The role of cytokinins in clubroot formation

Abstract

The occurrence of cytokinins in healthy turnips ( Brassica campestris L. var. rapa “Gelria”) and in turnips infected with Plasmodiophora brassicae has been studied. Extracts of clubroots were about three times as active in cytokinin bioassays as extracts of healthy turnips. Infected and healthy turnips contain compounds with the same R F values. Extracts from clubroots have been partially purified and cochromatographed on paper and on thin layer silica gel with zeatin and zeatin riboside. One of the active fractions upon silylation showed a gas chromatographic retention time which coincided with the retention time of silylated zeatin. Total cytokinin activity was estimated to be 0·1 to 1·0 mg zeatin equivalent/100 kg fresh weight clubroots. Healthy turnips contain 10 to 100 times lower amounts of cytokinins. These findings are consistent with the fact that explants infected with P. brassicae are independent of cytokinins and auxins for their growth, whereas callus of healthy turnip tissue requires these hormones for continued growth. However, infected callus did not grow very well on media deficient in both growth hormones. A considerable difference was found between the ratios of cytokinins and auxins which must be supplied to obtain maximum yield of infected and healthy callus tissue.