A cytokinin-binding 70-kD protein is localized predominantly in the root meristem

The contents of cytokinins (zeatin, zeatin riboside, zeatin nucleotide, and zeatin N9-glucoside), total protein, and a 70-kD cytokinin-binding protein (CBP70) were estimated in the root meristem and elongation zones of four-day-old etiolated maize seedlings. Earlier, it was shown that CBP70 is a cytokinin receptor involved in the hormone-induced activation of transcriptional elongation. A pair of monoclonal anti-CBP70 antibodies were selected; this permitted us to use a sandwich-type immunoenzyme analysis in combination with the streptovidin–biotin system, and this improved substantially the sensitivity of the CBP70 measurement. As calculated per cell, the elongating cells were enriched in all cytokinin forms, especially in transport cytokinins (zeatin riboside and zeatin nucleotide) as compared to the meristem cells, but the meristemic cell contained three times less total protein and two times more CBP70 than the cell in the elongation zone. The CBP70 concentration per milligram of total protein was six times higher in the cytosol of the meristemic cell as compared to the elongation zone. Predominant localization of the CBP70 in the meristem implies its involvement in the cytokinin control of cell division.