ИСТИНА

POSITIONAL CONTROL OF PLANT TISSUE DIFFERENTIATION Vladimir V. Choob Lomonosov Moscow State University, Moscow, Russia choob_v@mail.ru Plant tissue differentiation in ontogeny follows precise spatial patterns. The procambial strand position is tightly associated with subsequent leaf and stem venation. Vascular bundles exhibit their polarity by formation of phloem and xylem in the proper spatial and temporal mode. Leaf trichomes and guard cells display visible proximal order with prohibition of new trichome or stomata formation in close neighborhood to each other. All these examples demonstrate that plant cells should differentiate according to various positional signals, emitting by singular cells or cell groups. The term of positional information was invented by Lewis Wolpert (1969), who was attempted to solve the problem of differentiation of identical cells in concordance with their position. His famous model of French flag postulates the existence of some source of positional signal (morphogen). The morphogen concentration should decrease as it diffuses through the field of cells. Several threshold levels of morphogen concentration define the developmental fate of every cell, thus depending on the positional signal. Differentiation of phloem and xylem from procambial strand could be wellcircumscribed by the French flag model. The cells of the presumptive protophloem excrete a small peptide of CLE-family (TDIF), which plays the role of the positional signal. The concentration of TDIF decreases with the distance from the protophloem pole. Non-differentiated procambial cells expose the membrane receptor PXY, specifically binding to CLE-peptide (Fisher, Turner, 2007). The high level of CLE-peptide TDIF causes phloem differentiation, medium level of TDIF leads to cambial cell formation, whereas low level / absence of TDIF induces xylem development (Etchells, Turner, 2010). Thus mutations in TDIF or PXY drive toward xylem overproduction and defects in phloem differentiation, while the hyper-expression of TDIF has the opposite effect of massive phloem development. Extracellular position signals in the form of small cysteine-rich peptides are used in stomata patterning. In Arabidopsis the meristemoid cells synthesize EPIDERMAL PATTERNING FACTORS 1 and 2 (EPF1, EPF2), inhibiting new meristemoid formation in close proximity to the existing ones. As a consequence of dysfunction of this positional signaling system, multiple clustered stomata develop in mutants too many mouth, encoding the subunit of the membrane EPF-receptor 36 complex, whereas the EPF1/2 overproduction is accompanied in low stomata density (Zoulias et al., 2018). Positional signals may be distributed within plant cells through plasmodesmata. The establishment of radial patterning in root requires the interplay of central cylinder and cortical cell signals. In Arabidopsis, transcription factor SHORT ROOT (SHR) is transferred from pericycle cell lineage to the presumptive endodermis, where it induces expression of transcription factor SCARECROW (SCR). Mutual effect of SHR and SCR drives the cells to endodermal differentiation. Rapid cessation of positional signal toward cortex parenchyma cells occur due to block of transfer from endodermal lineage to cortex parenchyma via plasmodesmata (Helariutta et al., 2000). The mutation in SCR gene causes complete loss of endodermis in roots and starch sheath layer in shoots. These observations render the role of SCR in radial patterning both in roots and shoots (Wysocka-Diller et al., 2000). Recently, the involvement of SHR–SCR system of positional signaling, exploiting the same principles of plasmodesmatal transfer, was postulated for mesophyll–bundle sheath differentiation during the establishment of Kranz-anatomy in C4 leaves (Slewinski et al., 2012; Fouracre et al., 2014). All the listed examples suggest the important role of positional information in plant tissue differentiation, despite the diversity of the underlying molecular mechanisms. Therefore, the positional signaling orchestrates the proper spatial organization of plant tissues, necessary to fulfill their physiological functions. References Etchells J.P., Turner S.R. 2010. The PXY–CLE41 receptor ligand pair defines a multifunctional pathway that controls the rate and orientation of vascular cell division. Development 137: 767–774. Fisher K., Turner S.R. 2007. PXY, a receptor-like kinase essential for maintaining polarity during plant vascular-tissue development. Curr. Biol. 17: 1061–1066. Fouracre J.P., Ando S., Langdale J.A. 2014. Cracking the kranz enigma with systems biology. J. Exp. Bot. 65: 3327–3339. Helariutta Y., Fukaki H., Wysocka-Diller J., Nakajima K., Jung J., Sena G., Hauser M.T., Benfey P.N. 2000. The SHORT-ROOT gene controls radial patterning of the Arabidopsis root through radial signaling. Cell 101: 555–567. Slewinski T.L., Anderson A.A., Zhang C., Turgeon R. 2012. Scarecrow plays a role in establishing kranz anatomy in maize leaves. Plant Cell Physiol. 53: 2030–2037. Wolpert L. 1969. Positional information and the spatial pattern of cellular differentiation. J. Theor. Biol. 25: 1–47. Wysocka-Diller J.W., Helariutta Y., Fukaki H., Malamy J.E., Benfey P.N. 2000. Molecular analysis of SCARECROW function reveals a radial patterning mechanism common to root and shoot. Development 127: 595–603. Zoulias N., Harrison E.L., Casson S.A., Gray J.E. 2018. Molecular control of stomatal development. Biochem. J. 475: 441–454.