XYLEM INTERMIXED WITH PHLOEM1, a leucine-rich repeat receptor-like kinase required for stem growth and vascular development in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

The regulation of cell specification in plants is particularly important in vascular development. The vascular system is comprised two differentiated tissue types, the xylem and phloem, which form conductive elements for the transport of water, nutrients and signaling molecules. A meristematic layer, the procambium, is located between these two differentiated cell types and divides to initiate vascular growth. We report the identification of a receptor-like kinase (RLK) that is expressed in the vasculature. Histochemical analyses of mutants in this kinase display an aberrant accumulation of highly lignified cells, typical of xylem or fiber cells, within the phloem. In addition, phloem cells are sometimes located adjacent to xylem cells in these mutants. We, therefore, named this RLK XYLEM INTERMIXED WITH PHLOEM 1 (XIP1). Analyses of longitudinal profiles of xip1 mutant stems show malformed cell files, indicating defects in oriented cell divisions or cell morphology. We propose that XIP1 prevents ectopic lignification in phloem cells and is necessary to maintain the organization of cell files or cell morphology in conductive elements.     

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.     

OCTOPUS, a polarly localised membrane-associated protein, regulates phloem differentiation entry in Arabidopsis thaliana

Vascular development is embedded into the developmental context of plant organ differentiation and can be divided into the consecutive phases of vascular patterning and differentiation of specific vascular cell types (phloem and xylem). To date, only very few genetic determinants of phloem development are known. Here, we identify OCTOPUS (OPS) as a potentiator of phloem differentiation. OPS is a polarly localised membrane-associated protein that is initially expressed in provascular cells, and upon vascular cell type specification becomes restricted to the phloem cell lineage. OPS mutants display a reduction of cotyledon vascular pattern complexity and discontinuous phloem differentiation, whereas OPS overexpressers show accelerated progress of cotyledon vascular patterning and phloem differentiation. We propose that OPS participates in vascular differentiation by interpreting longitudinal signals that lead to the transformation of vascular initials into differentiating protophloem cells.     

Characterization of microRNAs Expressed during Secondary Wall Biosynthesis in Acacia mangium

MicroRNAs (miRNAs) play critical regulatory roles by acting as sequence specific guide during secondary wall formation in woody and non-woody species. Although thousands of plant miRNAs have been sequenced, there is no comprehensive view of miRNA mediated gene regulatory network to provide profound biological insights into the regulation of xylem development. Herein, we report the involvement of six highly conserved amg-miRNA families (amg-miR166, amg-miR172, amg-miR168, amg-miR159, amg-miR394, and amg-miR156) as the potential regulatory sequences of secondary cell wall biosynthesis. Within this highly conserved amg-miRNA family, only amg-miR166 exhibited strong differences in expression between phloem and xylem tissue. The functional characterization of amg-miR166 targets in various tissues revealed three groups of HD-ZIP III: ATHB8, ATHB15, and REVOLUTA which play pivotal roles in xylem development. Although these three groups vary in their functions, -psRNA target analysis indicated that miRNA target sequences of the nine different members of HD-ZIP III are always conserved. We found that precursor structures of amg-miR166 undergo exhaustive sequence variation even within members of the same family. Gene expression analysis showed three key lignin pathway genes: C4H, CAD, and CCoAOMT were upregulated in compression wood where a cascade of miRNAs was downregulated. This study offers a comprehensive analysis on the involvement of highly conserved miRNAs implicated in the secondary wall formation of woody plants.     

CLE Peptides in Vascular Development

[ Guodong Wang (Corresponding author)] The plant vascular system consists of two conductive tissues, phloem and xylem. The vascular meristem, namely the (pro‐)cambium, is a stem‐cell tissue that gives rise to both xylem and phloem. Recent studies have revealed that CLAVATA3/Embryo Surrounding Region‐related (CLE) peptides function in establishing the vascular system through interaction with phytohormones. In particular, TDIF/CLE41/CLE44, phloem‐derived CLE peptides, promote the proliferation of vascular cambium cells and prevent them from differentiating into xylem by regulating WOX4 expression through the TDR/PXY receptor. In this review article, we outline recent advances on how CLE peptides function in vascular development in concert with phytohormones through mediating cell‐cell communication. The perspective of CLE peptide signaling in vascular development is also discussed.