Xylogen makes plant channels

Zinnia are, according to a gardening website, “uncommonly beautiful” and “available in a riot of colors to satisfy any garden plan.” These plants from the daisy family are also a great model system for studying the formation of xylem—the woody channels in plants that conduct fluid and nutrients. Now, Hiroyasu Motose, Munetaka Sugiyama, and Hiroo Fukuda (University of Tokyo, Japan) have identified a protein called xylogen as an extracellular inducer of xylem formation. Like most mammalian growth factors, but unlike the chemical and peptide growth factors known to work in plants, xylogen is a large protein.

Xylem is not easy to study because the finished form is a series of reinforced but dead cells—the bones of a plant. Cells from Zinnia leaves can, however, be convinced to turn into xylem en masse using a combination of growth factors and cell extraction (possibly via a wound-related response). This artificial system does not, however, say much about how xylem forms in intact plants.

The Japanese group set about purifying an activity that enhanced xylem formation. The factor was in plant extracellular fluid and turned out to be an arabinogalactan protein that they named xylogen. It contains a GPI anchor that may be cleaved.

Xylogen accumulated during xylem formation on the apical sides of differentiating cells. Motose suggests the cells secrete the molecule in a polar fashion to recruit neighboring cells directionally. Plants lacking xylogen formed discontinuous and thicker veins, suggesting that xylogen does not act alone. ▪

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