Phytosterols, phytostanols, and their conjugates in foods: structural diversity, quantitative analysis, and health-promoting uses

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Analysis of biologically active compounds in potatoes (Solanum tuberosum), tomatoes (Lycopersicon esculentum), and jimson weed (Datura stramonium) seeds.

The potato steroidal glycoalkaloids (SGAs) are important components of plant resistance against pests and pathogens but can be toxic to humans at high levels. SGAs derive their toxicity from anticholinesterase activity affecting the central nervous system and the disruptive effects on cell membrane integrity affecting the digestive system and other organs. Accordingly, current safety regulations limit their content in the edible tuber to 20 mg per 100 g fresh weight. SGA composition and level are genetically determined, with unfavourable growth conditions and inappropriate postharvest management inducing the accumulation of SGAs at levels in the tubers of “safe” cultivars beyond the maximum level set by the industry. Hence, genetic alteration of potato to prevent toxic levels of SGAs in tubers is highly desirable. At the same time, maintaining high SGA levels in other plant organs will contribute to plant resistance against pathogen and pest attacks. To this end, SGA biosynthesis and degradation should be manipulated precisely to exploit tissue-specific expression rather than whole-plant suppression of SGA production, to produce potato cultivars with SGA content enriched in the foliage but diminished in the edible tubers. Only a few details are known about the SGA biosynthetic pathway, its genes and intermediates. Research on factors that regulate SGA biosynthesis and catabolism as well as searches for genetic markers linked to total and specific SGA levels have only recently been pursued. The present review summarizes current data on these issues to encourage further discussion on SGA manipulation for safer food products.

Potato Steroidal Glycoalkaloids: Biosynthesis and Genetic Manipulation

Metabolic compensation of steroidal glycoalkaloid biosynthesis in transgenic potato tubers: using reverse genetics to confirm the in vivo enzyme function of a steroidal alkaloid galactosyltransferase $

Transgenic potato (Solanum tuberosum cv Desiree) plants overexpressing a soybean (Glycine max) type 1 sterol methyltransferase (GmSMT1) cDNA were generated and used to study sterol biosynthesis in relation to the production of toxic glycoalkaloids. Transgenic plants displayed an increased total sterol level in both leaves and tubers, mainly due to increased levels of the 24-ethyl sterols isofucosterol and sitosterol. The higher total sterol level was due to increases in both free and esterified sterols. However, the level of free cholesterol, a nonalkylated sterol, was decreased. Associated with this was a decreased glycoalkaloid level in leaves and tubers, down to 41% and 63% of wild-type levels, respectively. The results show that glycoalkaloid biosynthesis can be down-regulated in transgenic potato plants by reducing the content of free nonalkylated sterols, and they support the view of cholesterol as a precursor in glycoalkaloid biosynthesis.

Reduction of Cholesterol and Glycoalkaloid Levels in Transgenic Potato Plants by Overexpression of a Type 1 Sterol Methyltransferase cDNA

Steroidal glycoalkaloids (SGAs) are produced following the general steroid biosynthesis pathway, starting from acetyl-coenzyme A and followed by the intermediates mevalonic acid, squalene, cycloartenol, and cholesterol. α-Chaconine and α-solanine are the main SGAs of the cultivated potato (Solanum tuberosum), whereas many other SGAs are known in the wild potato species. Low concentrations of SGAs improve the taste of potato, but concentrations greater than 200 mg/kg can have toxic effects on animals and humans. SGAs have antimicrobial activity and confer resistance to some insects, but many such pests of potato are not greatly affected. Certain environmental conditions and wounding enhance SGA accumulation in tubers in the field and storage. Low production of SGAs is a dominant character inherited in a relatively simple manner and can be selected for in potato-breeding programs, whereas the use of wild potato germplasm tends to increase the SGA accumulation in the breeding lines. Further efforts ...

Potato Glycoalkaloids: A Burden or a Blessing?

Protoplasts were fused to produce somatic hybrids between a triploid (2n=3x=32-34) interspecific somatohaploid betweenSolanum brevidens Phil. andS. tuberosum L., and a dihaploid (2n=2x=24) anther-derived line ofS. tuberosum cv. Van Gogh. A total of 265 plants were regenerated from protoplast fusion derived calli and their hybridity was verified using fusion partner specific RAPD markers. These “second generation” somatic hybrids were aneuploid pentaploids (2n=5x=51-65) with a 2C DNA content ranging from 3.36 to 4.43 pg, which corresponded to the sum of the 2C values of each of the fusion partners (somatohaploid: 2.22 pg; and the dihaploid line of cv. Van Gogh: 1.87 pg). Most of the “second generation” somatic hybrids were vigorous, but variable in morphology. They were extremely resistant to PLRV and they had tolerance to PVY infection derived from the somatohaploid fusion partner. Even though most of the “second generation” hybrids tuberized, the tuber morphology was variable and most were poorly shaped. InErwinia soft rot resistance tests, the tubers showed higher level of resistance than the tetraploidS. tuberosum cultivars, the dihaploidS. tuberosum fusion partners and the hexaploid somatic hybrids betweenS. brevidens andS. tuberosum. The “second generation” somatic hybrids were all male sterile and failed to produce berries or seeds.

Production and characterization of "second generation" somatic hybrids derived from protoplast fusion between interspecific somatohaploid and dihaploid Solanum tuberosum L.

Leena Pietilä

Papers

Potato Glycoalkaloids: A Burden or a Blessing?

Production and characterization of "second generation" somatic hybrids derived from protoplast fusion between interspecific somatohaploid and dihaploid Solanum tuberosum L.