Ultrastructural Localization of a Bean Glycine-Rich Protein in Unlignified Primary Walls of Protoxylem Cells.

Abstract

A polyclonal antibody was used to localize a glycine-rich cell wall protein (GRP 1.8) in French bean hypocotyls with the indirect immunogold method. GRP 1.8 could be localized mainly in the unlignified primary cell walls of the oldest protoxylem elements and also in cell corners of both proto-and metaxylem elements. In addition, GRP 1.8 was detected in phloem using tissue printing. The labeled primary walls of dead protoxylem cells showed a characteristically dispersed ultrastructure, resulting from the action of hydrolases during the final steps of cell maturation and from mechanical stress due to hypocotyl growth. Primary walls of living protoxylem and adjacent parenchyma cells were only weakly labeled. This was true also for the secondary walls of proto-and metaxylem cells, which in addition showed high background labeling. lnhibition of lignification with a specific and potent inhibitor of phenylalanine ammonia-lyase did not lead to enhanced labeling of secondary walls, showing that lignin does not mask the presence of GRP 1.8 in these walls. Dictyosomes of living proto-and metaxylem cells were not labeled, but dictyosomes of xylem parenchyma cells without secondary walls, adjacent to strongly labeled protoxylem elements, were clearly labeled. These observations suggest that GRP 1.8 is not produced by xylem vessels but by xylem parenchyma cells that export the protein to the wall of protoxylem vessels.

Expression patterns of three genes in the stem of lucerne (Medicago sativa)

“…Expression of the bean GRP gene family occurs almost exclusively within vascular tissue, primarily in xylem cells [30]. GRP1.8 accumulates in dying or dead protoxylem cells and synthesis continues by xylem parenchyma exporting GRP to the walls of dead protoxylem cells [47]. MsaS3 mRNA was detected in all stem internodes assayed, regardless of age.…”

A novel phloem-specific gene is expressed preferentially in aerial portions of Vicia faba

“…HRGPs have been localized to vascular meristematic cells as well as xylem and phloem fibers [20,25,26]. The PRPs and GRPs have been localized to the xylem and phloem [25,26,23,16]. In addition, a novel tyrosine-and lysine-rich protein has been localized to the lignified secondary cell walls of the xylem and sclerenchyma [5].…”

Common occurrence of homologues of petunia glycine-rich protein-1 among plants

“…GRP genes that contain coding sequences for a potential signal peptide are generally believed to code for structural proteins of the cell wall. To date, the only GRP that has been definitively shown, by electron microscopy immunolocalization, to be located within the cell wall is the French bean GRP1.8 protein [24]. In French bean and soybean this protein occurs primarily in protoxylem, but is also found in phloem tissue later in development [13,24,30,31].…”

“…To date, the only GRP that has been definitively shown, by electron microscopy immunolocalization, to be located within the cell wall is the French bean GRP1.8 protein [24]. In French bean and soybean this protein occurs primarily in protoxylem, but is also found in phloem tissue later in development [13,24,30,31]. RNA and protein homologs of GRP 1.8 are also present in petunia, tomato and tobacco and are localized in the vascular cambium, in addition to the protoxylem and phloem tissue [30].…”

“…Such a diversity of response to various stimuli suggests that this group of GRPs containing signal peptide sequences are not all functionally equivalent. Additionally, cell localization studies of this type of GRP, which have shown that individual GRPs can be absolutely cell-specific (the tapetum-specific tobacco GRP [17]), somewhat cell-specific (French bean GRP1.8 and its homologues [24,30,31 ]), to mainly vascular-specific (ptGRP1 [3,4]) also intimates that these genes may not all code for functional homologues. Indeed this diversity of GRP expression has brought forth the suggestion that the N-terminal signal peptides of some of these proteins may actually function as a stop-anchor signal.…”