Polarity, vascularization and auxin

Abstract

The recent cloning of the Arabidopsis MONOPTEROS (MP) gene has brought together in a dramatic way several experimental threads suggesting the nature of the relationship between auxin action, polarity and vascular differentiation[1Hardtke C.S. Berleth T. The Arabidopsis gene MONOPTEROS encodes a transcription factor mediating embryo axis formation and vascular development.EMBO J. 1998; 17: 1405-1411Crossref PubMed Scopus (799) Google Scholar]. MP has been shown to encode a transcription factor likely to bind to auxin response elements (AREs). This factor appears to allow a canalization of auxin flow, thereby defining the embryo axis.1. The importance of polar auxin transportThe polar transport of auxin from sources at distal points in the shoots in the direction of the roots has been implicated in the alignment of differentiation with the axis of this transport, beginning with the spatial organization of the embryo. Embryos in which auxin transport has been disturbed (by exogenous application of auxins or auxin transport inhibitors) exhibit defects in the spatial organization of post-globular-stage features relative to the embryo axis, such as cotyledons, the scutellum, and the apical meristems of both the shoot and the root[2Fischer C. Neuhaus G. Influence of auxin on the establishment of bilateral symmetry in monocots.Plant J. 1996; 9: 659-669Crossref Scopus (95) Google Scholar, 3Liu C. Xu Z. Chua N-H. Auxin polar transport is essential for the establishment of bilateral symmetry during early plant embryogenesis.Plant Cell. 1993; 5: 621-630PubMed Google Scholar]. During post-embryonic development, interference with auxin transport or modification of auxin concentrations can result in defects in inflorescence architecture, vascular anatomy, and a variety of other morphological and anatomical features normally oriented relative to the plant axis[4Sachs T. Cell polarity and tissue patterning in plants.Development. 1991; 91: 83-93Google Scholar]. In mature tissues, the redifferentiation of vascular strands following the severing of existing strands occurs along a path of auxin basipetal transport, which can be influenced by exogenous sources of auxin[5Sachs T. The control of patterned differentiation of vascular tissues.Adv. Bot. Res. 1981; 9: 152-255Google Scholar].2. Connecting polarity and axis formationDespite this convincing evidence of the central role of polar auxin transport, little is understood of the mechanism by which its polarity is translated into an axis for other events and processes. In part, this is because auxin influences so many physiological and developmental phenomena that experimental analysis is difficult. The polar transport of auxin appears to rely on membrane-bound carriers localized at the basal ends of transporting cells[6Lomax, T.L., Muday, G.K. and Rubery, P.H. (1995) Auxin transport, in Plant Hormones: Physiology, Biochemistry and Molecular Biology (Davies, P.J., ed.), pp. 509–530, KluwerGoogle Scholar]. Although several auxin-binding activities have been identified biochemically, it is still unclear which of these, if any, are responsible for polar transport, and a similar uncertainty exists as to which auxin-binding proteins act as receptors that initiate intracellular events such as changes in ion flux and gene expression. The analysis of auxin-response mutants has provided an alternative means of identifying key components in auxin signaling, as has the analysis of the transcriptional activation of early auxin-response genes. Much research has been done on the dependence of cell differentiation phenomena, such as cell expansion and programmed cell death, on the local concentration of auxin. Recent studies have attempted to correlate the distribution of auxin concentrations in the plant body with the distribution of events that appear to be auxin dependent[7Tuominen H. et al.A radial concentration gradient of indole-3-acetic acid is related to secondary xylem development in hybrid aspen.Plant Physiol. 1997; 115: 577-585PubMed Google Scholar]. For example, the differentiation of vascular tissues and surrounding cell types appears to occur in a spatial pattern of positions that coincide with concentration intervals in a downward gradient of auxin that begins at each developing vein. Is there a causal relationship? At present, we know far too little to accept or reject this possibility.Studies of the early auxin-response genes have revealed some of the cellular machinery through which auxin triggers gene expression. Several classes of early auxin-responsive genes have been identified, most encoding products of unknown function[8Abel S. Theologis A. Early genes and auxin action.Plant Physiol. 1996; 111: 9-17Crossref PubMed Scopus (588) Google Scholar]. The exceptions are one class of glutathione S-transferases and a second including ACC synthase, suggesting that at least some gene expression subsequent to auxin reception is associated with intercellular signalling. Members of the Aux/IAA class of early response proteins have putative DNA-binding domains, show nuclear-localization patterns and interact with one another in a yeast two-hybrid system[9Kim J. Harter K. Theologis A. Protein–protein interactions among the Aux/IAA proteins.Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 11786-11791Crossref PubMed Scopus (418) Google Scholar]. It has been suggested that members of this class include transcription factors that act in a combinatorial fashion through homo- and hetero-dimerization to activate later downstream genes. The transcription of the early auxin-responsive genes appears to rely on AREs in their promoters. Distinct AREs have been functionally defined for several of the genes, and appear to act as modules in combinations that are varied for individual genes[8Abel S. Theologis A. Early genes and auxin action.Plant Physiol. 1996; 111: 9-17Crossref PubMed Scopus (588) Google Scholar]. A nuclear factor, ARF1, that binds to AREs with a TGTCTC motif has been identified in Arabidopsis, and itself appears to be related to the Aux/IAA class[10Ulmasov T. Hagen G. Guilfoyle T.J. ARF1, a transcription factor that binds to auxin response elements.Science. 1997; 276: 1865-1868Crossref PubMed Scopus (705) Google Scholar].3. MONOPTEROS: a role in canalizationThe cloning of the MP gene of Arabidopsis has revealed that genetic studies of embryo axis formation and vascularization have begun to converge with molecular studies of gene activation by auxin[1Hardtke C.S. Berleth T. The Arabidopsis gene MONOPTEROS encodes a transcription factor mediating embryo axis formation and vascular development.EMBO J. 1998; 17: 1405-1411Crossref PubMed Scopus (799) Google Scholar]. The mp mutant was recovered in a screen for embryo pattern mutants, based on the lack of basal portions of the embryo axis and occasional fusion of cotyledons (monopteros is the Greek for single wing) in severe alleles[11Berleth T. Jürgens G. The role of the MONOPTEROS gene in organizing the basal body region of the Arabidopsis embryo.Development. 1993; 118: 575-587Google Scholar]. Less-severely phenotypic mp embryos can develop into plants that have defects in the differentiation and alignment of vascular cells, in the organization of inflorescences and in polar auxin transport through the stem[12Przemeck G.K.H. et al.Studies on the role of the Arabidopsis gene MONOPTEROS in vascular development and plant cell axialization.Planta. 1996; 200: 229-237Crossref PubMed Scopus (368) Google Scholar]. The leaves of mp mutants exhibit discontinuities in the vascular network. Among mp alleles, the severity of the embryo axis phenotype varies in parallel with those of the vascularization and inflorescence post-embryonic phenotypes, suggesting that these architectural features share an underlying molecular machinery. This also suggests that the primary defect in mp embryos is the failure to establish polarity, with the failure to organize a basal region (hypocotyl and radicle) as a consequence.Hardtke and Berleth isolated the MP gene by using positional cloning. The gene encodes a transcription factor (IAA24) recently identified by Ulmasov and co-workers as a factor that recognizes AREs (Ref. [10Ulmasov T. Hagen G. Guilfoyle T.J. ARF1, a transcription factor that binds to auxin response elements.Science. 1997; 276: 1865-1868Crossref PubMed Scopus (705) Google Scholar]). The Arabidopsis IAA24 is highly similar to ARF1, the functionally tested ARE-binding factor, and to VP1, a maize transcriptional activator associated with abscisic acid-related embryo dormancy[13McCarty D.R. et al.The Viviparous-1 developmental gene of maize encodes a novel transcriptional activator.Cell. 1991; 66: 895-905Abstract Full Text PDF PubMed Scopus (529) Google Scholar]. Based on in situ hybridization, the MP mRNA accumulates subepidermally throughout globular-stage embryos, and is gradually restricted to central (provascular) domains of later stages. In emerging organs of the shoot system, a similar pattern was observed, with accumulation throughout emerging primordia becoming restricted to provascular sites. The MP mRNA appeared only in the central vascular cylinder of the root.As the authors point out, this pattern is consistent with a potential role for the MP-encoded factor in the activation of downstream differentiation events in correspondence with the progressive `canalizing' of polar auxin flow into narrowing paths in the developing embryo and post-embryonic plant. Alternatively, the MP products might have a role in narrowing the polar flow itself. It will be intriguing to learn both how the MP product achieves its distribution in the developing embryo and plant body and how the downstream targets of the MP transcription factor enact polarity-dependent processes. If MP does indeed have the pivotal role that its expression pattern and mutant phenotypes suggest, relatively direct molecular genetic strategies should permit the identification of its up- and downstream partners in patterning. The recent cloning of the Arabidopsis MONOPTEROS (MP) gene has brought together in a dramatic way several experimental threads suggesting the nature of the relationship between auxin action, polarity and vascular differentiation[1Hardtke C.S. Berleth T. The Arabidopsis gene MONOPTEROS encodes a transcription factor mediating embryo axis formation and vascular development.EMBO J. 1998; 17: 1405-1411Crossref PubMed Scopus (799) Google Scholar]. MP has been shown to encode a transcription factor likely to bind to auxin response elements (AREs). This factor appears to allow a canalization of auxin flow, thereby defining the embryo axis. 1. The importance of polar auxin transportThe polar transport of auxin from sources at distal points in the shoots in the direction of the roots has been implicated in the alignment of differentiation with the axis of this transport, beginning with the spatial organization of the embryo. Embryos in which auxin transport has been disturbed (by exogenous application of auxins or auxin transport inhibitors) exhibit defects in the spatial organization of post-globular-stage features relative to the embryo axis, such as cotyledons, the scutellum, and the apical meristems of both the shoot and the root[2Fischer C. Neuhaus G. Influence of auxin on the establishment of bilateral symmetry in monocots.Plant J. 1996; 9: 659-669Crossref Scopus (95) Google Scholar, 3Liu C. Xu Z. Chua N-H. Auxin polar transport is essential for the establishment of bilateral symmetry during early plant embryogenesis.Plant Cell. 1993; 5: 621-630PubMed Google Scholar]. During post-embryonic development, interference with auxin transport or modification of auxin concentrations can result in defects in inflorescence architecture, vascular anatomy, and a variety of other morphological and anatomical features normally oriented relative to the plant axis[4Sachs T. Cell polarity and tissue patterning in plants.Development. 1991; 91: 83-93Google Scholar]. In mature tissues, the redifferentiation of vascular strands following the severing of existing strands occurs along a path of auxin basipetal transport, which can be influenced by exogenous sources of auxin[5Sachs T. The control of patterned differentiation of vascular tissues.Adv. Bot. Res. 1981; 9: 152-255Google Scholar]. The polar transport of auxin from sources at distal points in the shoots in the direction of the roots has been implicated in the alignment of differentiation with the axis of this transport, beginning with the spatial organization of the embryo. Embryos in which auxin transport has been disturbed (by exogenous application of auxins or auxin transport inhibitors) exhibit defects in the spatial organization of post-globular-stage features relative to the embryo axis, such as cotyledons, the scutellum, and the apical meristems of both the shoot and the root[2Fischer C. Neuhaus G. Influence of auxin on the establishment of bilateral symmetry in monocots.Plant J. 1996; 9: 659-669Crossref Scopus (95) Google Scholar, 3Liu C. Xu Z. Chua N-H. Auxin polar transport is essential for the establishment of bilateral symmetry during early plant embryogenesis.Plant Cell. 1993; 5: 621-630PubMed Google Scholar]. During post-embryonic development, interference with auxin transport or modification of auxin concentrations can result in defects in inflorescence architecture, vascular anatomy, and a variety of other morphological and anatomical features normally oriented relative to the plant axis[4Sachs T. Cell polarity and tissue patterning in plants.Development. 1991; 91: 83-93Google Scholar]. In mature tissues, the redifferentiation of vascular strands following the severing of existing strands occurs along a path of auxin basipetal transport, which can be influenced by exogenous sources of auxin[5Sachs T. The control of patterned differentiation of vascular tissues.Adv. Bot. Res. 1981; 9: 152-255Google Scholar]. 2. Connecting polarity and axis formationDespite this convincing evidence of the central role of polar auxin transport, little is understood of the mechanism by which its polarity is translated into an axis for other events and processes. In part, this is because auxin influences so many physiological and developmental phenomena that experimental analysis is difficult. The polar transport of auxin appears to rely on membrane-bound carriers localized at the basal ends of transporting cells[6Lomax, T.L., Muday, G.K. and Rubery, P.H. (1995) Auxin transport, in Plant Hormones: Physiology, Biochemistry and Molecular Biology (Davies, P.J., ed.), pp. 509–530, KluwerGoogle Scholar]. Although several auxin-binding activities have been identified biochemically, it is still unclear which of these, if any, are responsible for polar transport, and a similar uncertainty exists as to which auxin-binding proteins act as receptors that initiate intracellular events such as changes in ion flux and gene expression. The analysis of auxin-response mutants has provided an alternative means of identifying key components in auxin signaling, as has the analysis of the transcriptional activation of early auxin-response genes. Much research has been done on the dependence of cell differentiation phenomena, such as cell expansion and programmed cell death, on the local concentration of auxin. Recent studies have attempted to correlate the distribution of auxin concentrations in the plant body with the distribution of events that appear to be auxin dependent[7Tuominen H. et al.A radial concentration gradient of indole-3-acetic acid is related to secondary xylem development in hybrid aspen.Plant Physiol. 1997; 115: 577-585PubMed Google Scholar]. For example, the differentiation of vascular tissues and surrounding cell types appears to occur in a spatial pattern of positions that coincide with concentration intervals in a downward gradient of auxin that begins at each developing vein. Is there a causal relationship? At present, we know far too little to accept or reject this possibility.Studies of the early auxin-response genes have revealed some of the cellular machinery through which auxin triggers gene expression. Several classes of early auxin-responsive genes have been identified, most encoding products of unknown function[8Abel S. Theologis A. Early genes and auxin action.Plant Physiol. 1996; 111: 9-17Crossref PubMed Scopus (588) Google Scholar]. The exceptions are one class of glutathione S-transferases and a second including ACC synthase, suggesting that at least some gene expression subsequent to auxin reception is associated with intercellular signalling. Members of the Aux/IAA class of early response proteins have putative DNA-binding domains, show nuclear-localization patterns and interact with one another in a yeast two-hybrid system[9Kim J. Harter K. Theologis A. Protein–protein interactions among the Aux/IAA proteins.Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 11786-11791Crossref PubMed Scopus (418) Google Scholar]. It has been suggested that members of this class include transcription factors that act in a combinatorial fashion through homo- and hetero-dimerization to activate later downstream genes. The transcription of the early auxin-responsive genes appears to rely on AREs in their promoters. Distinct AREs have been functionally defined for several of the genes, and appear to act as modules in combinations that are varied for individual genes[8Abel S. Theologis A. Early genes and auxin action.Plant Physiol. 1996; 111: 9-17Crossref PubMed Scopus (588) Google Scholar]. A nuclear factor, ARF1, that binds to AREs with a TGTCTC motif has been identified in Arabidopsis, and itself appears to be related to the Aux/IAA class[10Ulmasov T. Hagen G. Guilfoyle T.J. ARF1, a transcription factor that binds to auxin response elements.Science. 1997; 276: 1865-1868Crossref PubMed Scopus (705) Google Scholar]. Despite this convincing evidence of the central role of polar auxin transport, little is understood of the mechanism by which its polarity is translated into an axis for other events and processes. In part, this is because auxin influences so many physiological and developmental phenomena that experimental analysis is difficult. The polar transport of auxin appears to rely on membrane-bound carriers localized at the basal ends of transporting cells[6Lomax, T.L., Muday, G.K. and Rubery, P.H. (1995) Auxin transport, in Plant Hormones: Physiology, Biochemistry and Molecular Biology (Davies, P.J., ed.), pp. 509–530, KluwerGoogle Scholar]. Although several auxin-binding activities have been identified biochemically, it is still unclear which of these, if any, are responsible for polar transport, and a similar uncertainty exists as to which auxin-binding proteins act as receptors that initiate intracellular events such as changes in ion flux and gene expression. The analysis of auxin-response mutants has provided an alternative means of identifying key components in auxin signaling, as has the analysis of the transcriptional activation of early auxin-response genes. Much research has been done on the dependence of cell differentiation phenomena, such as cell expansion and programmed cell death, on the local concentration of auxin. Recent studies have attempted to correlate the distribution of auxin concentrations in the plant body with the distribution of events that appear to be auxin dependent[7Tuominen H. et al.A radial concentration gradient of indole-3-acetic acid is related to secondary xylem development in hybrid aspen.Plant Physiol. 1997; 115: 577-585PubMed Google Scholar]. For example, the differentiation of vascular tissues and surrounding cell types appears to occur in a spatial pattern of positions that coincide with concentration intervals in a downward gradient of auxin that begins at each developing vein. Is there a causal relationship? At present, we know far too little to accept or reject this possibility. Studies of the early auxin-response genes have revealed some of the cellular machinery through which auxin triggers gene expression. Several classes of early auxin-responsive genes have been identified, most encoding products of unknown function[8Abel S. Theologis A. Early genes and auxin action.Plant Physiol. 1996; 111: 9-17Crossref PubMed Scopus (588) Google Scholar]. The exceptions are one class of glutathione S-transferases and a second including ACC synthase, suggesting that at least some gene expression subsequent to auxin reception is associated with intercellular signalling. Members of the Aux/IAA class of early response proteins have putative DNA-binding domains, show nuclear-localization patterns and interact with one another in a yeast two-hybrid system[9Kim J. Harter K. Theologis A. Protein–protein interactions among the Aux/IAA proteins.Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 11786-11791Crossref PubMed Scopus (418) Google Scholar]. It has been suggested that members of this class include transcription factors that act in a combinatorial fashion through homo- and hetero-dimerization to activate later downstream genes. The transcription of the early auxin-responsive genes appears to rely on AREs in their promoters. Distinct AREs have been functionally defined for several of the genes, and appear to act as modules in combinations that are varied for individual genes[8Abel S. Theologis A. Early genes and auxin action.Plant Physiol. 1996; 111: 9-17Crossref PubMed Scopus (588) Google Scholar]. A nuclear factor, ARF1, that binds to AREs with a TGTCTC motif has been identified in Arabidopsis, and itself appears to be related to the Aux/IAA class[10Ulmasov T. Hagen G. Guilfoyle T.J. ARF1, a transcription factor that binds to auxin response elements.Science. 1997; 276: 1865-1868Crossref PubMed Scopus (705) Google Scholar]. 3. MONOPTEROS: a role in canalizationThe cloning of the MP gene of Arabidopsis has revealed that genetic studies of embryo axis formation and vascularization have begun to converge with molecular studies of gene activation by auxin[1Hardtke C.S. Berleth T. The Arabidopsis gene MONOPTEROS encodes a transcription factor mediating embryo axis formation and vascular development.EMBO J. 1998; 17: 1405-1411Crossref PubMed Scopus (799) Google Scholar]. The mp mutant was recovered in a screen for embryo pattern mutants, based on the lack of basal portions of the embryo axis and occasional fusion of cotyledons (monopteros is the Greek for single wing) in severe alleles[11Berleth T. Jürgens G. The role of the MONOPTEROS gene in organizing the basal body region of the Arabidopsis embryo.Development. 1993; 118: 575-587Google Scholar]. Less-severely phenotypic mp embryos can develop into plants that have defects in the differentiation and alignment of vascular cells, in the organization of inflorescences and in polar auxin transport through the stem[12Przemeck G.K.H. et al.Studies on the role of the Arabidopsis gene MONOPTEROS in vascular development and plant cell axialization.Planta. 1996; 200: 229-237Crossref PubMed Scopus (368) Google Scholar]. The leaves of mp mutants exhibit discontinuities in the vascular network. Among mp alleles, the severity of the embryo axis phenotype varies in parallel with those of the vascularization and inflorescence post-embryonic phenotypes, suggesting that these architectural features share an underlying molecular machinery. This also suggests that the primary defect in mp embryos is the failure to establish polarity, with the failure to organize a basal region (hypocotyl and radicle) as a consequence.Hardtke and Berleth isolated the MP gene by using positional cloning. The gene encodes a transcription factor (IAA24) recently identified by Ulmasov and co-workers as a factor that recognizes AREs (Ref. [10Ulmasov T. Hagen G. Guilfoyle T.J. ARF1, a transcription factor that binds to auxin response elements.Science. 1997; 276: 1865-1868Crossref PubMed Scopus (705) Google Scholar]). The Arabidopsis IAA24 is highly similar to ARF1, the functionally tested ARE-binding factor, and to VP1, a maize transcriptional activator associated with abscisic acid-related embryo dormancy[13McCarty D.R. et al.The Viviparous-1 developmental gene of maize encodes a novel transcriptional activator.Cell. 1991; 66: 895-905Abstract Full Text PDF PubMed Scopus (529) Google Scholar]. Based on in situ hybridization, the MP mRNA accumulates subepidermally throughout globular-stage embryos, and is gradually restricted to central (provascular) domains of later stages. In emerging organs of the shoot system, a similar pattern was observed, with accumulation throughout emerging primordia becoming restricted to provascular sites. The MP mRNA appeared only in the central vascular cylinder of the root.As the authors point out, this pattern is consistent with a potential role for the MP-encoded factor in the activation of downstream differentiation events in correspondence with the progressive `canalizing' of polar auxin flow into narrowing paths in the developing embryo and post-embryonic plant. Alternatively, the MP products might have a role in narrowing the polar flow itself. It will be intriguing to learn both how the MP product achieves its distribution in the developing embryo and plant body and how the downstream targets of the MP transcription factor enact polarity-dependent processes. If MP does indeed have the pivotal role that its expression pattern and mutant phenotypes suggest, relatively direct molecular genetic strategies should permit the identification of its up- and downstream partners in patterning. The cloning of the MP gene of Arabidopsis has revealed that genetic studies of embryo axis formation and vascularization have begun to converge with molecular studies of gene activation by auxin[1Hardtke C.S. Berleth T. The Arabidopsis gene MONOPTEROS encodes a transcription factor mediating embryo axis formation and vascular development.EMBO J. 1998; 17: 1405-1411Crossref PubMed Scopus (799) Google Scholar]. The mp mutant was recovered in a screen for embryo pattern mutants, based on the lack of basal portions of the embryo axis and occasional fusion of cotyledons (monopteros is the Greek for single wing) in severe alleles[11Berleth T. Jürgens G. The role of the MONOPTEROS gene in organizing the basal body region of the Arabidopsis embryo.Development. 1993; 118: 575-587Google Scholar]. Less-severely phenotypic mp embryos can develop into plants that have defects in the differentiation and alignment of vascular cells, in the organization of inflorescences and in polar auxin transport through the stem[12Przemeck G.K.H. et al.Studies on the role of the Arabidopsis gene MONOPTEROS in vascular development and plant cell axialization.Planta. 1996; 200: 229-237Crossref PubMed Scopus (368) Google Scholar]. The leaves of mp mutants exhibit discontinuities in the vascular network. Among mp alleles, the severity of the embryo axis phenotype varies in parallel with those of the vascularization and inflorescence post-embryonic phenotypes, suggesting that these architectural features share an underlying molecular machinery. This also suggests that the primary defect in mp embryos is the failure to establish polarity, with the failure to organize a basal region (hypocotyl and radicle) as a consequence. Hardtke and Berleth isolated the MP gene by using positional cloning. The gene encodes a transcription factor (IAA24) recently identified by Ulmasov and co-workers as a factor that recognizes AREs (Ref. [10Ulmasov T. Hagen G. Guilfoyle T.J. ARF1, a transcription factor that binds to auxin response elements.Science. 1997; 276: 1865-1868Crossref PubMed Scopus (705) Google Scholar]). The Arabidopsis IAA24 is highly similar to ARF1, the functionally tested ARE-binding factor, and to VP1, a maize transcriptional activator associated with abscisic acid-related embryo dormancy[13McCarty D.R. et al.The Viviparous-1 developmental gene of maize encodes a novel transcriptional activator.Cell. 1991; 66: 895-905Abstract Full Text PDF PubMed Scopus (529) Google Scholar]. Based on in situ hybridization, the MP mRNA accumulates subepidermally throughout globular-stage embryos, and is gradually restricted to central (provascular) domains of later stages. In emerging organs of the shoot system, a similar pattern was observed, with accumulation throughout emerging primordia becoming restricted to provascular sites. The MP mRNA appeared only in the central vascular cylinder of the root. As the authors point out, this pattern is consistent with a potential role for the MP-encoded factor in the activation of downstream differentiation events in correspondence with the progressive `canalizing' of polar auxin flow into narrowing paths in the developing embryo and post-embryonic plant. Alternatively, the MP products might have a role in narrowing the polar flow itself. It will be intriguing to learn both how the MP product achieves its distribution in the developing embryo and plant body and how the downstream targets of the MP transcription factor enact polarity-dependent processes. If MP does indeed have the pivotal role that its expression pattern and mutant phenotypes suggest, relatively direct molecular genetic strategies should permit the identification of its up- and downstream partners in patterning.