Anatomy and cell wall ultrastructure of sunflower stalk rind

The anatomy and ultrastructure of sunflower stalk rind are closely related to its conversion and utilization. We studied systematically the anatomy and ultrastructure of the stalk rind using light, scanning electron, transmission electron and fluorescence microscopy. The results showed that the stalk rind consisted of phloem fibers (PF), xylem fibers (XF), vessel elements (V), ground parenchyma cells (GPC), axial parenchyma cells (APC), xylem ray parenchyma cells (XRPC), and pith ray parenchyma cells (PRPC). These cell walls were divided into the middle lamella, primary wall, and secondary wall (S). It was found that the S of PF, XF and V was further divided into three layers (S1–S3), while the S of APC, GPC, XRPC and PRPC showed a non-layered cell wall organization or differentiated two (S1, S2) to seven layers (S1–S7). Our research revealed the plasmodesmata characteristics in the pit membranes (PMs) between parenchyma cells (inter-GPCs, inter-XRPCs, and inter-PRPCs). The morphology of the plasmodesmata varied with the types of parenchyma cells. The thickness and diameter of PMs between the cells (inter-Vs, V–XF, V–APC, and V–XRPC) were greater than that of PMs between parenchyma cells. The cell corners among parenchyma cells were intercellular space. The lignification degree of vessels was higher than that of parenchyma cells and fibers. The results will provide useful insights into the biological structure, conversion and utilization of sunflower stalk rind.

Regeneration of vascular tissue through redifferentiation of interxylary phloem after complete girdling in Aquilaria sinensis

We studied the time-course of stem response for six months following complete girdling in branches of Aquilaria sinensis to determine the potential role of interxylary phloem (IP) in this response. It was found that the vascular cambium, as well as its derivative secondary xylem and phloem, regenerated fully through redifferentiation of IP. We confirmed that vascular cambium regenerated within one month after girdling based on observation of new vessels, IP, and secondary phloem fibers. The time-course study showed that IPs made connections with each other, merged, and became larger through the proliferation of IPs parenchyma cells and the cleaving of secondary xylem in a narrow zone 400 to 1000 μm deep inside the girdled edge. This led to the formation of a complete circular sheath of vascular cambium, followed by the regeneration of vascular tissue. It is worth noting that the secondary xylem is regenerated always following the formation of a thick belt of wound xylem.

Cell Wall Layer Induced in Xylem Fibers of Flax Upon Gravistimulation Is Similar to Constitutively Formed Cell Walls of Bast Fibers

In the fibers of many plant species after the formation of secondary cell walls, cellulose-enriched cell wall layers (often named G-layers or tertiary cell walls) are deposited which are important in many physiological situations. Flax (Linum usitatissimum L.) phloem fibers constitutively develop tertiary cell walls during normal plant growth. During the gravitropic response after plant inclination, the deposition of a cellulose-enriched cell wall layer is induced in xylem fibers on one side of the stem, providing a system similar to that of tension wood in angiosperm trees. Atomic force microscopy (AFM), immunochemistry, and transcriptomic analyses demonstrated that the G-layer induced in flax xylem fibers was similar to the constitutively formed tertiary cell wall of bast (phloem) fibers but different from the secondary cell wall. The tertiary cell walls, independent of tissue of origin and inducibility, were twice as stiff as the secondary cell walls. In the gravitropic response, the tertiary cell wall deposition rate in xylem was higher than that of the secondary cell wall. Rhamnogalacturonan I (RG-I) with galactan side chains was a prominent component in cellulose-rich layers of both phloem and xylem flax fibers. Transcriptomic events underlying G-layer deposition in phloem and xylem fibers had much in common. At the induction of tertiary cell wall deposition, several genes for rhamnosyltransferases of the GT106 family were activated in xylem samples. The same genes were expressed in the isolated phloem fibers depositing the tertiary cell wall. The comparison of transcriptomes in fibers with both inducible and constitutive tertiary cell wall deposition and xylem tissues that formed the secondary cell walls is an effective system that revealed important molecular players involved in the formation of cellulose-enriched cell walls.

Phylogenetic placement of enigmatic Astianthus (Bignoniaceae) based on molecular data, wood and bark anatomy

Background: Astianthus is a monospecific arborescent genus of Bignoniaceae that occur in the Pacific Coast of central Mexico and northern Central America, where it grows in dense populations along riversides. Its phylogenetic placement has remained controversial since Astianthus has unusual morphological characters such as a four-loculed ovary, and simple, pulvinate, verticillate leaves. Methods: Here we used three plastid markers ndhF, rbcL, and trnL-F, wood, and bark anatomical data to investigate the phylogenetic placement of Astianthus and assign it to one of Bignoniaceae’s main clades. Results: Our molecular phylogenetic analyses indicated that Astianthus belongs in tribe Tecomeae s.s., where other charismatic Neotropical Bignoniaceae genera such as Campsis and Tecoma are currently placed. Wood and bark anatomy support this placement, as Astianthus reunites a unique combination of features only known from members of Tecomeae s.s., such as storied axial parenchyma, the co-occurrence of homo- and heterocellular rays, septate fibers, and scattered phloem fibers in the bark. Conclusions: The placement of Astianthus within Tecomeae s.s. provides further support to previous proposals for the Neotropical origin of this Pantropical tribe.

Rearrangement of the Cellulose-Enriched Cell Wall in Flax Phloem Fibers over the Course of the Gravitropic Reaction

The plant cell wall is a complex structure consisting of a polysaccharide network. The rearrangements of the cell wall during the various physiological reactions of plants, however, are still not fully characterized. Profound changes in cell wall organization are detected by microscopy in the phloem fibers of flax (Linum usitatissimum) during the restoration of the vertical position of the inclined stems. To characterize the underlying biochemical and structural changes in the major cell wall polysaccharides, we compared the fiber cell walls of non-inclined and gravistimulated plants by focusing mainly on differences in non-cellulosic polysaccharides and the fine cellulose structure. Biochemical analysis revealed a slight increase in the content of pectins in the fiber cell walls of gravistimulated plants as well as an increase in accessibility for labeling non-cellulosic polysaccharides. The presence of galactosylated xyloglucan in the gelatinous cell wall layer of flax fibers was demonstrated, and its labeling was more pronounced in the gravistimulated plants. Using solid state NMR, an increase in the crystallinity of the cellulose in gravistimulated plants, along with a decrease in cellulose mobility, was demonstrated. Thus, gravistimulation may affect the rearrangement of the cell wall, which can enable restoration in a vertical position of the plant stem.

ADAPTIVE FEATURES OF THE VEGETATIVE ORGANS OF SOME COMMON RIPARIAN PLANTS IN THE LOWER HA THANH RIVER, NHONBINH WARD, QUYNHON CITY, BINHDINH PROVINCE

The plant species which live in the lower Ha Thanh river, Binhdinh province, have some morphological and anatomicaladaptations to the riparianenvironment. The research was carried out on 4 plant species, including trees and grasses in Magnoliopsida, namely Hibiscus tiliaceus L., Excoecaria agallocha L., Ludwigia octovalvis (Jacq.) Raven and Polygonum hydropiper L. The methods used consist of morphological comparison, microsurgery, double staining, measurement and microscopic photography of roots, stems and leaves. The results show suberized stem walls sloughing off easily, lenticels of secondary bark containing nodules or vertical crevices and the development of spongy tissue (33% to 39% of leaf thickness). These features help the plants increase gas exchange. There are small numbers of xylem vessels in the roots (10.33 ± 0.67 vessels/mm2) and stems (22.83 ± 0.75 vessels/mm2) with widths up to 179.17 ± 21.81µm in the roots and 75 ± 9.13µm in the stems. These features allow these species to adjust to the aqueous environment. The supporting tissues in the roots, stems and leaves are very well developed, including xylem and phloem fibers, sclereids and calcium oxalate crystals that help the plants stand firm in extreme conditions, including strong wind and storms.

Defoliation constrains xylem and phloem functionality

Insect defoliation contributes to tree mortality under drought conditions. Defoliation-induced alterations to the vascular transport structure may increase tree vulnerability to drought; however, this has been rarely studied. To evaluate the response of tree vascular function following defoliation, 2-year-old balsam poplar were manually defoliated, and both physiological and anatomical measurements were made after allowing for re-foliation. Hydraulic conductivity measurements showed that defoliated trees had both increased vulnerability to embolism and decreased water transport efficiency, likely due to misshapen xylem vessels. Anatomical measurements revealed novel insights into defoliation-induced alterations to the phloem. Phloem sieve tube diameter was reduced in the stems of defoliated trees, suggesting reduced transport capability. In addition, phloem fibers were absent, or reduced in number, in stems, shoot tips and petioles of new leaves, potentially reducing the stability of the vascular tissue. Results from this study suggest that the defoliation leads to trees with increased risk for vascular dysfunction and drought-induced mortality through alterations in the vascular structure, and highlights a route through which carbon limitation can influence hydraulic dysfunction.

Intrusive Growth of Phloem Fibers in Flax Stem: Integrated Analysis of miRNA and mRNA Expression Profiles

Phloem fibers are important elements of plant architecture and the target product of many fiber crops. A key stage in fiber development is intrusive elongation, the mechanisms of which are largely unknown. Integrated analysis of miRNA and mRNA expression profiles in intrusivelygrowing fibers obtained by laser microdissection from flax (Linum usitatissimum L.) stem revealed all 124 known flax miRNA from 23 gene families and the potential targets of differentially expressed miRNAs. A comparison of the expression between phloem fibers at different developmental stages, and parenchyma and xylem tissues demonstrated that members of miR159, miR166, miR167, miR319, miR396 families were down-regulated in intrusively growing fibers. Some putative target genes of these miRNA families, such as those putatively encoding growth-regulating factors, an argonaute family protein, and a homeobox-leucine zipper family protein were up-regulated in elongating fibers. miR160, miR169, miR390, and miR394 showed increased expression. Changes in the expression levels of miRNAs and their target genes did not match expectations for the majority of predicted target genes. Taken together, poorly understood intrusive fiber elongation, the key process of phloem fiber development, was characterized from a miRNA-target point of view, giving new insights into its regulation.