Living Cells in Wood 3. Overview; Functional Anatomy of the Parenchyma Network

The very different evolutionary pathways of conifers and angiosperms are very informative precisely because their wood anatomy is so different. New information from anatomy, comparative wood physiology, and comparative ultrastructure can be combined to provide evidence for the role of axial and ray parenchyma in the two groups. Gnetales, which are essentially conifers with vessels, have evolved parallel to angiosperms and show us the value of multiseriate rays and axial parenchyma in a vessel-bearing wood. Gnetales also force us to re-examine optimum anatomical solutions to conduction in vesselless gymnosperms. Axial parenchyma in vessel-bearing woods has diversified to take prominent roles in storage of water and carbohydrates as well as maintenance of conduction in vessels. Axial parenchyma, along with other modifications, has superseded scalariform perforation plates as a safety mechanism and permitted angiosperms to succeed in more seasonal habitats. This diversification has required connection to rays, which have concomitantly become larger and more diverse, acting as pathways for photosynthate passage and storage. Modes of growth such as rapid flushing, vernal leafing-out, drought deciduousness and support of large leaf surfaces become possible, advantaging angiosperms over conifers in various ways. Prominent tracheid-ray pitting (conifers) and axial parenchyma/ray pitting to vessels (angiosperms) are evidence of release of photosynthates into conductive cells; in angiosperms, this system has permitted vessels to survive hydrologic stresses and function in more seasonal habitats. Flow in ray and axial parenchyma cells, suggested by greater length/width ratios of component cells, is confirmed by pitting on end walls of elongate cells: pits are greater in area, more densely placed, and are often bordered. Bordered pit areas and densities on living cells, like those on tracheids and vessels, represent maximal contact areas between cells while minimizing loss of wall strength. Storage cells in rays can be distinguished from flow cells by size and shape, by fewer and smaller pits and by contents. By lacking secondary walls, the entire surfaces of phloem ray and axial phloem parenchyma become conducting areas across which sugars can be translocated. The intercontinuous network of axial parenchyma and ray parenchyma in woods is confirmed; there are no “isolated” living cells in wood when three-dimensional studies are made. Water storage in living cells is reported anatomically and also in the form of percentile quantitative data which reveal degrees and kinds of succulence in angiosperm woods, and norms for “typically woody” species. The diversity in angiosperm axial and ray parenchyma is presented as a series of probable optimal solutions to diverse types of ecology, growth form, and physiology. The numerous homoplasies in these anatomical modes are seen as the informative results of natural experiments and should be considered as evidence along with experimental evidence. Elliptical shape of rays seems governed by mechanical considerations; unusually long (vertically) rays represent a tradeoff in favor of flexibility versus strength. Protracted juvenilism (paedomorphosis) features redirection of flow from horizontal to vertical by means of rays composed predominantly or wholly of upright cells, and the reasons for this anatomical strategy are sought. Protracted juvenilism, still little appreciated, occurs in a sizeable proportion of the world’s plants and is a major source of angiosperm diversification.     

NEAR-VESSELLESSNESS IN EPHEDRA AND ITS SIGNIFICANCE

Wood of two specimens of Ephedra from high alpine localities is nearly vesselless: E. gerardiana from 4,750 m in Tibet, and E. rupestris from 4,300 m in Argentina. Ephedra gerardiana from lower elevations in the Himalayas has more vessels, although even there they are not abundant. Instances in other species of Ephedra in which extensive areas of latewood or even entire growth rings are vesselless are cited; these tend to occur in high deserts with climates somewhat less extreme than those of the alpine localities. Near-vessellessness is interpreted as an adaptive condition in which the proportion of tracheids, conductively safer than vessel elements, reaches a maximum. Near-vessellessness in Ephedra wood is probably reversible within limits, and should be likened to production of latewood to the exclusion of earlywood. Comparisons are offered to dicotyledons in which abundance of vessels is lowered with respect to ecological extremes.     

Thyroidal autoregulation: Iodide-induced suppression of thyrotropin-stimulated cyclic AMP production and iodinating activity in thyroid cells

In continuing our study of the thyroidal autoregulation phenomenon, we have investigated the effects of iodide on several thyroidal responses to thyrotropin. Thus, we have found that the 2–4-fold thyrotropin stimulation of protein iodination in beef thyroid cells was reduced about 30% by 4 h of preincubation with 10 μM iodide, and virtually abolished with 50 μM iodide. Similarly the 8-fold thyrotropin stimulation of cyclic AMP accumulation in the cells was reduced about 30% by 3 h of preincubation with 50 μM iodide. It appears therefore that the so-called autoregulation of the thyroid gland does include influences of iodide on the thyrotropin stimulation of cyclic AMP production, iodide transport, and protein iodination which can be demonstrated in vitro in the dispersed thyroid cell system. Two other effects of thyrotropin, namely, the stimulation of [¹⁴C]leucine incorporation into protein and of iodide efflux were not at all affected by treatment with excess iodide, and hence may not be subject to the autoregulatory influence of iodide.     

Fibre dimorphism: cell type diversification as an evolutionary strategy in angiosperm woods

Dimorphic fibres in angiosperm woods are designated when zones of two different kinds of fibres can be distinguished in transverse sections. The usage of most authors contrasts wider, thinner‐walled, shorter (sometimes storied) fibres with narrower, thicker‐walled fibres that have narrower lumina. The wider fibres can be distinguished in longitudinal sections from axial parenchyma, which usually consists of strands of two or more cells each surrounded by secondary walls (and thus different from septate fibres). This phenomenon occurs in some Araliaceae, Asteraceae, Fabaceae, Myrtales (notably Lythraceae), Sapindales (especially Sapindaceae), Urticales and even some Gnetales. Additional instances can doubtless be found, especially if instances of wide latewood fibres together with narrow earlywood fibres are included. There is little physiological evidence on differential functions of dimorphic fibres, except in Acer, in which hydrolysis of starch in the wide fibres is known to result in transfer of sugar into vessels early in the growing season. Starch storage in axial parenchyma may, in a complementary way, serve for embolism reversal and prevention and thus for maintenance of the water columns. Crystalliferous fibres (Myrtales, Sapindales) can be considered a form of fibre dimorphism that deters predation. Gelatinous fibres, often equated with tension wood, can also be considered as a form of fibre dimorphism. The evolutionary significance of fibre dimorphism is that a few small changes in fibre structure can result in the accomplishment of diversified functions. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174 , 44–67.     

Origins and nature of vessels in Monocotyledons. 14. Vessellessness in Orontioideae (Araceae): adaptation or relictualism?

SEM studies of tracheary elements of subfamily Orontioideae (Lysichiton, Orontium, Symplocarpus) of Araceae show unexpected features. The plants are entirely vesselless. There are small pores in pit membranes of end walls of tracheids in roots and stems, but pit membranes remain intact. End wall pit membranes of stems have a coarse fibrillar texture, somewhat reminiscent of (but different from) those of Nymphaeaceae and Cabombaceae. Acoraceae, which are also vesselless, represent the first branch of the monocot tree, according to phylogenies, and the orontioids form the next branch. Vessellessness is therefore a potentially plesiomorphic feature in monocots, but it may also be related to the highly mesic habitats of Acoraceae and the orontioids. Various other non‐submersed monocots have vesselless or near‐vesselless xylem. Sectioned xylem of Orontioideae is also very suggestive of stages in the development of the pit membranes of both end walls and lateral walls of tracheids: open networks of cellulosic fibrils apparently precede the addition of denser fibrillar meshes, key information in assessing to what extent perforations in scalariform perforation plates of vascular plants may stop formation at the open network stage, and to what extent a thicker pit membrane experiences lysis and disintegration as the vessel element matures.     

Crystal structures of DPP-IV (CD26) from rat kidney exhibit flexible accommodation of peptidase-selective inhibitors

Dipeptidyl peptidase IV (DPP-IV) belongs to a family of serine peptidases, and due to its indirect regulatory role in plasma glucose modulation, DPP-IV has become an attractive pharmaceutical target for diabetes therapy. DPP-IV inactivates the glucagon-like peptide (GLP-1) and several other naturally produced bioactive peptides that contain preferentially a proline or alanine residue in the second amino acid sequence position by cleaving the N-terminal dipeptide. To elucidate the details of the active site for structure-based drug design, we crystallized a natural source preparation of DPP-IV isolated from rat kidney and determined its three-dimensional structure using X-ray diffraction techniques. With a high degree of similarity to structures of human DPP-IV, the active site architecture provides important details for the design of inhibitory compounds, and structures of inhibitor-protein complexes offer detailed insight into three-dimensional structure-activity relationships that include a conformational change of Tyr548. Such accommodation is exemplified by the response to chemical substitution on 2-cyanopyrrolidine inhibitors at the 5 position, which conveys inhibitory selectivity for DPP-IV over closely related homologues. A similar conformational change is also observed in the complex with an unrelated synthetic inhibitor containing a xanthine core that is also selective for DPP-IV. These results suggest the conformational flexibility of Tyr548 is unique among protein family members and may be utilized in drug design to achieve peptidase selectivity.