Marcelo R. Pace

The rise and evolution of the cambial variant in Bignonieae (Bignoniaceae)

SUMMARY Cambial variants represent a form of secondary growth that creates great stem anatomical diversity in lianas. Despite the importance of cambial variants, nothing is known about the developmental mechanisms that may have led to the current diversity seen in these stems. Here, a thorough anatomical analysis of all genera along the phylogeny of Bignonieae (Bignoniaceae) was carried out in order to detect when in their ontogeny and phylogeny there were shifts leading to different stem anatomical patterns. We found that all species depart from a common developmental basis, with a continuous, regularly growing cambium. Initial development is then followed by the modification of four equidistant portions of the cambium that reduce the production of xylem and increase the production of phloem, the former with much larger sieve tubes and an extended lifespan. In most species, the formerly continuous cambium becomes disjunct, with cambial portions within phloem wedges and cambial portions between them. Other anatomical modifications such as the formation of multiples of four phloem wedges, multiple‐dissected phloem wedges, and included phloem wedges take place thereafter. The fact that each novel trait raised on the ontogenetic trajectory appeared in subsequently more recent ancestors on the phylogeny suggests a recapitulatory history. This recapitulation is, however, caused by the terminal addition of evolutionary novelties rather than a truly heterochronic process. Truly heterochronic processes were only found in shrubby species, which resemble juveniles of their ancestors, as a result of a decelerated phloem formation by the variant cambia. In addition, the modular evolution of phloem and xylem in Bignonieae seems to indicate that stem anatomical modifications in this group occurred at the level of cambial initials.

An overview of the anatomy, development and evolution of the vascular system of lianas

Background: Lianas present many interesting structural features that are linked to their climbing habit. Having lost substantial amounts of supporting tissue, these plants depend on external structures for support. Meanwhile, during their evolutionary history, they have gained additional conductive and storage tissues. The wood of lianas generally includes wider vessels, larger amounts of axial parenchyma, larger rays, and longer fibres than those of trees. Cambial variants represent another key anatomical feature of lianas. Aims: In this paper, we review various aspects of liana biology, including those associated with their vascular system and water conduction, secondary growth and seasonal responses to environmental variability, as well as aspects related to the evolution of their cambial variants. Methods: Examples from the Bignoniaceae and Leguminosae, the two most abundant liana taxa in the Neotropics, are presented in a series of case studies, bringing new data, such as the activity of the cambium during the dry seasons; the radial conducting elements that are associated with the habit; the cambial variant of Bignonieae that has evolved in a recapitulatory fashion; and the increased specialisation for photosynthate conduction by the phloem. Conclusions: Altogether, lianas represent an excellent model for studies on the convergent evolution of plants.

A new method to obtain good anatomical slides of heterogeneous plant parts.

A new method is presented to prepare anatomical slides of plant materials including a combination of soft and hard tissues, such as stems with cambial variants, arboreal monocotyledons, and tree bark. The method integrates previous techniques aimed at softening the samples and making them thereby more homogeneous, with the use of anti-tearing polystyrene foam solution. In addition, we suggest two other alternatives to protect the sections from tearing: adhesive tape and/or Mayer’s albumin adhesive, both combined with the polystyrene foam solution. This solution is cheap and easy to make by dissolving any packaging polystyrene in butyl acetate. It is applied before each section is cut on a sliding microtome and ensures that all the tissues in the section will hold together. This novel microtechnical procedure will facilitate the study of heterogeneous plant portions, as shown in some illustrated examples.

Evolution of disparity between the regular and variant phloem in Bignonieae (Bignoniaceae)

PREMISE OF THE STUDY The phloem is a plant tissue with a critical role in plant nutrition and signaling. However, little is still known about the evolution of this tissue. In lianas of the Bignoniaceae, two distinct types of phloem coexist: a regular and a variant phloem. The cells associated with these two phloem types are known to be anatomically different; however, it is still unclear what steps were involved in the evolution of such differences. METHODS Here we studied the anatomical development of the regular and variant phloem in representatives of all 21 genera of Bignonieae and used a phylogenetic framework to investigate the timing of changes associated with the evolution of each phloem type. KEY RESULTS We found that the variant phloem always appears in a determinate location, between the leaf orthostichies. Furthermore, the variant phloem was mostly occupied by very wide sieve tubes and generally included a higher concentration of fibers, indicating an increase in conduction and mechanical support. On the other hand, the regular phloem included much more parenchyma, more and wider rays, and tiny sieve tubes that resembled terminal sieve tubes from plants with seasonal formation of vascular tissues; these findings suggest reduced conduction and higher storage capacity in the regular phloem. CONCLUSIONS Overall, differences between the regular and variant phloem increased over time, leading to further specialization in conduction in the variant phloem and an increase in storage specialization in the regular phloem.

Wood Anatomy and Evolution: A Case Study in the Bignoniaceae

Wood (secondary xylem) is responsible for water transport and has been well-studied anatomically, ecologically, physiologically, and phylogenetically. Comparative methods can reveal patterns of evolution for xylem traits using knowledge from the phylogenetic history of the taxa and the branching pattern of phylogenies. Bignoniaceae (Lamiales) is a family of pantropical plants of various growth habits that includes trees, shrubs, and lianas, which display diverse wood anatomies and for which robust phylogenies are available. Here we review important aspects in classical wood anatomy and evolution and test hypotheses regarding patterns of wood evolution using the Bignoniaceae as a model. Altogether, 85% of the genera currently recognized in Bignoniaceae were sampled, and 30 characters were delimited and mapped onto a robust phylogeny of the family. Some patterns of wood evolution within the Bignoniaceae seem to have been shaped by ecophysiological and habit aspects in the family. For example, vessels increase in diameter in the lianoid lineages but decrease in trees and shrubs during evolution. Rays in trees have evolved from a mixture of homo- and heterocellular to exclusively homocellular and storied in some lineages, while in the lianas the opposite pattern was recorded. Other patterns are consistent with more general phylogenetic trends; for example, parenchyma increases in abundance from the most basal to the most derived nodes of the phylogeny. Other characters in the family that are delimited and discussed include growth rings, porosity, perforation plates, ray width, and height. This work provides evidence that wood evolution is rather labile and that the evolution of new habits and the occupation of new habitats greatly influence wood evolution.

Wood anatomy of major Bignoniaceae clades

The circumscription of Bignoniaceae genera and tribes has undergone major changes following an increased understanding of phylogenetic relationships within the family. While DNA sequence data have repeatedly reconstructed major clades within the family, some of the clades recovered still lack diagnostic morpho-anatomical features, complicating their recognition. In this study we investigated the wood anatomy of all major lineages of Bignoniaceae (except Tourrettieae) in search for anatomical synapomorphies for clades. We sampled 158 species of Bignoniaceae, representing 67 out of the 82 genera currently recognized. Detailed descriptions of quantitative and qualitative wood anatomical features are presented for each clade and interpreted in the light of a molecular phylogeny for the family. Jacarandae are characterized by a paratracheal winged-aliform parenchyma, with the traditional subdivision of Jacaranda into sections Monolobos and Dilobos supported by the uniseriate and homocellular rays of Monolobos versus the wide and heterocellular rays of Dilobos. Tecomeae s.s. are characterized by scanty paratracheal parenchyma, septate fibers, and heterocellular rays, traits also found in Delostoma, a genus previously included in Tecomeae s.l., but recently shown to represent a separate lineage. Crescentiina includes two sub-clades, the Tabebuia alliance and the Paleotropical clade, which share abundant aliform parenchyma, short and mainly homocellular rays, less commonly with heterocellular rays with body procumbent and one row of marginal square cells. Members of the Tabebuia alliance and the Paleotropical clade can be distinguished from each other by the narrow vessels with a widespread storied structure found in members of the Tabebuia alliance, versus the vessels with medium to wide width and a non-storied structure found in members of the Paleotropical clade. Oroxyleae are characterized by a combination of simple and foraminate perforation plates and homocellular rays, while Catalpeae are characterized by scanty paratracheal parenchyma, abundant tyloses and vessel-ray pits simple to semi-bordered. Bignonieae differ from all other clades by a variant secondary growth and a typically lianoid wood anatomy. Overall, wood anatomical characters are not very labile within the family, being distributed across clades in a very predictive manner. Several anatomical characters represent good anatomical synapomorphies and provide further support to clades identified in molecular phylogenetic studies.

Structure and ontogeny of the fissured stems of Callaeum (Malpighiaceae)

Stem ontogeny and structure of two neotropical twining vines of the genus Callaeum are described. Secondary growth in Callaeum begins with a typical regular cambium that gradually becomes lobed as a result of variation in xylem and phloem production rates in certain portions of the stem aligned with stem orthostichies. As development progresses, lignified ray cells of the initially formed secondary xylem detach on one side from the adjacent tissues, forming a natural fracture that induces the proliferation of both ray and axial nonlignified parenchyma. At the same time, parenchyma proliferation takes place around the pith margin and generates a ring of radially arranged parenchyma cells. The parenchyma generated in this process (here termed disruptive parenchyma) keeps dividing throughout stem development. As growth continues, the parenchyma finally cleaves the lignified axial parts of the vascular system into several isolated fragments of different sizes. Each fragment consists of xylem, phloem and vascular cambium and is immersed in a ground matrix of disruptive parenchyma. The cambium present in each fragment divides anticlinally to almost encircle each entire fragment and maintains its regular activity by producing xylem to the centre of the fragment and phloem to the periphery. Additionally, new cambia arise within the disruptive parenchyma and produce xylem and phloem in various polarities, such as xylem to the inside and phloem to the outside of the stem, or perpendicularly to the original cambium. Unlike the very distinctive stem anatomical architecture resulting from this cambial variant in Callaeum, its secondary xylem and phloem exhibit features typical of lianas. These features include very wide conducting cells, abundant axial parenchyma, high and heterocellular rays and gelatinous fibres.

Phylogenetic relationships of enigmatic Sphingiphila (Bignoniaceae) based on molecular and wood anatomical data

Sphingiphila is a monospecific genus, endemic to the Bolivian and Paraguayan Chaco, a semi-arid lowland region. The circumscription of Sphingiphila has been controversial since the genus was first described. Sphingiphila tetramera is perhaps the most enigmatic taxon of Bignoniaceae due to the presence of very unusual morphological features, such as simple leaves, thorn-tipped branches, and tetramerous, actinomorphic flowers, making its tribal placement within the family uncertain. Here we combined molecular and wood anatomical data to determine the placement of Sphingiphila within the family. The analyses of a large ndhF dataset, which included members of all Bignoniaceae tribes, placed Sphingiphila within Bignonieae. A second, smaller ndhF and pepC dataset, which included only members of tribe Bignonieae, placed the genus within Tanaecium. Unlike most macro-morphological traits, Sphingiphila is not unusual within Bignoniaceae from a wood anatomical point of view. Sphingiphila shares the presence of narrow vessels and vasicentric to aliform confluent parenchyma with the rest of the family. In addition, Sphingiphila has several specific wood anatomical traits, such as vessels in a diagonal to tangential arrangement, small intervessel pits, and non-storied heterocellular rays with occasional perforated ray cells. These features suggest that the genus is best placed either in Tecomeae s.str. or Bignonieae, with a better placement in Bignonieae due to its abundant parenchyma, despite lacking the cambial variant synapomorphic for this group. Sphingiphila and Tanaecium form a clade that is strongly supported by molecular characters, including two indels that are molecular synapomorphies of this clade. In addition, careful morphological inspections show that S. tetramera shares with Tanaecium the subulate, bromeliad-like prophylls, the most evident synapomorphy of this genus apart from long, tubular, villose corollas, and a lepidote ovary. Given the molecular phylogenetic placement of S. tetramera and its macro-morphological and wood anatomical similarities with Tanaecium, we propose the new combination Tanaecium tetramerum.

Diversity of metaxylem vessel elements in three Syagrus palms (Arecaceae) of different habits

Elementos de vasos em monocotiledoneas apresentam diferencas morfologicas de acordo com o orgao onde ocorrem. Tais diferencas tem sido explicadas sobretudo de um ponto de vista evolutivo, com poucos dados acerca de como a funcao de um orgao ou o habito de crescimento de uma planta influenciariam na morfologia dos elementos de vaso. Para responder tal pergunta, foram analisados os orgaos vegetativos de tres palmeiras do mesmo genero, Syagrus, crescendo em ambientes similares, mas com habitos diferentes. Assim, buscamos detectar se os elementos de vaso apresentariam caracteristicas similares em todas as especies ou se mudariam de acordo com os diferentes habitos. Tanto a largura quanto o tipo de placa de perfuracao variaram de maneira semelhante em todas as especies, ao passo que os comprimentos variaram de uma forma inedita. Primeiramente, todas as especies apresentaram elementos de vaso nas raizes tao ou mais longos que aqueles dos caules e folhas. Alem disso, os elementos de vaso dos caules variaram consideravelmente entre as especies. Especifi camente, em Syagrus romanzoffi ana os elementos de vaso apresentaram igual comprimento em todos os orgaos, enquanto tanto Syagrus fl exuosa quanto Syagrus petraea, vaso mais curtos foram encontrados nos caules. Provavelmente os elementos de vaso das raizes de palmeiras estao envolvidos na criacao de altos potenciais hidricos necessarios para evitar embolia no caule. O dimorfismo dos vasos no caule possivelmente reflete os habitos diferentes destas especies. Caules de maiores dimensoes, como os de palmeiras arboreas, apresentam elementos de vaso mais longos que aqueles de caules subterrâneos. Com base nos resultados anatomicos aqui expostos, sugerimos que as diferencas encontradas nos elementos de vaso da raiz, caule e folha teriam evoluido nao so devido a pressoes filogeneticas e ecologicas, mas tambem devido a especificidades inerentes aos diferentes habitos de crescimento presentes nas plantas.