Community Research and Development Information Service - CORDIS

Optimising nutritional quality of potato

The goal of our project was the nutritional improvement of potato, specifically the enhancement of lysine, cysteine and methionine content. For this purpose molecular breeding tools were combined with classical breeding techniques.

Potato is the most important non-cereal food crop. Transformation systems for potato relying on recovery of transgenic plants from different organs were developed in the past. As part of the process a selectable marker, generally an antibiotic resistance gene is co-introduced with the gene of interest. Recently, a large number of transgenic crops have been produced; however, there is still a public concern with respect to the safety of antibiotic resistant transgenic plants in agricultural application. To avoid this problem efficiency of marker-free transformation was tested in different potato varieties. Trangenic plants were isolated from Bintje and Hopehely. The frequency of transformation, however, was much lower than in Desiree (2%) used as a control. Thus we concluded that efficiency of transformation highly depends on cultivars.

The marker-free transformation system was applied for increasing the cysteine content of the Hungarian potato variety, White Lady. Two transgenic lines expressing the serine acetyltransferase (SAT) gene from the constitutive CaMV35S promoter were isolated. RP-HPLC analysis showed increased cysteine and glutathione contents in leaves.

A co-transformation experiment was carried out to enhance S flux towards methionine synthesis and storage. Double transgenic plants expressing the potato S-transporter, StST1, from the root-specific RIAGS promoter and the methionine-rich storage protein gene zein from the constitutive CaMV35S promoter were isolated. It was shown that these plants grew better under S-limited conditions, in vitro, than the non-transformed controls. Immunoprecipitation revealed that the selected transgenic lines exhibited zein expression at protein level.

In order to simultaneously increase lysine synthesis and storage in Desiree a feedback insensitive isoform of dihydrodipicolonate synthase (DHDPS) involved in lysine sythesis and the modified chymotrypsine inhibitor KDEL, as a lysine storage protein gene, were inserted into a binary vector suitable for cloning of multiple genes. Co-insertion of the genes was verified by PCR analysis and expression of the genes was tested on RNA gel blots. Interestingly, much more KDEL than DHDPS mRNA was detected in each line tested. Further studies are needed to detect KDEL and DHDPS expression at protein level.

Morphological studies on transgenic lines showed that increases in amino acid levels can lead to morphological alterations and fertility problems. However, the intensity of these changes was variable in the different transgenic lines even if they were obtained with the same construct. Thus it is feasible that transgenic lines with no phenotypical alterations but with a moderate increase in amino acid content can be selected in the future. An alternative solution might be the utilisation of storage proteins to decrease the level of free-amino acids that may serve as signal molecules for other biosynthetic pathways.

In contrast, SAT-expressing White Lady lines increased in cysteine content had no morphological alterations. Since these lines were obtained by marker-free transformation they are suitable for further breeding either by molecular or classical approaches.

Desiree lines with increased S-transport and zein production are valuable tools of basic studies on relation of S-transport, S-assimilation and storage.

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