**Abstract**

This study demonstrates the involvement of the cytoskeleton in the movement of cyanobacteria and fungal spores to their hosts to establish a state of symbiosis or pathogenicity. The term symbiosis sensu lato is referred not only to commensalism and mutualism but also to the parasitic aberrations. The establishment of association implies that the endohabitant can move on a wet surface until finding an entry point in the exohabitant surface. In aqueous media, the exohabitant secretes glycoproteins that form a chemoattraction gradient for the invading cells. In lichens, the gradient consists of fungal lectins whose function is to recognize a compatible green alga or cyanobacterium. In the case of pathogens, the secreted proteins usually are a mixture that includes false quorum and chemoattractant signals, and cell wall digestive enzymes. The results indicate that fungal lectins and defense proteins bind to specific cell wall receptors for signaling the activation of cytoskeleton, causing successive cycles of cell contraction-relaxation that permits the migration of the endohabitant. In this study, different biochemical and microscopy techniques have been used. The mechanisms through which the cytoskeleton carries out these cycles of cell contractionrelaxation are described, being this a remarkable advance compared to previous results.

**Keywords:** actin, chemotaxis, cytoskeleton, lichens, motility, myosin, *Nostoc*, pathogens, *Sporisorium*

## **1. Introduction**

The main interactions between plants include epiphytism, mutualism, commensalism, and parasitism, although the frontier between these types of association can be confusing [1]. For example, most epiphytes do not negatively influence their phytophores since they absorb water and nutrients directly from the atmosphere [2]. It is the case of many bromeliads or the crassulaceae *Aeonium arboreum*, growing on the *Phoenix dactylifera* stipe without damaging it (**Figure 1A**), although in some cases, drift toward parasitism is evident, as has been demonstrated by Montaña et al. [3] for epiphytic Bromeliads growing upon *Cercidium praecox*. Many lichens are also epiphytic, although they can behave as hemiparasitic if the phytophore is vitally weakened by environmental circumstances, such as drought or severe air pollution (**Figure 1B**). Examples include *Evernia prunastri* growing on *Quercus rotundifolia* [4] or on *Betula pendula* [5]. In other cases, nonlichenized fungi are decidedly parasites (**Figure 1C**).

Lichen thalli can be reproduced by propagules containing some compatible algal cells surrounded by fungal hyphae. But there is also the possibility that a free-living fungus may find compatible algal cells in its environment. These algae, living in an aqueous film that covers the substrate (soil, rock, tree trunk), can move toward the fungal mass that would envelop them after being recognized. A similar situation is established when a single-cell organism (bacteria, fungal spore) is deposited on the wet surface of a plant, and the higher organism must discriminate whether it is an epiphytic, potential endosymbiont, or decidedly pathogenic microorganism. In the latter two cases, the cells must move in the water film until a suitable point of

*Role of the Cytoskeletal Actomyosin Complex in the Motility of Cyanobacteria and Fungal Spores*

Therefore, two main problems arise to explain the mechanisms used to establish

Lichens generally secrete glycoproteins to the environment depending on the availability of water [7]. Since most of these glycoproteins were enzymes, it was long time assumed that secretion was a function of the chemical composition of the substrate. This secretion might be taken as a kind of exocellular digestion of the compounds in the medium in order to be internalized into the thallus as simpler structures. However, using the lichen *Xanthoria parietina* growing on different substrates, rock or tree branches, it was found that the composition of the substrate did not influence the secretion of particular enzymes, which resulted in an exclusive function of the water availability and the degree of hydration of the thalli [8].

The aim of this study is to investigate the mechanism by which both prokaryotic and eukaryotic cells that do not have motile organs can move in liquid media thanks

In the early stages of the establishment of lichen symbiosis, parasitic attack of the mycobiont (the fungal partner) against a variable number of photobiont cells (algae or cyanobacteria) can occur, which can be attenuated, according to Ahmadjian [9], by subjecting the neo-association to conditions of deprivation of organic nutrients. In this way, the fungus must keep a vital and active population of

green cells, on whose photosynthetic products it depends to maintain its

secretion and production of this enzyme must be avoided [11].

chemoorganic metabolism. This parasitic attack is carried out by invasion of the photosynthetic cells by fungal haustoria or by secretion of proteins that cause changes in genetic expression, structure disorganization, and cell death. These actions require proteins such as arginine methyltransferase, arginase, dioxygenases, or chitinases, according to Joneson et al. [10], secreted by the fungus *Cladonia grayi* in contact with the single-celled green alga *Asterochloris* sp. The appearance of chitinase as a secreted protein during the first stages of recognition has been explained as a defensive reaction of the algal partner against the fungus that attempts parasitism, which means that for the association to be successful, the

In the case of fungal recognition of an algae considered genetically incompatible, the contact ends with the disorganization of the photosynthetic apparatus and the enzymatic rupture of the cell wall, with the loss of protoplast and death of the cell [12]. When the fungal-secreted arginase does not find a specific receptor in the algal cell wall, the enzyme penetrates the cell wall and activates its own β-1,4-glucanase

this type of interspecific relationship: how unicellular organisms, potential endobionts, move toward the points of contact or entry and how they are recog-

nized by the potential exohabitant when it reaches this position.

to the properties of their actomyosin cytoskeleton.

penetration is found.

*DOI: http://dx.doi.org/10.5772/intechopen.81299*

**2. Secreted proteins**

**5**

In this respect, lichens, traditionally considered as an example of mutual symbiosis, exhibit a characteristic that can lead to a decided parasitism: the specificity between symbionts. The fungus selectively chooses individuals from an algal species from its surroundings to form the thallus, while those from other different species will be rejected. This implies that a fungus susceptible to lichenization is able to discriminate between compatible or incompatible algae: the former will form the association while the latter will be eliminated [6]. The argument can be further complicated: if the algae that make up the association split up inside an established thallus, the newly hatched algae may not be recognized as compatible and should therefore be removed (**Figure 1D**), unless they are able to set up the appropriate recognition systems in time.

#### **Figure 1.**

*(A)* Aeonium arboreum*, a crassulacean species epiphytically growing on the stipe of* Phoenix canariensis*. (B) Hemiparasitic action of a dense population of epiphytic lichens that defoliated branches of their oak substrate. (C) Red spots on the leaves of* Vitis vinifera*, symptoms of the disease called tinder. The causal agents are fungi from* Stereum hirsutum *and* Phellinus igniarius *species. (D) Parasitic drift of* Xanthoria parietina *mycobiont on its phycobiont,* Trebouxia*, devoid of the receptor for recognition lectin.*

#### *Role of the Cytoskeletal Actomyosin Complex in the Motility of Cyanobacteria and Fungal Spores DOI: http://dx.doi.org/10.5772/intechopen.81299*

Lichen thalli can be reproduced by propagules containing some compatible algal cells surrounded by fungal hyphae. But there is also the possibility that a free-living fungus may find compatible algal cells in its environment. These algae, living in an aqueous film that covers the substrate (soil, rock, tree trunk), can move toward the fungal mass that would envelop them after being recognized. A similar situation is established when a single-cell organism (bacteria, fungal spore) is deposited on the wet surface of a plant, and the higher organism must discriminate whether it is an epiphytic, potential endosymbiont, or decidedly pathogenic microorganism. In the latter two cases, the cells must move in the water film until a suitable point of penetration is found.

Therefore, two main problems arise to explain the mechanisms used to establish this type of interspecific relationship: how unicellular organisms, potential endobionts, move toward the points of contact or entry and how they are recognized by the potential exohabitant when it reaches this position.

Lichens generally secrete glycoproteins to the environment depending on the availability of water [7]. Since most of these glycoproteins were enzymes, it was long time assumed that secretion was a function of the chemical composition of the substrate. This secretion might be taken as a kind of exocellular digestion of the compounds in the medium in order to be internalized into the thallus as simpler structures. However, using the lichen *Xanthoria parietina* growing on different substrates, rock or tree branches, it was found that the composition of the substrate did not influence the secretion of particular enzymes, which resulted in an exclusive function of the water availability and the degree of hydration of the thalli [8].

The aim of this study is to investigate the mechanism by which both prokaryotic and eukaryotic cells that do not have motile organs can move in liquid media thanks to the properties of their actomyosin cytoskeleton.

## **2. Secreted proteins**

lichens are also epiphytic, although they can behave as hemiparasitic if the phytophore is vitally weakened by environmental circumstances, such as drought or severe air pollution (**Figure 1B**). Examples include *Evernia prunastri* growing on *Quercus rotundifolia* [4] or on *Betula pendula* [5]. In other cases, nonlichenized fungi

In this respect, lichens, traditionally considered as an example of mutual symbiosis, exhibit a characteristic that can lead to a decided parasitism: the specificity between symbionts. The fungus selectively chooses individuals from an algal species from its surroundings to form the thallus, while those from other different species will be rejected. This implies that a fungus susceptible to lichenization is able to discriminate between compatible or incompatible algae: the former will form the association while the latter will be eliminated [6]. The argument can be further complicated: if the algae that make up the association split up inside an established thallus, the newly hatched algae may not be recognized as compatible and should therefore be removed (**Figure 1D**), unless they are able to set up the appropriate

*(A)* Aeonium arboreum*, a crassulacean species epiphytically growing on the stipe of* Phoenix canariensis*. (B) Hemiparasitic action of a dense population of epiphytic lichens that defoliated branches of their oak substrate. (C) Red spots on the leaves of* Vitis vinifera*, symptoms of the disease called tinder. The causal agents are fungi from* Stereum hirsutum *and* Phellinus igniarius *species. (D) Parasitic drift of* Xanthoria parietina *mycobiont*

*on its phycobiont,* Trebouxia*, devoid of the receptor for recognition lectin.*

are decidedly parasites (**Figure 1C**).

*Parasitology and Microbiology Research*

recognition systems in time.

**Figure 1.**

**4**

In the early stages of the establishment of lichen symbiosis, parasitic attack of the mycobiont (the fungal partner) against a variable number of photobiont cells (algae or cyanobacteria) can occur, which can be attenuated, according to Ahmadjian [9], by subjecting the neo-association to conditions of deprivation of organic nutrients. In this way, the fungus must keep a vital and active population of green cells, on whose photosynthetic products it depends to maintain its chemoorganic metabolism. This parasitic attack is carried out by invasion of the photosynthetic cells by fungal haustoria or by secretion of proteins that cause changes in genetic expression, structure disorganization, and cell death. These actions require proteins such as arginine methyltransferase, arginase, dioxygenases, or chitinases, according to Joneson et al. [10], secreted by the fungus *Cladonia grayi* in contact with the single-celled green alga *Asterochloris* sp. The appearance of chitinase as a secreted protein during the first stages of recognition has been explained as a defensive reaction of the algal partner against the fungus that attempts parasitism, which means that for the association to be successful, the secretion and production of this enzyme must be avoided [11].

In the case of fungal recognition of an algae considered genetically incompatible, the contact ends with the disorganization of the photosynthetic apparatus and the enzymatic rupture of the cell wall, with the loss of protoplast and death of the cell [12]. When the fungal-secreted arginase does not find a specific receptor in the algal cell wall, the enzyme penetrates the cell wall and activates its own β-1,4-glucanase

up to 10 times above its normal physiological level, causing total digestion of specific areas of the cell wall. Such a drastic response contradicts the assertion of Wang et al. [13] when they state that *Endocarpon pusillum* mycobiont interacts with their photobiont, *Diplosphaera chodatii*, by means of secreted small proteins much weaker than those that produce pathogenic fungi.

Another model of interaction between individuals, studied in our laboratory, is the pathosystem *Saccharum officinarum*-*Sporisorium scitamineum*. Plant invasion by the pathogen causes the production of at least 5–6 defense proteins, among which a dirigent protein [14], secreted arginase, β-1,3- and β-1,4-glucanases, chitinase as well as a sixth protein that acts as a positive chemotactic factor have been identified [15]. The actions that these secreted proteins carry out on the spores of the pathogen are varied. On one hand, arginase secreted by the plant causes a false quorum effect on the fungal teliospore population. The quorum effect exists by itself. The teliospores themselves secrete authentic quorum signals to increase the population of cells at the points of invasion in such a way as to ensure the survival of a sufficient number of them in the event that the plant emits effective defense factors. The false quorum signal causes the teliospores to form large aggregates over which the hydrolytic enzymes of the plant, chitinase, and glucanases would act [16].

Therefore, the behavior of the former inhabitant against a process of recognition of compatibility in the symbiosis or defense against a pathogen presents molecular similarities, but a very different characteristic in each case. For lichens, the mycobiont secretes a protein (a lectin) able to discriminate between compatible and

> incompatible algae [17]. Only in the latter case, the secreted protein behaves as an aggressive factor (**Figure 2**). In the case of host-pathogen interactions, the proteins secreted by the host are always defense proteins (**Figure 3**). To carry out these actions, the potential endohabitant, symbiont or pathogen, must possess receptors for the secreted proteins that transmit the signal of compatibility or resistance to the

*Defense proteins produced by sugarcane cells are synthesized in the endoplasmic reticulum, glycosylated in the Golgi cisternae, and internalized in the* trans*-Golgi network (TGN) vesicles to be transported to periplasmic space, crossing the cell membrane, to deposit them on the inner surface of the cell wall or to be secreted outside*

*Role of the Cytoskeletal Actomyosin Complex in the Motility of Cyanobacteria and Fungal Spores*

*DOI: http://dx.doi.org/10.5772/intechopen.81299*

The nature of these receptors, both in lichen photobionts as well as in some sugarcane pathogens, has been investigated in our laboratory. The occurrence of a glycosylated urease located in the phycobiont cell wall of *X. parietina* has been identified as an arginase-lectin receptor [18]. This identity has also been extended to other lichen species, such as *E. prunastri* [6], *Leptogium corniculatum* [19], and *Peltigera canina* [20]. *X. parietina* and *E. prunastri* contain a green algae from the *Trebouxia* genus as chlorobiont, while *L. corniculatum* and *P. canina* are associated with *Nostoc* sp. (a cyanobacterium). Recently isolated photobionts from thalli of these four lichen species contained an active urease associated with the cell wall. However, this activity was completely inhibited when cell wall fractions isolated from phycobiont or cyanobiont cells were incubated for 2 h at 37°C with the corresponding, previously purified lectin. In addition, hydrolysis of the galactoside moiety of urease in intact algae with α-1,4-galactosidase releases high amounts of Dgalactose and impedes the binding of the lectin to the algal cell wall. However, the use of β-1,4-galactosidase releases low amounts of D-β-galactose from the algal cell wall and does not change the pattern of binding of the lectin to its ligand [21]. The production of glycosylated urease is restricted to the season in which algal cells

cell machinery when they receive the recognition protein.

**3. Receptors**

**7**

**Figure 3.**

*the cells.*

#### **Figure 2.**

*(A)* Trebouxia *cells isolated from* X. parietina *after the binding of the fluorescent lectin isolated from the compatible mycobiont. Fluorescence is superficially located on the algal cell wall. (B) The same algal cells lacking the specific lectin receptor. (C) Transmission electron micrograph of* Trebouxia *cells corresponding to (A). The integrity of the cells permits to distinguish the intact cell wall (CW), the chloroplast (CH) showing the complex lamellae system, the pyrenoid (PY), and one plastoglobuli (PG). (D) Transmission electron micrograph of* Trebouxia *cells corresponding to (B). The chloroplast has been disorganized, pyrenoid disappears, and the cell wall shows zones partially digested (black arrows).*

*Role of the Cytoskeletal Actomyosin Complex in the Motility of Cyanobacteria and Fungal Spores DOI: http://dx.doi.org/10.5772/intechopen.81299*

**Figure 3.**

up to 10 times above its normal physiological level, causing total digestion of specific areas of the cell wall. Such a drastic response contradicts the assertion of Wang et al. [13] when they state that *Endocarpon pusillum* mycobiont interacts with their photobiont, *Diplosphaera chodatii*, by means of secreted small proteins much

Another model of interaction between individuals, studied in our laboratory, is the pathosystem *Saccharum officinarum*-*Sporisorium scitamineum*. Plant invasion by the pathogen causes the production of at least 5–6 defense proteins, among which a dirigent protein [14], secreted arginase, β-1,3- and β-1,4-glucanases, chitinase as well as a sixth protein that acts as a positive chemotactic factor have been identified [15]. The actions that these secreted proteins carry out on the spores of the pathogen are varied. On one hand, arginase secreted by the plant causes a false quorum effect on the fungal teliospore population. The quorum effect exists by itself. The teliospores themselves secrete authentic quorum signals to increase the population of cells at the points of invasion in such a way as to ensure the survival of a sufficient number of them in the event that the plant emits effective defense factors. The false quorum signal causes the teliospores to form large aggregates over which the hydrolytic enzymes of the plant, chitinase, and glucanases would act [16].

Therefore, the behavior of the former inhabitant against a process of recognition of compatibility in the symbiosis or defense against a pathogen presents molecular similarities, but a very different characteristic in each case. For lichens, the

mycobiont secretes a protein (a lectin) able to discriminate between compatible and

*(A)* Trebouxia *cells isolated from* X. parietina *after the binding of the fluorescent lectin isolated from the compatible mycobiont. Fluorescence is superficially located on the algal cell wall. (B) The same algal cells lacking the specific lectin receptor. (C) Transmission electron micrograph of* Trebouxia *cells corresponding to (A). The integrity of the cells permits to distinguish the intact cell wall (CW), the chloroplast (CH) showing the complex lamellae system, the pyrenoid (PY), and one plastoglobuli (PG). (D) Transmission electron micrograph of* Trebouxia *cells corresponding to (B). The chloroplast has been disorganized, pyrenoid*

*disappears, and the cell wall shows zones partially digested (black arrows).*

weaker than those that produce pathogenic fungi.

*Parasitology and Microbiology Research*

**Figure 2.**

**6**

*Defense proteins produced by sugarcane cells are synthesized in the endoplasmic reticulum, glycosylated in the Golgi cisternae, and internalized in the* trans*-Golgi network (TGN) vesicles to be transported to periplasmic space, crossing the cell membrane, to deposit them on the inner surface of the cell wall or to be secreted outside the cells.*

incompatible algae [17]. Only in the latter case, the secreted protein behaves as an aggressive factor (**Figure 2**). In the case of host-pathogen interactions, the proteins secreted by the host are always defense proteins (**Figure 3**). To carry out these actions, the potential endohabitant, symbiont or pathogen, must possess receptors for the secreted proteins that transmit the signal of compatibility or resistance to the cell machinery when they receive the recognition protein.

#### **3. Receptors**

The nature of these receptors, both in lichen photobionts as well as in some sugarcane pathogens, has been investigated in our laboratory. The occurrence of a glycosylated urease located in the phycobiont cell wall of *X. parietina* has been identified as an arginase-lectin receptor [18]. This identity has also been extended to other lichen species, such as *E. prunastri* [6], *Leptogium corniculatum* [19], and *Peltigera canina* [20]. *X. parietina* and *E. prunastri* contain a green algae from the *Trebouxia* genus as chlorobiont, while *L. corniculatum* and *P. canina* are associated with *Nostoc* sp. (a cyanobacterium). Recently isolated photobionts from thalli of these four lichen species contained an active urease associated with the cell wall. However, this activity was completely inhibited when cell wall fractions isolated from phycobiont or cyanobiont cells were incubated for 2 h at 37°C with the corresponding, previously purified lectin. In addition, hydrolysis of the galactoside moiety of urease in intact algae with α-1,4-galactosidase releases high amounts of Dgalactose and impedes the binding of the lectin to the algal cell wall. However, the use of β-1,4-galactosidase releases low amounts of D-β-galactose from the algal cell wall and does not change the pattern of binding of the lectin to its ligand [21]. The production of glycosylated urease is restricted to the season in which algal cells

divide, and this assures the recognition of new phycobiont produced after cell division by its fungal partner [22]. This should be interpreted as meaning that the polypeptide sequence of arginase (the lectin produced by the mycobiont) possesses an amino acid domain capable of stereochemically recognizing the remains of D-βgalactose in β-1,3 bonds of the glycosylated, algal urease.

mid- (MMMG) and high molecular mass (HMMG). MMMG were preferentially desorbed from the bacterial cell wall with sucrose and galactitol, whereas HMMG were mainly desorbed with glucose and mannose [30]. This would indicate that, against this bacterium, MMMG behaves as signal molecules that bind to their receptor, or receptors, using their polysaccharide moiety, whereas, on the contrary, HMMG would use their peptide moiety for binding to different receptors, similar to

*Role of the Cytoskeletal Actomyosin Complex in the Motility of Cyanobacteria and Fungal Spores*

Surprisingly, receptors for both HMMG and MMMG do not behave as the typical adhesion receptors containing polysaccharides that bind by affinity to a specific peptide domain in the signaling molecule, the recognition of which implies the binding of this to selected carbohydrate moieties in their ligands [31]. In this case, the carbohydrate moiety of the signal molecule seems to be used to recognize a particular amino acid domain on the ligand (receptor) in an inverse way to that described for plant lectins and animal selectins. This fact suggests that HMMG and MMMG, with independence of their possible enzymatic activities [32], behave as true protein of resistance (PR), according to Su et al. [33], that would require

The cytoagglutinating effect of sugarcane glycoproteins on smut teliospores was

clearly reduced using invertase-digested glycoproteins. This suggested that the hydrolyzed glycidic moiety, which contains fructose residues polymerized as β-Dfructofuranosyl-1,2-β-D-fructose, could be involved in the process of binding since the extensive hydrolysis of β-(1 ! 2) bonds impeded cell adhesion. To obtain experimental evidence of the presence of such cell-wall receptor, or receptors, glycoproteins were isolated from the cell wall of the fungal pathogen. These glycoproteins were separated by affinity chromatography through activated agarose columns to which sugarcane glycoproteins from different cultivars had been previously bound. Fungal cell-wall receptors retained by sugarcane glycoproteins were then recovered, desorbed by certain monosaccharides used as eluents [35]. Sugarcane HMMG and MMMG fractions exhibited a high affinity for N-acetyl-Dglucosamine, component of the cell wall of filamentous fungi. Interestingly, this binding mechanism differed, for example, from that described by Blanco et al. [27] for the cell wall receptors of *G. diazotrophicus*. In this case, glycoproteins bound through a domain β-(1 ! 2)-fructofuranosyl fructose from its glycidic moiety to the bacterial cell wall receptors, which exhibited a binding site for this saccharide residue. Therefore, in the cases that HMMG or MMMG bound to their ligands using their polysaccharide moiety, either to bacterial cells or to fungal teliospores, they did not behave as lectins but as recognition factors using monosaccharide units or glycosidic bonds to bind to a particular domain of their ligands [36]. In addition, and as previously explained, HMMG and MMMG fractions behaved differently in their binding mechanisms to cell walls of *H. rubrisubalbicans*. These differences in the recognition mechanism could be interpreted as a discrimination factors between

**4. Cytoskeleton as the main responsible for displacement of** *Nostoc*

tions in cell shape and, generally, adhesion to the extracellular matrix [37]. Chemoattractive displacement is typically linked to the reorganization of actin filaments in cells, since polarization is the triggering event of cell migration [38]. A ligand on cell surface must activate a signaling pathway that leads to contraction/

Directed cell migration is a physical process that involves dramatic modifica-

the action mode of the lectin ConA [24], from *Canavalia ensiformis*.

*DOI: http://dx.doi.org/10.5772/intechopen.81299*

ligands similar to toll-like receptors (TLRs), studied in animals [34].

pathogens and endosymbionts.

**9**

**and** *Sporisorium scitamineum* **cells**

This mode of binding a lectin to the polysaccharide moiety of its ligand by an affinity reaction equals, at the level of action mechanism, the secreted lichen arginases with other, well-known lectins from higher plants, such as concanavalin A (ConA) from *Canavalia ensiformis*, and ricin A (RCA) from *Ricinus communis*. Studies carried out by using α-methyl-mannose as a ligand suggest that the sugar forms seven hydrogen bonds with the peptide of ConA, four with –NH groups of Lys99, Tyr100, Arg228 and Lys229, and three with amino acids interacting with Ca2+, Asn 14 and Asp208 [23]. On the other hand, Fontaniella et al. [24] showed that a commercial ConA was able to develop arginase activity that increased more than 40 times in the presence of 1.7 mM Mn2+. Another similarity between ConA and fungal arginases lies in the fact that their activity as enzymes requires Mn2+**,** while their activity as lectin is dependent on Ca2+ and both cations, at the level of biological activity, are mutually excluding. The comparison between crystalline structures of ConA-containing or not Ca2+ suggests that the cation pulls from Tyr12, Asp208, and Arg228 to conform the site to bind the specific sugar [25]. It is probable that the binding of Ca2+ to the specific domain for the cation changes the tertiary structure of the domain defined as site for the sugar binding and, for the same reason, the structure of the catalytic site for arginine. The ability to bind both cations together in order to develop their binding capacity to specific galactose ligands has been demonstrated for other lectins, such as that purified and crystallized from *Spatholobus parviflorus* [26].

According to this, Marx and Peveling [27] found that many cultured phycobionts isolated from several lichen species bind to commercial lectins, including Con A and RCA. In addition, Fontaniella et al. [24] found that ConA is able to bind to the cell wall of algal cells recently isolated from *E. prunastri* and *X. parietina* thalli. This binding involves a ligand, probably a glycoprotein containing mannose, which has been isolated by affinity chromatography. Analysis by SDS-PAGE of the purified ligand revealed that it is a dimeric protein composed by two monomers of 54 and 48 kDa. This ligand shows to be different from the receptor for natural lichen lectins, previously identified as a polygalactosylated urease.

The binding of sugarcane glycoproteins to their cell wall ligands in the bacterial endophyte *Gluconacetobacter diazotrophicus* [28] and in the bacterial pathogen *Xanthomonas albilineans* [29] results in cell recruitment (**Figure 4**) rather than a defense mechanism. Similar results on cytoagglutination were obtained using *Herbaspirillum rubrisubalbicans* treated with sugarcane glycoproteins of

#### **Figure 4.**

*Effect of secreted sugarcane glycoproteins on the cytoagglutination of* Xanthomonas albilineans*. (A) Bacterial cells immediately after the contact with plant defense glycoproteins and (B) 3 h after the contact.*

#### *Role of the Cytoskeletal Actomyosin Complex in the Motility of Cyanobacteria and Fungal Spores DOI: http://dx.doi.org/10.5772/intechopen.81299*

mid- (MMMG) and high molecular mass (HMMG). MMMG were preferentially desorbed from the bacterial cell wall with sucrose and galactitol, whereas HMMG were mainly desorbed with glucose and mannose [30]. This would indicate that, against this bacterium, MMMG behaves as signal molecules that bind to their receptor, or receptors, using their polysaccharide moiety, whereas, on the contrary, HMMG would use their peptide moiety for binding to different receptors, similar to the action mode of the lectin ConA [24], from *Canavalia ensiformis*.

Surprisingly, receptors for both HMMG and MMMG do not behave as the typical adhesion receptors containing polysaccharides that bind by affinity to a specific peptide domain in the signaling molecule, the recognition of which implies the binding of this to selected carbohydrate moieties in their ligands [31]. In this case, the carbohydrate moiety of the signal molecule seems to be used to recognize a particular amino acid domain on the ligand (receptor) in an inverse way to that described for plant lectins and animal selectins. This fact suggests that HMMG and MMMG, with independence of their possible enzymatic activities [32], behave as true protein of resistance (PR), according to Su et al. [33], that would require ligands similar to toll-like receptors (TLRs), studied in animals [34].

The cytoagglutinating effect of sugarcane glycoproteins on smut teliospores was clearly reduced using invertase-digested glycoproteins. This suggested that the hydrolyzed glycidic moiety, which contains fructose residues polymerized as β-Dfructofuranosyl-1,2-β-D-fructose, could be involved in the process of binding since the extensive hydrolysis of β-(1 ! 2) bonds impeded cell adhesion. To obtain experimental evidence of the presence of such cell-wall receptor, or receptors, glycoproteins were isolated from the cell wall of the fungal pathogen. These glycoproteins were separated by affinity chromatography through activated agarose columns to which sugarcane glycoproteins from different cultivars had been previously bound. Fungal cell-wall receptors retained by sugarcane glycoproteins were then recovered, desorbed by certain monosaccharides used as eluents [35]. Sugarcane HMMG and MMMG fractions exhibited a high affinity for N-acetyl-Dglucosamine, component of the cell wall of filamentous fungi. Interestingly, this binding mechanism differed, for example, from that described by Blanco et al. [27] for the cell wall receptors of *G. diazotrophicus*. In this case, glycoproteins bound through a domain β-(1 ! 2)-fructofuranosyl fructose from its glycidic moiety to the bacterial cell wall receptors, which exhibited a binding site for this saccharide residue. Therefore, in the cases that HMMG or MMMG bound to their ligands using their polysaccharide moiety, either to bacterial cells or to fungal teliospores, they did not behave as lectins but as recognition factors using monosaccharide units or glycosidic bonds to bind to a particular domain of their ligands [36]. In addition, and as previously explained, HMMG and MMMG fractions behaved differently in their binding mechanisms to cell walls of *H. rubrisubalbicans*. These differences in the recognition mechanism could be interpreted as a discrimination factors between pathogens and endosymbionts.
