Protophloem differentiation in early Arabidopsis thaliana development

During Arabidopsis embryogenesis, procambial cells undergo coordinated, asymmetric cell divisions, giving rise to vascular precursor cells (protophloem and protoxylem precursors). After germination, these cells terminally differentiate into specialized conducting cells, referred to as protophloem and protoxylem cells. Few readily identifiable markers of the onset of specification and differentiation are available, hampering the molecular genetic analysis of protophloem development. Confocal microscopy was used to investigate the patterning and differentiation of phloem cells during early plant development. Longitudinal divisions of phloem initials allowed the identification of protophloem precursor cells and adjacent metaphloem initials along the length of the plant. During germination, protophloem differentiation was observed at two independent locations, in the cotyledons and the hypocotyl. In both locations, differentiation was concomitant with cell elongation. We identified five gene-trap lines (PD1-PD5) with marker gene expression in immature protophloem elements. The spatio-temporal marker expression pattern of the lines divides them into two groups. The early specification markers PD4 and PD5 were expressed in developing organs before procambium formation and then became restricted to phloem initial cells. The protophloem precursor markers PD1-PD3 were expressed in differentiating protophloem cells at different stages of their development. All markers were expressed transiently and iteratively during the differentiation of protophloem in newly formed organs. Flanking genes were identified for four out of five gene-trap insertion lines. The possible function of these genes with respect to phloem differentiation is discussed.     

Cell differentiation in the longitudinal veins and formation of commissural veins in rice (Oryza sativa) and maize (Zea mays)

Vascular development is a central theme in plant science. However, little is known about the mechanism of vascular development in monocotyledons (compared with dicotyledons). Therefore, we investigated sequential processes of differentiation into various different vascular cells by carrying out detailed observations using serial sections of the bases of developing leaves of rice and maize. The developmental process of the longitudinal vascular bundles was divided into six stages in rice and five stages in maize. The initiation of differentiation into procambial progenitor cells forming the commissural vein arose in a circular layer cell that was adjacent to both a metaxylem vessel and one or a few phloem cells in stage V longitudinal vascular bundles. In most cases the differentiation of ground meristem cells into procambial progenitor cells extended in one direction, toward the next longitudinal vascular bundle, and subsequent periclinal divisions and further differentiation produced a vessel element, two companion cells and a sieve element to form a commissural vein. These results suggest the presence of an intercellular signal(s) that induces differentiation of the circular layer cell and the ground meristem cells into procambial progenitor cells, forming a commissural vein sequentially.     

Regulation of Vascular Development by CLE Peptide-receptor Systems

Cell division and differentiation of stem cells are controlled by non-cell-autonomous signals in higher organisms. The plant vascular meristem is a stem-cell tissue comprising procambial cells that produce xylem cells on one side and phloem cells on the other side. Recent studies have revealed that TDIF (tracheary element differentiation inhibitory factor)/CLE41/CLE44 peptide signal controls the procambial cell fate in a non-cell-autonomous manner. TDIF produced in and secreted from phloem cells is perceived by TDR/PXY, a leucine-rich repeat receptor kinase located in the plasma membrane of procambial cells. This signal suppresses xylem cell differentiation of procambial cells and promotes their proliferation. In addition to TDIF, some other CLE peptides play roles in vascular development. Here, we summarize recent advances in CLE signaling governing vascular development.     

Patterned cell determination in a plant tissue: the secondary phloem of trees

The secondary vascular tissues (xylem and phloem) of woody plants originate from a vascular cambium and develop as radially oriented files of cells. The secondary phloem is composed of three or four cell types, which are organised into characteristic recurrent cellular sequences within the radial cell files of this tissue. There is a gradient of auxin (indole acetic acid) across both the cambium and the immediately postmitotic cells within the xylem and phloem domains, and it is believed that this morphogen, probably in concert with other morphogenic factors, is closely associated with the determination and differentiation of the different cells types in each tissue. A hypothesis is developed that, in conjunction with the positional values conferred by the graded radial distribution of morphogen, cell divisions at particular positions within the cambium are sufficient to determine not only each of the phloem cell types but also their recurrent pattern of differentiation within each radial cell file.     

Genetic regulation of vascular tissue patterning in Arabidopsis

Plants transport water and nutrients through a complex vascular network comprised of interconnected, specialized cell types organized in discrete bundles. To identify genetic determinants of vascular tissue patterning, we conducted a screen for mutants with altered vascular bundle organization in Arabidopsis cotyledons. Mutations in two genes, CVP1 and CVP2 (for cotyledon vascular pattern), specifically disrupt the normal pattern of vascular bundles in cotyledons, mature leaves, and inflorescence stems. The spatial distribution of the procambium, the precursor to mature vascular tissue, is altered in cvp1 and cvp2 embryos, suggesting that CVP1 and CVP2 act at a very early step in vascular patterning. Similarly, in developing stems of cvp1 and leaves of cvp2, the pattern of vascular differentiation is defective, but the maturation of individual vascular cells appears to be normal. There are no discernible alterations in cell morphology in cvp2 mutants. In contrast, cvp1 mutants are defective in directional orientation of the provascular strand, resulting in a failure to establish uniformly aligned vascular cells, and they also show a reduction in vascular cell elongation. Neither cvp1 nor cvp2 mutants displayed altered auxin perception, biosynthesis, or transport, suggesting that auxin metabolism is not generally affected in these mutants.     

Primary phloem-specific expression of a Zinnia elegans homeobox gene.

Some plant homeobox genes are expressed specifically in vascular cells and are assumed to function in the differentiation of specific types of vascular cells. However, homeobox genes exhibiting primary phloem-specific expression have not been reported. To elucidate the molecular mechanisms of vascular development, we undertook to isolate from Zinnia elegans primary phloem-specific homeobox genes that may function in phloem development. An HD-Zip type homeobox gene, ZeHB3, was isolated. This gene encodes a class I HD-Zip protein, and constitutes a gene subfamily with the Daucus carota gene CHB6, and Arabidopsis thaliana genes Athb-5, Athb-6, and Athb-16. In situ hybridization of 1-, 14- and 50-day-old plants demonstrated that ZeHB3 mRNA accumulation is restricted to a few cells destined to differentiate into phloem cells and to the immature phloem cells surrounding the sieve elements and companion cells. ZeHB3 protein was also localized to immature phloem cells. These findings clearly indicate that ZeHB3 is a novel homeobox gene that marks, and may function in, the early stages of phloem differentiation.     

蚕豆籽粒内的~(14)C同化物的运转和卸出动态

通过向蚕豆叶片饲喂~(14)CO_2,应用液闪和显微放射性自显影技术表明标记同化物经叶脉和果荚韧皮部筛管快速运输至蚕豆种皮。种皮吸收营养、生长,后期逐步降解、供养子叶。种皮内的两类维管束系统同时输送营养并卸出到种皮内侧的质外体空间里。种皮里的反向维管束韧皮部卸出以共质体方式为主。并提供养分供种皮生长,而大部分的同化物由正向完整维管束韧皮部的筛分子一传递细胞进行质外体方式卸出。膨大中的子叶在早期即已成为生理上十分活跃的库。它对标记同化物的摄入随时间进程而急剧上升。     

Expression of the phloem lectin is developmentally linked to vascular differentiation in cucurbits

The conducting elements of phloem in angiosperms are a complex of two cell types, sieve elements and companion cells, that form a single developmental and functional unit. During ontogeny of the sieve element/companion cell complex, specific proteins accumulate forming unique structures within sieve elements. Synthesis of these proteins coincides with vascular development and was studied in Cucurbita seedlings by following accumulation of the phloem lectin (PP2) and its mRNA by RNA blot analysis, enzyme-linked immunosorbent assay, immunocytochemistry and in␣situ hybridization. Genes encoding PP2 were developmentally regulated during vascular differentiation in hypocotyls of Cucurbita maxima Duch. Accumulation of PP2 mRNA and protein paralleled one another during hypocotyl elongation, after which mRNA levels decreased, while the protein appeared to be stable. Both PP2 and its mRNA were initially detected during metaphloem differentiation. However, PP2 mRNA was detected in companion cells of both bundle and extrafascicular phloem, but never in differentiating sieve elements. At later stages of development, PP2 mRNA was most often observed in extrafascicular phloem. In developing stems of Cucurbita moschata L., PP2 was immunolocalized in companion cells but not to filamentous phloem protein (P-protein) bodies that characterize immature sieve elements of bundle phloem. In contrast, PP2 was immunolocalized to persistent ␣ P-protein bodies in sieve elements of the extrafascicular phloem. Immunolocalization of PP2 in mature wound sieve elements was similar to that in bundle phloem. It appears that PP2 is synthesized in companion cells, then transported into differentiated sieve elements where it is a component of P-protein filaments in bundle phloem and persistent P-protein bodies in extrafascicular phloem. This differential accumulation in bundle and extrafascicular elements may result from different functional roles of the two types of phloem. Received: 31 July 1996 / Accepted: 27 August 1996     

Analysis of early processes in wound-induced vascular regeneration using TED3 and ZeHB3 as molecular markers

Interruption of the vascular bundles of Zinnia internodes induced transdifferentiation of cells into tracheary elements (TEs) or sieve elements (SEs) within 4 d of wounding. The early stage of the regeneration processes was analyzed using two molecular marker genes, TED3 and ZeHB3, which are expressed specifically in TE precursor cells and immature phloem cells, respectively. An increase in the numbers of TED3 and ZeHB3 mRNA-expressing cells always preceded an increase in the numbers of TEs and SEs formed. The earliest sign of vascular differentiation was the appearance 24 h after wounding of a layer(s) of TED3 mRNA-expressing cells in the inter- and intrafascicular cambial-like regions along the severed vascular bundles. In contrast, the number of ZeHB3 mRNA-expressing cells decreased dramatically along the severed bundles 24 h after wounding, and increased again 36 h after wounding. These results clearly indicate that xylem and phloem differentiation are not synchronized during vascular regeneration. Treatment with 10(-3) M colchicine abolished the expression of ZeHB3 mRNA in pith parenchyma, but not TED3 mRNA; this suggests that cell division is a prerequisite for the transdifferentiation of pith parenchymal cells into immature phloem cells expressing ZeHB3. In contrast, transdifferentiation of pith parenchymal cells to TE precursor cells does not require preceding cell division. However, the inhibition of cell division prevented the formation of both radial files of TEs and the cambial-like layer(s) of TED3 mRNA-expressing cells, and, ultimately, vascular regeneration altogether. These results imply that wound-induced cambial-like activity in and between severed vascular bundles is essential for vascular regeneration.     

Regulation of Vascular Development by CLE Peptide-receptor Systems

Cell division and differentiation of stem cells are controlled by non-cell-autonomous signals in higher organisms. The plant vascular meristem is a stem-cell tissue comprising procambial cells that produce xylem cells on one side and phloem cells on the other side. Recent studies have revealed that TDIF (tracheary element differentiation inhibitory factor)/CLE41/CLE44 peptide signal controls the procambial cell fate in a non-cell-autonomous manner. TDIF produced in and secreted from phloem cells is perceive...     

Fine structure,pattern of division,and course of wound phloem in Coleus blumei

The wound phloem bridges which have developed six days after interrupting an internodal vascular bundle contain wound sieve-elements, companion cells, and phloem parenchyma cells. An analysis of the meristematic activity responding to the wounding clearly demonstrates that three consecutive divisions are prerequisite to the formation of phloem mother-cells. Companion cells are obligatory sister cells of wound sieve-elements, connected to the latter by specific plasmatic strands and provided with a dense protoplast. Six days after wounding most of the wound sieve-elements are still at a nucleate state of development, but already have characteristic P-protein bodies and plastids containing sieve-element starch. Their cytoplasmic differentiation corresponds to the changes recorded during maturation of ordinary sieve elements. Sieve-plate pores penetrate through preexisting parenchyma cell walls, only, and develop from primary pitfield-plasmodesmata. Wound sieve-elements do not connect to preexisting bundle sieve-elements, they open a new tier of young sieve elements produced by cambial activity.     

Histology and histochemistry of the cotyledons of pisum arvense L. during germination

Summary The mature cotyledon of Pisum arvense L. comprises several distinct tissue regions; these are the epidermis, hypodermis, storage parenchyma and procambium. The storage parenchyma includes two zones: an outer abaxial zone and an inner adaxial zone. The cells of both zones contain abundant starch grains and protein bodies. Scattered through the storage tissue but increasing in frequency towards the periphery are certain cells which differ to a slight extent from the majority of the parenchyma cells. They have a more opaque, granular cytoplasm and a higher level of cytoplasmic RNA. the cotyledon has a complex, reticulate vascular system. Differentiation of the conducting elements from the procambium appears to begin about 12 hours and to be completed 48 hours after the commencement of imbibition. Differentiation of phloem preceeds that of xylem. The relationship between the timing of vascular differentiation and various physiological events in the cotyledon is discussed.Mobilization of the reserves in the storage parenchyma is initiated at the periphery of the cotyledon and proceeds inwards. There appears to be a correlation between the breakdown of the reserves and changes in DNA and RNA content of the cells.     

Plasmalemmasomes in cotyledon leaves of germinating Vigna radiata L. (mung beans)

Abstract. Storage parenchyma, vascular parenchyma and phloem companion cells are found adjacent to sieve tubes in the vascular bundles of cotyledon leaves of mung bean ( Vigna radiata L.) seedlings. The paramural bodies of storage parenchyma cells are characterized by flask shaped invaginations of the plasmalemma whereas the plasmalemmasomes of the adjacent vascular parenchyma and companion cells consist of numerous finger-like evaginations which are not enclosed in plasmalemma pockets. Phloem associated transfer cells are not present and it is suggested that the fine tubular plasmalemmasomes may act as the interface between apoplast and symplast in the transport of rapidly mobilized reserves during germination. Tubular structures observed within the protoplasts of storage cells close to vascular tissue were also observed in vascular parenchyma and companion cells between the plasmalemmae and cell walls.