Structural and systematic study of an unusual tracheid type in cacti

Wide-band tracheids are a specialized tracheid type in which an annular or helical secondary wall projects deeply into the cell lumen. They are short, wide and spindle-shaped, and their bandlike secondary walls cover little of the primary wall, leaving most of it available for water diffusion. Wide-band tracheids appear to store and conduct water while preventing the spread of embolisms. They may be the most abundant tracheary element in the xylem, but they are always accompanied by at least a few vessels. Typically, fibers are absent wherever wide-band tracheids are present. Wide-band tracheids occur in the primary and secondary xylem of succulent stems, leaves and roots in genera of all three subfamilies of Cactaceae but were not found in the relictual genusPereskia, which lacks succulent tissues. In the large subfamily Cactoideae, wide-band tracheids occur only in derived members, and wide-band tracheids of North American Cactoideae are narrower and are aligned in a more orderly radial pattern than those of South American Cactoideae. Wide-band tracheids probably arose at least three times in Cactaceae.     

Molecular systematics of the Cactaceae

Bayesian, maximum‐likelihood, and maximum‐parsimony phylogenies, constructed using nucleotide sequences from the plastid gene region trnK‐matK, are employed to investigate relationships within the Cactaceae. These phylogenies sample 666 plants representing 532 of the 1438 species recognized in the family. All four subfamilies, all nine tribes, and 69% of currently recognized genera of Cactaceae are sampled. We found strong support for three of the four currently recognized subfamilies, although relationships between subfamilies were not well defined. Major clades recovered within the largest subfamilies, Opuntioideae and Cactoideae, are reviewed; only three of the nine currently accepted tribes delimited within these subfamilies, the Cacteae, Rhipsalideae, and Opuntieae, are monophyletic, although the Opuntieae were recovered in only the Bayesian and maximum‐likelihood analyses, not in the maximum‐parsimony analysis, and more data are needed to reveal the status of the Cylindropuntieae, which may yet be monophyletic. Of the 42 genera with more than one exemplar in our study, only 17 were monophyletic; 14 of these genera were from subfamily Cactoideae and three from subfamily Opuntioideae. We present a synopsis of the status of the currently recognized genera.
© The Willi Hennig Society 2011.     

Flavonols and C-Glycosylflavonoids of the caryophyllales

A survey of 112 species of the Caryophyllales showed the presence of flavonols in all eleven families and of C-glycosylflavonoids in nine families, being absent from the Aizoaceae and Cactaceae. 18% of the species contained both classes of compound. C-glycosylflavonoids are reported for the first time in the Amaranthaceae, Basellaceae, Didieraceae, Nyctaginaceae, Phytolaccaceae, Portulacaceae and Molluginaceae. The Caryophyllaceae contained prodominantly C-glycosylflavonoids, suggesting they are the most advanced family in the order.     

Comparative stem anatomy in the subfamily Cactoideae

Basic anatomical features of Cactaceae have been studied since the sixteenth century. This anatomical research has focused on selected features related to different external forms or on stem photosynthetic metabolism. Anatomical stem features, however, have rarely been taken into consideration in systematic studies. Recent work has focused on the subfamily Cactoideae because it is the largest and most diverse subfamily of Cactaceae. Molecular analyses support the monophyly of Cactoideae, but tribal and generic relationships are mostly unresolved. A major goal of this study was to synthesize the available information about anatomical stem features of Cactoideae and to evaluate their usefulness in phylogenetic analysis. Although dermal and vascular tissues have been studied for nearly 350 species of Cactoideae, comprehensive investigations are needed for most members of specific genera or tribes. Phylogenetic analysis based on structural data (morphology and anatomy) showed that the subfamily Cactoideae is monophyletic. This result supports molecular evidence and corroborates that highly reduced leaves are the synapomorphy of this clade. With the exception of Cacteae and Rhipsalideae, the tribes are not monophyletic. The morphological characters that have been used to define the tribes are not synapomorphies and have evolved independently in different lineages. Some anatomical features are unique characters that distinguish terminal taxa; for example, silica grains in dermal and hypodermal cells inStenocereus, prismatic crystals in dermal and hypodermal cells ofNeobuxbaumia, and lack of medullary bundles in members of Cacteae. Most anatomical features, however, behave in a highly homoplasious manner in the analysis of the subfamily. Other studies at the tribal or generic level show that anatomical features are informative and contribute to support different clades. Further studies of Cactoideae, at different taxonomic levels, that include anatomical features, are needed in order to understand their evolution.

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Resumen  Desde el siglo dieciseis se inició el estudio los caracteres anatómicos en Cactaceae. La investigación se ha enfocado a caracteres relacionados con las diferentes formas y el metabolismo fotosintético de los tallos. Sin embargo, en pocos estudios las estructuras anatómicas se han empleado en la sistemática de la familia. La investigación se ha centrado en la subfamilia Cactoideae porque es la más grande y diversa dentro de Cactaceae. Estudios moleculares apoyan la monofilia de Cactoideae; sin embargo, las relaciones tribales y genéricas son inciertas. Uno de los objetivos de este trabajo fue sintetizar la información sobre los caracteres anatómicos del tallo en Cactoideae y evaluarla desde una perspectiva filogenética. Aunque se ha estudiado el tejido dérmico o vascular de alrededor de 350 especies de Cactoideae, se requieren estudios que incluyan a la mayoría de las especies de géneros y tribus específicos. El análisis filogenético basado en datos estructurales (morfología y anatomia) mostró que la subfamilia Cactoideae es monofilética. Este resultado apoya las evidencias moleculares y corrobora que las hojas altamente reducidas son la sinapomorfia de este clado. Excepta por las tribus Cacteae y Rhipsalideae, las otras tribus no se recuperaron como monofiléticas. Los caracteres morfológicos que se han empleado para circunscribir las tribus no son sinapomorfias y se han adquirido en forma independiente en varios linajes. Varios caracteres anatómicos son únicos y distinguen a algunos taxa terminales como son la presencia de cuerpos de sílice en la epidermis e hipodermis deStenocereus, los cristales prismáticos en la epidermis e hipodermis deNeobuxbaumia y la ausencia de haces medulares en especies de Cacteae. Sin embargo, la mayoría de los caracteres anatómicos son homoplásicos en el análisis de la subfamilia, pero otros estudios a nivel tribal o genérico han mostrado que son informativos y contribuyen a diferenciar clados. Existe la necesidad de un mayor número de estudios a diferentes niveles taxonómicos que incluyan características anatómicas con la finalidad entender su evolución.

     

Phylogenetic relationships in the cactus family (Cactaceae) based on evidence from trnK/ matK and trnL-trnF sequences

Cacti are a large and diverse group of stem succulents predominantly occurring in warm and arid North and South America. Chloroplast DNA sequences of the trnK intron, including the matK gene, were sequenced for 70 ingroup taxa and two outgroups from the Portulacaceae. In order to improve resolution in three major groups of Cactoideae, trnL-trnF sequences from members of these clades were added to a combined analysis. The three exemplars of Pereskia did not form a monophyletic group but a basal grade. The well-supported subfamilies Cactoideae and Opuntioideae and the genus Maihuenia formed a weakly supported clade sister to Pereskia. The parsimony analysis supported a sister group relationship of Maihuenia and Opuntioideae, although the likelihood analysis did not. Blossfeldia, a monotypic genus of morphologically modified and ecologically specialized cacti, was identified as the sister group to all other Cactoideae. The tribe Cacteae was found to be sister to a largely unresolved clade comprising the genera Calymmanthium, Copiapoa, and Frailea, as well as two large and well-supported clades. Browningia sensu stricto (excluding Castellanosia), the two tribes Cereeae and Trichocereeae, and parts of the tribes Notocacteae and Rhipsalideae formed one clade. The distribution of this group is largely restricted to South America. The other clade consists of the columnar cacti of Notocacteae, various genera of Browningieae, Echinocereeae, and Leptocereeae, the tribes Hylocereeae and Pachycereeae, and Pfeiffera. A large portion of this latter group occurs in Central and North America and the Caribbean.     

Wide-band tracheids in leaves of genera in Aizoaceae: the systematic occurrence of a novel cell type and its implications for the monophyly of the subfamily Ruschioideae

 Stem and leaf anatomical characters of succulent plants in families of the order Caryophyllales were examined to determine phylogenetic and evolutionary trends within these families. Wide-band tracheids, novel tracheids having wide secondary walls hypothesized to prevent cell collapse under water stress, were found in three families–Aizoaceae, Cactaceae, and Portulacaceae. Wide-band tracheids are hypothesized to be a unique adaptation to severe drought conditions prevalent in the areas of origin for these families. Species of 89 genera of Aizoaceae were examined for the occurrence of these tracheids. If present, wide-band tracheids occurred only in leaves. Of the five subfamilies examined, these tracheids were found only in the subfamily Ruschioideae, but not in the putative basal genera of the ruschioids. The presence of wide-band tracheids is an apomorphic character for genera of Ruschioideae with the exception of the basal genera, and also suggests that further characters are needed to understand the alliance of the Apatesia and Cleretum groups with other ruschioid groups. Received April 19, 2000 Accepted December 26, 2000     

Effects of light and temperature on seed germination of cacti of Brazilian ecosystems

Environmental factors are used by plants as spatio‐temporal indicators of favorable conditions for seed germination. Thus, the objective of this study was to determine the effect of light and temperature on seed germination of 30 taxa of Cactaceae occurring in northeastern Brazil and to evaluate whether fluctuations in temperature are capable of altering light sensitivity. The seeds were tested for germination under two light conditions (12 h photoperiod and continuous darkness) and 10 temperature treatments: eight constant temperatures (10, 15, 20, 25, 30, 35, 40 and 45°C) and two alternating temperatures (30/20°C and 35/25°C). The species studied showed two photoblastic responses. All cacti from the Cactoideae subfamily (22 taxa) were classified as positive photoblastic (i.e., no germination in darkness), regardless of the temperature treatment used. Likewise, temperature fluctuation did not alter the seed sensitivity to light. On the other hand, the species of the Opuntioideae (five taxa) and Pereskioideae (three taxa) subfamilies are indifferent to light (i.e., germinated both in the presence and absence of light). The cacti from the areas of Caatinga and Cerrado showed an optimal germination temperature of 30°C, while the species from Atlantic Forest and Restinga areas showed an optimal germination temperature of 25°C.     

Deletions in the plastid trnT–trnL intergenic spacer define clades within Cactaceae subfamily Cactoideae

 The phylogenetic distribution of two deletions, of about 350 and 250 bp respectively, within the chloroplast trnT–trnL intergenic spacer was examined. One deletion was found in all members of Cactaceae subfamily Cactoideae sampled, totaling 37 species, but not in taxa from other subfamilies or closely related families. The second was shared by a subset of Cactoideae comprising members of tribes Cereeae, Trichocereeae, and Browningieae (in part), as well as Harrisia. Close links among the former three South American tribes have been previously hypothesized. This distribution suggested that Browningieae, a tribe defined largely by shared primitive features, were not monophyletic, and that Harrisia may have been incorrectly placed outside the Browningieae–Cereeae–Trichocereeae group. Received June 12, 2001 Accepted October 26, 2001     

The closest relatives of cacti: insights from phylogenetic analyses of chloroplast and mitochondrial sequences with special emphasis on relationships in the tribe Anacampseroteae

Recent molecular and morphological systematic investigations revealed that the cacti are most closely related to Anacampseroteae, Portulaca and Talinum of the family Portulacaceae (ACPT clade of suborder Portulacineae). A combined analysis of ndhF, matK, and nad1 sequence data from the chloroplast and the mitochondrial genomes indicates that the tribe Anacampseroteae is the sister group of the family Cactaceae. This clade, together with Portulaca, is well characterized by the presence of axillary hairs or scales. Relationships within Anacampseroteae are characterized by a grade of five species of Grahamia s.l. from North and South America, and Grahamia australiana is found to be sister to the genera Anacampseros and Avonia. A comparison of vegetative characteristics indicates an evolutionary transition from woody subshrubs to dwarf perennial and highly succulent herbs during the diversification of Anacampseroteae. Available evidence from the present investigation as well as from previously published studies suggests that a revised classification of Portulacineae on the basis of inferred phylogenetic relationships might consist of a superfamily that includes Cactaceae and the three genera Anacampseros s.l. (including Avonia and Grahamia s.l.), Portulaca, and Talinum (including Talinella), either referred to three monogeneric families or to a paraphyletic family Portulacaceae*.     

Vivipary in coastal cacti: a potential reproductive strategy in halophytic environments

Vivipary, the germination of seeds within the fruit prior to abscission from the maternal plant, is an important event in plants. Two main types of vivipary are known in vascular plants: true vivipary and pseudovivipary. In crop grasses, pseudovivipary is an undesirable character as it results in lower yields. To date, vivipary in the Cactaceae has been reported for less than 20 species, most of which are cultivated. Here, we report viviparous (cryptoviviparous-a subcategory of true vivipary) cacti in nature in members of the tribes Cacteae and Pachycereeae (subfamily Cactoideae). We present four species inhabiting coastal plains in areas subject to periodic flooding, namely, Ferocactus herrerae, Stenocereus alamosensis, S. thurberi, and Pachycereus schottii. These species from localities in northwestern Mexico had viviparous fruits and offspring in different stages of development. A potential trend in the data indicates that the overall proportion of viviparous plants is higher in coastal flooding areas relative to halophytic, nonflooding areas. In our view, vivipary is a reproductive strategy that has evolved to provide a more efficient mechanism favoring germination and new avenues for survival by contributing to population maintenance and short-distance dispersal on halophytic substrates.     

Effect of light on seed germination and seedling shape of succulent species from Mexico

Aims Light requirements for cactus seed germination have been considered to be associated with their adult plant height and seed mass, but this has not been thoroughly studied for other succulent species. In order to understand seed photosensitivity from desert species belonging to Asparagaceae (subfamily Agavoideae) and Cactaceae, we performed a germination experiment with and without light for 12 species and 2 varieties from 1 species from the Southern Chihuahuan Desert. We also determined if adult growth is totally determined by seedling 'growth form' in cacti.Methods We performed a germination experiment using light and darkness for 13 species from Southern Chihuahuan Desert: 10 rosette species (Asparagaceae), as well as 1 globose, 1 columnar and 2 varieties from 1 depressed-globose species (Cactaceae). The response variables were seed germination percentage and relative light germination (RLG). In addition, in order to determine if adult-globose cacti could have cylindrical seedlings, we calculated the shape index (height/width ratio) for Coryphanta clavata and Mammillaria compressa .Important findings All species were considered neutral photoblastic. Eleven species had similar seed germination in both light and dark conditions, and three taxa (M. compressa and the two varieties of Ferocactus latispinus) showed higher germination with light than without it. Agave salmiana, M. compressa and the two varieties of F. latispinus had higher RLG than the other species. Seed mass was an important factor because with higher seed mass there was lower dependence to light. These findings support the hypothesis that small seed mass and light requirements have coevolved as an adaptation to ensure germination. One adult-globose cactus species, M. compressa, and one adult-columnar species, C. clavata, had small seeds and neutral fotoblasticism. Seedlings from these two species exposed to light were cylindrical and those under darkness conditions were columnar. Perhaps seeds from this species are able to germinate in the dark because they produce columnar seedlings with the ability to emerge from greater soil depths where sunlight cannot penetrate.     

Wide-band tracheids in genera of Portulacaceae: novel, non-xylary tracheids possibly evolved as an adaptation to water stress

Wide-band tracheids (WBTs) are novel tracheids with wide, lignified secondary walls that intrude deeply into the cell lumen when viewed in transverse sections. These tracheids are found in a few genera in related families in the order Caryophyllales: Aizoaceae, Cactaceae, and Portulacaceae. WBTs in these three families vary in (1) systematic occurrence (found in more highly derived genera in each family), (2) location in plant organs, and (3) structure and dimensions. In addition, an analysis was conducted of WBT cell walls to test the hypothesis that WBTs evolved as an adaptation to water stress (i.e., the wide secondary walls should prevent collapse of the primary wall during water stress). The cell wall data show that primary cell walls in WBTs cannot inwardly collapse to occlusion, thus providing support for the water stress hypothesis of WBT evolution. With consideration of their systematic occurrence, the molecular phylogenetic data, and data here showing support for a water stress adaptive origin, it is logical to assume that WBTs evolved in genera that were adapting to environments undergoing a rapid trend toward aridification.     

Wide-band tracheids are present in almost all species of Cactaceae

Wide-band tracheids (WBTs) have been found in seedlings of most species of cacti that have fibrous wood in their adult bodies. Consequently, this cell type is now known to be present in almost all cacti. Earlier studies of adult plants revealed WBTs to be present only in cacti with globose or short, broad bodies, whereas all species with large columnar or long slender bodies had fibrous wood without WBTs. However, even these species produce WBTs during the first several months after germination. In species with fibrous wood in their adult bodies (species with large or slender bodies), seedlings undergo a phase transition in wood morphogenesis after a few months and stop producing the juvenile (WBT) wood and begin producing adult (fibrous) wood. If adult plants have an intermediate size, the phase transition is delayed and the plant produces WBT wood for several years. Species with globose bodies repress the phase transition completely and never switch to producing adult (fibrous) wood. Because WBTs are so widespread, they probably originated only once in Cactaceae, not multiple times as suggested earlier, or there may have been just a single origin in the Cactaceae/Portulacaceae clade.     

Vivipary in the Cactaceae: Its taxonomic occurrence and biological significance

Vivipary, a rare reproductive strategy in flowering plants, has been previously reported in only four species of the Cactaceae. In this paper, eight additional examples (Cleistocactus smaragdiflorus, Disocactus martianus, the hybrid Epiphyllum X Fern la Borde, Harrisia martinii, Lepismium ianthothele, L. monacanthum, Rhipsalis micrantha forma micrantha, and R. baccifera subsp. horrida) of vivipary in cultivated cacti from tribes Hylocereeae, Rhipsalideae, and Trichocereeae are documented. The plants had fully-developed and mature fruits containing seeds at different stages of germination, including tiny embryos emerging from the seed coat to young, healthy seedlings with well-developed organs, such as cotyledons and radicles. Several features, though not unique, distinguish cactus fruits with viviparous seedlings. Foremost, the fruits are fleshy with abundant mucilage and a relatively thick, often transparent pericarp. The seeds are generally numerous and embedded in a transparent, white or red pulp. The occurrence of vivipary in taxa from four phylogenetically distinct tribes of the subfamily Cactoideae suggests an independent origin and indicates that this reproductive strategy may be more widespread in the family than originally thought. Vivipary in the Cactaceae is regarded as a form of cryptovivipary, a condition in which the zygote develops inside the fruit without penetrating the ovary wall for dispersal purposes, which in turn is considered a subcategory of true vivipary. Further, vivipary is interpreted as an adaptive reproductive strategy that enables seedlings to establish more rapidly.     

Novel seed coat lignins in the Cactaceae: structure,distribution and implications for the evolution of lignin diversity

We have recently described a hitherto unsuspected catechyl lignin polymer (C‐lignin) in the seed coats of Vanilla orchid and in cacti of one genus, Melocactus (Chen et al., Proc. Natl. Acad. Sci. USA. 2012, 109, 1772‐1777.). We have now determined the lignin types in the seed coats of 130 different cactus species. Lignin in the vegetative tissues of cacti is of the normal guaiacyl/syringyl (G/S) type, but members of most genera within the subfamily Cactoidae possess seed coat lignin of the novel C‐type only, which we show is a homopolymer formed by endwise β–O–4‐coupling of caffeyl alcohol monomers onto the growing polymer resulting in benzodioxane units. However, the species examined within the genera Coryphantha, Cumarinia, Escobaria and Mammillaria (Cactoideae) mostly had normal G/S lignin in their seeds, as did all six species in the subfamily Opuntioidae that were examined. Seed coat lignin composition is still evolving in the Cactaceae, as seeds of one Mammillaria species (M. lasiacantha) possess only C‐lignin, three Escobaria species (E. dasyacantha, E. lloydii and E. zilziana) contain an unusual lignin composed of 5‐hydroxyguaiacyl units, the first report of such a polymer that occurs naturally in plants, and seeds of some species contain no lignin at all. We discuss the implications of these findings for the mechanisms that underlie the biosynthesis of these newly discovered lignin types.     

Basal cactus phylogeny: implications of Pereskia (Cactaceae) paraphyly for the transition to the cactus life form

The cacti are well-known desert plants, widely recognized by their specialized growth form and essentially leafless condition. Pereskia, a group of 17 species with regular leaf development and function, is generally viewed as representing the "ancestral cactus," although its placement within Cactaceae has remained uncertain. Here we present a new hypothesis of phylogenetic relationships at the base of the Cactaceae, inferred from DNA sequence data from five gene regions representing all three plant genomes. Our data support a basal split in Cactaceae between a clade of eight Pereskia species, centered around the Caribbean basin, and all other cacti. Two other Pereskia clades, distributed mostly in the southern half of South America, are part of a major clade comprising Maihuenia plus Cactoideae, and Opuntioideae. This result highlights several events in the early evolution of the cacti. First, during the transition to stem-based photosynthesis, the evolution of stem stomata and delayed bark formation preceded the evolution of the stem cortex into a specialized photosynthetic tissue system. Second, the basal split in cacti separates a northern from an initially southern cactus clade, and the major cactus lineages probably originated in southern or west-central South America.     

Molecular phylogenetics of suborder Cactineae (Caryophyllales), including insights into photosynthetic diversification and historical biogeography

? Premise of the study: Phylogenetic relationships were investigated among the eight families (Anacampserotaceae, Basellaceae, Cactaceae, Didiereaceae, Halophytaceae, Montiaceae, Portulacaceae, Talinaceae) that form suborder Cactineae (= Portulacineae) of the Caryophyllales. In addition, photosynthesis diversification and historical biogeography were addressed. ? Methods: Chloroplast DNA sequences, mostly noncoding, were used to estimate the phylogeny. Divergence times were calibrated using two Hawaiian Portulaca species, due to the lack of an unequivocal fossil record for Cactineae. Photosynthetic pathways were determined from carbon isotope ratios (δ(13)C) and leaf anatomy. ? Key results: Maximum likelihood and Bayesian analyses were consistent with previous studies in that the suborder, almost all families, and the ACPT clade (Anacampserotaceae, Cactaceae, Portulacaceae, Talinaceae) were strongly supported as monophyletic; however, relationships among families remain uncertain. The age of Cactineae was estimated to be 18.8 Myr. Leaf anatomy and δ(13)C and were congruent in most cases, and inconsistencies between these pointed to photosynthetic intermediates. Reconstruction of photosynthesis diversification showed C(3) to be the ancestral pathway, a shift to C(4) in Portulacaceae, and five independent origins of Crassulacean acid metabolism (CAM). Cactineae were inferred to have originated in the New World. ? Conclusions: Although the C(3) pathway is inferred as the ancestral state in Cactineae, some CAM activity has been reported in the literature in almost every family of the suborder, leaving open the possibility that CAM may have one origin in the group. Incongruence among loci could be due to internal short branches, which possibly represent rapid radiations in response to increasing aridity in the Miocene.     

Seed anatomy and water uptake in relation to seed dormancy in Opuntia tomentosa (Cactaceae, Opuntioideae)

BACKGROUND AND AIMS: There is considerable confusion in the literature concerning impermeability of seeds with 'hard' seed coats, because the ability to take up (imbibe) water has not been tested in most of them. Seeds of Opuntia tomentosa were reported recently to have a water-impermeable seed coat sensu lato (i.e. physical dormancy), in combination with physiological dormancy. However, physical dormancy is not known to occur in Cactaceae. Therefore, the aim of this study was to determine if seeds of O. tomentosa are water-permeable or water-impermeable, i.e. if they have physical dormancy. METHODS: The micromorphology of the seed coat and associated structures were characterized by SEM and light microscopy. Permeability of the seed-covering layers was assessed by an increase in mass of seeds on a wet substrate and by dye-tracking and uptake of tritiated water by intact versus scarified seeds. KEY RESULTS: A germination valve and a water channel are formed in the hilum-micropyle region during dehydration and ageing in seeds of O. tomentosa. The funicular envelope undoubtedly plays a role in germination of Opuntia seeds via restriction of water uptake and mechanical resistance to expansion of the embryo. However, seeds do not exhibit any of three features characteristic of those with physical dormancy. Thus, they do not have a water-impermeable layer(s) of palisade cells (macrosclereids) or a water gap sensu stricto and they imbibe water without the seed coat being disrupted. CONCLUSIONS: Although dormancy in seeds of this species can be broken by scarification, they have physiological dormancy only. Further, based on information in the literature, it is concluded that it is unlikely that any species of Opuntia has physical dormancy. This is the first integrative study of the anatomy, dynamics of water uptake and dormancy in seeds of Cactaceae subfamily Opuntioideae.