Regulation of Legumin Levels in Developing Pea Seeds under Conditions of Sulfur Deficiency

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Abstract
When peas (P. sativum L.) are grown with suboptimal S supply, the level of legumin (the more S-rich of the 2 major seed storage proteins) in the mature seed is selectively reduced (Randall, Thomson and Schroeder, 1979). A study of the cellular mechanisms involved in regulating legumin synthesis under these conditions is reported. Pulse and pulse-chase labeling experiments were carried out with excised, immature cotyledons from normal and S-deficient plants. Legumin was isolated from cotyledon extracts by immunochromatography and the proportion of legumin synthesis relative to total protein synthesis was determined. Reduced legumin accumulation could largely be accounted for by a greatly reduced level of legumin synthesis (80-88% reduction) rather than by a major increase in legumin breakdown. Legumin mRNA levels were assayed by 2 methods. In vitro translation of polysomal RNA from cotyledons of normal and S-deficient plants indicated a reduction of 60-70% in synthesis of legumin-related products by preparations from S-deficient plants. A legumin cDNA clone was constructed, characterized and used to measure the levels of legumin mRNA in polysomal and total RNA preparations from developing cotyledons. Legumin mRNA levels were reduced by 90% in preparations from S-deficient plants. When restored to an adequate S supply, S-deficient plants (or pods taken from such plants) recovered normal levels of legumin synthesis (in vivo and in vitro) and of legumin mRNA. Apparently, reduced legumin accumulation under conditions of S deficiency is primarily a consequence of reduced levels of legumin mRNA.