Abstract
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.
Highlights
Proteins of the extracellular matrix in animal cells or the cell wall in plant cells have only evolved after the divergence of plants and animals, about 1 billion years ago during the Precambrian
3D visualization of the glycine-rich proteins (GRPs) network with confocal laser scanning microscopy (CLSM) Enzymatically softened tissue pieces from different regions of the hypocotyl were incubated with bean GRP1.8 polyclonal antibodies and stained with Alexa Fluor 488 secondary antibody conjugates
In contrast to the apical region of the hypocotyl, where the GRPcontaining cell-wall material (GRPCWM) spans only the distance between adjacent secondary wall thickenings (SW), long, continuous filaments or sheets are typically observed in the mature regions of the hypocotyl. (G-I) Basal region of the hypocotyl
Summary
Proteins of the extracellular matrix in animal cells or the cell wall in plant cells have only evolved after the divergence of plants and animals, about 1 billion years ago during the Precambrian. The cell walls of seed plants are mainly composed of polysaccharides such as cellulose, hemicelluloses and pectins, they contain variable amounts of structural proteins. These proteins have traditionally been classified as glycine-rich proteins (GRPs), proline-rich proteins (PRPs) and hydroxyproline-rich proteins (HRGPs). Dividing and elongating plant cells contain very little cross-linked structural cell-wall proteins. Cell-typeand tissue-specific cross-linking of these proteins into the cell wall occurs towards the end of elongation growth. It is thought that they strengthen the cell walls in normal plant development (Fry, 1988; Carpita and Gibeaut, 1993) and are involved in incompatible plant-pathogen interactions (Lamb and Dixon, 1997; Delledonne et al, 1998)
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