2004/06/01 by Eric M. Engstrom, Anat Izhaki, John L. Bowman · 2 citations
Agricultural and Biological Sciences · Biochemistry, Genetics and Molecular Biology · #Light effects on plants #Plant Molecular Biology Research #Plant Reproductive Biology
paper · doi:10.1104/pp.104.040394
openalex publication_date 2004/06/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/30
The bodies of seed plants are comprised of two classes of organs with contrasting growth and symmetry attributes. Stems and roots are indeterminate organs that exhibit apical growth at apical meristems and radial growth at the vascular cambium, a pattern of growth that results in primarily radially symmetric organs. Lateral organs of the shoot, for example cotyledons, leaves, and floral organs, are determinate and exhibit localized planer growth resulting in breaking of radial symmetry and asymmetric development. Localized planer growth in the leaf generates the leaf blade, the principle site of photosynthesis in most plants. Four axes of development in seed plants. The apical-basal axis (1) of the plant represents a polarity established in embryogenesis with the shoot apical meristem being at the apical end and the root meristem residing at basal tip shown here in a longitudinal section of an Arabidopsis shoot (A). The other 3 axes depicted are defined with reference to this apical-basal axis. The central (ce)-peripheral (pe) axis (2) in the stem (B) is analogous to the adaxial (ad)-abaxial (ab) axis (4) of lateral organs (D) with the central/adaxial end being adjacent to the center of the shoot and the peripheral/abaxial end being away from the center of the shoot meristem (m). Lateral organs such as leaves also exhibit a proximal-distal axis (3) that can also be defined with respect to the stem, with the distal end away from the stem and the proximal end attached to the stem, as shown in the Acer leaf in C. Positions of tissues within vascular bundles develop with respect to the central-peripheral axis with the phloem (ph) positioned peripherally, the xylem (xy) positioned centrally in the stem (B), and the xylem positioned adaxially and phloem abaxially in lateral organs (E). In addition to polar differentiation of vascular tissues, leaf asymmetry is evident with palisade mesophyll (pm) differentiating adaxially and spongy mesophyll (sp) positioned abaxially, as shown in E, a longitudinal section of an Arabidopsis leaf. Lateral organs of the shoot arise from the flanks of shoot meristems and consequently possess an intrinsic positional relationship with the meristem from which they are derived. Two axes of lateral organs, the proximal-distal (Fig. 1C, Axis 3) and adaxial-abaxial (Fig. 1D, Axis 4) axes, may be readily defined with respect to the position of the lateral organ in relationship to the meristem from which the organ is derived. Both these axes are exploited as references for the regulation of asymmetrical development. The proximal-distal axis runs from the base of the organ (proximal, i.e. nearest to the meristem) to the tip (distal, i.e. furthest from the meristem; Fig. 1C). Asymmetric development along the proximal-distal axis is most evident in leaves, in that the amount of lateral growth frequently correlates with location along the proximal-distal axis in a manner that is highly predictable for a given species. The adaxial-abaxial axis runs from the surface of the lateral organ closest to the meristem (ad-adjacent) to the surface of the organ furthest from the meristem (ab-away; Fig. 1D). Asymmetric development in the adaxial-abaxial axis is most evident in many species in the leaf blade, where the adaxial epidermis and underlying mesophyll differentiate to develop characteristics specialized for efficient light capture, while the abaxial epidermis and mesophyll develop characteristics specialized for efficient gas exchange (Fig. 1E). Proper asymmetric development along the adaxial-abaxial axis is therefore often critical for development of a leaf architecture optimized for photosynthesis. It should be noted that the central-peripheral axis of radially symmetric organs and the adaxial-abaxial axis of shoot lateral organs are equivalent. Both axes are defined with respect to the same reference, the apical-basal axis at the center of the shoot meristem. Moreover, the pattern of vascular tissue placement in lateral organs (xylem-adaxial, phloem-abaxial; Fig. 1E) is equivalent to that in radially symmetric organs (xylem-central, phloem-peripheral; Fig. 1B). The ultimate basis of asymmetrical development in all multicellular organisms is the ability of cells to interpret their position with respect to an external reference and transduce this information into asymmetric patterns of cellular differentiation, a process termed polarity establishment. The positional relationship of lateral organs to the meristem from which they are derived indicates that the meristem could in principle serve as the reference for polarity establishment in lateral organs. That this is in fact the case is demonstrated by surgical experiments performed roughly 50 years ago (Warlaw, 1949; Sussex, 1955; Snow and Snow, 1959). When an incision is placed between the meristem and the site where a lateral organ primordia will next emerge (the P0 site), a lateral organ primordia emerges but develops as a radially symmetrical structure with apparently abaxial characteristics. Several tentative conclusions may be drawn from this simple experiment. First, some form of signal emanating from the meristem, transmission of which is interrupted by the presence of the incision, is required for interpretation of the adaxial-abaxial axis. Second, adaxial-abaxial polarity establishment is necessary for blade growth. Third, in the absence of adaxial-abaxial polarity establishment, abaxial identity may be the default state. The surgical experiments provide a conceptual framework in which the results of subsequent genetic analyses of polarity establishment may be interpreted. Beginning in 1995 with characterization of the role of PHANTASTICA (PHAN) in promoting adaxial cell fate in Antirrhinum, the genetic basis of adaxial-abaxial polarity establishment in lateral organs has begun to be elucidated in Antirrhinum, maize, and Arabidopsis (Waites and Hudson, 1995; Tsiantis et al., 1999; Bowman et al., 2002). Results of these studies largely reinforce the conclusions derived from the early surgical experiments. Genetic studies in Arabidopsis have identified several families of genes that play a role in promoting proper adaxial-abaxial development (Table I Partial list of Arabidopsis genes referenced in the text with expression profiles Partial list of Arabidopsis genes referenced in the text with expression profiles Conspicuously missing from the field of Arabidopsis polarity mutants is the equivalent of the phan mutant of Antirrhinum, in which a single loss-of-function mutation generates a dramatic adaxial to abaxial conversion in lateral organs (Waites and Hudson, 1995). Loss-of-function mutations of the Arabidopsis PHAN ortholog, ASYMMETRIC LEAVES1 (AS1), result in only a weakly abaxialized leaf phenotype (Byrne et al., 2000). What is the underlying basis for this apparent mechanistic divergence in polarity establishment between Arabidopsis and Antirrhinum? In this update, we discuss several recent publications that provide insight into the questions stated above and consider how the new findings might best be incorporated into a framework of adaxial-abaxial polarity establishment in Arabidopsis. The class III HD-ZIP genes in Arabidopsis are transcription factors that belong to a family composed of five members: PHB, PHV, and REV which comprise one clade and ATHB8 and ATHB15 which form a separate clade (Emery et al., 2003). All five proteins are characterized by an amino-terminal homeodomain/Leu zipper (HD-ZIP) followed by a region exhibiting sequence similarity to mammalian sterol/lipid-binding domain (START domain) (McConnell et al., 2001). Previous work has shown that PHB, PHV, and REV are involved in establishment of adaxial identity of lateral organs, as well as in the development of the apical meristem and the vascular bundles (McConnell and Barton, 1998; McConnell et al., 2001; Emery et al., 2003). All three genes exhibit similar mRNA expression patterns in apical and floral meristems, vasculature, and the adaxial domain of lateral organ primordia (McConnell et al., 2001; Otsuga et al., 2001; Emery et al., 2003). Gain-of-function mutations in PHB, PHV, and REV suggest that these genes might play slightly different roles in plant development, with PHB and PHV more important for lateral organ patterning and REV in vascular patterning. The other two family members, ATHB8 and ATHB15, are also likely to direct vascular development, although their precise roles are yet to be uncovered (Baima et al., 2001; Ohashi-Ito and Fukuda, 2003). Dominant phb and phv gain-of-function mutations result in an apparent abaxial to adaxial conversion and disruption of lateral growth manifested by filamentous and radially symmetric leaves (McConnell and Barton, 1998; McConnell et al., 2001). In contrast, dominant rev-10d mutations result in defects in vascular patterning in the stem, characterized by radialized amphivasal bundles consisting of xylem surrounding the phloem (Emery et al., 2003). Since these changes in vascular patterning are in the stem, they are independent of patterning events in the leaves. Single loss-of-function mutations in PHB or PHV show no conspicuous effects on lateral organ polarity (McConnell et al., 2001; Emery et al., 2003), while rev loss-of-function plants fail to generate axillary meristems but exhibit normal leaf polarity and stem vascular patterning (Talbert et al., 1995). Since PHB, PHV, and REV exhibit a similar expression pattern it was postulated that these genes act redundantly to promote meristem development and adaxial fate in lateral organs. Indeed, the homozygous triple loss of function mutant, phb phv rev, exhibits a severe loss-of-polarity phenotype, with the apical part of the plant consisting of a single radialized cotyledon and lacking an apical meristem (Emery et al., 2003). Consistent with the hypothesis that these genes promote adaxial fates in lateral organs, the vascular pattern in the radialized cotyledon represents a mirror image of that observed in rev-10d gain-of-function mutant, with phloem surrounding the xylem, the predicted arrangement in abaxialized organs. Molecular and genetic evidence suggests that PHB and KAN genes mutually repress each other's activity in leaf primordia giving rise to two adjacent distinct regions of expression (Eshed et al., 2001). The juxtaposition of adaxial-abaxial domains is proposed to promote lamina growth (Waites and Hudson, 1995). KANADIs and PHABs exhibit a complementary expression pattern not only in the leaf primordia but also in the vasculature. PHABs are expresses in the developing xylem (previously interpreted as the adaxial domain of the vascular bundle), while KANADIs are expressed in the phloem (the abaxial domain; Baima et al., 2001; Emery et al., 2003; Ohashi-Ito and Fukuda, 2003). Complementary loss- and gain-of-function leaf phenotypes of KANADI and PHAB genes indicate that these expression patterns represent functional domains. Also consistent with expression patterns, the stem vasculature phenotype of rev-10d gain-of-function and KANADI triple loss-of-function, kan1 kan2 kan3 are similar. In both mutants the stem vascular bundles are radialized with xylem surrounding phloem (Emery et al., 2003). Thus, the same genetic program controlling polarity in leaves generated from the apical meristem also patterns vascular tissues generated from the procambium in the stem. Since the spatial position of these tissues relative to the central axis of the shoot system is similar, a signal derived from the meristem could activate the PHBs both in the adaxial regions of the leaves and the central region of the stem, thus patterning tissues generated from both meristems (Emery et al., 2003). All identified gain-of-function mutations in PHB, PHV, and REV occur in the START domain, the of this region in the expression of these The START domain is predicted on the basis of sequence to a domain, McConnell et to that PHB may be of a a of the meristem to leaf to this of PHB are expressed but as the leaf primordia a emanating from the meristem PHB in the adaxial of which in adaxial leaf to PHB activity as well as the the is the developing leaf is and not on the meristem for PHB and polarity this the gain-of-function mutations in the START domain proposed to the proteins in the absence of the (McConnell et al., 2001). recent work has identified a different basis of the phb gain-of-function mutant regulation by are to in that have the to expression at the and 2003; and 2003). identified in plants by different et al., et al., et al., et al., 2002). work has shown that the PHABs are the genes that are by et from Arabidopsis by at to and derived from the genes and et for the Arabidopsis that complementary to identified the predicted all five of the class III HD-ZIP with PHB, PHV, and ATHB8 and ATHB15 Since both and exhibit identity to a region within the START domains of all five it is not yet the above are The region with and rev gain-of-function that these genes could be by and that in the site are for the gain-of-function by et the PHV and PHB with are at the sequence complementary to while the dominant gain-of-function phv mRNA of the mutant mRNA can be only a with to the mutant sequence is to the that the of the to the mutant mRNA was a direct of plants provide for the role of in the regulation of PHABs in Arabidopsis plants with a REV mutations the complementary site but not the REV exhibit a phenotype similar to that of the rev-10d gain-of-function mutant (Emery et al., 2003). It was that the disruption of the site to of REV expression that in the Loss-of-function of are in several in Arabidopsis and are weakly et al., 2003). In a kan1 mutations in the loss of abaxial identity with the triple mutant characterized by development adaxial tissues, and in abaxial in the (Eshed et al., 2001). a similar to the mammalian a et al., 2003). it has demonstrated that efficient of from the to the where they are by III family to the et al., 2003; et al., Thus, one for the phenotype of mutants is that to promote abaxial identity postulated role in In this loss of activity to a in and of the to the family consisting of predicted in Arabidopsis. in the family might the polarity phenotype in the single mutant and the conspicuous phenotype in the triple kan1 In the absence of other genes promoting abaxial such as the polarity in is new regulation of the PHAB genes the START domain, the be is no the function of the START domain as a the the START domain might part in both and serve as a for a emanating from the meristem, as well as an which regulation of the The role of the genes in promoting adaxial-abaxial polarity establishment is that of the PHAB and KANADI may be largely the result of apparent functional family members, which to has of a loss of function mutant, and to the fact that expression of genes generates only a adaxial to abaxial conversion of cellular identity et al., 1999; et al., The Arabidopsis family of Two of and show being to the and and 1999; et al., of the and are expressed more in abaxial regions of the shoot lateral organs et al., 1999; et al., and are genes and possess identity not only in their but also a of their In a recent and to provide a of a for of a mutant phenotype, is to the three most abaxial cell of leaf primordia with expression slightly in the a pattern similar to that observed from in studies et al., 1999; et al., In of expression to abaxial cell could be by several distinct in the abaxial in the adaxial or of into the abaxial region in to a and to to several and the structure of the a of adaxial abaxial of the as elucidated by and The in the is at the of the is above the the two and the are by The is defined by the of all regions are depicted to What is the adaxial of the regulation of the to be to the abaxial regions of the that axis establishment early in the establishment of the and Consistent with this of the results in the of expression within the and other are more likely to play a role in asymmetrical differentiation in to a established adaxial-abaxial axis to play a role in establishment of the axis a where of in adaxial regions is the of a that is adaxially localized to the of expression is as early as the organ in the gain-of-function mutant et al., PHB, expression domain to the region of primordia in which is an for adaxial of and suggest that the is similar to of the and may be the of an Arabidopsis and mutants have no conspicuous leaf phenotype, mutants exhibit loss of abaxial most evident in the abaxial and of floral organs et al., 1999; et al., 2002). of mutants also exhibit and of shoot meristems, at the site of or on the adaxial leaf most in the more proximal regions of the blade and to the leaf of meristems on the leaf lamina is not a normal of adaxial or abaxial leaf is it observed in the most severe loss-of-polarity indicate that of meristems in the leaf is not a polarity but a loss of organ that meristems arise only at of or on the adaxial surface and not on the abaxial leaf surface suggests a between organ polarity and organ is consistent with that of adaxial identity meristem while of abaxial identity is to meristem and (McConnell and Barton, 1998; et al., 2001; et al., 2001). of meristems on the adaxial leaf surface is also of plants that class I genes et al., et al., 2000). In the class I family is comprised of the and I genes are transcription factors that promote meristem and are in the organ et al., et al., et al., 2002). The phenotypes of and plants indicate that class I genes promote and genes repress meristem in the leaf. The that class I genes are by genes is the of a recent by et transcription to of three class I and in leaves of and plants et al., 2002). of and are not in leaves but are in and leaves. of are at in leaves but at in and leaves. The leaves for not possess meristems, a fact by the absence of the that meristems are the of results are consistent with a and of meristem in the leaf is by of class I The et also of at of function between and is in leaves at in leaves and at the in leaves. is at in relative to leaves and at the in leaves. is at similar in both and leaves but at a in leaves. these results in the they indicate that in the of genes act with only a in these is that et not of expression and therefore these be as of relative by and repress I expression leaf primordia or only within their domain of abaxial of a into a mutant in expression to the abaxial of leaf consistent with of class I genes being to the expression domain and in a That proteins may act by from the abaxial to the adaxial regions of the leaf et al., et al., 2003), and that a of and PHAB activity may be required for the of shoot meristems (Emery et al., 2003), could meristems develop from the adaxial leaf surface although expression in plants is to the abaxial regions of leaves. which we have noted as the Arabidopsis of a family transcription and is expressed organ primordia (Byrne et al., 2000). a family transcription and is expressed in the adaxial region of leaf primordia et al., 2002). and mutants have similar leaf that and act in the same genetic has by experiments that and in that they act in a single et al., 2003). expression of class I genes in the leaf is of both and and as in mutants meristems on the adaxial leaf surface (Byrne et al., although with a in the effects of to and on leaf are with plants exhibiting leaves with lateral and and plants exhibiting leaves with an What is the of class I in leaf primordia being the of localized to repress class I genes the basis of the and leaf phenotype or is along the of the to a proposed is the basis of the apparent mechanistic divergence between the PHAN of and in on the two questions has by results that promote adaxial identity in the leaf. studies have expression of the regulation of the to the function of in leaf development et al., et al., 2003; et al., 2003). the of functional interpretation of loss-of-function plants exhibit a of with some exhibiting to kan1 kan2 plants (Eshed et al., radialized and leaves and the of on the abaxial leaf the of KANADI by severe phenotypes exhibit of shoot and root The arrangement of the vascular tissues in the leaf is in plants with xylem cells largely surrounding are also evident in the mesophyll where cells palisade to the of cells adjacent to the adaxial are the of the leaf. these phenotypes are consistent with an of adaxial leaf a role for in adaxial-abaxial leaf cells of the adaxial and abaxial leaf of and of and leaf the and of cells in the leaves and of plants are is no abaxial to adaxial pattern of tissue conversion is the of that observed in where adaxial to abaxial are most apparent in the epidermis but are not conspicuous in the mesophyll et al., results may be interpreted as that the epidermis is more to activity tissues and that tissues are more to activity Consistent with the phenotype of expression of class I genes in and et observed of and in by in expression are not apparent in of regulation of class I genes with studies expression of and but not in leaves et al., 2001). results a divergence between and of class I in that to be required for of in the leaf while we noted is an apparent between polarity and regulation of leaf in that meristems are on the adaxial but not the abaxial leaf of meristem to the adaxial leaf surface may a for PHB (Emery et al., 2003), to the adaxial domain of the in meristem interpretation is consistent with the of axillary meristems in the abaxial base of the leaf in gain-of-function mutants (McConnell and Barton, et the expression of and PHB in the mutant and in plants. changes in of expression are for of the polarity genes in relative to that is for normal regulation of these polarity a consistent with the polarity phenotypes of and In contrast, expression of the abaxial and is in while the expression of the adaxial promoting PHB is consistent with the interpretation of the phenotype as expression of is therefore to abaxial to adaxial conversion at both the and one for the abaxialized phenotype of phan mutants and the weakly abaxialized phenotype of mutants is that of adaxial identity is not a function of is by the results of the et that the as a of adaxial and for the polarity phenotype of and mutants is that activity is for the of adaxial identity with other for this role the PHAB which along with are predicted to repress the the of PHB is in of the KANADI is observed in Arabidopsis. factors redundantly with may to be identified or as in promoting adaxial The given that the loss-of-function phenotype of such genes be predicted to be only weakly framework of adaxial-abaxial polarity establishment in lateral organs of Arabidopsis. arise from the flanks of shoot The organ is by the expression of and of class I The apical meristem is the of a of the signal the PHB PHB transcription and transcription of the and KANADI PHB in is at the and by the KANADI genes and The KAN to the hypothesis that regulation of PHB expression by KANADIs could occur in part or in KANADI of PHB and KANADI are predicted to into adaxial and abaxial domains of and this may the adaxial-abaxial axis of the lateral of genes is predicted on the structure of the to be localized to the abaxial domain of the organ by a of the with in the adaxial genes can promote differentiation of abaxial cell is predicted to be expressed in the adaxial organ domain where a with to promote of KANADI and differentiation of adaxial cell between and other of polarity establishment as the to which axis establishment, polar differentiation, or both Both genes and repress expression of class I of adaxial and abaxial organ domains is a for the establishment of lateral growth. The into the of adaxial-abaxial polarity establishment in vascular plants from experiments surgical of the apical meristem (Warlaw, 1949; Sussex, 1955; Snow and Snow, 1959). experiments have an important conceptual framework for the results of genetic The is also is of to interpret the surgical experiments in the framework of the recent and et al., 2003). et and of meristem regions with of the expression of the and genes to the basis of meristem and the function of the meristem in meristem growth and organ of meristem cells with expression could be to important questions of adaxial-abaxial polarity establishment. PHB and PHV are as of a signal and are predicted to their of cells between the meristem and the P0 to adaxial-abaxial polarity establishment, PHB expression and PHB of the between meristem and the P0 site to adaxial-abaxial polarity establishment or are meristem the radially abaxialized organs that result from incision between the meristem and P0 exhibit expression of KANADI The may be for the of surgical and to the of lateral organ and for to