Physiological and metabolic origin of sulphur for the synthesis of seed storage proteins

Wheat plants were grown with adequate sulphur (S) during vegetative growth but the supply of nutrients (including S) was terminated during generative growth. The grain yield and the S content of plants that did not receive nutrients after anthesis were similar to plants that received S throughout generative growth. When the S supply was terminated at anthesis the S for grain growth was derived principally from sulphate in the root and glutathione in the flag leaf; insoluble S was not an important source. Plants that received inadequate S prior to terminating the S supply at anthesis produced a lower yield of grain with a lower S content indicating the production of low-S storage proteins. The internal pool of soluble S in these plants was negligible. Furthermore, the S for the synthesis of grain proteins was derived principally from protein-S in the flag leaf rather than from soluble sources. Since glutathione was the main source of S in the endosperm cavity the protein-S must have been metabolised to glutathione in the flag leaf. Endosperm extracts from S-inadequate plants catalysed the hydrolysis of glutathione at rates consistent with those required to supply S for grain growth. The extracts also contained enzymes of methionine synthesis and reductive sulphate assimilation. Extracts from S-adequate plants also exhibited these activities but the enzymes of sulphate assimilation were more active, consistent with the production of S-rich proteins in these plants. Soybeans, unlike wheat, acquired most of the S for grain growth during generative growth. Prior to the onset of grain growth, sulphate and homoglutathione accumulated in the pod. Sulphate declined rapidly when grain growth commenced but homoglutathione showed a transitory increase. Developing grains contained homoglutathione but negligible sulphate. The most likely explanation is that sulphate is metabolised to homoglutathione in the pod but uptake of sulphate and assimilation within the cotyledons cannot be ruled out.