Differential Expression of Proteins and mRNAs from Border Cells and Root Tips of Pea

Many plants release large numbers of metabolically active root border cells into the rhizosphere. We have proposed that border cells, cells produced by the root cap meristem that separate from the rest of the root upon reaching the periphery of the cap, are a singularly differentiated part of the root system that modulates the environment of the plant root by producing specific substances to be released into the rhizosphere. Proteins synthesized in border cells exhibit profiles that are very distinct from those of the root tip (root cap, root meristem, and adjacent cells). In vivo-labeling experiments demonstrate that 13% of the proteins that are abundant in preparations from border cells are undetectable in root tip preparations. Twenty-five percent of the proteins synthesized by border cells in a 1-h period are rapidly excreted into the incubation medium. Quantitative variation in levels of specific marker proteins, including glutamine synthetase, heat-shock protein 70, and isoflavone reductase, also occurs between border cells and cells in the root tip. mRNA differential-display assays demonstrate that these large qualitative and quantitative differences in protein expression are correlated with similarly distinct patterns of gene expression. These observations are consistent with the hypothesis that a major switch in gene expression accompanies differentiation into root border cells, as expected for cells with specialized functions in plant development.