Available online at www.sciencedirect.corn Current Opinion in
%%;'ScienceDirect Plant Biology
ELSEVIER
Cell-to-cell communication in vascular morphogenesis
Satu J Lehesranta, Raffael Lichtenberger and Yka Helariutta
The plant vascular system consists of two conductive cell types, xylem and phloem, which are both produced by procambial cells. Recently, several novel regulatory mechanisms that control the specification of vascular patterning and differentiation have been uncovered. The non-cell-autonomous TDIF/CLE signalling mediates phloem-xylem cross-talk and cambial maintenance; a flowering-related long-distance signal governs secondary development; and novel genetic players such as LHW regulate vascular morphogenesis. A future challenge is to conflate data on the various genetic, hormonal and other factors to understand the networks underlying vascular tissue formation.
Address
Department of Biological and Environmental Sciences/Institute of Biotechnology, University of Helsinki, P.O. Box 65, 00014 Helsinki, Finland
Corresponding author: Helariutta, Yka (yrjo.helariutta@helsinki.fi)
Current Opinion in Plant Biology 2010, 13:59-65
This review comes from a themed issue on Growth and Development
Edited by Dominique C. Bergmann and Andrew J. Fleming Available online 25th September 2009
1369-5266/$ - see front matter
© 2009 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.pbi.2009.09.004
Introduction
The plant vascular tissues form bundles that connect all parts of the plant and serve two important functions: they give mechanical support, and distribute water and nutrients as well as other substances needed for growth and defence. Individual vascular bundles consist of two differentiated, highly specialised conductive tissue types, xylem and phloem, and intervening procambial cells of pluripotent nature that can differentiate into both conductive cell types (Figure 1). Developmental decisions require precise spatial and temporal organisation to produce continuous vascular strands.
Novel genomic and molecular techniques using several model plant systems (such as Arabidopsis, Populus and Zinnia) have recently provided insights into the regulation of vascular development. Signals controlling aspects ofvascular morphogenesis include hormones (auxin, cyto-kinins and brassinosteroids), other small regulatory mol-
ecules, their transporters, receptors, and various temporal and spatial regulators of gene expression. Several recent studies suggest that the control of xylem and phloem development is highly integrated. Many of the regulatory factors have been shown to act on cell autonomously; however, an emerging theme in these processes is the movement of signals controlling vascular development and cross-talk between cell types and tissues.
This review will focus on the emerging concepts of how these components and long-range and short-range signals might function to set up a regulatory network governing vascular development.
Formation of vascular strands
Continuous procambial strands need to be established before vasculature differentiates. This has long been thought to be caused by the accumulation of the phyto-hormone auxin through polar transport mechanisms [1,2]. Indeed, expression of the synthetic auxin reporter DR5 and the auxin-induced pre-procambial marker AtHB8 precedes formation of vascular strands in leaves [3]. To achieve this, auxin is channelled to the provascular regions by the auxin transporter PIN1, which is already expressed before procambium formation (Figure 2a).
Radial patterning and maintenance of proliferation
Besides the formation of vascular strands, auxin may also play a role in radial patterning ofvascular bundles, such as the differentiation of adaxial and abaxial sides of the bundle. The radial pattern is initially set up during embryogenesis by the main agents of radial patterning, KANADI (KAN) and class III HD-ZIP (PHABULOSA [PHB], PHAVOLUTA [PHV], REVOLUTA [REV] and CORONA [CNA]) transcription factors [4-6]. The KAN signalling and auxin transport pathways appear to integrate, as PIN1 localisation is affected in kan mutants [7]. Conversely, auxin is likely to influence the spatial and temporal expression patterns of these transcription factors. The triple mutant phb phv rev has radialised, abax-ialised leaves and vascular bundles [8](Figure 2b). In contrast, gain-of-function HD-ZIP III mutants with faulty microRNA (miRNA) regulation and kan1 kan2 kan3 have radialised, adaxialised leaves and bundles. In addition, the HD-ZIP III genes also appear to regulate vascular tissue proliferation [9].
Another important hormone, cytokinin (CK), is required for vascular patterning and the differentiation of all cell except protoxylem. Mutants with impaired CK signalling, such as wooden leg (wol) and the triple CK receptor mutant
www.sciencedirect.com
Current Opinion in Plant Biology 2010, 13:59-65
60  Growth and Development
Figure 1
Photo-synthetic products
Water, nutrients
Current Opinion in Plant Biology
A schematic outline showing the basic vascular patterning, (a) Cross-section of a developing leaf, Vascular bundles develop as pre-procambial strands. (b) Procambial cells give rise to phloem (abaxial side of the leaf) and xylem (adaxial side) that differentiate asymmetrically within the vascular strands. (c) Arrangement of vascular strands within the stem. (d) During the secondary phase of vascular development in the stem and root, cambial cells form a cylindrical meristem that produces secondary xylem and phloem. (e) Cross-section of a root tip shows the structure of primary vascular tissue within the stele.
ahk2 ahk3 ahk4 (for ARABIDOPSIS HISTIDINE KINASE), or with depleted levels of CK, such as transgenic lines overexpressing CYTOKININ OXIDASE (CKX), all lead to an increased number of protoxylem cell files and loss of other cell types in the root vasculature [10,11]. The CK signalling inhibitor ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6 (AHP6) restricts the domain of CK activity and thereby allows for protoxylem differentiation in a spatially specific manner. CK also appears to be important for the maintenance and proliferation of cambial cells [12,13](Figure 2c).
Having fewer cells sometimes leads to altered cell type ratios, sometimes not; this relationship is not currently understood. Recently, the bHLH-domain-containing protein LONESOME HIGHWAY (LHW) has been identified as a key factor in establishing bilateral symmetry [14]. LHW is required to promote proliferation of cell files in the stele and to maintain longitudinal growth. Whilst Ihw mutants still show distinct cell types, they have fewer cells in the centre of the root. This leads to roots with only single xylem and phloem poles. It remains to be seen whether it relates to CK signalling or other known mechanisms.
Also, the brassinosteroid (BR) class of plant hormones are involved in long-distance signalling to establish the vas-culature [15]. Mutations in the BR receptor BR-INSEN-SITIVE 1 (BRI1) cause an abnormal ratio in the differentiation of phloem to xylem cells, with a disproportionately high number of phloem cells.
Differentiation of vascular cells
Besides AHP6 which promotes protoxylem identity [11], several genes have been described to be specifically involved in specifying xylem or phloem identity. These genes primarily encode transcription factors, but putative signalling proteins and cell death regulators also play a role. It is currently not known what regulates the specific expression of these genes.
Fukuda [16] has previously proposed a temporal model where the balance of auxin and cytokinin is important to specify procambial cells that will adopt xylem fate. Cyto-kinin signalling must be counteracted early by AHP6 to allow for protoxylem differentiation [11]. Downstream of these factors, the differentiation ofxylem cells is conferred by the action of a transcriptional network, the first members of which were described as the VASCULAR-RELATED NAC-DOMAIN (VND) transcription factors VND6 and VND7 [17]. VND6 promotes metaxylem identity, whereas VND7 promotes protoxylem identity. Our understanding of the xylem transcriptional network has been expanded to include a cascade involving several MYB and NAC transcription factors [18*,19*]. Furthermore, the spermine biosynthesis gene ACAULIS5 (ACL5) controls xylem specification. In ad5 mutant, fibre elements and metaxylem vessel elements fail to differentiate [20*]. This is due to premature cell death, which is normally inhibited by ACL5. Vascular continuity is mediated by xylogen, a small proteoglycan-like protein that is secreted by immature tracheary elements [21]. Mutants lacking xylogen fail to form continuous xylem strands.
The only currently known factor for phloem specification is ALTERED PHLOEM DEVELOPMENT (APL)[22]. This MYB-coiled-coil transcription factor is essential for the proper differentiation of companion cells and
Current Opinion in Plant Biology 2010, 13:59-65
www.sciencedirect.com
Cell-to-cell communication in vascular morphogenesis Lehesranta, Lichtenberger and Helariutta 61
Figure 2
Signalling and genetic components during different stages of vascular tissue development. (a) Model for polar auxin transport visualises vascular bundle formation in the leaf midvein (adapted from [28"]). First, polar localisation of PIN1 (red) directs auxin flow (arrows; auxin concentration shown in blue) in the surface layer of the meristem (green cells) to form an auxin maximum. Subsequently, auxin flux is directed from this maximum towards inner tissue. A procambial strand forms at the site of the auxin flux. (b) The HD-ZIP III and KANADI genes are important regulators of vascular patterning and have an antagonistic relationship. In shoot vascular bundles, they determine the adaxial and abaxial localisation of xylem and phloem, respectively. In phb phv rev triple mutants, phloem surrounds xylem, and in gain-of-function HD-ZIP III mutants or kan1 kan2 kan3 triple mutants, xylem surrounds phloem. HD-ZIP III genes are regulated by miR165/166 and apparently interact with polar auxin transport and brassinosteroids. (c) Cambium, the vascular meristem, produces both xylem and phloem cells. Polar auxin transport and cytokinins have been suggested to regulate cambial activity. A currently unknown flowering-related signal from the shoot is required for secondary cambial activity. (d) A model according to Hirakawa et a/.[37""] suggests that a CLE receptor/ligand system maintains cambial cell activity in a non-cell-autonomous manner. The TDIF/CLE41/44 peptide is secreted from the phloem and perceived by the receptor PXY/TDR in procambial cells. This signal promotes proliferation of cambium and inhibits its differentiation into xylem.
www.sciencedirect.com
Current Opinion in Plant Biology 2010, 13:59-65
62  Growth and Development
sieve elements, and contributes to spatially limiting the differentiation of xylem.
Long-range and short-range signalling molecules are involved in vascular development
Results from several recent studies suggest that the control of differentiation of xylem and phloem requires short-range and long-range intercellular transport of signalling molecules.
Auxin has been shown to regulate a number of key developmental events such as organisation of meris-tems.Thisispossiblebecause short-range morphogenic auxin gradients are regulated by intricate mechanisms to provide positional information; for an in-depth update on polar auxin transport, we refer to recent reviews [23,24]. The asymmetric localisation of PIN proteins directs auxin within tissues [2,25,26]. This polarity and the fact that auxin can redirect PIN1 transporter in pre-procambial cells explain how auxin flow can channel and amplify itself in a self-organising process to establish a new auxin maximum [3,27] (Figure 2a). Computer simulations give further support to this canalisation hypothesis during leaf midvein formation [28*]. Thereisalsoevidencefor thelong-range transport of auxin in the phloem from source to sink tissues [29-31]. Mechanisms for short-range or long-range transport of other hormones have not yet been described. Cytokinin probably acts both as a local and a long-distance signal and may be transported in the vasculature — by both phloem and xylem — but the mechanisms involved are so far poorly characterised
[32].
Short-range signalling by TDIF/CLE41/44 and PXY mediates phloem-xylem cross-talk
The control of stem cell maintenance in the shoot apical meristem of Arabidopsis involves an interaction between the CLV3 peptide and receptor components CLV1 and CLV2. The CLV pathway induces downstream components that limit the expression domain of the WOX family homeodomain transcription factor WUSCHEL; WUS acts non-cell-autonomously to maintain stem cell identity and promote CLV3 expression [33]. Another feedback regulon maintains the root apical meristem and involves the peptide CLE40 and the WUS-related WOX5 [34*,35].
A similar cross-talk model between phloem and xylem has been suggested as the regulator for vascular patterning. The CLE peptide TDIF was originally identified in the Zinnia cell culture system and shown to be a dodecapeptide that could suppress the differentiation of cultured cells to tracheary elements at very low levels but did not inhibit meristem function in plants [36]. TDIF inhibits xylem differentiation and increases
proliferation of procambial cells in leaves in planta [37**]. These results suggested that a CLV-like signalling mechanism operates in the vasculature. More evidence for this kind of regulon came from the identification of PXY, a CLV-like receptor kinase which is required for normal cambial cell divisions and spatial organisation of xylem and phloem in Arabidopsis [38**]. In pxy mutants (for phloem intercalated with xylem), the organisation of vascular bundles in stems is severely disrupted in both transverse and longitudinal directions: cell divisions occur irregularly, and xylem and phloem are no longer neatly separated. The model for the functions of these two genes came together when TDIF/CLE41/44/42 was shown to act as ligand for PXY, also named TDR for putative TDIF receptor [37**]. The direct binding of TDIF was demonstrated by a biochemical approach, where photoaffinity labelling was used on recombinant TDR produced in tobacco cells. CLE41/44 encoded peptides are secreted from the phloem and bind their receptor in the cambium (Figure 2d). Combined, these data strongly suggest that the CLE receptor/ligand system maintains cambialstemcellactivityandregulatesitsdifferen-tiation into xylem. Whether there is a downstream factor similar to WUS that would constitute a complete regulatory loop remains to be seen.
miRNAs on the move?
Small RNAs can influence many aspects of gene regulation. Whilst the well-characterised classes of small interfering RNAs (siRNAs) act mainly in defence, miR-NAs and trans-acting RNAs have important functions in growth and development by regulating endogenous target genes. For instance, miR165/166 has been shown to regulate HD-ZIPIII patterning [39,40](Figure 2b). Is it possible that these miRNAs would provide positional information by moving short distances? There is plenty of evidence of short-range and systemic movement of siRNAs [41] but miRNAs have been considered to function cell autonomously [42]. Various miRNAs, including miR165/166, are present in phloem [43*], although it is debatable whether they could act as long-range developmental signals [44]. It has been suggested that short-range miRNA movement could have important developmental consequences [45*] but direct functional evidence for miRNA movement remains to be demonstrated.
Flowering triggers secondary development
An interesting concept for the long-range control of vascular patterning comes from work on natural variation. There is a transition of secondary growth in Arabidopsis hypocotyl and roots that leads to expansion of xylem and fibre differentiation, reminiscent of trunk formation in trees [46**]. Analyses of quantitative trait loci analyses revealed FLOWERING LOCUS C as the major locus controlling this transition, suggesting that flowering
Current Opinion in Plant Biology 2010, 13:59-65
www.sciencedirect.com
Cell-to-cell communication in vascular morphogenesis Lehesranta, Lichtenberger and Helariutta 63
induction is required for the onset of xylem expansion (Figure 2c). This implicates a flowering-related signal from the shoot but it is not currently known what this signal might be.
How do molecules move between cells?
Xylem and phloem are the primary conduits of long-distance transport within the plant. Furthermore, short-range non-cell-autonomous developmental signals such as transcription factor movement are well established in plants. Plasmodesmata are probably the main mediators for sym-plastic transport of non-cell-autonomous transcription factors and small RNAs [47] and especially important for phloem as they regulate the loading of molecules into the phloem stream (reviewed in [48]). Phloem sieve elements are connected to each other by sieve plate pores, plasmodesmatal complexes, the functions of which are currently poorly known. Furthermore, plasmodesmata can fine-tune cell-to-cell trafficking by setting size exclusion limits [49]. Because of this complexity and subtle regulation, novel mutants with defects in plasmodesmata may shed light on their specific roles in phloem continuity, differentiation and/or transport function. Few have so far been described, but recently, a family of novel plasmo-desmata-located proteins that affect cell-to-cell communication has been identified [50*]. Another mutant with increased callose deposition and decreased plasmodesmal transport has severe defects in development and phloem unloading [51*].
Protein movement may in some cases also involve targeted regulated transport. SHORT-ROOT (SHR), an important transcription factor for ground tissue patterning, is also implicated in vascular development [52]. SHR is expressed in vascular tissues in the root but moves into the endo-dermis, where it promotes endodermal cell identity. SHR requires both cytoplasmic and nuclear localisation in order to be transported in cell-specific manner, and therefore this is unlikely to occur simply through diffusion [53*]. The TDIF/CLE41/42/44 peptide is likely to be secreted and distributed in the apoplast [37**].
Conclusions
Many different factors involved in vascular development have been described, but so far there has been relatively little knowledge on how they interact and regulate each other and how positional information is provided. Morphogenesis requires the integration of a multitude of signals and gene functions coming from outside and within the vascular system. Recently, novel concepts of signalling between cells and tissues such as the TDIF/ CLE regulon mediating phloem-xylem cross-talk and miRNA movement have been proposed. In the future, it will be of great interest to see how these signalling networks fit in with other known players and how cell-to-cell communication occurs to produce this complex system.
Acknowledgements
We would like to thank Dr Anthony Bishopp for a critical reading of the manuscript. Financial support was provided the Academy of Finland, Tekes and the European Science Foundation.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
* of special interest
** of outstanding interest
1. Sachs T: The control of the patterned differentiation of vascular tissues. Adv Bot Res 1981, 9:151-262.
2. Friml J, Yang X: A PINOID-dependent binary switch in apical-basal PIN polar targeting directs auxin efflux.
Science 2004, 306:862-865.
3. Scarpella E, Marcos D, Friml J, Berleth T: Control of leaf vascular patterning by polar auxin transport. Genes Dev 2006, 20:1015-1027.
4. McConnell JR, Barton MK: Leaf polarity and meristem formation in Arabidopsis. Development 1998, 125:2935-2942.
5. McConnell JR, Emery J, Eshed Y, Bao N, Bowman J, Barton MK: Role of PHABULOSA and PHAVOLUTA in determining radial patterning in shoots. Nature 2001, 411:709-713.
6. Kerstetter RA, Bollman K, Taylor RA, Bomblies K, Poethig RS: KANADI regulates organ polarity in Arabidopsis. Nature 2001,
411 :706-709.
7. Izhaki A, Bowman JL: KANADI and class III HD-Zip gene families regulate embryo patterning and modulate auxin flow during embryogenesis in Arabidopsis. Plant Cell 2007,
19:495-508.
8. Emery JF, Floyd SK, Alvarez J, Eshed Y, Hawker NP, Izhaki A, Baum SF, Bowman JL: Radial patterning of Arabidopsis shoots by class III HD-ZIP and KANADI genes. CurrBiol 2003, 13:1768-1774.
9. Prigge MJ, Otsuga D, Alonso JM, Ecker JR, Drews GN, Clark SE: Class III homeodomain-leucine zipper gene family members have overlapping, antagonistic, and distinct roles in Arabidopsis development. Plant Cell 2005, 17:61-76.
10. Mahonen AP, Bonke M, Kauppinen L, Riikonen M, Benfey N, Helariutta Y: A novel two-component hybrid molecule regulates vascular morphogenesis of the Arabidopsis root.
Genes Dev 2000, 14:2938-2943.
11. Mahonen AP, Bishopp A, Higuchi M, Nieminen KM, Kinoshita K, Tormakangas K, Ikeda Y, Oka A, Kakimoto T, Helariutta Y: Cytokinin signaling and its inhibitor AHP6 regulate cell fate during vascular development. Science 2006, 311:94-98.
12. Matsumoto-Kitano M, Kusumoto T, Tarkowski P, Kinoshita-Tsujimura K, Vaclavikova K, Miyawaki K, Kakimoto T: Cytokinins are central regulators of cambial activity. Proc Natl Acad Sci U S A 2008, 105:20027-20031.
13. Nieminen K, Immanen J, Laxell M, Kauppinen L, Tarkowski P, Dolezal K, Tahtiharju S, Elo A, Decourteix M, Ljung K et al.: Cytokinin signaling regulates cambial development in poplar.
Proc Natl Acad Sci USA 2008, 105:20032-20037.
14. Ohashi-Ito K, Bergmann DC: Regulation of the Arabidopsis root vascular initial population by LONESOME HIGHWAY. Development 2007, 134:2959-2968.
15. Cano-Delgado A, Yin Y, Yu C, Vafeados D, Mora-Garcia S, Cheng J-C, Nam KH, Li J, Chory J: BRL1 and BRL3 are novel brassinosteroid receptors that function in vascular differentiation in Arabidopsis. Development 2004,131:5341-5351.
16. Fukuda H: Signals that control plant vascular cell differentiation. Nat Rev Mol Cell Biol 2004, 5:379-391.
17. Kubo M, Udagawa M, Nishikubo N, Horiguchi G, Yamaguchi M, Ito J, Mimura T, Fukuda H, Demura T: Transcription switches for
www.sciencedirect.com
Current Opinion in Plant Biology 2010, 13:59-65
64  Growth and Development
protoxylem and metaxylem vessel formation. Genes Dev 2005, 19:1855-1860.
18. Zhong R, Lee C, Zhou J, McCarthy RL, Ye ZH: A battery of
• transcription factors involved in the regulation of secondary cell wall biosynthesis in Arabidopsis. Plant Cell 2008, 20:2763-2782.
19. Zhou J, Lee C, Zhong R, Ye ZH: MYB58 and MYB63 are
• transcriptional activators of the lignin biosynthetic pathway during secondary cell wall formation in Arabidopsis.
Plant Cell 2009, 21:248-266.
These two papers describe a network of MYB, NAC and other transcription factors that are involved in secondary cell wall formation important for xylem cell development.
20. Muiniz L, Minguet EG, Singh SK, Pesquet E, Vera-Sirera F, Moreau-
• Courtois CL, Carbonell J, Blazquez MA, Tuominen H: ACAULIS5 controls Arabidopsis xylem specification through the prevention of premature cell death. Development 2008, 135:2573-2582.
The putative spermine biosynthetic gene ACAULIS5 is required for proper specification of late-developing pitted xylem vessel elements. This may be due to the gene preventing premature death of the developing vessel elements.
21. Motose H, Sugiyama M, Fukuda H: A proteoglycan mediates inductive interaction during plant vascular development.
Nature 2004, 429:873-878.
22. Bonke M, Thitamadee S, Mahonen AP, Hauser MT, Helariutta Y: APL regulates vascular tissue identity in Arabidopsis. Nature 2003, 426:181-186.
23. Vanneste S, Friml J: Auxin: a trigger for change in plant development. Cell 2009, 136:1005-1016.
24. Robert HS, Friml J: Auxin and other signals on the move in plants. Nat Chem Biol 2009, 5:325-332.
25. Wisniewska J, Xu J, Seifertova D, Brewer PB, RuZicka K, Blilou I, Rouquie D, Benkova E, Scheres B, Friml J: Polar PIN localization directs auxin flow in plants. Science 2006, 312:883.
26. Michniewicz M, Zago MK, Abas L, Weijers D, Schweighofer A, Meskiene I, Heisler MG, Ohno C, Zhang J, Huang F et al.: Antagonistic regulation of PIN phosphorylation by PP2A and PINOID directs auxin flux. Cell 2007, 130:1044-1056.
27. Sauer M, Balla J, Luschnig C, Wisniewska J, Reinohl V, Friml J, Benkova E: Canalization of auxin flow by Aux/IAA-ARF-dependent feedback regulation of PIN polarity. Genes Dev
2006, 20:2902-2911.
28. Bayer EM, Smith RS, Mandel T, Nakayama N, Sauer M,
• Prusinkiewicz P, Kuhlemeier C: Integration of transport-based models for phyllotaxis and midvein formation. Genes Dev 2009,
23:373-384.
The authors provide further support to the canalisation hypothesis. Computer simulations of auxin fluxes during leaf midvein initiation are consistent with the idea that auxin can redirect PIN1 and amplify small auxin maxima to trigger de novo vascular strand formation.
29. Cambridge AP, Morris DA: Transfer of exogenous auxin from the phloem to the polar auxin transport pathway in pea (Pisum sativum L). Planta 1996, 199:583-588.
30. Marchant A, Bhalerao R, Casimiro I, Eklof J, Casero PJ, Bennett M, Sandberg G: AUX1 promotes lateral root formation by facilitating indole-3-acetic acid distribution between sink and source tissues in the Arabidopsis seedling. Plant Cell 2002,
14:589-597.
31. Ljung K, Hull AK, Celenza J, Yamada M, Estelle M, Normanly J, Sandberg G: Sites and regulation of auxin biosynthesis in Arabidopsis roots. Plant Cell 2005, 17:1090-1104.
32. Hirose N, Takei K, Kuroha T, Kamada-Nobusada T, Hayashi H: Regulation of cytokinin biosynthesis, compartmentalization and translocation. J Exp Bot 2008, 59:75-83.
33. Schoof H, Lenhard M, Haecker A, Mayer KF, JiJrgens G, Laux T: The stem cell population of Arabidopsis shoot meristems is maintained by a regulatory loop between the CLAVATA and WUSCHEL genes. Cell 2000, 100:635-644.
34. Stahl Y, Wink RH, Ingram GC, Simon R: A signalling module
• controlling the stem cell niche in Arabidopsis root meristems.
CurrBiol 2009, 19:909-914. A CLE40/WOX5 signalling module similar to the TDIF/CLE41/44 system controls the root stem cell niche maintenance. This spatiotemporal communication appears to be a recurring theme in the maintenance of meristematic cells.
35. Sarkar AK, Luijten M, Miyashima S, Lenhard M, Hashimoto T, Nakajima K, Scheres B, Heidstra R, Laux T: Conserved factors regulate signalling in Arabidopsis thaliana shoot and root stem cell organizers. Nature 2007, 446:811-814.
36. Ito Y, Nakanomyo I, Motose H, Iwamoto K, Sawa S, Dohmae N, Fukuda H: Dodeca-CLE peptides as suppressors of plantstem cell differentiation. Science 2006, 313:842-845.
37. Hirakawa Y, Shinohara H, Kondo Y, Inoue A, Nakanomyo I, ••   Ogawa M, Sawa S, Ohashi-Ito K, Matsubayashi Y, Fukuda H:
Non-cell-autonomous control of vascular stem cell fate by a CLE peptide/receptor system. Proc Natl Acad Sci U S A 2008, 105:15208-15213.
A signal peptide, TDIF (CLE41/CLE44), is produced and secreted by phloem. The authors propose a role for TDIF/CLE41/44 signalling as a mobile mediator for phloem-xylem cross-talk and show that TDIF binds to its receptor PXY/TDR.
38. Fisher K, Turner S: PXY, a receptor-like kinase essential for ••   maintaining polarity during plant vascular-tissue
development. CurrBiol 2007, 17:1061-1066. Procambial cells perceive the TDIF signal through the PXY (TDR) receptor. A role for PXY/TDR is supported by data on its expression in the cambium and the identification of the pxy mutant with severely impaired vascular development.
39. McHale NA, Koning RE: MicroRNA-directed cleavage of Nicotiana sylvestris PHAVOLUTA mRNA regulates the vascular cambium and structure of apical meristems.
Plant Cell 2004, 16:1730-1740.
40. Juarez MT, Kul JS, Thomas J, Heller BA, Timmermans MC: MicroRNA-mediated repression of rolled leaf1 specifies maize leaf polarity. Nature 2004, 428:84-88.
41. Kalantidis K, Schumacher HT, Alexiadis T, Helm JM: RNA silencing movement in plants. Biol Cell 2008, 100:13-26.
42. Voinnet O: Origin, biogenesis and activity of plant microRNAs.
Cell 2009, 136:669-687.
43. Buhtz A, Springer F, Chappell L, Baulcombe DC, Kehr J:
• Identification and characterization of small RNAs from the phloem of Brassica napus. Plant J 2008, 53:739-749.
Besides transporting photosynthetic products, phloem carries various molecules with signalling functions. Analysis of small RNAs in phloem of Brassica napus demonstrates that many miRNAs and other small RNA species are present in the sap and may convey long-distance information.
44. Tretter EM, Alvarez JP, Eshed Y, Bowman JL: Activity range of Arabidopsis small RNAs derived from different biogenesis pathways. Plant Physiol 2008, 147:58-62.
45. Chitwood DH, Nogueira FTS, Howell MD, Montgomery TA,
• Carrington JC, Timmermans MCP: Pattern formation via small RNA mobility. Genes Dev 2009, 23:549-554.
The authors demonstrate incongruities between expression patterns of precursor and mature small RNAs in a developmental context and provide evidence for the movement of trans-acting siRNAs. This suggests the intriguing idea that small RNA movement combined with the activity of spatially restricting effectors could form morphogenic patterns.
46. Sibout R, Plantegenet S, Hardtke CS: Flowering as a condition ••   for xylem expansion in Arabidopsis hypocotyl and root.
CurrBiol 2008, 18:458-463. The authors propose the concept that flowering is a condition required to trigger the transition to secondary vascular development, suggesting a currently unknown long-distance signal from the shoot.
47. Kobayashi K, Zambryski P: RNA silencing and its cell-to-cell spread during Arabidopsis embryogenesis. Plant J 2007,
50:597-604.
48. Turgeon R, Wolf S: Phloem transport: cellular pathways and molecular trafficking. Annu Rev Plant Biol 2009,
60:207-221.
Current Opinion in Plant Biology 2010, 13:59-65
www.sciencedirect.com
Cell-to-cell communication in vascular morphogenesis Lehesranta, Lichtenberger and Helariutta 65
49. Kim I, Cho E, Crawford K, Hempel FD, Zambryski PC: Cell-to-cell movement of GFP during embryogenesis and early seedling development in Arabidopsis. Proc Natl Acad Sci USA 2005, 102:2227-2231.
50. Thomas CL, Bayer EM, Ritzenthaler C, Fernandez-Calvino L,
• Maule AJ: Specific targeting of a plasmodesmal
protein affecting cell-to-cell communication. PLoS Biol 2008, 6:e7.
Few mutants with defects in plasmodesmal trafficking have been described. This paper describes a family of novel plasmodesmata-located proteins that have a role in cell-to-cell communication.
51. Benitez-Alfonso Y, Cilia M, San Roman A, Thomas C, Maule A,
• Hearn S, Jackson D: Control of Arabidopsis meristem development by thioredoxin-dependent regulation of intercellular transport. Proc Natl Acad Sci USA 2009, 106:3615-3620.
This paper describes a mutant with increased callose deposition and decreased plasmodesmal transport, resulting in severe defects in development and phloem unloading.
52. Levesque MP, Vernoux T, Busch W, Cui H, Wang JY, Blilou I, Hassan H, Nakajima K, Matsumoto N, Lohmann JU et al.: Whole-genome analysis of the SHORT-ROOT developmental pathway in Arabidopsis. PLoS Biol 2006, 4:e143.
53. Gallagher KL, Benfey PN: Both the conserved GRAS domain •    and nuclear localization are required for SHORT-ROOT
movement. Plant J 2009, 57:785-797. SHORT-ROOT is a master regulator of root development and also implied in vascular morphogenesis. The movement of the SHR transcription factor from the vascular cylinder to surrounding tissues is used as a tool to study cell-to-cell trafficking. The movement of the protein is required for proper root patterning, and both nuclear and cytoplasmic localisation signals appear to be important for this phenomenon.
www.sciencedirect.com
Current Opinion in Plant Biology 2010, 13:59-65