**3. Chiral processes and structures in multicellular organisms**

The axiality of multicellular organism is similar to the polarity of a single cell but on hierarchically higher level of organization. The first evolutionarily step towards axiality of the integrated biological system composed of many cells represent 1D filaments of cyanobacteria, eukaryotic algae and fungi or moss protonemata. Higher plants maintain this ancestral condition as a kind of atavistic trait at the embryonic stage of their development - linear suspensor, which originates from the basal cell of already polarized and divided zygote, transports nutrients to the 3D globular embryo, developing from the apical cell [18]. Many forms of animals with such model organisms as tiny worm *Coenorhabditis elegans* [49] or fruit fly *Drosophila melanogaster* and finally ourselves, exhibit axiality, metamerism and L/R symmetry.

In this section the short survey of the most interesting cases of mirror symmetry in multicellular plants and animals will be made and the mechanisms that stay behind them will be discussed.

#### **3.1 Changing chirality in the thalli of charophytes**

Architecture of these green algae resembles that of the horsetails. The thallus of *Chara* is composed of the giant internodal cells typically enveloped by the sheet of cortical cells, which take an origin from the adjacent nodal cells. From the nodes the 1-st order branchlets grow out horizontally, on which the reproductive structures are positioned: ovaloid oogonia and spherical antheridia. They may be treated as the 2-nd order axial outgrowths of the branching thallus. The peculiar, to date unexplained transition takes place from the Z orientation of enveloping cortical cells in the main axis of the thallus, through their mostly parallel alignment in the branchlets, to the S orientation in oogonia (**Figure 3**). The developmental sequence of these changes in the chiral structure of extant *Chara* species is always the same, although gyrogonites (fossilized oogonia) of charophytes show that in the late Devonian period the right-handed (Z) oogonia were also present. They belonged to the charophyte family Trochiliscaceae and got extinct with the onset of Mesozoic

#### **Figure 3.**

*Details of* Chara *sp. thallus architecture. Left photo shows the main axis covered by Z oriented cortical cells; on the right photo two S chiral oogonia sit on the branchlets enveloped by the cortical cells, which run parallel to their axes.*

**169**

**Figure 4.**

*photo they are numbered decreasingly.*

*Mirror Symmetry of Life*

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

These problems remain unresolved.

oriented cell divisions.

era [50–52]. We will probably never know if they had the same sequence of chiral

The cortical cells of *Chara* parallel orientation of the cytoplasmic streaming in the central internodal cell they envelop. How do they read this direction? How is this information translated from the interface between ecto- and endoplasm, where the cellular engine of cyclosis is located [39, 53, 54], to the surface of the cell wall, on which the enveloping cells slide? Finally, how and why does it change in the *Chara* branching system, from the right in the main axis to the left in oogonia? Is actin – motor protein involved? Actin microfilaments must then have orientation of their alignment determined by the position of the cell in branching thallus. How?

The control over orientation of the cell division plane is of particular importance in plant tissues. Plant protoplasts are "imprisoned" within the boxes of their cell walls. They cannot migrate as freely as do the animal cells during embryonic stages of ontogenetic development. Morphogenesis of plants relies entirely on the properly

In the simple multicellular filaments of green algae we encounter for the first time the manifestation of L/R symmetry. The plane of cell divisions may be inclined relative to the filament axis either to the left or to the right as exemplified by the filamentous green alga *Coleochaete nitellarum* [55]. Also in planar (2D) gametophytes of ferns the pyramidal apical cell (AC) with rectangular base divides alter-

Precision in controlling chiral configuration of cell divisions is even more striking in 3D leafy gametophores of mosses. Their tetrahedral AC, watched from its triangular base, divides either CW or CCW and this direction is randomly established in the development of the gametophore main axis. However, it is not so in the case of its lateral branches – the chirality of their AC is always opposite to that of the supporting axis [5]. It is possible that this antidromic correlation triggers the horizontal gradient of some putative signals vertically transported from the neighboring leaves of gametophore (**Figure 5**). Two genes of the model moss *Physcomitrella patens* were identified to be engaged in a process of cell divisions in leafy gametophore: *PpTONE1* controlling intracellular organization of MT cyto-

*Typically heart shaped fern gametophyte. This planar structure develops due to activity of AC (red arrow) located atop of its symmetry axis and cleaving derivatives alternately to the left and to the right. On the right* 

changes, but *in reverso*, like the extant *Chara* branching thalli.

**3.2 Oriented cell divisions in apical cells of mosses and ferns**

nately to the left and to the right in a regular sequence (**Figure 4**).

skeleton and *PpNOG1* assuring proper development of AC [56, 57].

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

*Current Topics in Chirality - From Chemistry to Biology*

behind them will be discussed.

**3.1 Changing chirality in the thalli of charophytes**

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

The axiality of multicellular organism is similar to the polarity of a single cell but on hierarchically higher level of organization. The first evolutionarily step towards axiality of the integrated biological system composed of many cells represent 1D filaments of cyanobacteria, eukaryotic algae and fungi or moss protonemata. Higher plants maintain this ancestral condition as a kind of atavistic trait at the embryonic stage of their development - linear suspensor, which originates from the basal cell of already polarized and divided zygote, transports nutrients to the 3D globular embryo, developing from the apical cell [18]. Many forms of animals with such model organisms as tiny worm *Coenorhabditis elegans* [49] or fruit fly *Drosophila melanogaster* and finally ourselves, exhibit axiality, metamerism and L/R symmetry. In this section the short survey of the most interesting cases of mirror symmetry

in multicellular plants and animals will be made and the mechanisms that stay

Architecture of these green algae resembles that of the horsetails. The thallus of *Chara* is composed of the giant internodal cells typically enveloped by the sheet of cortical cells, which take an origin from the adjacent nodal cells. From the nodes the 1-st order branchlets grow out horizontally, on which the reproductive structures are positioned: ovaloid oogonia and spherical antheridia. They may be treated as the 2-nd order axial outgrowths of the branching thallus. The peculiar, to date unexplained transition takes place from the Z orientation of enveloping cortical cells in the main axis of the thallus, through their mostly parallel alignment in the branchlets, to the S orientation in oogonia (**Figure 3**). The developmental sequence of these changes in the chiral structure of extant *Chara* species is always the same, although gyrogonites (fossilized oogonia) of charophytes show that in the late Devonian period the right-handed (Z) oogonia were also present. They belonged to the charophyte family Trochiliscaceae and got extinct with the onset of Mesozoic

*Details of* Chara *sp. thallus architecture. Left photo shows the main axis covered by Z oriented cortical cells; on the right photo two S chiral oogonia sit on the branchlets enveloped by the cortical cells, which run parallel to* 

known about L/R symmetry regulation in multicellular plant organisms.

**3. Chiral processes and structures in multicellular organisms**

**168**

**Figure 3.**

*their axes.*

era [50–52]. We will probably never know if they had the same sequence of chiral changes, but *in reverso*, like the extant *Chara* branching thalli.

The cortical cells of *Chara* parallel orientation of the cytoplasmic streaming in the central internodal cell they envelop. How do they read this direction? How is this information translated from the interface between ecto- and endoplasm, where the cellular engine of cyclosis is located [39, 53, 54], to the surface of the cell wall, on which the enveloping cells slide? Finally, how and why does it change in the *Chara* branching system, from the right in the main axis to the left in oogonia? Is actin – motor protein involved? Actin microfilaments must then have orientation of their alignment determined by the position of the cell in branching thallus. How? These problems remain unresolved.

### **3.2 Oriented cell divisions in apical cells of mosses and ferns**

The control over orientation of the cell division plane is of particular importance in plant tissues. Plant protoplasts are "imprisoned" within the boxes of their cell walls. They cannot migrate as freely as do the animal cells during embryonic stages of ontogenetic development. Morphogenesis of plants relies entirely on the properly oriented cell divisions.

In the simple multicellular filaments of green algae we encounter for the first time the manifestation of L/R symmetry. The plane of cell divisions may be inclined relative to the filament axis either to the left or to the right as exemplified by the filamentous green alga *Coleochaete nitellarum* [55]. Also in planar (2D) gametophytes of ferns the pyramidal apical cell (AC) with rectangular base divides alternately to the left and to the right in a regular sequence (**Figure 4**).

Precision in controlling chiral configuration of cell divisions is even more striking in 3D leafy gametophores of mosses. Their tetrahedral AC, watched from its triangular base, divides either CW or CCW and this direction is randomly established in the development of the gametophore main axis. However, it is not so in the case of its lateral branches – the chirality of their AC is always opposite to that of the supporting axis [5]. It is possible that this antidromic correlation triggers the horizontal gradient of some putative signals vertically transported from the neighboring leaves of gametophore (**Figure 5**). Two genes of the model moss *Physcomitrella patens* were identified to be engaged in a process of cell divisions in leafy gametophore: *PpTONE1* controlling intracellular organization of MT cytoskeleton and *PpNOG1* assuring proper development of AC [56, 57].

#### **Figure 4.**

*Typically heart shaped fern gametophyte. This planar structure develops due to activity of AC (red arrow) located atop of its symmetry axis and cleaving derivatives alternately to the left and to the right. On the right photo they are numbered decreasingly.*

#### **Figure 5.**

*Scheme of the split open surface of cylindrical moss leafy gametophore. It explains horizontal gradient of the hypothetical signal (green) originating from the leaf segments cleaved CCW (upper red arrow) by the tetrahedral AC of the main axis. The lateral branch AC (red), reading the gradient, starts cleaving its segments CW (upper blue arrow). Main axis segments are numbered increasingly.*

#### **3.3 "Music of trees"**

Anticlinal, pseudotransverse divisions of elongated stem cells in cambium, cylindrical meristem located in trees between the outer bark and the inner secondary xylem, are chiral. Their partitions, while watched from outside of the tree, are inclined to the right (Z divisions) or to the left (S divisions). Cambium therefore is a plant tissue that exhibits clear L/R symmetry. Also subsequent growth of the cells shortened by the divisions is oriented. Cambial cellular events are not randomly distributed over the surface of the meristem but orderly segregated into domains of opposite chirality. The S and Z domains alternate along the vertical axis of the cambial cylinder [58, 59]. This leads to emergence of structural waviness within which the cells assume alternately the opposite S and Z orientation. Because cambial structure is replicated every year in the annual wood increment, the history of all these developmental changes is recorded in the wood and may be extracted for the periods equaling the age of a tree. The domain pattern and resulting structural waviness are propagated vertically in cambium thus the cells in a particular location undergo the cycle of inclination change (**Figure 6**). This barely known biological rhythm is the longest in nature. Its period approximates 20 years although in some cases may be shorter. Theoretical model predicts that propagation of cell oscillations associated with the domain pattern motion may lead to development of the spiral grain in a tree trunk [14]. Handedness of the spiral grain should depend, according to the model, on the nature of the wave front i.e. the direction of the first change in stem cells inclination during the initial period (**Figure 7**).

Neither the molecular mechanisms, nor the nature of domain positional information for dividing and growing cells have been elucidated so far. The first suspect

**171**

**3.4 Twinning vines**

*Mirror Symmetry of Life*

**Figure 6.**

**Figure 7.**

*right photo.*

*on the right side of the photo.*

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

is the polar auxin transport changing directions due to redistribution of the hormone carriers in plasma membrane of cambial stem cells. The intrinsic cell chirality resulting from unknown nature of the intracellular oscillator cannot be excluded. In one tree more than one domain pattern may be present – they usually differ in the domain size and propagation velocity. According to the hypothesis put forward by their discoverer, the domain patterns result from morphogenetic waves traveling in the tissue and capable of superposition [59]. This means that the trees play silent

*Scheme explaining how morphogenetic wave propagated upward and exciting oscillations of cambial cells may lead to development of spiral grain in such trees as majestic sugi tree (*Cryptomeria japonica*) on the* 

*Scheme of the relationship between hypothetical cambial morphogenetic wave and the domain pattern composed of S (blue) and Z (red) domains. They lead to development of wavy cambium and subsequently to wavy wood. (A) Tangential face of the beech wood, (B) radial split face of the oak wood showing dynamics of the wavy pattern: inclined ripples indicate upward movement of the structural waviness in cambium - bark is* 

The helical growth of plant organs is not uncommon [15]. Some plants developed quite effective strategy to grow quickly towards the light source relying on the support, provided sometimes even by another plant. This way they do not have to spend too much energy for building sturdy skeleton composed of mechanical tissues. Finding support is possible thanks to circumnutation of the shoot tip, caused by differential growth along the circumference [60]. Some vines are heterochiral, i.e. capable of twinning CW and CCW. Around 90% of the homochiral species twirl CCW [61, 62] but in some genera the direction of twinning is a

music, the beauty of which is mostly unknown even to the scientists.

#### **Figure 6.**

*Current Topics in Chirality - From Chemistry to Biology*

**170**

inclination during the initial period (**Figure 7**).

**3.3 "Music of trees"**

**Figure 5.**

Anticlinal, pseudotransverse divisions of elongated stem cells in cambium, cylindrical meristem located in trees between the outer bark and the inner secondary xylem, are chiral. Their partitions, while watched from outside of the tree, are inclined to the right (Z divisions) or to the left (S divisions). Cambium therefore is a plant tissue that exhibits clear L/R symmetry. Also subsequent growth of the cells shortened by the divisions is oriented. Cambial cellular events are not randomly distributed over the surface of the meristem but orderly segregated into domains of opposite chirality. The S and Z domains alternate along the vertical axis of the cambial cylinder [58, 59]. This leads to emergence of structural waviness within which the cells assume alternately the opposite S and Z orientation. Because cambial structure is replicated every year in the annual wood increment, the history of all these developmental changes is recorded in the wood and may be extracted for the periods equaling the age of a tree. The domain pattern and resulting structural waviness are propagated vertically in cambium thus the cells in a particular location undergo the cycle of inclination change (**Figure 6**). This barely known biological rhythm is the longest in nature. Its period approximates 20 years although in some cases may be shorter. Theoretical model predicts that propagation of cell oscillations associated with the domain pattern motion may lead to development of the spiral grain in a tree trunk [14]. Handedness of the spiral grain should depend, according to the model, on the nature of the wave front i.e. the direction of the first change in stem cells

*Scheme of the split open surface of cylindrical moss leafy gametophore. It explains horizontal gradient of the hypothetical signal (green) originating from the leaf segments cleaved CCW (upper red arrow) by the tetrahedral AC of the main axis. The lateral branch AC (red), reading the gradient, starts cleaving its segments* 

*CW (upper blue arrow). Main axis segments are numbered increasingly.*

Neither the molecular mechanisms, nor the nature of domain positional information for dividing and growing cells have been elucidated so far. The first suspect *Scheme of the relationship between hypothetical cambial morphogenetic wave and the domain pattern composed of S (blue) and Z (red) domains. They lead to development of wavy cambium and subsequently to wavy wood. (A) Tangential face of the beech wood, (B) radial split face of the oak wood showing dynamics of the wavy pattern: inclined ripples indicate upward movement of the structural waviness in cambium - bark is on the right side of the photo.*

#### **Figure 7.**

*Scheme explaining how morphogenetic wave propagated upward and exciting oscillations of cambial cells may lead to development of spiral grain in such trees as majestic sugi tree (*Cryptomeria japonica*) on the right photo.*

is the polar auxin transport changing directions due to redistribution of the hormone carriers in plasma membrane of cambial stem cells. The intrinsic cell chirality resulting from unknown nature of the intracellular oscillator cannot be excluded. In one tree more than one domain pattern may be present – they usually differ in the domain size and propagation velocity. According to the hypothesis put forward by their discoverer, the domain patterns result from morphogenetic waves traveling in the tissue and capable of superposition [59]. This means that the trees play silent music, the beauty of which is mostly unknown even to the scientists.

## **3.4 Twinning vines**

The helical growth of plant organs is not uncommon [15]. Some plants developed quite effective strategy to grow quickly towards the light source relying on the support, provided sometimes even by another plant. This way they do not have to spend too much energy for building sturdy skeleton composed of mechanical tissues. Finding support is possible thanks to circumnutation of the shoot tip, caused by differential growth along the circumference [60]. Some vines are heterochiral, i.e. capable of twinning CW and CCW. Around 90% of the homochiral species twirl CCW [61, 62] but in some genera the direction of twinning is a

species specific trait. *Wisteria sinensis* and Japanese *Wisteria floribunda,* both the attractive ornamental vines, are opposite in this respect. Darwin, who was very interested in twinning plants and made observations on *W. sinensis* stated: "*I have seen no instance of two species of the same genus twinning in opposite directions, and such cases must be rare"* [61]. He could say so because in his times Japanese *Wisteria* has not yet been introduced to England.

The confusing descriptions of *Wisteria* in botanical literature [63] show clearly that there is a certain problem with definition of a chiral configuration of structures or processes with mirror symmetry in biological systems. *W. sinensis* ascending shoot twins CW when looked at from its base and CCW when looked at from above. The same configuration of this plant twining, in some sources is claimed to be CCW [64] in others CW [65]. In Darwin's words the plant "*moves against the sun*" [61]. Compton and Lack [63] claim that *W. floribunda*:" … has climbing woody stems twining from left to right…", which should not be if it is truly opposite to *W. sinensis*. It all shows the importance of clear convention how the chiral configuration is determined. Moving along the S helix upward is a CW motion whereas descending along the same line we move CCW. While looked at from outside the *W. sinensis* twins from the left to right (Z configuration). It is opposite (S configuration) when looked at from the inside of the growing shoot's helical structure. The same necessity of defining chiral configuration according to specific convention applies to the cellulose microfibrils rotated in the layers of the plant cell secondary wall, to the spiral grain in a tree trunk or to the cells enveloping charophyte oogonia. It seems that definition of the helix chiral configuration, looked at from its outside, as being S or Z is the most reasonable and unequivocal.

Molecular mechanism responsible for the direction of plant climbers twinning is not known. The results of the studies on the *lefty* and *spiral* mutants of the model plant *Arabidopsis thaliana* [40–42] suggest the involvement of the genetic factor. It is possible that the species specific behavior depends on distinct and constitutive gene expression patterns established differently for each species.

#### **3.5 Aestivation**

The petal folding in a flower bud, in most of the flowering plants, is clearly chiral. Petals overlap either CCW or CW and this chiral configuration is often later maintained in fully developed flower (**Figure 8**). The direction of petals

#### **Figure 8.**

*The chiral CW folding (aestivation) of petals in Hawaiian plumeria's flower bud (left) is maintained in a pinwheel-like corolla of an open flower (right).*

**173**

**Figure 9.**

*numbers and orientations shown below.*

*Mirror Symmetry of Life*

individual plant.

**3.6 Phyllotaxis**

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

folding may be, like in the case of circumnutation, the species specific trait*.* For instance*, Anagallis arvensis* petals always twist CCW, whereas in Hawaiian plumerias they do it otherwise. Common European weed *Malva neglecta* in turn is heterochiral, capable of producing CW and CCW buds on the same

Among best known chiral phenomena and investigated since the ancient times [66] is helical phyllotaxis – the regular distribution of lateral organs such as leaves or flowers on a plant shoot. Their consecutive primordia, circumferentially equidistant, emerge on the vertically growing shoot apex in the regular intervals. The primordia may be connected with an imaginary line called ontogenetic helix. The helix S or Z configuration depends on whether the process of primordia initiation proceeds CW or CCW. The plantlets growing from seeds have this configuration established at random in the main axis. It is not so, however, in the axes of lateral branches. Their ontogenetic helix may be either concordant (a homodromy case) or discordant (an antidromy case) with that of the supporting axis. It has been found, that even when both phyllotactic correlations occur with the same frequency [67] the supporting axis and the laterals may have the same chirality of vascular sympodia - elements of the

The sympodia follow the course of one set of superficial secondary helices - phyllotactic prastichies. Two sets of parastichies running in opposite directions constitute a phyllotactic lattice. This is why even when ontogenetic helices in two axes making up one branching unit are discordant, the axes still may be concordant on the level of their vasculature. The numbers of parastichies in the sets of opposite chiral configuration belong to the mathematical series, the quality of which is associated with the size of circumferential distance between successive primordia. This distance, usually given in an angular measure, is known as divergence angle. The most common is the main Fibonacci series (1,1,2,3,5,8,13…) present in the system with the divergence angle approximating 137,5 degrees or Lucas series (1,3,4,7,11 …) with the angle close to 99,5 degrees. There are also many other divergencies and phyllotactic patterns [68].

*Scheme on the left shows how, in the laterals of one coniferous branching shoot, ontogenetic helix may be either S (blue) or Z (red) but orientation of vascular sympodia the same in the whole system. The sympodia chiral configuration depends on their number, which is one of the mathematical series shown below the scheme. H- homodromic, A - antidromic correlations of chiral configurations. Upper right photo shows the righthanded and lefthanded whirls of needles in two coniferous shoots with the same S Fibonacci phyllotaxis. Their opposite chiral configurations, resulting likely from growing shoot rotation, are caused by the different sympodia* 

internal transport system strongly related to phyllotaxis (**Figure 9**).

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

folding may be, like in the case of circumnutation, the species specific trait*.* For instance*, Anagallis arvensis* petals always twist CCW, whereas in Hawaiian plumerias they do it otherwise. Common European weed *Malva neglecta* in turn is heterochiral, capable of producing CW and CCW buds on the same individual plant.
