Abstract
Potatoes (Solanum tuberosum) contain α-solanine and α-chaconine, two well-known toxic steroidal glycoalkaloids (SGAs). Sprouts and green tubers accumulate especially high levels of SGAs. Although SGAs were proposed to be biosynthesized from cholesterol, the biosynthetic pathway for plant cholesterol is poorly understood. Here, we identify sterol side chain reductase 2 (SSR2) from potato as a key enzyme in the biosynthesis of cholesterol and related SGAs. Using in vitro enzyme activity assays, we determined that potato SSR2 (St SSR2) reduces desmosterol and cycloartenol to cholesterol and cycloartanol, respectively. These reduction steps are branch points in the biosynthetic pathways between C-24 alkylsterols and cholesterol in potato. Similar enzymatic results were also obtained from tomato SSR2. St SSR2-silenced potatoes or St SSR2-disrupted potato generated by targeted genome editing had significantly lower levels of cholesterol and SGAs without affecting plant growth. Our results suggest that St SSR2 is a promising target gene for breeding potatoes with low SGA levels.
Highlights
Steroidal glycoalkaloids (SGAs) are famous as toxic compounds in solanaceous plants that include the world’s major food crops such as potato (Solanum tuberosum) and tomato (Solanum lycopersicum) (Friedman, 2002, 2006). a-Chaconine and a-solanine account for most of the steroidal glycoalkaloids (SGAs) in cultivated potato and are especially abundant in sprouts, the green peel of tubers, and other aerial parts
Genes associated with SGA biosynthesis in potato and tomato were reported to be clustered on chromosomes 7 and 12 (Itkin et al, 2013), SSR genes in potato and tomato are positioned on chromosome 2 and are more than 10 Mbp away from each other (St SSR1, PGSC0003DMG400011801, and St sterol side chain reductase 2 (SSR2), PGSC0003DMG400021142, in Xu et al, 2011; Sl SSR1, Solyc02g030170.2, and Sl SSR2, Solyc02g069490.2, in Tomato Genome Consortium, 2012)
The data presented here show that St SSR2 is a key enzyme in the biosynthesis of toxic SGAs derived from cholesterol in potato
Summary
Steroidal glycoalkaloids (SGAs) are famous as toxic compounds in solanaceous plants that include the world’s major food crops such as potato (Solanum tuberosum) and tomato (Solanum lycopersicum) (Friedman, 2002, 2006). a-Chaconine and a-solanine account for most of the SGAs in cultivated potato and are especially abundant in sprouts, the green peel of tubers, and other aerial parts. Steroidal glycoalkaloids (SGAs) are famous as toxic compounds in solanaceous plants that include the world’s major food crops such as potato (Solanum tuberosum) and tomato (Solanum lycopersicum) (Friedman, 2002, 2006). In potato breeding, introducing wild potato species as a genetic resource could increase the SGA level (Jacobsen and Rousselle, 1993). Reducing the SGA level is an important target for potato breeding. Feeding studies with isotope-labeled substrates suggested that SGAs are biosynthesized via cholesterol as a common precursor (Figure 1) Plants produce C-24 alkylsterols including campesterol, a precursor of the phytohormone brassinolide (Figure 1). The primary structural difference between the C-24 alkylsterols and cholesterol is the presence of an alkyl group at the C-24 position of the side chain (Benveniste, 1986). The C-24 alkylsterols have a methyl or an ethyl group at C-24 resulting from the addition of one
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