Germin-Like Polypeptides Increase in Barley Roots during Salt Stress

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Abstract
As part of the study of the possible role(s) of CBF-1, a cytokinin-binding protein abundant in wheat embryo, a cytokinin oxidase was found in wheat (Triticum aestivum L.) germ and partially purified by conventional purification techniques and high performance chromatofocusing. This preparation catalyzes conversion of N6-(Δ2-isopentenyl)adenosine to adenosine at a Vmax of 0.4 nanomol per milligram protein per minute at 30°C and pH 7.5, the Km being 0.3 micromolar. This high affinity and the apparent molecular weight of 40,000 estimated by high performance gel permeation on a Spherogel TSK-3000 SW column indicate that this enzyme is different from other cytokinin oxidases previously reported. Oxygen is required for the reaction, as for other cytokinin oxidases already described. N6-(Δ2-isopentenyl)adenine and zeatin riboside are also degraded, but N6-(Δ2-isopentenyl)adenosine-5′-monophosphate is apparently not a substrate. Benzyladenine is degraded, but to a small extent, and it inhibits slightly the degradation of N6-(Δ2-isopentenyl)adenosine. The degradation of N6-(Δ2-isopentenyl)adenosine is strongly inhibited by diphenylurea and its highly active derivative N-(2-chloro-4-pyridyl)-N′-phenylurea.