Translocation of zeatin riboside and zeatin in soybean explants

Abstract

Soybean explants consisting of a leaf, one or more young pods, and a subtending piece of stem were given a 1-h pulse of3H (ring-labeled)-zeatin riboside (ZR) or -zeatin (Z), via the base of the stem, followed by a 24-h incubation. At the end of the pulse, about 55% of the soluble3H was in the leaf blades, 11% in the petiole, 30% in the stem, 2% in the carpels, 0.1% in the seed coats, and 0.08% in the embryos. After 24 h, the percentages were 58, 7, 26, 6, 2, and 0.3, respectively. During this period, the total soluble3H decreased by 84%, the remainder being bound to “insoluble” material. The3H-cytokinin was rapidly converted to diverse metabolites including adenosine and adenine. At the end of the 1-h pulse, appreciable percentages (1–16%) of the total soluble3H in the seed coats chromatographed with ZR (or dihydro ZR) and with the 5′-phosphate of ZR, but these percentages declined markedly at 24 h. No distinct peaks of3H corresponded to known metabolites in the soluble extracts of embryos, and at 24 h, the3H equivalent to ZR must have been less than 0.0006% of the3H-ZR supplied. The bound3H corresponded to adenine and guanine in DNA and RNA. In contrast to cytokinin,3H-adenosine given as a pulse was readily translocated into the seed coats and embryos. Thus, cytokinin (ZR and Z) flowing up through the xylem from the root system does not readily enter the embryo (though metabolites such as adenosine could), and the seeds clearly do not compete with the leaves for this supply of cytokinin.