Independent roles of methionine andO-acetyl-l-serine in the regulation of the β subunit gene of β-conglycinin

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Abstract
The gene encoding the β subunit of β-conglycinin, one of the major seed storage proteins of soybean (Glycine max [L.] Merr.) , is downregulated by methionine (Met) and upregulated by O-acetyl-l-serine (OAS). We examined the interaction between Met and OAS in the regulation of the expression of the β subunit gene in in vitro-cultured immature soybean cotyledons. Application of Met reduced the accumulation of the β subunit both in the presence and absence of OAS in the media: Application of OAS increased the accumulation of the β subunit both in the presence and absence of Met. We also studied the regulation of the β subunit gene promoter by sulfur in Arabidopsis thaliana (L.) Heynh. mutant mto1-1, that over-accumulates soluble Met, using the β-glucuronidase (GUS) gene as a reporter. When grown under sulfur-deficient conditions, the GUS activity in seeds of the transgenic plants carrying the mto1-1 mutation was lower than that of the plants which do not carry the mutation, suggesting that the β subunit gene promoter was downregulated by the mto1-1 mutation under sulfur deficiency. When the transgenic plants carrying the mto1-1 mutation were grown under control and sulfur-deficient conditions, the GUS activity was higher under sulfur-deficient conditions, suggesting that the β subunit gene promoter was upregulated by sulfur deficiency even in the mto1-1 mutant background. Furthermore, OAS contents in immature siliques were elevated both in the mto1-1 mutant and wild-type plants under sulfur deficiency. Met contents in immature siliques of the mto1-1 mutant plants increased even under sulfur deficiency. There was a strong positive correlation between the GUS activities in seeds and the endogenous [OAS]/[Met] ratio. These results suggest that OAS and Met regulate the β subunit accumulation through independent pathways.