**1. Introduction**

Polarized environment, from the beginning of life on our planet, imposes on living organisms a necessity to read directions. Light and gravity are the two most important oriented signals that come from well-defined sources. The response to these primary polar signals allowed for development of the secondary, more sophisticated reactions to the network of other polar signals like gradients of chemical molecules or mechanical stresses. The signals of chemical and physical nature provide the extrinsic but also an intrinsic information for biological systems.

One of the basic features of the organism developing in polar environment is its axiality. This brings in consequence following possibilities: 1) multiplication of repetitive units of the body and their special alignment along the axis (segmentation, metamerism) and 2) deviations of structures from the axis to the left or to the right (development of L/R symmetry).

The main axis in the motionless plants, fastened to the ground, is mostly vertical, extending between the apical and basal poles. The animal main axis, regardless its position in the gravity field, connects the anterior and posterior poles. The axis

formation starts at the very early stages of development. The identity of segments formed iteratively along the axis in both plants and animals is genetically controlled and their evolutionary multiplication creates a great potential for morphological and functional diversity through many useful modifications. This process is in a sense similar to the effects of gene duplication on the molecular level.

Subsequent emergence of L/R symmetry may be observed on all, hierarchically different levels of body organization. Some general principles, like minimum energy rule, are universal in nature leading to the identical solutions on all these levels. Good example represent spherical geodesic shapes. The structure of carbon allotrope - C60 closed fullerene, is also present in coated endocytic vesicles reinforced by the clathrin cage [1], in regularly sculptured surface of pollen grains [2] and in a cellular pattern on the surface of the plant paraboloid apical meristem [3].

Other universal basic forms are chiral helices and spirals commonly observed on molecular, cellular and organismal levels. They are of particular interest here because of distinct mirror symmetry they have, which is the main focus of this chapter. Chirality of many macromolecules: nucleic acids, proteins or cellulose fibers [4], coiled coils of collagen, or such structures as tubulin cytoskeleton, thickenings of plant cell wall, plant tendrils, spiral snail shells or narwhal tusks are but a few examples. Not only structures but also some developmental processes may be chiral. The apical cell divisions in moss gametophores [5], cell cleavage in the embryos of snails [6, 7] or lateral organ initiation on plant shoot apical meristem (SAM) proceed clockwise (CW) or counterclockwise (CCW). Two interesting problems may be addressed while considering chiral structures in biological systems – mechanism of their formation and proportion between the two chiral configurations.

The aim of this chapter is to provide the readers with the overview of some examples of bio-chirality discovered over the years both in animals and plants. The stronger accent will be placed on the latter because they are less known and because they have always been in a focus of the author's research. The mechanisms of many cases of mirror-symmetry presence in plants are yet to be elucidated.

#### **2. Mirror symmetry on cellular level**

Cell chirality or handedness is a newly discovered phenomenon, which nowadays is intensely studied, mostly in animal cells [7–10]. It is manifested in the presence of chiral structures within the cells but also in the cell behavior that may lead to directional movements or assuming L or R orientation of cell alignment. In animals it affects organs laterality [10], in plants results in development of spiral, helical or wavy patterns [3, 11–15].

During primary axis formation on the cellular level the polarity of the cell is manifested in an uneven distribution of receptors, ion channels and hormone carriers on plasma membrane, and internally in the ion currents and cytoskeletal fibers parallel to the developing axis but also in the polar distribution of ultrastructural components like cell organelles or nutrients. All these sophisticated processes have been investigated mainly in plant egg cells or fucoid zygotes [16–18] and animal oocytes [19, 20]. However, even in the integrated system of multicellular organism, singular cell polarity is often a case. In animal body the epithelial cells of intestines constituting a planar 2D barrier, have their polarity unified. It is manifested in nonrandom distribution of glucose transporters which facilitates the oriented transepithelial sugar transport [21]. In plants the polar distribution of the auxin influx (AUX) and efflux (PIN) carriers in plasma membrane results in polar transport of the hormone between the cells [22]. In L1 layer of SAM auxin is transported

**165**

**Figure 1.**

*watched from the inside of the cell.*

*Mirror Symmetry of Life*

of the roots [24].

undiscovered.

*DOI: http://dx.doi.org/10.5772/intechopen.96507*

acropetally, whereas inside of the plant body, in the provascular tissues and later in cambium, the hormone transport is basipetal. Change in the distribution of carriers and thus of the cell polarity redirects the transport, often affecting the directions of plant organ growth [23]. This, for instance, has been noted in gravitropic response

Many elements of the cell ultrastructure are spectacularly chiral. In some green algae exemplified by multicellular filamentous *Spirogyra* or unicellular *Spirotaenia* and *Chlamydomonas spirogyroides* the chloroplasts are of considerable length, flat and ribbon-like. They assume helical course in the cortical cytoplasm of the cell. Not much is known about the chiral configurations of their coiling. Images available in various data bases suggest that in *Spirogyra* both configurations may be present in different filaments [25], in different cells of the same filament or even within the same cell [26]. However, the error resulting from improper focusing during microscopic observations cannot be excluded. The sample taken for analysis from the aquarium of the Botanical Garden of Wrocław showed under light microscope hundreds of cells of the same S configuration of chloroplasts coiling from the right to the left (**Figure 1**). Mechanisms by which the configuration is regulated remain

Another clearly chiral component of the cell is basal body. In eukaryotic, plant and animal cells some identical structures bear different names although they look the same. Two centrioles of the centrosome, basal bodies or kinetosomes in the motile or ciliary epithelial cells have the same architecture. Composed of 9 triplets of microtubules (MT), overlapping on all available images either CW or CCW, they resemble a pinwheel toy. It is unclear, however, whether both configurations, being a mirror-image of one another, are indeed present in all different types of the cells. Transmission electron micrographs (TEM) of tangentially sectioned cell surface show that in a particular cell all basal bodies underlying cilia are of the same chirality [27]. However, unless it is clearly stated like in [28], it is not known whether

*The same filament of* Spirogyra *photographed under fluorescent microscope at different optical levels: at its upper surface (left) and well below, close to its opposite side (right). S helix of the chloroplast, visualized here by the red autofuorescence of chlorophyll, may be falsely interpreted as Z, when due to the changing focus is* 

#### *Mirror Symmetry of Life DOI: http://dx.doi.org/10.5772/intechopen.96507*

*Current Topics in Chirality - From Chemistry to Biology*

formation starts at the very early stages of development. The identity of segments formed iteratively along the axis in both plants and animals is genetically controlled and their evolutionary multiplication creates a great potential for morphological and functional diversity through many useful modifications. This process is in a

Subsequent emergence of L/R symmetry may be observed on all, hierarchically different levels of body organization. Some general principles, like minimum energy rule, are universal in nature leading to the identical solutions on all these levels. Good example represent spherical geodesic shapes. The structure of carbon allotrope - C60 closed fullerene, is also present in coated endocytic vesicles reinforced by the clathrin cage [1], in regularly sculptured surface of pollen grains [2] and in a cellular pattern on the surface of the plant paraboloid apical meristem [3]. Other universal basic forms are chiral helices and spirals commonly observed on molecular, cellular and organismal levels. They are of particular interest here because of distinct mirror symmetry they have, which is the main focus of this chapter. Chirality of many macromolecules: nucleic acids, proteins or cellulose fibers [4], coiled coils of collagen, or such structures as tubulin cytoskeleton, thickenings of plant cell wall, plant tendrils, spiral snail shells or narwhal tusks are but a few examples. Not only structures but also some developmental processes may be chiral. The apical cell divisions in moss gametophores [5], cell cleavage in the embryos of snails [6, 7] or lateral organ initiation on plant shoot apical meristem (SAM) proceed clockwise (CW) or counterclockwise (CCW). Two interesting problems may be addressed while considering chiral structures in biological systems – mechanism of their formation and proportion between the two chiral

The aim of this chapter is to provide the readers with the overview of some examples of bio-chirality discovered over the years both in animals and plants. The stronger accent will be placed on the latter because they are less known and because they have always been in a focus of the author's research. The mechanisms of many

Cell chirality or handedness is a newly discovered phenomenon, which nowadays is intensely studied, mostly in animal cells [7–10]. It is manifested in the presence of chiral structures within the cells but also in the cell behavior that may lead to directional movements or assuming L or R orientation of cell alignment. In animals it affects organs laterality [10], in plants results in development of spiral,

During primary axis formation on the cellular level the polarity of the cell is manifested in an uneven distribution of receptors, ion channels and hormone carriers on plasma membrane, and internally in the ion currents and cytoskeletal fibers parallel to the developing axis but also in the polar distribution of ultrastructural components like cell organelles or nutrients. All these sophisticated processes have been investigated mainly in plant egg cells or fucoid zygotes [16–18] and animal oocytes [19, 20]. However, even in the integrated system of multicellular organism, singular cell polarity is often a case. In animal body the epithelial cells of intestines constituting a planar 2D barrier, have their polarity unified. It is manifested in nonrandom distribution of glucose transporters which facilitates the oriented transepithelial sugar transport [21]. In plants the polar distribution of the auxin influx (AUX) and efflux (PIN) carriers in plasma membrane results in polar transport of the hormone between the cells [22]. In L1 layer of SAM auxin is transported

cases of mirror-symmetry presence in plants are yet to be elucidated.

**2. Mirror symmetry on cellular level**

helical or wavy patterns [3, 11–15].

sense similar to the effects of gene duplication on the molecular level.

**164**

configurations.

acropetally, whereas inside of the plant body, in the provascular tissues and later in cambium, the hormone transport is basipetal. Change in the distribution of carriers and thus of the cell polarity redirects the transport, often affecting the directions of plant organ growth [23]. This, for instance, has been noted in gravitropic response of the roots [24].

Many elements of the cell ultrastructure are spectacularly chiral. In some green algae exemplified by multicellular filamentous *Spirogyra* or unicellular *Spirotaenia* and *Chlamydomonas spirogyroides* the chloroplasts are of considerable length, flat and ribbon-like. They assume helical course in the cortical cytoplasm of the cell. Not much is known about the chiral configurations of their coiling. Images available in various data bases suggest that in *Spirogyra* both configurations may be present in different filaments [25], in different cells of the same filament or even within the same cell [26]. However, the error resulting from improper focusing during microscopic observations cannot be excluded. The sample taken for analysis from the aquarium of the Botanical Garden of Wrocław showed under light microscope hundreds of cells of the same S configuration of chloroplasts coiling from the right to the left (**Figure 1**). Mechanisms by which the configuration is regulated remain undiscovered.

Another clearly chiral component of the cell is basal body. In eukaryotic, plant and animal cells some identical structures bear different names although they look the same. Two centrioles of the centrosome, basal bodies or kinetosomes in the motile or ciliary epithelial cells have the same architecture. Composed of 9 triplets of microtubules (MT), overlapping on all available images either CW or CCW, they resemble a pinwheel toy. It is unclear, however, whether both configurations, being a mirror-image of one another, are indeed present in all different types of the cells. Transmission electron micrographs (TEM) of tangentially sectioned cell surface show that in a particular cell all basal bodies underlying cilia are of the same chirality [27]. However, unless it is clearly stated like in [28], it is not known whether

#### **Figure 1.**

*The same filament of* Spirogyra *photographed under fluorescent microscope at different optical levels: at its upper surface (left) and well below, close to its opposite side (right). S helix of the chloroplast, visualized here by the red autofuorescence of chlorophyll, may be falsely interpreted as Z, when due to the changing focus is watched from the inside of the cell.*

basal body is seen on TEM from the surface of the cell or from its inside. This is the reason for chiral configurations of basal bodies being uncertain. The image of *Paramaecium micronucleatum* by Dennis Kunkel [27] shows CCW overlapping triplets, whereas in *Paramecium tetraurelia* [29] the triplets overlap CW.

In flagellar apparatus of *Chlamydomonas* or *Acrosiphonia* gamete both chiral configurations of basal bodies are present on the same electron micrograph [30, 31] but in these cases it is certainly an effect of opposite orientation of flagella and their two basal bodies facing each other horizontally. The bodies are in fact of the same chirality. This case shows again how careful one must be analyzing and interpreting the examples of mirror-symmetry of the chiral ultrastructural components of the cell on TEM images, that can be easily flipped and show the same structure either from above or from the bottom side. There is one additional aspect of mirror symmetry presence in the locomotion apparatus of green algae. The flagellar roots of the two basal bodies are slightly rotated one with respect to the other – CW in representatives of Chlorophyceae and CCW in Ulvophyceae [30, 32, 33]. This finding, among the others, was the foundation of the profound revision of green algae taxonomy [30, 33].

The process of cilia beating is chiral. Both ciliates, motile spermatozoids and stationary epithelial cells readily change the direction of cilia movement either to navigate or alter the current of surrounding fluids [34]. Interestingly, the latter has been employed by unicellular *Stentor rosei*, to avoid, in quite deliberate and calculated manner, the irritating particles experimentally added to the medium [35]. The cell, however, is not always in full control over the cilia beating. The doublets of some ciliates, having notably the same chirality of basal bodies, show that in the form being a mirror image of the typical one, food particles are expelled from the oral apparatus instead of being directed towards it [28]. Typical chiral form of *Paramaecium* swims forward employing leftward rotation. Stressful conditions like low temperature or heavy metals force the ciliate to spin in opposite direction [36].

Among fibrillar elements constituting cytoskeleton, a tensegral structure of eukaryotic cytoplasm it is actin microfilament (MF) that is chiral. Mammalian cells exhibit specific, actin dependent L/R asymmetry which is different in normal and cancerous cells and changes when inhibitors of actin function are applied [37]. In bacteria changeable chirality of actin homologs MreB has impact on their growth and cell shape [38]. Actin is also responsible for directional, chiral movement of cytoplasm in the cells of charophytes [39]. Another important component of cytoskeleton, MTs. *per se* are not chiral. However, their arrangement in the cortical cytoplasm, beneath plasma membrane, is often oriented in plant cells (**Figure 2**). The elongating cells in the axial organs of the model plant *Arabidopsis thaliana* apparently have the chirality of their cortical cytoskeleton genetically controlled. In *spiral 1* and *spiral 2* mutants the cortical MTs, watched from the cell surface, form an ascending S helix, whereas in *lefty* mutant the helix is Z. The mutations lead to abnormal growth of the plant axial organs which, strangely enough, become twisted oppositely to the configuration of MT helix in their cells – Z in the *spiral*, S in the *lefty* mutants [40–42].

Configuration of cortical cytoskeleton, below plasma membrane, in some differentiating plant cells may change with time causing the development of interlocked pattern of the cellulose microfibrils deposited in the secondary wall [43]. Both chiral systems: intracellular and extracellular, parallel each other in consecutive stages of the secondary cell wall formation. The cycle of changes in microfibrils orientation always starts from the ascending S helix in S1 layer of the secondary wall (watched from the outside of the cell), it alters to Z in the S2 layer and returns to S helix in the S3 layer. No exception from this rule has been found

**167**

*Mirror Symmetry of Life*

**Figure 2.**

from breaking (**Figure 2**).

recently shown in chicken embryos [48].

function [45].

*DOI: http://dx.doi.org/10.5772/intechopen.96507*

*surface of the thickenings seen from their outside.*

even though, at least theoretically, the opposite sequence of changes in microfibrils orientation is possible. Moreover, the cycle of changes in the chirality of microfibrillar helix appears to be independent of the overall orientation of the differentiating cell within figured wood, which also exhibits L/R symmetry [44]. This astounding regularity points to existence of yet undiscovered mechanism that must precisely regulate the phases sequence in the full cycle of changes in the cortical MTs orientation. Must be also independent of the other, hypothetical one, which controls L/R symmetry of cambial cellular events such as oriented anticlinal divisions and intrusive growth. This assumption is supported even further by the S helix of the secondary wall thickenings in differentiating protoxylem. The first, S1 deposit of this wall, not yet completely covering the primary wall surface prevents the cells, exposed to mechanical stress caused by longitudinal expansion of growing shoot,

*Immunofluorescent visualization of Z helical cortical cytoskeleton in the elongated cambial cell of*  Cinnamomum camphora *(left) and two S helices of the secondary cell wall thickenings isolated from the protoxylem cells of* Scindapsus *sp. photographed at two different optical levels. The middle photo shows the* 

The discovery of cell intrinsic chirality resulted from the fundamental question how the laterality of organs within the animal body is accomplished. Over the years much attention has been paid to this phenomenon and its connection with the development of L/R symmetry of the whole multicellular organism. It was found that blood neutrophiles polarity, defined by position of centrosomes with regard to the cell nucleus, makes them capable of directional movements in absence of polar external signals. This property disappears after application of drugs affecting MT

The model invertebrate organism *Drosophila melanogaster* provided evidence that myosin encoding gene mutation switches cell chirality and results in the development of *situs inversus* phenotype of the hindguts or genitalia [7, 8, 10, 46]. In vertebrates the development of typical L/R asymmetry from anteceding state of embryo bilateral symmetry is generated by various mechanisms. One of them is based on function of axonemal dynein. This motor-protein is responsible for appropriate beating of cilia in nodal epithelial cells, causing the directed ion current. Defects in the gene structure encoding for the dynein results in random selection of heart position in mouse embryo [47]. The involvement of C kinase signaling pathway in the reversal of cells chirality leading to mirrored position of heart was

#### **Figure 2.**

*Current Topics in Chirality - From Chemistry to Biology*

Z in the *spiral*, S in the *lefty* mutants [40–42].

taxonomy [30, 33].

basal body is seen on TEM from the surface of the cell or from its inside. This is the reason for chiral configurations of basal bodies being uncertain. The image of *Paramaecium micronucleatum* by Dennis Kunkel [27] shows CCW overlapping

In flagellar apparatus of *Chlamydomonas* or *Acrosiphonia* gamete both chiral configurations of basal bodies are present on the same electron micrograph [30, 31] but in these cases it is certainly an effect of opposite orientation of flagella and their two basal bodies facing each other horizontally. The bodies are in fact of the same chirality. This case shows again how careful one must be analyzing and interpreting the examples of mirror-symmetry of the chiral ultrastructural components of the cell on TEM images, that can be easily flipped and show the same structure either from above or from the bottom side. There is one additional aspect of mirror symmetry presence in the locomotion apparatus of green algae. The flagellar roots of the two basal bodies are slightly rotated one with respect to the other – CW in representatives of Chlorophyceae and CCW in Ulvophyceae [30, 32, 33]. This finding, among the others, was the foundation of the profound revision of green algae

The process of cilia beating is chiral. Both ciliates, motile spermatozoids and stationary epithelial cells readily change the direction of cilia movement either to navigate or alter the current of surrounding fluids [34]. Interestingly, the latter has been employed by unicellular *Stentor rosei*, to avoid, in quite deliberate and calculated manner, the irritating particles experimentally added to the medium [35]. The cell, however, is not always in full control over the cilia beating. The doublets of some ciliates, having notably the same chirality of basal bodies, show that in the form being a mirror image of the typical one, food particles are expelled from the oral apparatus instead of being directed towards it [28]. Typical chiral form of *Paramaecium* swims forward employing leftward rotation. Stressful conditions like low temperature or heavy metals force the ciliate to spin in opposite direction [36]. Among fibrillar elements constituting cytoskeleton, a tensegral structure of eukaryotic cytoplasm it is actin microfilament (MF) that is chiral. Mammalian cells exhibit specific, actin dependent L/R asymmetry which is different in normal and cancerous cells and changes when inhibitors of actin function are applied [37]. In bacteria changeable chirality of actin homologs MreB has impact on their growth and cell shape [38]. Actin is also responsible for directional, chiral movement of cytoplasm in the cells of charophytes [39]. Another important component of cytoskeleton, MTs. *per se* are not chiral. However, their arrangement in the cortical cytoplasm, beneath plasma membrane, is often oriented in plant cells (**Figure 2**). The elongating cells in the axial organs of the model plant *Arabidopsis thaliana* apparently have the chirality of their cortical cytoskeleton genetically controlled. In *spiral 1* and *spiral 2* mutants the cortical MTs, watched from the cell surface, form an ascending S helix, whereas in *lefty* mutant the helix is Z. The mutations lead to abnormal growth of the plant axial organs which, strangely enough, become twisted oppositely to the configuration of MT helix in their cells –

Configuration of cortical cytoskeleton, below plasma membrane, in some differentiating plant cells may change with time causing the development of interlocked pattern of the cellulose microfibrils deposited in the secondary wall [43]. Both chiral systems: intracellular and extracellular, parallel each other in consecutive stages of the secondary cell wall formation. The cycle of changes in microfibrils orientation always starts from the ascending S helix in S1 layer of the secondary wall (watched from the outside of the cell), it alters to Z in the S2 layer and returns to S helix in the S3 layer. No exception from this rule has been found

triplets, whereas in *Paramecium tetraurelia* [29] the triplets overlap CW.

**166**

*Immunofluorescent visualization of Z helical cortical cytoskeleton in the elongated cambial cell of*  Cinnamomum camphora *(left) and two S helices of the secondary cell wall thickenings isolated from the protoxylem cells of* Scindapsus *sp. photographed at two different optical levels. The middle photo shows the surface of the thickenings seen from their outside.*

even though, at least theoretically, the opposite sequence of changes in microfibrils orientation is possible. Moreover, the cycle of changes in the chirality of microfibrillar helix appears to be independent of the overall orientation of the differentiating cell within figured wood, which also exhibits L/R symmetry [44]. This astounding regularity points to existence of yet undiscovered mechanism that must precisely regulate the phases sequence in the full cycle of changes in the cortical MTs orientation. Must be also independent of the other, hypothetical one, which controls L/R symmetry of cambial cellular events such as oriented anticlinal divisions and intrusive growth. This assumption is supported even further by the S helix of the secondary wall thickenings in differentiating protoxylem. The first, S1 deposit of this wall, not yet completely covering the primary wall surface prevents the cells, exposed to mechanical stress caused by longitudinal expansion of growing shoot, from breaking (**Figure 2**).

The discovery of cell intrinsic chirality resulted from the fundamental question how the laterality of organs within the animal body is accomplished. Over the years much attention has been paid to this phenomenon and its connection with the development of L/R symmetry of the whole multicellular organism. It was found that blood neutrophiles polarity, defined by position of centrosomes with regard to the cell nucleus, makes them capable of directional movements in absence of polar external signals. This property disappears after application of drugs affecting MT function [45].

The model invertebrate organism *Drosophila melanogaster* provided evidence that myosin encoding gene mutation switches cell chirality and results in the development of *situs inversus* phenotype of the hindguts or genitalia [7, 8, 10, 46]. In vertebrates the development of typical L/R asymmetry from anteceding state of embryo bilateral symmetry is generated by various mechanisms. One of them is based on function of axonemal dynein. This motor-protein is responsible for appropriate beating of cilia in nodal epithelial cells, causing the directed ion current. Defects in the gene structure encoding for the dynein results in random selection of heart position in mouse embryo [47]. The involvement of C kinase signaling pathway in the reversal of cells chirality leading to mirrored position of heart was recently shown in chicken embryos [48].

All the above studies show how intrinsic cell polarity may be translated onto the higher level of multicellular organism organization in animals. Very little though is known about L/R symmetry regulation in multicellular plant organisms.
