Late Metaxylem Elements Research Articles

The Aux/IAA gene rum1 involved in seminal and lateral root formation controls vascular patterning in maize (Zea mays L.) primary roots.

The maize (Zea mays L.) Aux/IAA protein RUM1 (ROOTLESS WITH UNDETECTABLE MERISTEMS 1) controls seminal and lateral root initiation. To identify RUM1-dependent gene expression patterns, RNA-Seq of the differentiation zone of primary roots of rum1 mutants and the wild type was performed in four biological replicates. In total, 2 801 high-confidence maize genes displayed differential gene expression with Fc ≥2 and FDR ≤1%. The auxin signalling-related genes rum1, like-auxin1 (lax1), lax2, (nam ataf cuc 1 nac1), the plethora genes plt1 (plethora 1), bbm1 (baby boom 1), and hscf1 (heat shock complementing factor 1) and the auxin response factors arf8 and arf37 were down-regulated in the mutant rum1. All of these genes except nac1 were auxin-inducible. The maize arf8 and arf37 genes are orthologues of Arabidopsis MP/ARF5 (MONOPTEROS/ARF5), which controls the differentiation of vascular cells. Histological analyses of mutant rum1 roots revealed defects in xylem organization and the differentiation of pith cells around the xylem. Moreover, histochemical staining of enlarged pith cells surrounding late metaxylem elements demonstrated that their thickened cell walls displayed excessive lignin deposition. In line with this phenotype, rum1-dependent mis-expression of several lignin biosynthesis genes was observed. In summary, RNA-Seq of RUM1-dependent gene expression in maize primary roots, in combination with histological and histochemical analyses, revealed the specific regulation of auxin signal transduction components by RUM1 and novel functions of RUM1 in vascular development.

Silicon mitigates the Cd toxicity in maize in relation to cadmium translocation, cell distribution, antioxidant enzymes stimulation and enhanced endodermal apoplasmic barrier development

The objective of this study was to assess the effect of different Cd and Si concentrations on the maize plants. The following Cd and/or Si treatments were used: 5 Cd; 10 Cd; 100 Cd; 5 Cd + 0.08 Si; 10 Cd + 0.08 Si; 100 Cd + 5 Si treatments (Cd concentration in μM, Si concentration in mM). The plant growth, photosynthetic pigments content, antioxidant enzymes activities (POX, SOD, CAT), Cd and Si accumulation, translocation and cell wall deposition of the maize plants was observed. Changes in the endodermal cell walls development and late metaxylem elements lignification due to Cd and/or Si treatment were also evaluated. The negative effect of Cd (5 and 10 μM) on the growth parameters was alleviated by Si at 0.08 mM. The positive effect of Si was not observed at higher Cd and Si concentrations. This indicates that the alleviating effect of Si on Cd toxicity depends on the Cd and Si concentrations. Plants responded to Cd toxicity by an increase of antioxidant enzyme activity. Silicon addition in Cd + Si treatment stimulated an increase in the activity of antioxidant enzymes in comparison with the Cd treatment. Chlorophyll and carotenoid content in the Cd treated plants was not significantly affected by Si. The young maize plants retained much more Cd in their roots as they translocated into the shoots. 5 Cd + 0.08 Si and 10 Cd + 0.08 Si treatments correlated with an increase in Cd concentration in the roots and shoots, and in the cell walls. Silicon caused a slight decrease of the Cd translocation into the shoots in 5 Cd + 0.08 Si and 10 Cd + 0.08 Si treatments. Negative correlation between the root Cd cell wall deposition and Cd translocation was observed. Cadmium and/or Si altered root anatomy. Cadmium enhanced suberin lamellae development and late metaxylem lignification; silicon in Cd + Si treatments accelerated suberin lamellae deposition and enhanced the tertiary endodermal cell walls formation in comparison with Cd treatments. Negative correlation between the endodermal cell walls development and Cd translocation was observed.

Perturbation of Polyamine Catabolism Can Strongly Affect Root Development and Xylem Differentiation

Spermidine (Spd) treatment inhibited root cell elongation, promoted deposition of phenolics in cell walls of rhizodermis, xylem elements, and vascular parenchyma, and resulted in a higher number of cells resting in G(1) and G(2) phases in the maize (Zea mays) primary root apex. Furthermore, Spd treatment induced nuclear condensation and DNA fragmentation as well as precocious differentiation and cell death in both early metaxylem and late metaxylem precursors. Treatment with either N-prenylagmatine, a selective inhibitor of polyamine oxidase (PAO) enzyme activity, or N,N(1)-dimethylthiourea, a hydrogen peroxide (H(2)O(2)) scavenger, reverted Spd-induced autofluorescence intensification, DNA fragmentation, inhibition of root cell elongation, as well as reduction of percentage of nuclei in S phase. Transmission electron microscopy showed that N-prenylagmatine inhibited the differentiation of the secondary wall of early and late metaxylem elements, and xylem parenchymal cells. Moreover, although root growth and xylem differentiation in antisense PAO tobacco (Nicotiana tabacum) plants were unaltered, overexpression of maize PAO (S-ZmPAO) as well as down-regulation of the gene encoding S-adenosyl-l-methionine decarboxylase via RNAi in tobacco plants promoted vascular cell differentiation and induced programmed cell death in root cap cells. Furthermore, following Spd treatment in maize and ZmPAO overexpression in tobacco, the in vivo H(2)O(2) production was enhanced in xylem tissues. Overall, our results suggest that, after Spd supply or PAO overexpression, H(2)O(2) derived from polyamine catabolism behaves as a signal for secondary wall deposition and for induction of developmental programmed cell death.

Branch Roots of Zea. V. Structural Features that may Influence Water and Nutrient Transport

AbstractFirst‐order branch roots of field‐grown Zea mays L. were examined by optical and electron microscopy. They were small‐scale versions of nodal roots except for the usual retention of a live epidermis throughout their length. The Casparian strips and suberized lamellae of hypodermis and endodermis developed closer to the root tip than reported for main roots (in the zone 0.5 to 5.5 cm from the tip for the hypodermis, and 0.5 to 4 cm for the endodermis), in branches retaining an apical meristem. The hydrophobic deposits were in place to the distal ends of determinate branches. All hydrophobic deposits were fully formed before the late metaxylem elements were mature. Gaps in the suberized lamellae of both hypodermis and endodermis may permit apoplastic diffusion of solutes through these layers. Pit frequency in the outer tangential walls of the hypodermis and endodermis was 0.3 per 100 μm2, and 0.6 to 0.7 per 100 μm2, respectively, in both branch and main roots. Numbers of plasmodesmata per pit in the branches were 60 and 30 in the hypodermis and endodermis, respectively. Water fluxes from published data were used to calculated the possible flux through plasmodesmata on a symplastic path. Values up to 0.2 pl h−1 for the hypodermis and twice this for the endodermis were obtained.

Hydraulic propagation of pressure along immature and mature xylem vessels of roots of Zea mays measured by pressure-probe techniques

Turgor pressure was measured in cortical cells and in xylem elements of excised roots and roots of intact plants of Zea mays L. by means of a cell pressure probe. Turgor of living and hence not fully differentiated late metaxylem (range 0.6–0.8 MPa) was consistently higher than turgor of cortical cells (range 0.4–0.6 MPa) at positions between 40 and 180 mm behind the root tip. Closer to the tip, no turgor difference between the cortex and the stele was measured. The turgor difference indicated that late-metaxylem elements may function as nutrient-storage compartments within the stele. Excised roots were attached to the root pressure probe to precisely manipulate the xylem water potential. Root excision did not affect turgor of cortical cells for at least 8 h. Using the cell pressure probe, the propagation of a hydrostatic pressure change effected by the root pressure probe was recorded in mature and immature xylem elements at various positions along the root. Within seconds, the pressure change propagated along both early and late metaxylems. The half-times of the kinetics, however, were about five times smaller for the early metaxylem, indicating they are likely the major pathway of longitudinal water flow. The hydraulic signal dissipated from the source of the pressure application (cut end of the root) to the tip of the root, presumably because of radial water movement along the root axis. The results demonstrate that the water status of the growth zone and other positions apical to 20 mm is mainly uncoupled from changes of the xylem water potential in the rest of the plant.

Different populations of bacteria associated with sheathed and bare regions of roots of field-grown maize

It has been previously shown that soil sheaths cling tightly to some portions of all axile roots and cover all but the growing tips of the young roots of field-grown maize. These sheaths overlie immature regions of the roots which have intact epidermal cells with root hairs, and living, thus non-conducting, late metaxylem elements. Loss of the soil sheath in the proximal region coincides with the opening of these large metaxylem vessels. Now, total, and viable counts have been recorded of bacteria associated with the root surface and adhering soil of sheathed and bare regions. These showed some common features, in that populations of similar size were associated with the two root regions in plants beginning to flower. Each population included about the same numbers of bacteria that were viable on each of three selective media (nitrogen-free, Pseudomonas F or MacConkey). However, more spore-formers capable of growth on nitrogen-free media and more fluorescent bacteria were isolated from the sheathed regions. Actinomycetes were absent from sheathed but plentiful on bare regions. The high numbers of diverse types of bacteria associated with both root surfaces can be related to the previously demonstrated similarity in amounts of organic carbon released from each region. The proliferation of actionomycetes on the bare roots and their exclusion from sheathed roots may in part be due to the lower water status of the bare region, which is related to its greater axial conducting capacity. Thus the distribution of the two types of root surface within an individual root system has important implications for the choice of root and rhizosphere sampling techniques and for root bacterization work.

Accumulation of potassium by differentiating metaxylem elements of maize roots

The previous demonstration that the large late metaxylem vessels of field‐grown maize (Zea mays L. cv. Rosella) roots do not lose their crosswalls until they are 20–30 cm from the tip, and that the presence of a soil sheath outside the root was indicative of immature vessels within, greatly strengthened the hypothesis that ion accumulation into these roots was by uptake into living xylem element vacuoles. Proposals that salt movement into the xylem was by leakage or secretion into dead vessels became much less plausible. Potassium concentration in the vacuoles of late metaxylem elements was measured by X‐ray microanalysis in unetched fracture faces of bulk, frozen‐hydrated pieces of sheathed roots, and found to be in the range 150–400 mM. Potassium concentration in open vessels of bare roots, measured both with the microprobe and by spectrophotometry of aspirated sap, was in the range of 5 to 25 mM. It is concluded that uptake of potassium (and possibly other ions) is into living xylem elements, and that its release to the transpiration stream occurs by the breakdown of their crosswalls and the addition of their vacuoles to the solution in the vessels above.

On the Elongation of Root Hairs of Rice Plants

A few studies on the elongation of root hairs of rice plants in relation to ecological conditions (nitrogen fertilizer, water percolation, etc.) had been carried out by the authors. But the study on the elongation of root hairs in relation to physiological conditions of roots has not been ascertained. The present work was carried out to study the elongation of root hairs in relation to the speed of root growth, root diameter, and thickening of vessel element. The following results were obtained: Generally, at the root apex, papilla bears firstly and then elongation of root hair begins; in this case, at the early stage the growing speed is very slow. Gradually the growing speed of root hairs becomes speedy with time, but at the later stage it decreases and finally stops. As regards the elongation of root hairs in relation to the diameter and the growing speed of the roots, it was found that the thicker roots have longer root apex-papilla distance (i.e. the distance from the root apex excluding root cap to the first papilla), longer root-hair-growing zone and longer root hairs as compared with the thinner roots, and that the roots which grow speedy have longer root hairs than the roots which grow slowly. It was also found that there is close correlation between the elongation of root hairs and the thickening of vessel elements; at the point that papilla were ascertained, matured protoxylem elements (PX) were seen, and at the point that elongation of root hair becomes maximum, matured early metaxylem elements (MXV I) were seen, and at the point that elongation of root hairs stopped, matured late metaxylem elements (MXV II) were observed.

On the Structure of the Crown Root and the Lateral Root, and the Vessel Connection Between Them, in Rice Plants

It is known that the roots of rice plants branch out from the secondary to the sixth order lateral roots, and there are, generally, two kinds of lateral roots can be seen at any order, namely, thick and thin lateral roots. Their structures differ in that the thin lateral roots are absent in the late metaxylem elements (MX II) and in the cortex which forms lysigenous aerenchyma (Fig. 1, B and C). The vessel connection between the crown root and the secondary lateral root, between the secondary lateral root and the tertiary lateral root, and so on, are alike. In other words, the protoxylem elements (PX) of the lateral root are connected with the protoxylem elements or early metaxylem elements of the parent root, and the late metaxylem elements (MXII) of the lateral root are connected with the late metaxylem elements of the parent root, independently of the order (Figs. 3, 4 and 5). Between the vssels of the lateral root and the parent root, at the connecting region, tracheary elements (TE) were observed which may be modified forms of the stelar parenchyma cells, and these tracheary elements were jointed with the side wall with each other (Figs. 3, 4 and 5). The connection between the protoxylem elements (or the early metaxylem elements in the case of the crown roots) and the late metaxylem elements in the parent root was observed at the region where the lateral roots were formed. The connection might be undergone by modification into tracheary elements of the stelar parenchyma cells between them (Fig. 6). Some of these tracheary elements had perforations and others did not (Fig. 7).

Ontogeny of xylem transfer cells inHieracium floribundum

The ontogeny of transfer cells associated with tracheary elements in the rhizome of the rosette plant,Hieracium floribundum is described. The events preceding the initiation of wall ingrowths in parenchymatous cells associated with tracheary elements determining them as transfer cells vary with the type of tracheary element and the position within the rhizome. Parenchyma cells adjacent to protoxylem and metaxylem elements close to the rhizome apex usually divide before wall ingrowth formation whereas this tendency is decreased in the basipetal direction in the rhizome. When parenchyma cells do not divide before wall ingrowth formation, the wall ingrowths are poorly developed and the cells remain vacuolated. If cell division does occur, the daughter cell next to the tracheary element always becomes a transfer cell and the other develops as a xylem parenchyma cell. Within a vascular bundle there is a gradient in intensity of wall ingrowth formation, with the transfer cells associated with protoxylem elements having the most elaborate wall ingrowths, and the transfer cells associated with the late metaxylem elements having few or no wall ingrowths. Parenchyma cells associated with secondary xylem elements lack wall ingrowths. At the end of primary growth and the establishment of secondary growth within a bundle, successive cytological changes occur in the xylem transfer cells, leading ultimately to the loss of wall ingrowths, the thickening of cell walls and the complete vacuolation of the once-richly cytoplasmic cells. Although the thickened walls of other primary xylem parenchyma cells become lignified at this time, the transfer cell walls remain unlignified until the rhizome is very old.

The extent and differences in mitotic activity of the root tip ofVicia faba L.

Mitotic activity does not stop for different meristematic cells of the root apex at the same distance from the initials. The differences are connected with the functional heterogeneity of the apical meristem of the root. The arrangement of vascular bundles,i.e. the alternation of independent xylem and phloem groups, is of major importance. In broad bean roots, the protophloem sieve elements stop dividing first. The centre of the stelei. e. late metaxylem elements stop dividing next. Division in the stele gradually ceases centrifugally, while it ceases centripetally in the peripheral part of the root. The cylindrical region with prolonged cell division includes internal layers of the cortex including endodermis, pericycle and adjoining cells of the stele. Proximally apical meristem is reduced to isolated strands of cells adjacent to the protoxylem poles. Pericycle cells stop dividing last at a distance of approx. 9–10 mm from the initials. The number of the division cycles is limited and is specific for individual cell types. Epidermal and cortical cells divide in broad bean roots transversely approximately seven times, cells of late metaxylem approximately five times. Root apical meristem is an asynchronous cell population with a different duration of the mitotic cycle. We determined local variations in the duration of the mitotic cycle in the apical meristem of broad bean root by means of colchicine-induced polyploidy. The cells of the quiescent centre had the longest mitotic cycle after colchicine treatment. The region of the proper root adjacent to the quiescent centre was mixoploid (2n and 4n). Isolated cells with a long cycle occurred also in the cortex and in the central cylinder. Cells with a division cycle of 18h were found in the root cap, in the epidermis, in the cortex and in the central cylinder. Relatively numerous cells with the shortest division cycle, approx. 12 h, occurred farther of the quiescent centre in the epidermis, in the cortex, in the pericycle, and in adjacent layers of the stele through-out the entire meristematic region. The results derived from the analysis of the apical meristem are discussed in connection with the ontogenesis of different types of cells taking part in the primary structure of the root.

On the First Lateral Root Primordium Formation Closer to Crown Root Apex of Rice Plants

In a crown root of rice plants (fig. 24), protophloem elements (PP) were the first of all vascular elements to show maturation usually at about 1 mm from the apex, and secondary wall layer starts covering protoxylem elements (PX) at a distance of 4-5 mm region from the apex. At about 7-8 mm from the apex, nuclei and cytoplasm in the protoxylem elements have degenerated and successively the secondary thickening ends. A first lateral primordium (I) of the crown root seems to originate in pericyclic cells between two protoxylems in which nuclei are degenerating and secondary thickening is going on. While covering by secondary wall in early metaxylem elements (MXI), anticlinal divisions of the first lateral root primordium (II) in the pericyclic cells begin to take place. Before and after periclinal division of the pericyclic cells of the first lateral root primordium, a certain grains (fig. 17 and 18, G) gather near the end wall as well as along protoplasmic membrane of metaphloem elements (fig. 24, MP) and a rounded shape of nucleolus in the metaphloem has changed to a chestnut-in-burs like shape (fig, 19) and the nucleolus is found near the sieve plate of the mature metaphloem as an extruded nucleolus. At a half-way penetration of the cortex (fig. 24, IV), secondary thickening of the early metaxylem completes and maturation of the metaphloem takes place. Before the first lateral root emerges on the surface of crown roots (V), late metaxylem elements (MXII) begin to be covered by secondary wall and all vascular elements except the late metaxylem have matured. A certain causal relation between the first lateral root primordium formation and the development of the vascular element, especially the protoxylem, may be estimated generally in any roots of rice plants.