Protoxylem Elements Research Articles

Stem and root anatomy of Zanonia indica L. (Cucurbitaceae) and significant adaptations of the aerial roots

Summary The physiological and ecological adaptions of higher plants are driven by anatomical changes that contribute to the evolution of the climbing habit. This not only makes them able to survive in a dry environment but also supports them to undergo secondary growth and complete their reproduction. Some plants even form aerial roots as a survival strategy. In the present study, the anatomy of both aerial and underground roots and stems in Zanonia indica L. (Cucurbitaceae) is investigated. In young stems, vascular bundles (VB) had sieve elements on the inner margin of the protoxylem elements (i.e., bicollateral VB). In stems as well as aerial and underground roots, the axial elements of the secondary xylem were arranged in radial plates separated by wide medullary rays. Aerial and underground roots shared similar features in having exarch protoxylem and distinct narrow piths. Structurally, the secondary xylem was composed of vessels, tracheids, fibres, axial and ray parenchyma cells while extra-fascicular sieve elements were observed only in stems and aerial roots. The formation of aerial roots is a survival mechanism whose significance is discussed with reference to the available literature.

Isolation and utilization of cellulosic elements from the plant cell wall

Two very different types of cellulosic materials that may be isolated from plant-based sources are reviewed herein. We have shown that long helical cellulosic coils may be isolated by gentle chemical treatment of leaf petioles. The coils stem from protoxylem elements and are readily distinguished from other cellulosic components by polarized light microscopy, forming single- or multistranded left-handed helices. More widely known are the nanometer thickness cellulosic elements that may also be isolated from plant cell walls. These nanocelluloses can be subdivided into two main families. Cellulose nanocrystals (CNCs) are less than 250 nm long and are usually made by sulfuric acid hydrolysis. CNCs form stable liquid crystalline suspensions in water that dry to give iridescent coloured films, analogous to those found in some tropical plants. Cellulose nanofibrils (CNFs) are much longer and may have carboxyl groups on the surface. Both CNCs and CNFs are intrinsically hydrophilic and form useful aqueous gels and viscosity modifiers. By altering the surface chemistry, the nanocelluloses can also be incorporated as strengthening agents in composite materials. A wide range of commercial applications for these renewable materials has been proposed.

Seed development of Jatropha curcas L. (Euphorbiaceae): integrating anatomical, ultrastructural and molecular studies

This work provides a detailed histological analysis of the development of Jatropha curcas seeds, together with an assessment of the role of programmed cell death in this process. Seeds of Jatropha curcas are a potential source of raw material for the production of biodiesel, but very little is known about how the architecture of the seeds is shaped by the coordinated development of the embryo, endosperm and maternal tissues, namely integuments and nucellus. This study used standard anatomical and ultrastructural techniques to evaluate seed development and programmed cell death (PCD) in the inner integument was monitored by qPCR. In these studies, we found that the embryo sac formation is of the Polygonum type. We also found that embryogenesis is a slow process and the embryo is nourished by the suspensor at earlier stages and by nutrients remobilized from the lysis of the inner integument at later stages. Two types of programmed cell death contribute to the differentiation of the inner integument that begins at early stages of seed development. In addition, the mature embryo presents features of adaptation to dry environments such as the presence of four seminal roots, water absorbing stomata in the root zone and already differentiated protoxylem elements. The findings in this study fill in gaps related to the ontogeny of J. curcas seed development and provide novel insights regarding the types of PCD occurring in the inner integument.

Cadmium-induced changes in antioxidative systems and differentiation in roots of contrasted Medicago truncatula lines.

Defense pathways and stress responses induced under Cd stress were illustrated in roots of hydroponically grown Medicago truncatula seedlings. Actually, the ascorbate-glutathione and antioxidative system, secondary metabolism events including peroxidases, phenolic compounds, and lignification launching, and developmental modifications were described. Cd (100μM) initially increased reactive oxygen species, enhanced antioxidative (total SOD, CAT, and PRX) and ascorbate-glutathione-related metabolism enzymes (APX and MDAR), except in A17 and TN1.11. In agreement with peroxidase enhancement, physiological measurement and in situ observation illustrated soluble phenolic compound accumulation under Cd treatment. However, lignification was restricted to recently created protoxylem elements established in the root tip area, usually constituting the elongation zone. Cell death was increased. In the absence of necrotic reactions, developmental changes including lignin deposition, increase in cellulose and pectin contents, intercellular meatus, and condensed and deformed hairs were noticed in Cd-treated roots.

Primer registro de un estípite de Coryphoideae: Palmoxylon kikaapoa de la Formación Olmos del Cretácico Superior, Coahuila, México

In sediments of the Olmos Formation from the Late Cretaceous (upper Campanian) a diverse flora represented by flowers, fruits, leaves and wood of angiosperms, gymnosperms and polypodiales has been collected, these records document the presence of a flora with paratropical affinities. Here we describe a new fossil palm species, Palmoxylon kikaapoa. The permineralized palm trunk is characterized by an atactostele with fibro-vascular bundles simple with reniform shape; with 1-4 vessels of metaxylem with scalariform and oblique perforation plates; 1-5 protoxylem elements with simple perforation plates; paravascular parenchyma developed; phloem in one phloem strad; tabular parenchyma with 2-8 layers of elongated cells; ground tissue consisting of thin-walled cells forming a not compact tissue with intercellular spaces, type A; numerous fibrous bundles in the fundamental tissue; spherical stegmata occur in discontinuous files next to fibro-vascular bundles or fibrous bundles. These anatomical characters allowed us to taxonomically relate the fossil to the family Arecaceae, subfamily Coryphoideae, and tribe Trachycarpeae. Taking into account paleobotanical and botanical evidence, we propose a tropical affinity and an arboreal habit for this fossil species.

Cellular pathways of source leaf phloem loading and phloem unloading in developing stems of Sorghum bicolor in relation to stem sucrose storage.

Cellular pathways of phloem loading in source leaves and phloem unloading in stems of sweet Sorghum bicolor (L.) Moench were deduced from histochemical determinations of cell wall composition and from the relative radial mobilities of fluorescent tracer dyes exiting vascular pipelines. The cell walls of small vascular bundles in source leaves, the predicted site of phloem loading, contained minimal quantities of lignin and suberin. A phloem-loaded symplasmic tracer, carboxyfluorescein, was retained within the collection phloem, indicating symplasmic isolation. Together, these findings suggested that phloem loading in source leaves occurs apoplasmically. Lignin was restricted to the walls of protoxylem elements located in meristematic, elongating and recently elongated regions of the stem. The apoplasmic tracer, 8-hydroxypyrene-1,3,6-trisulfonic acid, moved radially from the transpiration stream, consistent with phloem and storage parenchyma cells being interconnected by an apoplasmic pathway. The major phase of sucrose accumulation in mature stems coincided with heavy lignification and suberisation of sclerenchyma sheath cell walls restricting apoplasmic tracer movement from the phloem to storage parenchyma apoplasms. Phloem unloading at this stage of stem development followed a symplasmic route linking sieve elements and storage parenchyma cells, as confirmed by the phloem-delivered symplasmic tracer, 8-hydroxypyrene-1,3,6-trisulfonic acid, moving radially from the stem phloem.

Overexpression of the cytosolic cytokinin oxidase/dehydrogenase (CKX7) from Arabidopsis causes specific changes in root growth and xylem differentiation

Degradation of the plant hormone cytokinin is catalyzed by cytokinin oxidase/dehydrogenase (CKX) enzymes. The Arabidopsis thaliana genome encodes seven CKX proteins which differ in subcellular localization and substrate specificity. Here we analyze the CKX7 gene, which to the best of our knowledge has not yet been studied. pCKX7:GUS expression was detected in the vasculature, the transmitting tissue and the mature embryo sac. A CKX7-GFP fusion protein localized to the cytosol, which is unique among all CKX family members. 35S:CKX7-expressing plants developed short, early terminating primary roots with smaller apical meristems, contrasting with plants overexpressing other CKX genes. The vascular bundles of 35S:CKX7 primary roots contained only protoxylem elements, thus resembling the wol mutant of the CRE1/AHK4 receptor gene. We show that CRE1/AHK4 activity is required to establish the CKX7 overexpression phenotype. Several cytokinin metabolites, in particular cis-zeatin (cZ) and N-glucoside cytokinins, were depleted stronger in 35S:CKX7 plants compared with plants overexpressing other CKX genes. Interestingly, enhanced protoxylem formation together with reduced primary root growth was also found in the cZ-deficient tRNA isopentenyltransferase mutant ipt2,9. However, different cytokinins were similarly efficient in suppressing 35S:CKX7 and ipt2,9 vascular phenotypes. Therefore, we hypothesize that the pool of cytosolic cytokinins is particularly relevant in the root procambium where it mediates the differentiation of vascular tissues through CRE1/AHK4. Taken together, the distinct consequences of CKX7 overexpression indicate that the cellular compartmentalization of cytokinin degradation and substrate preference of CKX isoforms are relevant parameters that define the activities of the hormone.

An extensin-rich matrix lines the carinal canals in Equisetum ramosissimum, which may function as water-conducting channels.

The anatomy of Equisetum stems is characterized by the occurrence of vallecular and carinal canals. Previous studies on the carinal canals in several Equisetum species suggest that they convey water from one node to another. Cell wall composition and ultrastructure have been studied using immunocytochemistry and electron microscopy, respectively. Serial sectioning and X-ray computed tomography were employed to examine the internode-node-internode transition of Equisetum ramosissimum. The distribution of the LM1 and JIM20 extensin epitopes is restricted to the lining of carinal canals. The monoclonal antibodies JIM5 and LM19 directed against homogalacturonan with a low degree of methyl esterification and the CBM3a probe recognizing crystalline cellulose also bound to this lining. The xyloglucan epitopes recognized by LM15 and CCRC-M1 were only detected in this lining after pectate lyase treatment. The carinal canals, connecting consecutive rings of nodal xylem, are formed by the disruption and dissolution of protoxylem elements during elongation of the internodes. Their inner surface appears smooth compared with that of vallecular canals. The carinal canals in E. ramosissimum have a distinctive lining containing pectic homogalacturonan, cellulose, xyloglucan and extensin. These canals might function as water-conducting channels which would be especially important during the elongation of the internodes when protoxylem is disrupted and the metaxylem is not yet differentiated. How the molecularly distinct lining relates to the proposed water-conducting function of the carinal canals requires further study. Efforts to elucidate the spatial and temporal distribution of cell wall polymers in a taxonomically broad range of plants will probably provide more insight into the structural-functional relationships of individual cell wall components or of specific configurations of cell wall polymers.

Putative dual pathway of auxin transport in organogenesis of Arabidopsis

In Arabidopsis, damage to the superficial acropetal polar auxin transport (PAT) inhibits generative but not vegetative organ initiation. In order to verify whether in a vegetative phase auxin can be transported to the meristem in a different way, the research on wild-type and plants with defective PAT was performed. Distance from the differentiated vascular elements to the shoot apical meristem (SAM) was measured for Arabidopsis cultured in different experimental systems. The influence of this distance on the ability to induce organogenesis as well as transport of the fluorescent dye to the SAM, and the LEAFY gene expression were analyzed. The youngest protoxylem elements were used as a marker of the vascular tissues. The distance of protoxylem to the SAM and organogenesis were interrelated. Organ initiation occurred only when protoxylem was localized near to the SAM. Experimental elongation of internodes in a vegetative rosette caused an increase in the distance between protoxylem and the SAM organogenic zone. Thus, the inhibition of organ initiation took place already during the vegetative phase. The results suggest the presence of at least two pathways of acropetal transport of auxin inducing organogenesis: one superficial way through PAT, and the second, putative one, internal through the vascular system. Possibly, organogenesis is completely blocked only when both these pathways are dysfunctional.

Characterization of SHORT-ROOT function in the Arabidopsis root vascular system.

Development of the vascular tissues is a dynamic process that integrates extrinsic and intrinsic factors to control vascular tissue formation throughout the plant life cycle. During vascular tissue formation in Arabidopsis roots, radial and longitudinal signals, including nuclear factors and plant hormones, control the developmental processes involved in the specification, differentiation, and maintenance of the correct cell types. SHR, a GRAS transcription factor, has been known to regulate the specification of the stem cell niche and ground tissue identity in the root meristem in a non-cell-autonomous manner. However, the role of SHR in the root vasculature is relatively overlooked, despite localization of its mRNA and protein in the stele. Here, we investigated the role of SHR in the vascular system of the primary root using a reverse genetic approach and detailed phenotypic analysis. A novel, loss-of-function null mutant, shr-6, was isolated in the Columbia background, and vascular patterning was characterized in detail. Our results reveal that shr mutants have developmental defects in both protophloem and protoxylem elements. Our study also suggests that SHR plays a central role in the root vascular system to control patterning processes, possibly regulated by longitudinal and radial signals.

Characterization of Adventitious Root Development in Sweetpotato

Adventitious roots of ‘Beauregard’ and ‘Georgia Jet’ sweetpotato were observed and anatomically characterized over a period of 60 days of storage root development. The majority of ‘Beauregard’ and ‘Georgia Jet’ adventitious roots sampled at 5 to 7 days after transplanting (DAT) possessed anatomical features (five or more protoxylem elements) associated with storage root development. The majority of ‘Beauregard’ (86%) and ‘Georgia Jet’ (89%) storage roots sampled at 60 to 65 DAT were traced directly to adventitious roots extant at 5 to 7 DAT. The two varieties, however, differed in the timing in which regular and anomalous cambia were formed. Regular vascular cambium development, i.e., initiation and completion, was observed in both varieties at 19 to 21 DAT. Formation of complete regular vascular cambium was negligible for ‘Beauregard’ (4%) in comparison with ‘Georgia Jet’ (32%) at 26 to 28 DAT. However, anomalous cambia development adjacent to xylem elements was greater in ‘Beauregard’ (30%) in comparison with ‘Georgia Jet’ (13%). Nearly 40% to 50% of samples in both varieties showed extensive lignification in the stele region. At 32 to 35 DAT, 62% to 70% of the adventitious roots for both varieties had either been initiated (developed anomalous cambium) or were lignified. The remaining adventitious roots showed intermediate stages of vascular cambium development. The adventitious root count increased up to 19 to 21 DAT and then remained constant up to 32 to 35 DAT. These accumulated results suggest that the initial stages of adventitious root development are critical in determining storage root set in sweetpotato.

Effect of xylem blockage on stem resistance and water potential in Triplochiton scleroxylon K. Schum. cuttings

SummaryCuttings of the tropical hardwood species Triplochiton scleroxylon K. Schum. may be extremely difficult to root, depending on the physiological status of the parent plant and propagation conditions. This study examined the importance of xylem blockage in limiting the water supply to unrooted stem cuttings. Stem resistance was greatest in Node 1 cuttings (youngest node) of stock plants, and decreased basipetally. Resistance increased to a maximum, and water potential reached a minimum, 6 d after transfer to rooting medium.The primary site of blockage occurred within 1 mm of the stem base, suggesting an excision-induced wounding response. Histological studies revealed that gums of carbohydrate origin were deposited in some protoxylem elements near the stem base. The influence of these factors in determining cutting survival is discussed.

Gleichenia appianensis sp. nov. (Gleicheniaceae): A Permineralized Rhizome and Associated Vegetative Remains from the Eocene of Vancouver Island, British Columbia

A permineralized gleicheniaceous rhizome with an attached stipe base has been identified from the Appian Way locality on Vancouver Island, British Columbia, Canada. The specimens are preserved in Middle Eocene marine calcareous concretions that are studied using the cellulose acetate peel technique. The marginally mesarch rhizome has a vitalized protostele, protoxylem elements with helical wall thickenings, and scalariform metaxylem tracheids that occur in clusters. A continuous band of phloem and pericycle surrounds the xylem. The inner cortex consists of a layer of small‐diameter sclerenchyma fibers, and the outer cortex is composed of larger parenchyma cells. A nodal island of sclerenchyma is present between the leaf trace and protostele at the level of frond‐trace divergence. Long metaxylem tracheids in the frond trace show septa in longitudinal section that likely represent tyloses. Roots diverge from all sides of the rhizome with traces that run obliquely through the cortex. Numerous associated but isolated frond segments are found in the Appian Way nodules that show a pinched and inrolled C‐shaped vascular trace and pseudodichotomous bifurcation. The frond traces have numerous protoxylem strands, and the general anatomy is comparable to that of gleicheniaceous stipes. The rhizome is described as a new species of Gleichenia, Gleichenia appianensis Mindell, Stockey, Rothwell, et Beard sp. nov., and represents the first record of Gleicheniaceae in the Tertiary of North America.

Anatomia da raiz de Cyperus giganteus Vahl (Cyperaceae) em desenvolvimento

Cyperus giganteus or piri, constitutes a vegetable community denominated Pirizal, in the Pantanal. It is considered an amphibious or mesophyte species, because it grows in ponds or dry soils. In the tip root there is the calyptrogen, from which the root cap originates; the promeristem, the protoderm, the procambium and the fundamental meristem are also evident in this part. During the differentiation of the protoderm and of the fundamental meristem from promeristem, the initial cell of both tissues can be observed. The external cortex is constituted by the hypodermis and the inner cortex is formed from the precursory layer of the endodermis. The endodermis presents Casparian strip, suberin lamella, thickened walls and phenolic compounds. In successive phases the inner cortex presents schizo-lysigenous aerenchyma. Precocious metaxylem elements are observed in the vascular cylinder followed by the protoxylem elements differentiation. During the development of the rhizome, the roots arise from the pericycle, that constitutes the innermost layer of the primary thickening meristem (PTM). The disposition of the aerenchyma, the multiple hypodermis and idioblasts with phenolic componds are the most representative characters in the root organization for the family Cyperaceae.

A new structural element containing glycine-rich proteins and rhamnogalacturonan I in the protoxylem of seed plants.

The water pipes of elongating plant organs are the result of programmed cell death and are formed by the walls of dead and empty protoxylem elements. These protoxylem elements are passively elongated many times by the surrounding tissue before they are replaced and collapse. Well-known adaptations for this unique task include the characteristic secondary wall thickenings, forming rings and helices. A new, clearly distinct structural element containing glycine-rich proteins is now visualized for the first time, using confocal laser scanning microscopy in the mature protoxylem of elongating organs of seed plants. This structural element is arranged along the longitudinal axis of the protoxylem elements. It interconnects the secondary wall thickenings within and between protoxylem elements, as well as the protoxylem with other cell types such as xylem parenchyma cells and metaxylem elements. The structural element is stable against detergent extractions, proteinase, pectinase and cellulase hydrolysis, and is closely associated with rhamnogalacturonan-I, a pectic polysaccharide. The results clearly demonstrate that the cell wall of protoxylem cells is a highly dynamic and complex structure. The typical polysaccharide-rich primary wall of living and elongating plant cells is progressively modified and finally replaced by a protein-rich wall in the dead and passively stretched protoxylem elements. These glycine-rich walls originated early in the evolution of the seed plants as confirmed by the analysis of genomic information.

Exogenously applied jasmonic acid induces changes in apical meristem morphology of potato stolons.

Hooked apex stolons and initial swelling stolons of potato plants were treated with 3 x 10-8 mol l-1 jasmonic acid (JA) to study the effect of this compound on histology, cell expansion and tissue differentiation. In hooked apex stolons, JA application increased the meristem thickness and reduced the length of the leaf primordia, whereas in initial swelling stolons narrowing of the apical region, absence of leaf primordia and swelling of the subapical meristem were evident. Early vascular tissue differentiation was observed in response to JA treatment, especially of xylem elements from regions proximal to the tunic. Protoxylem elements, such as tracheal elements, were present with thin primary cell walls. The cell area was measured in two zones: zone I, central mother cells situated immediately under the tunic; and zone II, rib meristem cells. JA caused a four- and six-fold increase in cell area in both zones in hooked apex stolons and initial swelling stolons, respectively. Thus, tuber formation is concluded to occur as a consequence of increased cell expansion, a reduction in the length of leaf primordia, enlargement of meristems, and early vascular tissue differentiation.

Protoxylem: the deposition of a network containing glycine-rich cell wall proteins starts in the cell corners in close association with the pectins of the middle lamella.

Antibodies were used to localise polysaccharide and protein networks in the protoxylem of etiolated soybean (Glycine max L.) hypocotyls. The deposition of glycine-rich proteins (GRPs) starts in the cell corners between protoxylem elements and xylem parenchyma cells. Finally, the GRPs form a network between two mature protoxylem elements. The network also interconnects the ring- and spiral-shaped secondary wall thickenings, as well as the thickenings with the middle lamellae of living xylem parenchyma cells. In addition to the GRP network, a polysaccharide network composed mainly of pectins is involved in the attachment of the secondary wall thickenings to the middle lamellae of xylem parenchyma cells.

Cadmium-induced changes in antioxidative systems, hydrogen peroxide content, and differentiation in Scots pine roots.

To investigate whether Cd induces common plant defense pathways or unspecific necrosis, the temporal sequence of physiological reactions, including hydrogen peroxide (H(2)O(2)) production, changes in ascorbate-glutathione-related antioxidant systems, secondary metabolism (peroxidases, phenolics, and lignification), and developmental changes, was characterized in roots of hydroponically grown Scots pine (Pinus sylvestris) seedlings. Cd (50 microM, 6 h) initially increased superoxide dismutase, inhibited the systems involved in H(2)O(2) removal (glutathione/glutathione reductase, catalase [CAT], and ascorbate peroxidase [APX]), and caused H(2)O(2) accumulation. Elongation of the roots was completely inhibited within 12 h. After 24 h, glutathione reductase activities recovered to control levels; APX and CAT were stimulated by factors of 5.5 and 1.5. Cell death was increased. After 48 h, nonspecific peroxidases and lignification were increased, and APX and CAT activities were decreased. Histochemical analysis showed that soluble phenolics accumulated in the cytosol of Cd-treated roots but lignification was confined to newly formed protoxylem elements, which were found in the region of the root tip that normally constitutes the elongation zone. Roots exposed to 5 microM Cd showed less pronounced responses and only a small decrease in the elongation rate. These results suggest that in cells challenged by Cd at concentrations exceeding the detoxification capacity, H(2)O(2) accumulated because of an imbalance of redox systems. This, in turn, may have triggered the developmental program leading to xylogenesis. In conclusion, Cd did not cause necrotic injury in root tips but appeared to expedite differentiation, thus leading to accelerated aging.

Hydrophobic interactions of the structural protein GRP1.8 in the cell wall of protoxylem elements.

The glycine-rich structural protein GRP1.8 of French bean (Phaseolus vulgaris) is specifically localized in the modified primary cell walls of protoxylem elements. Continuous deposition of GRP1.8 into the cell walls during elongation growth of the plant suggests that GRP1.8 is part of a repair mechanism to strengthen the protoxylem. In this work, a reporter-protein system was developed to study the interaction of GRP1.8 with the extracellular matrix. Fusion proteins of a highly soluble chitinase with different domains of GRP1.8 were expressed in the vascular tissue of tobacco (Nicotiana tabacum), and the chemical nature of the interaction of these fusion proteins in the cell wall compartment was analyzed. In contrast with chitinase that required only low-salt conditions for complete extraction, the different chitinase/GRP1.8 fusion proteins were completely extracted only by a nonionic or ionic detergent, indicating hydrophobic interactions of GRP1.8. The same interactions were found with the endogenous GRP1.8 in bean hypocotyls. In addition, in vitro experiments indicate that oxidative cross-linking of tyrosines might account for the insolubilization of GRP1.8 observed in later stages of protoxylem development. Our data suggest that GRP1.8 forms a hydrophobic protein-layer in the cell wall of protoxylem vessels.

Effects of salinity on xylem structure and water use in growing leaves of sorghum

Within the growth zone, salt‐affected leaves of sorghum (Sorghum bicolor) had narrower protoxylem and metaxylem cells than controls. Leaf width and cross‐sectional area were also reduced, so that the salt treatment had no effect on the area of protoxylem per area of leaf cross section. Dye uptake studies suggested that in controls most of the veins, but in salt‐affected leaves only half of the veins, are functional in water transport. Volumetric water flow was greatly diminished in the salt‐affected plants. The reduced flow rate was largely explained by the salt‐induced decrease in leaf surface area. Some decreases in flow rates per unit leaf mass or area were also produced by salinity, particularly in late developmental stages. Not surprisingly, leaf conductance measured with a diffusion porometer did not appear to be correlated with the diameter of the protoxylem or metaxylem elements. By contrast, published values of water deposition rates are strongly related to the size of the protoxylem elements: the rates of water deposition into the growing leaf tissue are proportional to the square of the protoxylem radius. Thus environmentally produced change of the hydraulic architecture of monocot leaves may cause change in local growth rates.