Shared body plans of lycophytes inferred from root formation of Lycopodium clavatum

Late Silurian to early Devonian lycophytes had prostrate aerial axes, while subordinate organs or subterranean axes were formed around the dichotomies of the axes. The subterranean axes are hypothesized to have evolved into root-bearing axes (rhizophores) and roots in extant Selaginellaceae and Lycopodiaceae, respectively. Consistent with this hypothesis, rhizophores are formed on the dichotomies of shoots in Selaginellaceae. However, it has remained unclear whether roots are borne in the same position in Lycopodiaceae. In addition, roots form endogenously in the stem, but no data are available regarding the tissues in stem from which they arise. In this study, we tracked the root development in the clubmoss, Lycopodium clavatum, based on anatomical sections and 3D reconstructed images. The vascular tissue of the stem is encircled by ground meristem, which supplies cortical cells outwardly by periclinal divisions. A linear parenchymatous tissue is present on the ventral side of vascular cylinder, which we call “ventral tissue” in this study. We found that root primordia are formed endogenously on the ventral side of stem, possibly from the ventral tissue. In addition, roots always initiate at positions close to dichotomies of stem. The root-initiating position supports the suggestion that Lycopodium roots share a body plan with the subterranean organs of the hypothesized ancestry.

Elevated mutation rates underlie the evolution of the aquatic plant family Podostemaceae

Molecular evolutionary rates vary among lineages and influence the evolutionary process. Here, we report elevated genome-wide mutation rates in Podostemaceae, a family of aquatic plants with a unique body plan that allows members to live on submerged rocks in fast-flowing rivers. Molecular evolutionary analyses using 1640 orthologous gene groups revealed two historical increases in evolutionary rates: the first at the emergence of the family and the second at the emergence of Podostemoideae, which is the most diversified subfamily. In both branches, synonymous substitution rates were elevated, indicating higher mutation rates. On early branches, mutations were biased in favour of AT content, which is consistent with a role for ultraviolet light-induced mutation and habitat shift. In ancestors of Podostemoideae, DNA-repair genes were enriched in genes under positive selection, which may have responded to the meristem architectural changes.

Lycopodium root meristem dynamics supports homology between shoots and roots in lycophytes

Roots have played a pivotal role in the conquest of land by vascular plants, yet their origin has remained enigmatic. Palaeobotanical evidence suggests that roots may have originated from subterranean shoots in some lycophyte species. If this hypothesis is correct, it would follow that the roots and shoots of extant lycophytes share fundamental developmental mechanisms. We tracked meristem dynamics in root and shoot apices of Lycopodium clavatum using a thymidine analogue and expression patterns of histone H4, respectively. Then we compared the meristem dynamics of roots and shoots to identify developmental similarities. Both apical meristems contained a quiescent tissue characterised by a low frequency of cell division. Actively dividing cells appeared in the quiescent tissue during dichotomous branching of both roots and shoots. As a result, the parental meristem divides into two daughter meristems, which give rise to new root or shoot apices. These striking similarities in meristem dynamics provide new neobotanical data that support the shoot-origin hypothesis of lycophyte roots. Although Lycopodium roots may have originated from subterranean shoots of Devonian lycophytes, these shoots may have changed into root-bearing axes in other extant lycophyte lineages.

Root apical meristem diversity and the origin of roots: insights from extant lycophytes

The independent origin of roots in lycophytes and euphyllophytes has been proposed, mainly based on paleobotanical records. However, the question of how roots evolved within these lineages remains unresolved. Root apical meristem (RAM) organization in lycophytes would provide a clue toward understanding the early evolution of roots. Recently, we examined RAM organization in lycophytes (Lycopodiaceae, Isoetaceae, and Selaginellaceae) in terms of cell division activity and anatomy, comparing RAM among vascular plants. Lycophyte RAM exhibited four organization types (I, II, III, and apical); thus, RAM organization in extant lycophytes was more diverse than expected. Type I RAM contained a region with very low cell division frequency, reminiscent of the quiescent center (QC) in seed plant RAM. Although some euphyllophyte RAMs were structurally similar to types II and III and apical cell-type RAM, lycophyte RAM of types II and III had no QC-like area. These results support the paleobotanical predictions that roots evolved several times in lycophytes, as well as in euphyllophytes. In this review, we also introduce recent findings on RAM organization in extant lycophytes and discuss the origin of roots in vascular plants.

Fern gametophytes of Angiopteris lygodiifolia and Osmunda japonica harbor diverse Mucoromycotina fungi

Mycorrhizal symbiosis between plants and fungi is ubiquitous, and has been played key roles in plant terrestrialization and diversification. Although arbuscular mycorrhizal (AM) symbioses with Glomeromycotina fungi have long been recognized as both ancient and widespread symbionts, recent studies showed that Mucoromycotina fungi were also ancestral symbionts and would thus be expected to co-exist with many land plants. To explore whether Mucoromycotina colonize fern gametophytes, we subjected fungal associations with gametophytes of two distantly related ferns, Angiopteris lygodiifolia (Marattiales) and Osmunda japonica (Osmundales), to molecular analysis. Direct PCR amplification from intracellular hyphal coils was also performed. We detected Mucoromycotina sequences in the gametophytes of A. lygodiifolia and O. japonica at rates of 41% (7/17) and 50% (49/98) of gametophytes, respectively, and assigned them to 10 operational taxonomic units of Endogonales lineages. In addition, we used AM fungal-specific primers and detected Glomeromycotina sequences in all individuals examined. The results suggest that Glomeromycotina and Mucoromycotina colonized fern gametophytes simultaneously. We found that Mucoromycotina were present in fern gametophytes of Marratiales and Osmundales, which implies that a variety of fern taxa have Mucoromycotina associations.

Expression pattern of CUC3 ortholog in Zeylanidium tailichenoides (Podostemaceae) infers organization of a unique distichous shoot in Podostemoideae

Shoots of the aquatic eudicot family, Podostemaceae, exhibit unusual organogenesis with mixed leaf and stem identities. New shoots arise at the base of the older shoot with shoot apical meristem (SAM) identity but the entire SAM differentiates into a "leaf" as it develops in the Podostemoideae subfamily. The "leaves" are tightly arranged in a zigzag manner to form an apparent distichous shoot as a whole. Although previous studies have suggested that Podostemoideae shoots have evolved by modifying the ancestral sympodial branching system in the basal Tristichoideae subfamily, this evolutionary scenario requires elucidation at the molecular level. To confirm that the shoots arise as axillary shoots, in the present study, we examined gene expression patterns in plumular shoots of Zeylanidium tailichenoides using CUP-SHAPED COTYLEDON 3 (CUC3) and SHOOT MERISTEMLESS (STM) orthologs, which are involved in the determination of axils and meristem formation in model plants. Expression of the CUC3 ortholog was detected at the adaxial base of cotyledons and parental shoots where the new shoots are initiated, while STM ortholog was expressed at the initiation site and in the young shoot primordia throughout early shoot development. The results demonstrate that each Z. tailichenoides shoot arises as an axillary bud in a manner similar to axillary meristem formation in model plants involving CUC3 and STM genes. Considering that each of the two cotyledons produces an axillary bud that in turn continues to form its own axillary bud independently, the apparent distichous shoot in Z.tailichenoides is not a single shoot, but a composite of two sympodially branched shoots.

Evolution of root apical meristem structures in vascular plants: plasmodesmatal networks

PREMISE OF THE STUDY

The apical meristem generates indeterminate apical growth of the stem and root of vascular plants. Our previous examination showed that shoot apical meristems (SAMs) can be classified into two types based on plasmodesmatal networks (PNs), which are important elements in symplasmic signaling pathways within the apical meristem. Here, we examined the PNs of root apical meristems (RAMs) in comparison with those of SAMs.

METHODS

Root apical meristems of 18 families and 22 species of lycophytes and euphyllophytes were analyzed. Plasmodesmata (PD) in cell walls in median longitudinal sections of RAMs were enumerated using transmission electron micrographs, and the PD density per 1 μm² of each cell wall was calculated.

KEY RESULTS

Root apical meristems with prominent apical cells of monilophytes (euphyllophytes) and Selaginellaceae (lycophytes) had high PD densities, while RAMs with plural initial cells of gymnosperms and angiosperms (euphyllophytes), and of Lycopodiaceae and Isoetaceae (lycophytes) had low PD densities. This correlation between structures of apical meristems and PD densities is identical to that in SAMs already described.

CONCLUSIONS

Irrespective of their diversified structures, the RAMs of vascular plants can be classified into two types with respect to PNs: the fern (monilophyte) type, which has a lineage-specific PN with only primary PD, and the seed-plant type, which has an interspecific PN with secondary PD in addition to primary PD. PNs may have played a key role in the evolution of apical meristems in vascular plants.

Root apical meristem diversity in extant lycophytes and implications for root origins

Root apical meristem (RAM) organization in lycophytes could be a key to understanding the early evolution of roots, but this topic has been insufficiently explored. We examined the RAM organization of lycophytes in terms of cell division activities and anatomies, and compared RAMs among vascular plants. RAMs of 13 species of lycophytes were semi-thin-sectioned and observed under a light microscope. Furthermore, the frequency of cell division in the RAM of species was analyzed using thymidine analogs. RAMs of lycophytes exhibited four organization types: type I (Lycopodium and Diphasiastrum), II (Huperzia and Lycopodiella), III (Isoetes) and RAM with apical cell (Selaginella). The type I RAM found in Lycopodium had a region with a very low cell division frequency, reminiscent of the quiescent center (QC) in angiosperm roots. This is the first clear indication that a QC-like region is present in nonseed plants. At least four types of RAM are present in extant lycophytes, suggesting that RAM organization is more diverse than expected. Our results support the paleobotanical hypothesis that roots evolved several times in lycophytes, as well as in euphyllophytes.

Arbuscular mycorrhizal colonization in field-collected terrestrial cordate gametophytes of pre-polypod leptosporangiate ferns (Osmundaceae, Gleicheniaceae, Plagiogyriaceae, Cyatheaceae)

To determine the mycorrhizal status of pteridophyte gametophytes in diverse taxa, the mycorrhizal colonization of wild gametophytes was investigated in terrestrial cordate gametophytes of pre-polypod leptosporangiate ferns, i.e., one species of Osmundaceae (Osmunda banksiifolia), two species of Gleicheniaceae (Diplopterygium glaucum, Dicranopteris linearis), and four species of Cyatheales including tree ferns (Plagiogyriaceae: Plagiogyria japonica, Plagiogyria euphlebia; Cyatheaceae: Cyathea podophylla, Cyathea lepifera). Microscopic observations revealed that 58 to 97% of gametophytes in all species were colonized with arbuscular mycorrhizal (AM) fungi. Fungal colonization was limited to the multilayered midrib (cushion) tissue in all gametophytes examined. Molecular identification using fungal SSU rDNA sequences indicated that the AM fungi in gametophytes primarily belonged to the Glomeraceae, but also included the Claroideoglomeraceae, Gigasporaceae, Acaulosporaceae, and Archaeosporales. This study provides the first evidence for AM fungal colonization of wild gametophytes in the Plagiogyriaceae and Cyatheaceae. Taxonomically divergent photosynthetic gametophytes are similarly colonized by AM fungi, suggesting that mycorrhizal associations with AM fungi could widely occur in terrestrial pteridophyte gametophytes.

Habitat specificity enhances genetic differentiation in two species of aquatic Podostemaceae in Japan

PREMISE OF THE STUDY

Species with habitat specificity show restricted distribution, and the limited dispersal provides opportunity for long-term isolation. Aquatic Podostemaceae grow on rocks in river rapids, which are extreme habitats for angiosperms. To infer the diversification process of the podostemad species in such specific habitats, we investigated environmental factors shaping their distribution and the relationship between their distribution and genetic structure.

METHODS

We used Hydrobryum japonicum and Cladopus doianus, which have contrasting distributions (i.e., narrower and wider, respectively) in southern Kyushu, Japan. Environmental factors contributing to their distribution were estimated by ecological niche modeling. Using polymorphisms in chloroplast DNA, we performed population genetic analyses of 13 H. japonicum populations occurring in nearby river basins and eight C. doianus populations in distant rivers.

KEY RESULTS

Estimation of distribution probability showed great contributions of geology and temperature to the distribution of these populations, suggesting that the species grow on volcanic rocks in relatively warm areas. Genetic analyses revealed higher interpopulational genetic diversity than intrapopulational diversity and strong differentiation between rivers in both species. No correlation between genetic and geographic distances was detected among the C. doianus populations, in contrast to the significant correlation observed in the H. japonicum populations.

CONCLUSIONS

The high-level genetic differentiations among populations of the two species may result from their limited dispersal. Their restricted habitats, which are primarily characterized by volcanic rocks in rapids and lower temperatures in winter, may enhance isolation among populations in distant rivers.

Developmental morphology of flattened shoots in Dalzellia ubonensis and Indodalzellia gracilis with implications for the evolution of diversified shoot morphologies in the subfamily Tristichoideae (Podostemaceae)

PREMISE OF THE STUDY

Podostemaceae is a unique family of aquatic angiosperms that grow in swift-running water on rock surfaces in the tropics. Their plant bodies show a remarkable adaptation: the main plant body is mostly creeping or flattened, or in extreme cases foliose, functioning as an adhering and photosynthetic organ. In the subfamily Podostemoideae, the root is foliose, whereas in the subfamily Tristichoideae, the shoot is foliose. An evolutionary scenario for the foliose root has already been proposed, but that for the foliose shoot remains to be addressed.

METHODS

Shoots of Indodalzellia gracilis and Dalzellia ubonensis (subfamily Tristichoideae) were observed using light microscopy and scanning electron microscopy. Gene expression patterns of orthologs of marker genes for the shoot apical meristem, i.e., SHOOT MERISTEMLESS and WUSCHEL, in D. ubonensis were analyzed.

KEY RESULTS

When very young, the phyllotaxis is tristichous in both genera: a set of one dorsal and two marginal leaves forms. When the shoot branches, extra-axillary buds of two subsequent marginal leaves form as new (lateral) shoots, and the original shoot stops growing; this growth pattern is called sympodial branching. Due to zonal growth in the common zone just below the original and lateral shoot apices, flattened or foliose shoots result. The expression patterns of DuSTM and DuWUS in the shoot apices of Dalzellia were similar to those published for Terniopsis.

CONCLUSIONS

The foliose shoots of Indodalzellia and Dalzellia evolved as a result of congenital fusion among several original and lateral branches, each of which grows separately in other Tristichoideae.

Developmental morphology of the typical cordate gametophyte of a homosporous leptosporangiate fern, Lygodium japonicum (Lygodiaceae), focusing on the initial cell behavior of two distinct meristems

PREMISE OF THE STUDY

Understanding the origin and early evolution of vascular plants requires thorough consideration of the gametophyte generation of ferns and lycophytes. Unfortunately, information about this generation is quite limited. To reveal the origin and evolution of varied gametophyte shapes, we used comparative morphological studies of meristem behavior of gametophytes of Lygodium japonicum, which exhibit the typical cordate shape.

METHODS

Microscopic images of epi-illuminated growing gametophytes cultured from spores were captured periodically using a metallurgical microscope equipped with a digital camera to analyze the cell lineage in the meristem.

KEY RESULTS

Gametophytes form from two meristems: the apical-cell-based meristem and the multicellular meristem. The triangular apical cell produces six to eight derivatives from two lateral facets, then disappears. Subsequently, the multicellular meristem, with a row of several rectangular cells, forms in the notch. These rectangular cells divide asynchronously in the periclinal and anticlinal walls to produce cells to both lateral sides and downward. Usually two, and sometimes three, cells located at the center of the meristem divide at a slower pace in the periclinal and anticlinal planes than others at the periphery. The cells at the periphery are pushed away and become involved in the wing base.

CONCLUSIONS

The triangular apical cell behaves as a permanent initial cell. In the multicellular meristem, however, two or three central cells behave as initial cells that are transient and regulated in a position-dependent manner. The organization and behavior of both meristems are shared with the ribbon-shaped gametophytes of Colysis.

A survey of the fern gametophyte flora of Japan: Frequent independent occurrences of noncordiform gametophytes

PREMISE OF THE STUDY

Ferns and lycophytes are the only extant land plants with two free-living generations (sporophytes and gametophytes); hence, a single species may have two different distributions. The distribution of the gametophytes of most fern species, which are much smaller in size than sporophytes, are almost unknown due to the difficulty of identifying gametophytes using morphological characters.

METHODS

Twelve quadrats (1 m(2) or 0.25 m(2)), each subdivided into a grid of 100 (10 × 10) or 25 (5 × 5) squares, were used to survey gametophytes in the Japanese Archipelago, where distribution data of sporophytes and "DNA barcodes" for identification of gametophytes have fully been established in previous studies. Collected gametophytes were identified using the plastid rbcL-a region.

KEY RESULTS

In total, gametophytes of 38 species in two broad morphological categories (28 cordiform and 10 noncordiform species) were identified among 407 collections. The cordiform gametophytes discovered are without exception accompanied by their conspecific sporophytes at the periphery of the quadrats. On the other hand, the sporophytic counterparts of the noncordiform gametophytes are often not found or are rare around the sites.

CONCLUSIONS

This study demonstrates with a regional flora that fern gametophytes do not always co-occur with sporophytes of the same species. In particular, noncordiform gametophytes tended to occur independently of conspecific sporophytes. This pattern may be due to the capability for indeterminate growth and vegetative reproduction by gemmae in noncordiform gametophytes.

Arbuscular mycorrhiza formation in cordate gametophytes of two ferns, Angiopteris lygodiifolia and Osmunda japonica

Mycorrhizal symbiosis is common among land plants including pteridophytes (monilophytes and lycophytes). In pteridophytes with diplohaplontic life cycle, mycorrhizal formations were mostly reported for sporophytes, but very few for gametophytes. To clarify the mycorrhizal association of photosynthetic gametophytes, field-collected gametophytes of Angiopteris lygodiifolia (Marattiaceae, n = 52) and Osmunda japonica (Osmundaceae, n = 45) were examined using microscopic and molecular techniques. Collected gametophytes were mostly cut into two pieces. One piece was used for light and scanning microscopic observations, and the other for molecular identification of plant species (chloroplast rbcL sequences) and mycorrhizal fungi (small subunit rDNA sequences). Microscopic observations showed that 96 % (50/52) of Angiopteris and 95 % (41/43) of Osmunda gametophytes contained intracellular hyphae with arbuscules and/or vesicles and fungal colonization was limited to the inner tissue of the thick midribs (cushion). Fungal DNA analyses showed that 92 % (48/52) of Angiopteris and 92 % (35/38) of Osmunda have sequences of arbuscular mycorrhizal fungi, which were highly divergent but all belonged to Glomus group A. These results suggest that A. lygodiifolia and O. japonica gametophytes consistently form arbuscular mycorrhizae. Mycorrhizal formation in wild fern gametophytes, based on large-scale sampling with molecular identification of host plant species, was demonstrated for the first time.

Comparative development of heavily asymmetric-cordate gametophytes of Anemia phyllitidis (Anemiaceae) focusing on meristem behavior

Development of heavily asymmetric cordate gametophytes of Anemia phyllitidis (Anemiaceae), one of the schizaeoid ferns, was examined using a sequential observation technique; epi-illuminated light micrographs of the same growing gametophytes were taken approximately every 24 h. The apical cell-like wedge-shaped cell was produced once from the terminal cell of a germ filament, but it stopped dividing soon after production of one or two derivative cells. Without a functional apical cell, the gametophyte developed by intercalary growth until the early stage of wing formation, and then the multicellular (pluricellular) meristem arose from the lower lateral side of the gametophyte. This was in sharp contrast to the observation that the multicellular meristem forms in place of the apical cell in typical cordate gametophytes. Loss of the functional apical cell probably caused a site-shift in the multicellular meristem of the Anemia phyllitidis gametophyte during evolution from apical to lateral. The results suggest that apical cell-based and multicellular meristems are primarily independent of each other. The multicellular meristem produced cells equally in the distal and proximal directions to form wings in both directions but proximally produced cells divided much less frequently. As a result, a heavily asymmetric gametophyte was formed.

Dimorphic chloroplasts in the epidermis of Podostemoideae, a subfamily of the unique aquatic angiosperm family Podostemaceae

Plants of the Podostemoideae, a subfamily of the unique aquatic angiosperm family Podostemaceae, which are found in rapids and waterfalls of the tropics and subtropics, have two different sizes of chloroplasts in their epidermis. These small and large chloroplasts are located separately in each epidermal cell along its upper and inner tangential walls, respectively. This is the first case of the chloroplast dimorphism in a single epidermal cell of angiosperms. While the large chloroplasts have well developed starch grains, the small chloroplasts have a normal granal ultrastructure but very few starch grains. This suggests that the small chloroplasts mainly function in CO(2) uptake for photosynthesis from torrential water.

Ancestral expression patterns and evolutionary diversification of YABBY genes in angiosperms

Lateral organ growth in seed plants is controlled in part by members of the YABBY (YAB) and class III homeodomain/leucine zipper (HD-ZIPIII) families of transcription factors. HD-ZIPIII genes appear to play a conserved role in such organs, but YAB genes have diversified, with some members of the family having specialized functions in leaves, carpels or ovule integuments. The ancestral expression patterns and timing of divergence of the various classes of YAB genes remain to be established. We isolated and evaluated the expression of one HD-ZIPIII and five YAB genes representing the five major YAB gene classes from Cabomba caroliniana, a member of the earliest-diverging angiosperms. Consistent with observations in eudicots, the FILAMENTOUS FLOWER (FIL) and YABBY5 (YAB5) genes of C. caroliniana were expressed in the abaxial regions of the leaf where new laminar segments arise, and the patterns of expression were mutually exclusive to those of HD-ZIPIII, indicating that these expression patterns are ancestral. Expression of CRABS CLAW (CRC) in the abaxial carpel wall, and of INNER NO OUTER (INO) in the abaxial outer integument of ovules was also conserved between eudicots and C. caroliniana, indicating that these patterns are primitive. However, the CRC gene was also expressed in other floral organs in C. caroliniana, and expression in stamens was also observed in another early-diverging species, Amborella trichopoda, indicating that carpel-specific expression was acquired after divergence of the Nymphaeales. The expression data and phylogeny for YAB genes suggest that the ancestral YAB gene was expressed in proliferating tissues of lateral organs.

Developmental morphology of strap-shaped gametophytes of Colysis decurrens: a new look at meristem development and function in fern gametophytes

BACKGROUND AND AIMS

The gametophytes of most homosporous ferns are cordate-thalloid in shape. Some are strap- or ribbon-shaped and have been assumed to have evolved from terrestrial cordate shapes as an adaptation to epiphytic habitats. The aim of the present study was to clarify the morphological evolution of the strap-shaped gametophyte of microsoroids (Polypodiaceae) by precise analysis of their development.

METHODS

Spores of Colysis decurrens collected in Kagoshima, Japan, were cultured and observed microscopically. Epi-illuminated micrographs of growing gametophytes were captured every 24 h, allowing analysis of the cell lineage of meristems. Light microscopy of resin-sections and scanning electron microscopy were also used.

KEY RESULTS

Contrary to previous assumptions that strap-shaped Colysis gametophytes have no organized meristem, three different types of meristems are formed during development: (1) apical-cell based - responsible for early growth; (2) marginal - further growth, including gametophyte branching; and (3) multicellular - formation of cushions with archegonia. The cushion is two or three layers thick and intermittent. The apical-cell and multicellular meristems are similar to those of cordate gametophytes of other ferns, but the marginal meristem is unique to the strap-shaped gametophyte of this fern.

CONCLUSIONS

The strap-shaped gametophytes of C. decurrens may have evolved from ancestors with a cordate shape by insertion of the marginal meristem phase between the first apical-cell-based meristem and subsequent multicellular meristem phases. Repeated retrieval of the marginal meristem at the multicellular meristem phase would result in indefinite prolongation of gametophyte growth, an ecological adaptation to epiphytic habitats.

Developmental anatomy of Terniopsis malayana (Podostemaceae, subfamily Tristichoideae), with implications for body plan evolution

The developmental morphology of Terniopsis malayana, an unusual aquatic angiosperm from Thailand, was examined. A unique vegetative structure called the "ramulus" arises endogenously in the root tissue. The ramulus has an actively growing apical meristem. The ramulus branches several times to form a "ramulus complex" consisting of up to six ramuli, which are distichously arranged in almost a single plane. In a ramulus complex, the new ramulus (ramulus branch) is initiated on the adaxial side of the first (the basalmost) scale in the first ramulus, but at a site lateral to the first scale in later ramuli, suggesting that the new ramulus arises from axillary or extra-axillary buds of the immediately older ramulus. Ramulus growth is terminated in association with the loss of the apical meristem, and its axillary or extra-axillary buds begin to grow to form the next new ramulus instead. The flower occurs in place of the youngest ramulus, when reproductive. It seems likely that the Terniopsis ramulus and its scale are comparable to the shoot and leaf, and thus a ramulus complex is interpreted as a sympodially branched shoot.

AINTEGUMENTA homolog expression in Gnetum (gymnosperms) and implications for the evolution of ovulate axes in seed plants

The expression of GpANTL1, a homolog of AINTEGUMENTA (ANT) found in the gymnosperm Gnetum parvifolium, was analyzed by RT-PCR and in situ hybridization. GpANTL1 was expressed in the leaf primordia, root tips, and young ovules. In the ovulate axis, expression was detected as four distinct rings around the outer, middle, and inner envelope primordia as well as around the nucellar tip. This pattern of expression is similar to that of ANT in Arabidopsis thaliana. A comparison of the expression of GpANTL1 with that of PtANTL1 in the conifer Pinus thunbergii suggests that the integrated expression of PtANTL1 may have been caused by congenital fusion of the integument, ovuliferous scale, and bract.

Taxonomic status of Monotropastrum humile, with special reference to M. humile var. glaberrimum (Ericaceae, Monotropoideae)

Taxonomic treatment of the achlorophyllous monotropoid plant Monotropastrum humile is still unclear and confusing because of the lack of detailed morphological analyses and molecular phylogeny. In particular, the taxonomic status of a glabrous variety, M. humile var. glaberrimum, is under debate. Our detailed examination of the morphological characteristics of living plants revealed that M. humile var. glaberrimum can be easily distinguished from the putative conspecific taxon M. humile var. humile by characteristics not previously recognized, namely the shape and color of the floral disc. Most morphological features characterizing Cheilotheca were also found in M. humile var. glaberrimum. Moreover, there was considerable nucleotide differentiation in the internal transcribed spacer (ITS)2 sequences of M. humile var. humile and var. glaberrimum. Molecular analysis of the phylogenetic relationship of M. humile var. humile, var. glaberrimum, and other monotropoids using ITS2 sequences showed that two varieties of M. humile formed a monophyletic clade with a member of a different genus, Monotropa L., but obvious phylogenetic relationships among these three taxa were not obtained. Thus we conclude that Monotropastrum humile var. glaberrimum should be treated as a distinct species. However, the generic affiliation of M. humile var. glaberrimum could not be determined because of its intermediate character state combination and the insufficient characterization of related species. We strongly suggest that Monotropastrum as a whole needs re-evaluation.

Evolution of shoot apical meristem structures in vascular plants with respect to plasmodesmatal network

Vascular plants have evolved shoot apical meristems (SAMs), whose structures differ among plant groups. To clarify the evolutionary course of the different structural types of SAMs, we compared plasmodesmatal networks in the SAMs for 17 families and 24 species of angiosperms, gymnosperms, and pteridophytes, using transmission electron microscopy (TEM). The plasmodesmata (PD) in almost all cell walls in median longitudinal sections of SAMs were counted, and the PD density per unit area was calculated for each cell wall. Angiosperm and gymnosperm SAMs have low densities, with no difference between stratified (tunica-corpus) and unstratified structures. SAMs of ferns, including Psilotum and Equisetum, have average densities that are more than three times higher than those of seed plants. Interestingly, microphyllous lycopods have both the fern and seed-plant types of PD networks; Selaginellaceae SAMs with single apical cells have high PD densities, while SAMs of Lycopodiaceae and Isoetaceae with plural initial cells have low PD densities, equivalent to those of seed plants. In summary, PD networks are strongly correlated to SAM organizations-SAMs with single and plural initial cells have the fern and seed-plant types of PD, respectively. The two SAM organizations may have evolved separately in lycophytes and euphyllophytes and may be associated with gain or loss of the ability to form secondary PD.

Expression patterns of class I KNOX and YABBY genes in Ruscus aculeatus (Asparagaceae) with implications for phylloclade homology

STM (RaSTM) and YAB2 (RaYAB2) homologues were isolated from Ruscus aculeatus (Asparagaceae, monocots), and their expressions were analyzed by real-time polymerase chain reaction (PCR) to assess hypotheses on the evolutionary origin of the phylloclade in the Asparagaceae. In young shoot buds, RaSTM is expressed in the shoot apex, while RaYAB2 is expressed in the scale leaf subtending the shoot bud. This expression pattern is shared by other angiosperms, suggesting that the expression patterns of RaSTM and RaYAB2 are useful as molecular markers to identify the shoot and leaf, respectively. RaSTM and RaYAB2 are expressed concomitantly in phylloclade primordia. These results suggest that the phylloclade is not homologous to either the shoot or leaf, but that it has a double organ identity.

Leaf-shape variation of Paederia foetida in Japan: reexamination of the small, narrow leaf form from Miyajima Island

Variations in Paederia foetida L. leaf shape were examined to evaluate the taxonomic validity of the small, narrow leaf form of P. foetida f. microphylla Honda from Miyajima Island, Honshu, Japan. There is considerable variation in P. foetida individuals in terms of leaf size and leaf index (leaf length:leaf width ratio). On Miyajima Island, some individuals have narrow leaves with a high leaf index value, a phenotype represented by the type specimen of P. foetida f. microphylla, and some do not. Given that the leaf size of individuals from Miyajima Island is smaller than that of individuals from other localities in Japan, and that the small leaf phenotype is stable even under cultivation, P. foetida f. microphylla is classified as the form having the smallest leaf size. Anatomical examination of leaf blades revealed that the large variation in leaf size was attributable to variation in the number of leaf cells but not to differences in cell size or cell shape. Based on these results, we discuss the endemism of P. foetida f. microphylla.

Comparative anatomy of root meristem and root cap in some species of Podostemaceae and the evolution of root dorsiventrality

In the unusual aquatic Podostemaceae, the root is the leading organ of the plant body and is variously compressed and submerged as it adheres to rock surfaces in rapid water. In an anatomical comparison of the root apical meristems and root caps of 33 species that represent the major lineages of the family, the dorsiventrality of root meristems varied and was classified into four patterns: (1) The root cap is produced outward from a nearly radially symmetrical meristem. (2) The meristem and root cap are markedly dorsiventral; the outermost cells of the hood-shaped cap are acroscopic derivatives from bifacial initials on the ventral side, while the pattern on the dorsal side is similar to pattern 1. (3) Bifacial initials are on both the dorsal and ventral sides. (4) No root cap is present. An evolutionary polarity may be evident from pattern 1 to 2 and then to 3. Pattern 2 arose in the early evolution of the subfamily Podostemoideae and subsequently, pattern 3 arose in species with crustose roots, while the least specialized pattern 1 is retained in Tristichoideae and Weddellinoideae. Pattern 4 characterized by caplessness may have appeared recurrently in Tristichoideae and Podostemoideae. These evolutionary changes in the meristem preceded the specialization of external root morphologies.

Development and structure of trichotomous branching in Edgeworthia chrysantha (Thymelaeaceae)

We studied the development and structure of the unusual trichotomous branching of Edgeworthia chrysantha. Three "branch primordia" are formed sequentially on the shoot apex of a main axis and develop into trichotomous branching. The branch primordia are clearly distinguishable from the typical axillary buds of other angiosperms; they develop much more rapidly than axillary buds, and the borders between the branch primordia and shoot apex of the main axis are anatomically unclear. Furthermore, at a later stage, leaves subtending the branch primordia produce typical axillary buds. These results suggest that the trichotomous branching in this species involves the division of the shoot apical meristem. Expression analysis of genes involved in branching or maintenance of the shoot apical meristem in this species should clarify the control mechanism of this novel branching pattern in angiosperms. We also observed the phyllotactic patterns in trichotomous branching and have related these patterns to the shoot system as a whole.

Leaf development in the absence of a shoot apical meristem in Zeylanidium subulatum (Podostemaceae)

BACKGROUND AND AIMS

The Podostemaceae are a family of unusual aquatic angiosperms that live in rapids and waterfalls. To adapt to such extreme habitats, the family shows unusual morphologies. This study investigated the developmental anatomy of the shoot of Zeylanidium subulatum borne on the prostrate root attached to submerged rock surfaces. *

METHODS

Shoots of Z. subulatum were observed under the microscope using resin-sections. *

KEY RESULTS

The shoot has no shoot apical meristem (SAM) and, without it, forms leaves distichously dorsiventrally facing the immediately older leaf. A new leaf forms on the adaxial side of a pre-existing leaf and also on the abaxial side of a leaf on flowering shoots. In both cases, the young leaf is endogenous below the older leaf and maintains histological continuity with it. Shortly after internal initiation, the leaf primordia become separate from each other due to cleavage between adjacent leaves of opposite ranks. The cleavage is caused by intercellular separation as well as by degeneration of vacuolated cells. Loss of the SAM is probably linked with the speculated shift of the site of leaf formation to the root. *

CONCLUSIONS

The 'shoot' of Z. subulatum is characterized by the absence of a SAM, endogenous leaf formation in the absence of a SAM, cleavage between leaf primordia, and adventitious leaf formations. These innovations occur in some Podostemaceae that have become increasingly adapted to extreme aquatic habitats.

Developmental morphology of the Asian one-leaf plant, Monophyllaea glabra (Gesneriaceae) with emphasis on inflorescence morphology

We examined the developmental morphology of the tropical Asian one-leaf plant Monophyllaea glabra, which is believed to have diverged first in the phylogenetic tree of the genus. The embryo within the seed consists of two cotyledons and a hypocotyl with no shoot or root apical meristems. The endogenous root meristem is formed nearer the hypocotyl end than in other examined Monophyllaea species. One of the cotyledons grows to form the macrocotyledon by means of the basal meristem. The groove meristem arises between the anisocotyledons, shifts toward the macrocotyledon, and is transformed to the inflorescence apex, which produces inflorescence axes in the axils of all ventral bracts of two rows, and secondary inflorescences in the axils of the lower dorsal bracts of the other two rows. The macrocotyledon may act as a ventral bract for the first inflorescence axis at the reproductive stage. This organization suggests that a common ancestor of Monophyllaea and Whytockia with decussate inflorescences diverged in one direction to become Monophyllaea and in another to become Whytockia.

The outer integument and funicular outgrowth complex in the ovule of Magnolia grandiflora (Magnoliaceae)

The development of the outer integument and funicular outgrowth in the ovule of Magnolia grandiflora was examined by microtomy and scanning electron microscopy to reveal the morphology and evolution of the outer integument, a novel angiosperm structure. Early in development the outer integument is semiannular, decurrent to the lateral sides of the funiculus, and extends downwards beyond the funicular outgrowth that forms in the gap of the outer integument, and is transverse to the funiculus. The outer integument then overgrows the funicular outgrowth perpendicularly to the funiculus to form a micropyle together. The hood-shaped outer integument and the funicular outgrowth compose an envelope complex, and the interpretation of a single cupular outer integument is not supported. This envelope complex may differ from the cupular outer integument of other angiosperms, e.g., Nymphaeaceae, suggesting independent origin of apparently cupular outer integuments and hood-shaped outer integuments. Anatropous curving is due mainly to differential growth of the chalaza. The bistomic micropyle of Magnoliaceae seems to represent a derived character state, compared to an endostomic micropyle.

Developmental morphology of ovules and seeds of Nymphaeales

Ovule and seed development in six species of Nymphaeales was examined. In the Cabombaceae the two species studied resemble some extant basal angiosperms by having a hood-shaped outer integument. A micropyle-hilum complex results when the outer integument and derived testa are lacking between the micropyle and the funiculus, thus the hood-shaped appearance. In the Nymphaeaceae the outer integument is annular at an early stage and then cup-shaped though it is semiannular at initiation in Nupar japonicum and Nymphaea alba. The micropyle and hilum are separated by an intervening testa. Developmental data on the formation of the outer integument, from semiannular to hood-shaped vs. from annular to cup-shaped, are useful for inferring the morphology of the outer integument from the relative position of the micropyle to the hilum in seed fossils. The oldest (early Cretaceous) probable nymphaealean seeds had the micropyle-hilum complex, suggesting that the hood-shaped outer integument may be primitive in the Nymphaeales. This needs to be tested by examination of this feature in other groups of basal angiosperms.

Developmental morphology of the thalloid Hydrobryum japonicum (Podostemaceae)

We describe the unique development and branching of lobed thalli in Hydrobryum japonicum. Lobe formation begins with meristem initiation at random sites near the thallus margin fringed by protective tissues. As the protective tissues are successively peeled off particularly in the growing new lobes, the lobes become naked and then become fringed again by new protective tissues that develop from the marginal part of the new meristems. Subsequently the meristems become less active and are differentiated into parenchymatous ground tissue at maturity. The random pattern of meristem formation during the sporadic development gives rise to a nonorderly branching pattern of the thalli. Some other lobes (∼10%) are regenerated from injured parts of the thalli. The vegetative shoots arise endogenously near the thallus margin and are enclosed by the nonvascular strand nets. The rudimentary shoot apices remain embedded in the thalli. The thalli, though remarkably different from typical roots of other angiosperms, might be extremely transformed roots.

Cell division patterns in the apices of subterranean axis and aerial shoot of Psilotum nudum (Psilotaceae): morphological and phylogenetic implications for the subterranean axis

The cell division pattern in the apical meristem of Psilotum nudum was examined using epi-illumination microscopy and a paraffin method. In the subterranean axis, about half of the derivative cells of the apical cell produce tetrahedral daughter apical cells by the first three or more oblique divisions. Roughly half of these apical cells give rise to the apical meristems of axes, whereas the other half do not. Various relative activities of the mother and daughter apical cells give rise to disordered branching patterns. In the ill-organized apical meristem as well as the leafless and capless structure, the Psilotum subterranean axis differs from the basic organs of vascular plants such as stem and root and seems to be an independent organ. The cell division pattern characteristic of the subterranean axis persists in the young unbranched aerial shoots, although fewer daughter apical cells are produced. Dichotomous branching of the aerial shoots, as in a variety of organs of pteridophytes, involves loss of the mother apical cell followed by appearance of two daughter apical cells.

Morphology of the outer integument in three primitive angiosperm families

Scanning electron microscope examinations were performed to characterize the developmental morphology of the outer integument in several species of the three families Annonaceae (Cananga odorata var. fruticosa, Goniothalamus macrophyllus, and Stelechocarpus burahol), Eupomatiaceae (Eupomatia laurina), and Winteraceae (four Tasmannia species). In all species examined, the inner integument arises as an annular ring, but the outer integument initiates as a semiannular outgrowth interrupted on the concave side of the funiculus; the outer integument then becomes hood-shaped. The inner integument overtops the outer one at maturity, and the micropyle consists only of an endostome. Data from the present and previous studies support the hypothesis that the outer integument is hood-shaped in magnolialean angiosperms and that the bilaterally symmetrical, bladelike outer integument is homologous with the ovuliferous sporophyll of the glossopterids. The micropyle consisting of an endostome might be in a primitive state, compared with a bistomic one. Key words: developmental morphology, integument, micropyle, phylogenesis, primitive angiosperms, scanning electron microscopy.

Leaf anatomy of tropical fern rheophytes, with its evolutionary and ecological implications

Rheophytes are restricted to stream beds that are regularly flooded by swift-running water after rains and are morphologically characterized by having narrow, (ob)lanceolate leaves – leaflets (stenophylls) and other features that are adapted to the unique habitat, decreasing resistance to the swift-running water. The present study characterized anatomically the leaves of fern rheophytes. Generally, the mesophyll cells of rheophytes are less expanded, and therefore their intercellular spaces are smaller than those of related dryland species. Furthermore, frequency of occurrence of stomata per unit leaf area is greater, the cuticular layer is thicker, and the epicuticular wax deposits on the leaf epidermis are denser in at least some rheophytes than in related dryland species. It can be assumed that the stenophylls of the rheophytes are produced by developmental events, including weaker cell expansion than in dryland species, and that a phylogenetic decrease in cell expansion in leaves was involved in the origin of stenophylls from broader leaves of ancestral dryland species. The leaf anatomical features are discussed in relation to the ecological specificity of the rheophytes. Key words: evolution, flood resistance, intercellular space, leaf anatomy, rheophytic ferns, stenophyll.

Developmental anatomy of the shoot apex of leptosporangiate ferns. II. Leaf ontogeny of Adiantum capillus-veneris (Adiantaceae)

Early leaf development of Adiantum capillus-veneris L. was examined with special reference to the origin of constituent cells of the leaf. At the earliest stage of leaf development, an enlarged superficial cell (leaf initial cell) occurs in the fourth or fifth cell packet derived from the shoot apical cell and divides to form a leaf apical cell. At the same time, cells surrounding the enlarged cell, which are not derivatives of it, also divide to form the basal part of a leaf primordium. Unlike the situation in leaves of other ferns, the leaf apical cell does not divide actively during early development, while the basal cells divide frequently from the beginning. The major part of a fairly developed leaf primordium therefore consists of derivatives of the basal cells. The leaf primordium is multicellular in origin in the sense that its distal part is derived from the enlarged superficial cell and its basal part from the cells surrounding the enlarged superficial cell.