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Phloem specific expression of target genes

Among the proteinogenic amino acids cysteine and methionine display plenty of essential direct or indirect functions in cellular metabolism. Met is an essential amino acid required in the diet of non-ruminant animals. Major crops, such as cereals and legumes, are low in Met. The aim of this work was to improve the nutritional quality of crops by either expressing or down-regulating target genes in the whole plant or phloem specific manner, respectively. Target genes of interest such as cystathionine gamma-synthase (CgS) and homoserine kinase (HSK) were expressed in companion cells under the control of a cell specific promoter or in whole plants by using the 35S promoter to manipulate the flux towards end product amino acids such as cysteine and methionine or the cysteine homeostasis was misbalanced by down-regulation of O-acetylserine(thiol)lyase (OASTL).

Results obtained during the project indicate that first; OASTL activity does not only regulate cysteine de novo synthesis but also its homeostasis, second; the provision of the carbon backbone in form of O-phosphohomoserine in the whole plants is probably only successful if TS activity is down regulated, thirdly; potato CgS catalyses a near-equilibrium reaction and, more importantly, does not display features of a pathway regulating enzyme and supports data obtained for the HSK over-expressing plants, fourthly; preliminary data suggest that expression of HSK in plastids had no effect on amino acids of the aspartate family while several plants with the cytosolicly expressed HSK contained higher levels of methionine, threonine and isoleucine. Thus, the cell type specific expression has a much stronger effect on methionine and threonine levels as if expressed under a ubiquitous promoter and has to be investigated in more detail in the future.

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14476 GOLM
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