henkel, P.A.; Yuzhakova, L.A. Nitrogen-fixing bacteria in lichens. Izv. Biol. Inst. Permsk. Gos. Univ. 1936,10, 9–10.
38. Iskina, R.Y. On nitrogen fixing bacteria in lichens. Isv. Biol. Inst. Permsk. 1938,11, 133–139.
39.
González, I.; Ayuso-Sacido, A.; Anderson, A.; Genilloud, O. Actinomycetes isolated from lichens: Evaluation of their diversity
and detection of biosynthetic gene sequences. FEMS Microbiol. Ecol. 2005,54, 401–415. [CrossRef]
40.
Amann, R.I.; Ludwig, W.; Schleifer, K.-H. Phylogenetic identification and in situ detection of individual microbial cells without
cultivation. Microbiol. Rev. 1995,59, 143–169. [CrossRef]
41. Rappé, M.S.; Giovannoni, S.J. The Uncultured Microbial Majority. Annu. Rev. Microbiol. 2003,57, 369–394. [CrossRef]
42.
Almendras, K.; García, J.; Carú, M.; Orlando, J. Nitrogen-fixing bacteria associated with Peltigera cyanolichens and Cladonia
chlorolichens. Molecules 2018,23, 3077. [CrossRef]
43.
Zúñiga, C.; Leiva, D.; Carú, M.; Orlando, J. Substrates of Peltigera Lichens as a Potential Source of Cyanobionts. Microb. Ecol.
2017,74, 561–569. [CrossRef]
44.
Graham, L.E.; Trest, M.T.; Will-Wolf, S.; Miicke, N.S.; Atonio, L.M.; Piotrowski, M.J.; Knack, J.J. Microscopic and metagenomic
analyses of peltigera ponojensis (Peltigerales, ascomycota). Int. J. Plant Sci. 2018,179, 241–255. [CrossRef]
45. Grube, M.; Cardinale, M.; De Castro, J.V., Jr.; Müller, H.; Berg, G. Species-specific structural and functional diversity of bacterial
communities in lichen symbioses. ISME J. 2009,3, 1105–1115. [CrossRef] [PubMed]
46.
Hodkinson, B.P.; Lutzoni, F. A microbiotic survey of lichen-associated bacteria reveals a new lineage from the Rhizobiales.
Symbiosis 2009,49, 163–180. [CrossRef]
47.
Bates, S.T.; Cropsey, G.W.G.; Caporaso, J.G.; Knight, R.; Fierer, N. Bacterial communities associated with the lichen symbiosis.
Appl. Environ. Microbiol. 2011,77, 1309–1314. [CrossRef] [PubMed]
48.
Krassowski, M.; Das, V.; Sahu, S.K.; Misra, B.B. State of the Field in Multi-Omics Research: From Computational Needs to Data
Mining and Sharing. Front. Genet. 2020,11, 610798. [CrossRef] [PubMed]
49.
Grube, M.; Cernava, T.; Soh, J.; Fuchs, S.; Aschenbrenner, I.; Lassek, C.; Wegner, U.; Becher, D.; Riedel, K.; Sensen, C.W.; et al.
Exploring functional contexts of symbiotic sustain within lichen-associated bacteria by comparative omics. ISME J.
2015
,9,
412–424. [CrossRef] [PubMed]
50.
Cernava, T.; Erlacher, A.; Aschenbrenner, I.A.; Krug, L.; Lassek, C.; Riedel, K.; Grube, M.; Berg, G. Deciphering functional
diversification within the lichen microbiota by meta-omics. Microbiome 2017,5, 82. [CrossRef]

Encyclopedia 2022,21430
51.
Lendemer, J.C.; Keepers, K.G.; Tripp, E.A.; Pogoda, C.S.; McCain, C.M.; Kane, N.C. A taxonomically broad metagenomic survey
of 339 species spanning 57 families suggests cystobasidiomycete yeasts are not ubiquitous across all lichens. Am. J. Bot.
2019
,106,
1090–1095. [CrossRef]
52.
Kono, M.; Kon, Y.; Ohmura, Y.; Satta, Y.; Terai, Y.
In vitro
resynthesis of lichenization reveals the genetic background of
symbiosis-specific fungal-algal interaction in Usnea hakonensis. BMC Genom. 2020,21, 671. [CrossRef]
53. Tuovinen, V.; Ekman, S.; Thor, G.; Vanderpool, D.; Spribille, T.; Johannesson, H. Two Basidiomycete Fungi in the Cortex of Wolf
Lichens. Curr. Biol. 2019,29, 476–483.e5. [CrossRef]
54.
Noh, H.-J.; Park, Y.; Hong, S.G.; Lee, Y.M. Diversity and Physiological Characteristics of Antarctic Lichens-Associated Bacteria.
Microorganisms 2021,9, 607. [CrossRef] [PubMed]
55.
Cardinale, M.; Vieira De Castro Jr., J.; Müller, H.; Berg, G.; Grube, M. In situ analysis of the bacterial community associated with
the reindeer lichen Cladonia arbuscula reveals predominance of Alphaproteobacteria. FEMS Microbiol. Ecol.
2008
,66, 63–71.
[CrossRef]
56.
Aschenbrenner, I.A.; Cernava, T.; Berg, G.; Grube, M. Understanding microbial multi-species symbioses. Front. Microbiol.
2016
,
7, 180. [CrossRef]
57.
Spribille, T.; Tagirdzhanova, G.; Goyette, S.; Tuovinen, V.; Case, R.; Zandberg, W.F. 3D biofilms: In search of the polysaccharides
holding together lichen symbioses. FEMS Microbiol. Lett. 2020,367, fnaa023. [CrossRef]
58.
Spribille, T.; Tuovinen, V.; Resl, P.; Vanderpool, D.; Wolinski, H.; Aime, M.C.; Schneider, K.; Stabentheiner, E.; Toome-Heller, M.;
Thor, G.; et al. Basidiomycete yeasts in the cortex of ascomycete macrolichens. Science 2016,353, 488–492. [CrossRef]
59.
ˇ
Cernajová, I.; Škaloud, P. The first survey of Cystobasidiomycete yeasts in the lichen genus Cladonia; with the description of
Lichenozyma pisutiana gen. nov., sp. nov. Fungal Biol. 2019,123, 625–637. [CrossRef]
60.
Millanes, A.M.; Diederich, P.; Wedin, M. Cyphobasidium gen. nov., a new lichen-inhabiting lineage in the Cystobasidiomycetes
(Pucciniomycotina, Basidiomycota, Fungi). Fungal Biol. 2016,120, 1468–1477. [CrossRef]
61. Oberwinkler, F. Yeasts in Pucciniomycotina. Mycol. Prog. 2017,16, 831–856. [CrossRef]
62.
Wilkinson, D.M.; Creevy, A.L.; Kalu, C.L.; Schwartzman, D.W. Are heterotrophic and silica-rich eukaryotic microbes an important
part of the lichen symbiosis? Mycology 2015,6, 4–7. [CrossRef]
63.
Petrzik, K.; Koloniuk, I.; Sehadová, H.; Sarkisova, T. Chrysoviruses inhabited symbiotic fungi of lichens. Viruses
2019
,11, 1120.
[CrossRef]
64.
Simon, J.-C.; Marchesi, J.R.; Mougel, C.; Selosse, M.-A. Host-microbiota interactions: From holobiont theory to analysis. Microbiome
2019,7, 5. [CrossRef] [PubMed]
65.
Hawksworth, D.L.; Grube, M. Lichens redefined as complex ecosystems. New Phytol.
2020
,227, 1281–1283. [CrossRef] [PubMed]
66.
Honegger, R. The symbiotic phenotype of lichen-forming ascomycetes and their endo- and epibionts. In Fungal Associations,
2nd ed.; Springer Berlin Heidelberg; Institute of Plant Biology, University of Zürich: Zürich, Switzerland, 2012; Volume 9, pp.
287–339, ISBN 9783642308260.
67.
Grimm, M.; Grube, M.; Schiefelbein, U.; Zühlke, D.; Bernhardt, J.; Riedel, K. The Lichens’ Microbiota, Still a Mystery? Front.
Microbiol. 2021,12, 714. [CrossRef] [PubMed]
68.
Ahmadjian, V.; Jacobs, J.B. Relationship between fungus and alga in the lichen Cladonia cristatella Tuck. Nature
1981
,289, 169–172.
[CrossRef]
69.
Athukorala, S.N.P.; Huebner, E.; Piercey-Normore, M.D. Identification and comparison of the 3 early stages of resynthesis for the
lichen Cladonia rangiferina. Can. J. Microbiol. 2014,60, 41–52. [CrossRef] [PubMed]
70.
Honegger, R. Morphogenesis. In Lichen Biology; Nash, T.H., III, Ed.; Cambridge University Press: Cambridge, UK, 2008; pp. 69–93,
ISBN 9780521871624.
71.
Meeßen, J.; Ott, S. Recognition mechanisms during the pre-contact state of lichens: I. Mycobiont-photobiont interactions of the
mycobiont of Fulgensia bracteata. Symbiosis 2013,59, 121–130. [CrossRef]
72.
Joneson, S.; Armaleo, D.; Lutzoni, F. Fungal and algal gene expression in early developmental stages of lichen-symbiosis.
Mycologia 2011,103, 291–306. [CrossRef] [PubMed]
73.
Ahmadjian, V.; Jacobs, J.B.; Russell, L.A. Scanning Electron Microscope Study of Early Lichen Synthesis. Science
1978
,200,
1062–1064. [CrossRef] [PubMed]
74.
Galun, M. Lichenization. In CRC Handbook of Lichenology; Galun, M., Ed.; CRC Press: Boca Raton, FL, USA, 1988; Volume II, pp.
153–169.
75.
Armaleo, D. Experimental Microbiology of Lichens—Mycelia Fragmentation, a Novel Growth Chamber, and the Origins of
Thallus Differentiation. Symbiosis 1991,11, 163–177.
76.
Joneson, S.; Lutzoni, F. Compatibility and thigmotropism in the lichen symbiosis: A reappraisal. Symbiosis
2009
,47, 109–115.
[CrossRef]
77.
Yoshimura, I.; Kurokawa, T.; Yamamoto, Y.; Kinoshita, Y. Development of Lichen Thalli in Vitro. Bryologist
1993
,96, 412–421.
[CrossRef]
78.
Kon, Y.; Kashiwadani, H.; Masada, M.; Tamura, G. Artificial Syntheses of Mycobionts of Usnea confusa ssp. kitamiensis and
Usnea orientalis with Their Natural and Non-Natural Phycobiont. J. Jpn. Bot. 1993,68, 129–137.
79.
Stocker-Wörgötter, E. Experimental studies of the lichen symbiosis: DNA-analyses, differentiation and secondary chemistry of
selected mycobionts, artificial resynthesis of two- and tripartite symbioses. Symbiosis 2001,30, 207–227.

Encyclopedia 2022,21431
80.
Trembley, M.L.; Ringli, C.; Honegger, R. Morphological and molecular analysis of early stages in the resynthesis of the lichen
Baeomyces rufus. Mycol. Res. 2002,106, 768–776. [CrossRef]
81.
Guzow-Krzemi´nska, B.; Stocker-Wörgötter, E.
In vitro
culturing and resynthesis of the mycobiont Protoparmeliopsis muralis
with algal bionts. Lichenologist 2013,45, 65–76. [CrossRef]
82.
Ahmadjian, V.; Russell, L.A.; Hildreth, K.C. Artificial Reestablishment of Lichens. I. Morphological Interactions Between the
Phycobionts of Different Lichens and the Mycobionts Cladonia Cristatella and Lecanora Chrysoleuca. Mycologia
1980
,72, 73–89.
[CrossRef]
83. Bubrick, P.; Galun, M. Spore to spore resynthesis of Xanthoria Parietina. Lichenologist 1986,18, 47–49. [CrossRef]
84.
Culberson, C.F.; Culberson, W.L.; Johnson, A. Genetic and environmental effects of growth and production of secondary
compounds in Cladonia cristatella. Biochem. Syst. Ecol. 1983,11, 77–84. [CrossRef]
85.
Stocker-Wörgötter, E.; Türk, R. Artificial cultures of the cyanobacterial lichenPeltigera didactyla (Peltigeraceae) in the natural
environment. Plant Syst. Evol. 1989,165, 39–48. [CrossRef]
86.
Stocker-Wörgötter, E.; Türk, R. The resynthesis of thalli of Dermatocarpon miniatum under laboratory conditions. Symbiosis
1989
,
7, 37–50.
87.
Athukorala, S.N.P.; Piercey-Normore, M.D. Recognition-and defense-related gene expression at 3 resynthesis stages in lichen
symbionts. Can. J. Microbiol. 2014,61, 1–12. [CrossRef]
88.
Insarova, I.D.; Blagoveshchenskaya, E.Y. Lichen symbiosis: Search and recognition of partners. Biol. Bull.
2016
,43, 408–418.
[CrossRef]
89.
Gaßmann, A.; Ott, S. Growth strategy and the gradual symbiotic interactions of the lichen Ochrolechia frigida. Plant Biol.
2000
,2,
368–378. [CrossRef]
90.
Meeßen, J.; Eppenstein, S.; Ott, S. Recognition mechanisms during the pre-contact state of lichens: II. Influence of algal exudates
and ribitol on the response of the mycobiont of Fulgensia bracteata. Symbiosis 2013,59, 131–143. [CrossRef]
91.
Díaz, E.M.; Vicente-Manzanares, M.; Sacristan, M.; Vicente, C.; Legaz, M.-E. Fungal lectin of Peltigera canina induces
chemotropism of compatible Nostoc cells by constriction-relaxation pulses of cyanobiont cytoskeleton. Plant Signal. Behav.
2011
,
6, 1525–1536. [CrossRef] [PubMed]
92.
Munzi, S.; Gouveia, C.; Cruz, C.; Branquinho, C.; Coelho, A.V. Proteomic analysis contributes to unveil the mechanisms of
nitrogen tolerance in the lichen Xanthoria parietina. Not. Della Soc. Lichenol. Ital. 2018,31, 23.
93.
Crosino, A.; Moscato, E.; Blangetti, M.; Carotenuto, G.; Spina, F.; Bordignon, S.; Puech-Pagès, V.; Anfossi, L.; Volpe, V.; Prandi,
C.; et al. Extraction of short chain chitooligosaccharides from fungal biomass and their use as promoters of arbuscular mycorrhizal
symbiosis. Sci. Rep. 2021,11, 3798. [CrossRef]
94.
Volpe, V.; Carotenuto, G.; Berzero, C.; Cagnina, L.; Puech-Pagès, V.; Genre, A. Short chain chito-oligosaccharides promote
arbuscular mycorrhizal colonization in Medicago truncatula. Carbohydr. Polym. 2020,229, 115505. [CrossRef]
95.
Feng, F.; Sun, J.; Radhakrishnan, G.V.; Lee, T.; Bozsóki, Z.; Fort, S.; Gavrin, A.; Gysel, K.; Thygesen, M.B.; Andersen, K.R.; et al. A
combination of chitooligosaccharide and lipochitooligosaccharide recognition promotes arbuscular mycorrhizal associations in
Medicago truncatula. Nat. Commun. 2019,10, 5047. [CrossRef]
96.
Diédhiou, I.; Diouf, D. Transcription factors network in root endosymbiosis establishment and development. World J. Microbiol.
Biotechnol. 2018,34, 37. [CrossRef]
97.
Omoarelojie, L.O.; Van Staden, J. Plant-endophytic fungi interactions: A strigolactone perspective. S. Afr. J. Bot.
2020
,134, 280–284.
[CrossRef]
98.
Peláez-Vico, M.A.; Bernabéu-Roda, L.; Kohlen, W.; Soto, M.J.; López-Ráez, J.A. Strigolactones in the Rhizobium-legume symbiosis:
Stimulatory effect on bacterial surface motility and down-regulation of their levels in nodulated plants. Plant Sci.
2016
,245,
119–127. [CrossRef]
99.
Maclean, A.M.; Bravo, A.; Harrison, M.J. Plant signaling and metabolic pathways enabling arbuscular mycorrhizal symbiosis.
Plant Cell 2017,29, 2319–2335. [CrossRef] [PubMed]
100.
Kuga, Y.; Schläppi, K.; Reinhardt, D. From Imaging to Functional Traits in Interactions between Roots and Microbes. In Methods
in Rhizosphere Biology Research; Reinhardt, D., Sharma, A.K., Eds.; Springer: Singapore, 2019; pp. 227–239, ISBN 978-981-13-5767-1.
101.
Liu, C.-W.; Breakspear, A.; Stacey, N.; Findlay, K.; Nakashima, J.; Ramakrishnan, K.; Liu, M.; Xie, F.; Endre, G.; de Carvalho-Niebel,
F.; et al. A protein complex required for polar growth of rhizobial infection threads. Nat. Commun.
2019
,10, 2848. [CrossRef]
[PubMed]
102.
Radhakrishnan, G.V.; Keller, J.; Rich, M.K.; Vernié, T.; Mbadinga Mbadinga, D.L.; Vigneron, N.; Cottret, L.; Clemente, H.S.;
Libourel, C.; Cheema, J.; et al. An ancestral signalling pathway is conserved in intracellular symbioses-forming plant lineages.
Nat. Plants 2020,6, 280–289. [CrossRef] [PubMed]
103.
Boschiero, C.; Dai, X.; Lundquist, P.K.; Roy, S.; de Bang, T.C.; Zhang, S.; Zhuang, Z.; Torres-Jerez, I.; Udvardi, M.K.; Scheible,
W.-R.; et al. MtSSPDB: The Medicago truncatula Small Secreted Peptide Database. Plant Physiol.
2020
,183, 399–413. [CrossRef]
[PubMed]