trends in plant science Reviews Vascular continuity and auxin signals Thomas Berleth, Jim Mattsson and Christian S. Hardtke Plant vascular tissues form systems of interconnected cell files throughout the plant body. Vascular tissues usually differentiate at predictable positions but the wide range of func- tional patterns generated in response to abnormal growth conditions or wounding reveals partially self-organizing patterning mechanisms. Signals ensuring aligned cell differentiation within vascular strands are crucial in self-organized vascular patterning, and the apicalÐbasal flow of indole acetic acid has been suspected to act as an orienting signal in this process. Several recent advances appear to converge on a more precise definition of the role of auxin flow in vascular tissue patterning. ranched cellular systems exist in most multicellular organ- auxin sources can induce the formation of new vascular strands isms and the principles underlying their reticulate patterns from parenchymatic cells3. Vascular differentiation in response to Bhave long intrigued philosophers, mathematicians and ex- IAA application does not occur readily in all genotypes, suggesting perimental biologists. In particular, the exposed and often beautiful that further factors are often required. These factors probably include vascular system of plants has fascinated many, yet Ð as for other other plant hormones, and a vascular-differentiation-promoting branched systems Ð the patterning cues have remained elusive. influence of cytokinins, ethylene, gibberellins and brassinosteroids The plant vascular system is a network of interconnected cells has indeed been reported4,10Ð12. for the transport of water and dissolved materials throughout the The role of IAA is nevertheless unique because the position of plant1,2. Vascular tissues are typically organized in bundles or IAA application can define the site of vascular differentiation: a strands that contain two kinds of conducting tissues, phloem and new functional vascular strand will extend basally from a local xylem, each comprising a variety of distinguishable cell types. IAA source3 (Fig. 2). Remarkably, IAA does not just trigger vas- Dissolved photoassimilates from source organs in the shoot are cular differentiation per se but also induces the formation of a transported in the phloem, and the transfer of water and minerals continuous vascular strand Ð a cellular response with peculiar geo- from roots occurs in the xylem. metrical properties. First, the response is polar: local IAA appli- Vascular patterns are typically both variable in the course and cation typically induces vascular strand formation towards the arrangement of vascular strands and reproducible in their inte- basal pole of the plant. Second, the responding cells differentiate gration into the local tissue context (Fig. 1). These seemingly con- in a continuous area to form a file of interconnected cells. Third, flicting features suggest that genetic controls of tissue patterns in the differentiation zone is restricted in the radial and tangential plant organs leave room for variable vascular strand arrangements dimensions, because differentiation occurs only within a narrow and that vascular tissues have partially self-organizing capacities strip of cells rather than isotropically around the IAA source. to ensure tissue continuity irrespective of the particular routes and The capacity of a simple signal to trigger a complex and oriented arrangements of vascular strands. cellular response suggests that the signaling mechanism co-opts At present, neither the molecular mechanisms underlying over- directional cues already present in plant tissues. Not surprisingly, all tissue patterning within plant organs nor the specific signals therefore, it is the polar (apicalÐbasal) transport of the same mol- ensuring vascular continuity within variable networks are known. ecule, IAA, which has been postulated to integrate cell polarity However, auxin and its apicalÐbasal flow have long been impli- and aligned differentiation across the entire plant. In normal plant cated in promoting continuous vascular differentiation3,4. In this growth, IAA is predominantly produced in apical regions, such as article, we briefly summarize evidence that auxin has a role in young leaves or flowers, from which it is transported basally. Auxin continuous vascular differentiation and discuss several recent transport is thought to proceed in a cell-to-cell fashion through the findings that together have generated an experimental basis for action of specific membrane-bound influx and efflux carriers13Ð15 exploring auxin functions in vascular development at the cellular (Fig. 2). Although the molecular details remain hypothetical, and molecular levels. We finally discuss several newly isolated the apicalÐbasal transport of IAA itself is experimentally well Arabidopsis mutants that might identify auxin-independent mech- established and its properties could account for the geometrical anisms in vascular strand formation. The focus is on mechanisms peculiarities of vascular strand formation (Fig. 2). underlying aligned cell differentiation leading to the formation of First, the polar effect of auxin application can be explained by the vascular strands; further aspects of vascular development and integration of the applied IAA in the general apicalÐbasal flow of research in a broader spectrum of plant species have been thought- IAA. Second, differentiation in response to a transported signaling fully reviewed in Refs 4Ð6. molecule would be inherently continuous. Third, the restriction of vascular differentiation to a narrow zone could be because of effi- Auxin and vascular differentiation cient drainage of IAA through incipient provascular strands. This Several lines of evidence have implicated indole acetic acid (IAA) drainage would prevent auxin accumulation and high IAA exposure (the predominant auxin in higher plants) in vascular development. of all cells outside the narrow ÔcanalÕ region. This interpretation IAA (supported by cytokinins) can induce xylem tracheary ele- forms the basis of the Ôauxin canalization hypothesisÕ, which ment differentiation in suspension culture cells of suitable species7. postulates a positive feedback by which IAA-conducting cells dif- Auxin-overproducing transgenic plants have increased amounts of ferentiate towards increased IAA conductivity3 (Figs 2 and 3). An vascular tissues8, IAA application can replace leaf primordia in auxin-triggered feedback mechanism is an attractive explanation inducing vascular (Ôleaf traceÕ) connections in stems9, and local for the reproducible position of the induced vascular strand relative 1360 - 1385/00/$ Ð see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(00)01725-8 September 2000, Vol. 5, No. 9 387 trends in plant science Reviews orienting cell differentiation and in restricting vas- cular differentiation to narrow zones, possibly by mediating efficient auxin drainage3. Inhibition of auxin transport also profoundly affected leaf venation patterns. Arabidopsis leaf venation is normally pinnate, characterized by several distinguishable vein size orders, with sec- ondary veins branching laterally from a single prominent midvein6 (Fig. 3). Under the influence of auxin transport inhibitors, vascular strands along the leaf margin became more prominent, associated with increased numbers of secondary veins and multiple parallel vascular strands in the center. Remarkably, this pattern shift was already visible at low inhibitor concentrations that had no detectable effects on overall leaf morphology. Thus, several alternative functional venation patterns can be generated in a given genetic back- ground, depending on the overall auxin transport Fig. 1. Levels of vascular tissue organization. Vascular tissues connect organs across properties of a leaf primordium. the plant body and are simultaneously integrated into the local tissue context. Their This finding implicates auxin signaling in the organization therefore appears to involve at least two types of control. First, direc- genetic control of leaf venation patterns in Ara- tional signals involving indole acetic acid in combination with self-organizing bidopsis (and three other dicot species in one of the strand-forming capacities generate continuous strands; these link to form variable 17 strand networks. Second, genetic cues integrate vascular and non-vascular tissue studies ). Stronger inhibition of auxin transport patterns in plant organs and might also constrain the variability of vascular strand progressively restricts vascular differentiation to patterns. Self-organizing capacities are reflected in the formation of perfectly aligned the leaf margin, suggesting that this region harbors strands along unpredictable routes. Integration into a larger tissue context is reflected major auxin sources critical for the formation of in the organ-specific position and internal organization of vascular bundles and in the major (primary and secondary) veins (Fig. 3). reproducible differentiation of non-vascular cell types in fixed spatial relationship Veins of different hierarchical orders emerge at to vascular bundles. (a) Alignment of cells in a provascular strand of a young leaf successive stages of leaf development17Ð19. When primordium. The strand emerges as a row of narrow cells, generated by aligned leaf primordia were exposed to auxin transport divisions and the elongation of previously isodiametric cells. The differentiation