The Regulation of Pea Seed Storage Protein Genes by Sulfur Stress

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Abstract
In this review the role of sulfur in regulating the expression of genes for pea seed storage proteins in both peas and transgenic tobacco is discussed. The levels of the sulfur-containing proteins, legumin and pea albumin 1 (PA1), are reduced in the seeds of peas grown under mild sulfur nutrient stress. In contrast, the levels of sulfur-poor proteins such as pea lectin and vicilin are either unaffected or increased slightly under the same conditions. The levels of all the proteins are a direct reflection of the levels of their respective mRNAs. The reduced levels of legumin and PA1 mRNAs under sulfur stress is known to be due largely to increased mRNA turnover rather than decreased transcription. The advent of gene transfer procedures for plants has allowed re-examination of the mechanism of regulation of mRNA stability under conditions of sulfur stress. A pea albumin 1 gene was engineered for leaf expression and transferred to tobacco and the transgenic plants were grown on normal and low levels of sulfur. Sulfur stress reduces total leaf protein in tobacco by about 20% and there are minor qualitative changes in the total protein profile. In contrast, PA1 levels are reduced by over 90% compared with the controls when the transgenic tobaccos are grown under sulfur stress. Thus, it is clear that sequences responsible for recognising the sulfur status have been included in the transgene. A number of gene constructs have been designed to test where the sulfur-responsive sequences are located in the PA1 gene and some of the preliminary findings are reported.