Chemotaxonomy of the tuber-bearing Solanum species, subsection Potatoe (Solanaceae)

Recent advances in steroidal glycoalkaloid biosynthesis in the genus Solanum

Steroidal glycoalkaloids (SGAs) are specialized metabolites found in members of Solanum species, and are also known as toxic substances in Solanum food crops such as tomato (Solanum lycopersicum), potato (Solanum tuberosum), and eggplant (Solanum melongena). SGA biosynthesis can be divided into two main parts: formation of steroidal aglycones, which are derived from cholesterol, and glycosylation at the C-3 hydroxy group. This review focuses on recent studies that shed light on the complete process of the aglycone formation in SGA biosynthesis and structural diversification of SGAs by duplicated dioxygenases, as well as the development of non-toxic potatoes through genome editing using these findings.

Current Advances in the Biosynthesis, Metabolism, and Transcriptional Regulation of α-Tomatine in Tomato

Steroid glycoalkaloids (SGAs) are a class of cholesterol-derived metabolites commonly found in the Solanaceae plants. α-Tomatine, a well-known bitter-tasting compound, is the major SGA in tomato, accumulating extensively in all plant tissues, particularly in the leaves and immature green fruits. α-Tomatine exhibits diverse biological activities that contribute to plant defense against pathogens and herbivores, as well as conferring certain medicinal benefits for human health. This review summarizes the current knowledge on α-tomatine, including its molecular chemical structure, physical and chemical properties, biosynthetic and metabolic pathways, and transcriptional regulatory mechanisms. Moreover, potential future research directions and applications of α-tomatine are also discussed.

Characterization of C-26 aminotransferase, indispensable for steroidal glycoalkaloid biosynthesis

Steroidal glycoalkaloids (SGAs) are toxic specialized metabolites found in members of the Solanaceae, such as Solanum tuberosum (potato) and Solanum lycopersicum (tomato). The major potato SGAs are α-solanine and α-chaconine, which are biosynthesized from cholesterol. Previously, we have characterized two cytochrome P450 monooxygenases and a 2-oxoglutarate-dependent dioxygenase that function in hydroxylation at the C-22, C-26 and C-16α positions, but the aminotransferase responsible for the introduction of a nitrogen moiety into the steroidal skeleton remains uncharacterized. Here, we show that PGA4 encoding a putative γ-aminobutyrate aminotransferase is involved in SGA biosynthesis in potatoes. The PGA4 transcript was expressed at high levels in tuber sprouts, in which SGAs are abundant. Silencing the PGA4 gene decreased potato SGA levels and instead caused the accumulation of furostanol saponins. Analysis of the tomato PGA4 ortholog, GAME12, essentially provided the same results. Recombinant PGA4 protein exhibited catalysis of transamination at the C-26 position of 22-hydroxy-26-oxocholesterol using γ-aminobutyric acid as an amino donor. Solanum stipuloideum (PI 498120), a tuber-bearing wild potato species lacking SGA, was found to have a defective PGA4 gene expressing the truncated transcripts, and transformation of PI 498120 with functional PGA4 resulted in the complementation of SGA production. These findings indicate that PGA4 is a key enzyme for transamination in SGA biosynthesis. The disruption of PGA4 function by genome editing will be a viable approach for accumulating valuable steroidal saponins in SGA-free potatoes.

Identification of α-Tomatine 23-Hydroxylase Involved in the Detoxification of a Bitter Glycoalkaloid

Tomato plants (Solanum lycopersicum) contain steroidal glycoalkaloid α-tomatine, which functions as a chemical barrier to pathogens and predators. α-Tomatine accumulates in all tissues and at particularly high levels in leaves and immature green fruits. The compound is toxic and causes a bitter taste, but its presence decreases through metabolic conversion to nontoxic esculeoside A during fruit ripening. This study identifies the gene encoding a 23-hydroxylase of α-tomatine, which is a key to this process. Some 2-oxoglutarate-dependent dioxygenases were selected as candidates for the metabolic enzyme, and Solyc02g062460, designated Sl23DOX, was found to encode α-tomatine 23-hydroxylase. Biochemical analysis of the recombinant Sl23DOX protein demonstrated that it catalyzes the 23-hydroxylation of α-tomatine and the product spontaneously isomerizes to neorickiioside B, which is an intermediate in α-tomatine metabolism that appears during ripening. Leaves of transgenic tomato plants overexpressing Sl23DOX accumulated not only neorickiioside B but also another intermediate, lycoperoside C (23-O-acetylated neorickiioside B). Furthermore, the ripe fruits of Sl23DOX-silenced transgenic tomato plants contained lower levels of esculeoside A but substantially accumulated α-tomatine. Thus, Sl23DOX functions as α-tomatine 23-hydroxylase during the metabolic processing of toxic α-tomatine in tomato fruit ripening and is a key enzyme in the domestication of cultivated tomatoes.

Characterization of steroid 5α-reductase involved in α-tomatine biosynthesis in tomatoes

α-tomatine and dehydrotomatine are steroidal glycoalkaloids (SGAs) that accumulate in the mature green fruits, leaves, and flowers of tomatoes (Solanum lycopersicum) and function as defensive compounds against pathogens and predators. The aglycones of α-tomatine and dehydrotomatine are tomatidine and dehydrotomatidine (5,6-dehydrogenated tomatidine), and tomatidine is derived from dehydrotomatidine via four reaction steps: C3 oxidation, isomerization, C5α reduction, and C3 reduction. Our previous studies (Lee et al. 2019) revealed that Sl3βHSD is involved in the three reactions except for C5α reduction, and in the present study, we aimed to elucidate the gene responsible for the C5α reduction step in the conversion of dehydrotomatidine to tomatidine. We characterized the two genes, SlS5αR1 and SlS5αR2, which show high homology with DET2, a brassinosteroid 5α reductase of Arabidopsis thaliana. The expression pattern of SlS5αR2 is similar to those of SGA biosynthetic genes, while SlS5αR1 is ubiquitously expressed, suggesting the involvement of SlS5αR2 in SGA biosynthesis. Biochemical analysis of the recombinant proteins revealed that both of SlS5αR1 and SlS5αR2 catalyze the reduction of tomatid-4-en-3-one at C5α to yield tomatid-3-one. Then, SlS5αR1- or SlS5αR2-knockout hairy roots were constructed using CRISPR/Cas9 mediated genome editing. In the SlS5αR2-knockout hairy roots, the α-tomatine level was significantly decreased and dehydrotomatine was accumulated. On the other hand, no change in the amount of α-tomatine was observed in the SlS5αR1-knockout hairy root. These results indicate that SlS5αR2 is responsible for the C5α reduction in α-tomatine biosynthesis and that SlS5αR1 does not significantly contribute to α-tomatine biosynthesis.

Identification of a 3β-Hydroxysteroid Dehydrogenase/ 3-Ketosteroid Reductase Involved in α-Tomatine Biosynthesis in Tomato

α-Tomatine and dehydrotomatine are major steroidal glycoalkaloids (SGAs) that accumulate in the mature green fruits, leaves and flowers of tomato (Solanum lycopersicum), and function as defensive compounds against bacteria, fungi, insects and animals. The aglycone of dehydrotomatine is dehydrotomatidine (5,6-dehydrogenated tomatidine, having the Δ5,6 double bond; the dehydro-type). The aglycone of α-tomatine is tomatidine (having a single bond between C5 and C6; the dihydro-type), which is believed to be derived from dehydrotomatidine via four reaction steps: C3 oxidation, isomerization, C5 reduction and C3 reduction; however, these conversion processes remain uncharacterized. In the present study, we demonstrate that a short-chain alcohol dehydrogenase/reductase designated Sl3βHSD is involved in the conversion of dehydrotomatidine to tomatidine in tomato. Sl3βHSD1 expression was observed to be high in the flowers, leaves and mature green fruits of tomato, in which high amounts of α-tomatine are accumulated. Biochemical analysis of the recombinant Sl3βHSD1 protein revealed that Sl3βHSD1 catalyzes the C3 oxidation of dehydrotomatidine to form tomatid-4-en-3-one and also catalyzes the NADH-dependent C3 reduction of a 3-ketosteroid (tomatid-3-one) to form tomatidine. Furthermore, during co-incubation of Sl3βHSD1 with SlS5αR1 (steroid 5α-reductase) the four reaction steps converting dehydrotomatidine to tomatidine were completed. Sl3βHSD1-silenced transgenic tomato plants accumulated dehydrotomatine, with corresponding decreases in α-tomatine content. Furthermore, the constitutive expression of Sl3βHSD1 in potato hairy roots resulted in the conversion of potato SGAs to the dihydro-type SGAs. These results demonstrate that Sl3βHSD1 is a key enzyme involved in the conversion processes from dehydrotomatidine to tomatidine in α-tomatine biosynthesis.

Generation of α-solanine-free hairy roots of potato by CRISPR/Cas9 mediated genome editing of the St16DOX gene

Potato (Solanum tuberosum) is a major food crop, while the most tissues of potato accumulates steroidal glycoalkaloids (SGAs) α-solanine and α-chaconine. Since SGAs confer a bitter taste on human and show the toxicity against various organisms, reducing the SGA content in the tubers is requisite for potato breeding. However, generation of SGA-free potato has not been achieved yet, although silencing of several SGA biosynthetic genes led a decrease in SGAs. Here, we show that the knockout of St16DOX encoding a steroid 16α-hydroxylase in SGA biosynthesis causes the complete abolition of the SGA accumulation in potato hairy roots. Nine candidate guide RNA (gRNA) target sequences were selected from St16DOX by in silico analysis, and the two or three gRNAs were introduced into a CRISPR/Cas9 vector designated as pMgP237-2A-GFP that can express multiplex gRNAs based on the pre-tRNA processing system. To establish rapid screening of the candidate gRNAs that can efficiently mutate the St16DOX gene, we used a potato hairy root culture system for the introduction of the pMgP237 vectors. Among the transgenic hairy roots, two independent lines showed no detectable SGAs but accumulated the glycosides of 22,26-dihydroxycholesterol, which is the substrate of St16DOX. Analysis of the two lines with sequencing exhibited the mutated sequences of St16DOX with no wild-type sequences. Thus, generation of SGA-free hairy roots of tetraploid potato was achieved by the combination of the hairy root culture and the pMgP237-2A-GFP vector. This experimental system is useful to evaluate the efficacy of candidate gRNA target sequences in the short-term.

Biosynthesis of steroidal alkaloids in Solanaceae plants: involvement of an aldehyde intermediate during C-26 amination

The C-26 amino group of steroidal alkaloids, such as tomatine, is introduced during an early step of their biosynthesis from cholesterol. In the present study, the mechanism of C-26 amination was reinvestigated by administering stable isotope labeled compounds, such as (26,26,26,27,27,27-(2)H6)cholesterol during biosynthesis of tomatine, solanine and solasonine. The chemical compositions of tomatine and solanine so obtained were analyzed by LC-MS after administering the d6-cholesterol to a tomato seedling and a potato shoot, respectively. The resulting spectra indicated that two deuterium atoms were eliminated from C-26 of cholesterol during biosynthesis. Furthermore, administration of (6-(13)C(2)H3)mevalonate in combination with lovastatin to an eggplant seedling, followed by GC-MS analysis of solasodine after TMS derivatization established that two deuterium atoms were eliminated from C-26 of cholesterol during solasonine biosynthesis. These findings are in contrast to an earlier observation that one hydrogen atom was lost from C-26 during tomatidine biosynthesis, and suggest that C-26 nitrogen atom addition involves an aldehyde intermediate. Thus, it is proposed that the C-26 amination reaction that occurs during steroidal alkaloid biosynthesis proceeds by way of a transamination mechanism.

Potato glycoalkaloids and metabolites: roles in the plant and in the diet

Potatoes, members of the Solanaceae plant family, serve as major, inexpensive low-fat food sources providing energy (starch), high-quality protein, fiber, and vitamins. Potatoes also produce biologically active secondary metabolites, which may have both adverse and beneficial effects in the diet. These include glycoalkaloids, calystegine alkaloids, protease inhibitors, lectins, phenolic compounds, and chlorophyll. Because glycoalkaloids are reported to be involved in host-plant resistance and to have a variety of adverse as well as beneficial effects in cells, animals, and humans, a need exists to develop a clearer understanding of their roles both in the plant and in the diet. To contribute to this effort, this integrated review presents data on the (a) history of glycoalkaloids; (b) glycoalkaloid content in different parts of the potato plant, in processed potato products, and in wild, transgenic, and organic potatoes; (c) biosynthesis, inheritance, plant molecular biology, and glycoalkaloid-plant phytopathogen relationships; (d) dietary significance with special focus on the chemistry, analysis, and nutritional quality of low-glycoalkaloid potato protein; (e) pharmacology and toxicology of the potato glycoalkaloids comprising alpha-chaconine and alpha-solanine and their hydrolysis products (metabolites); (f) anticarcinogenic and other beneficial effects; and (g) possible dietary consequences of concurrent consumption of glycoalkaloids and other biologically active compounds present in fresh and processed potatoes. An enhanced understanding of the multiple and overlapping aspects of glycoalkaloids in the plant and in the diet will benefit producers and consumers of potatoes.

Effect of light exposure on the glycoalkaloid content of Solanum phureja tubers

The individual glycoalkaloid contents of tubers from eleven Solanum phureja genotypes have been determined prior to and following exposure to light. In all genotypes, light exposure resulted in a statistically significant increase in total glycoalkaloid content. In nine of the genotypes studied, this was not only due to an increase in the levels of the solanidine-based glycoalkaloids, alpha-solanine and alpha-chaconine, but also due to the light-induced synthesis of a tomatidenol-based glycoalkaloid, alpha-solamarine. Those genotypes that accumulated alpha-solamarine in their tubers also contained tomatidenol-based glycoalkaloids in their leaves, but only solanidine-based glycoalkaloids were detected in the sprouts.

Photo-induced synthesis of tomatidenol-based glycoalkaloids in Solanum phureja tubers

The effect of light exposure on the steroidal glycoalkaloid content of Solanum phureja tubers has been investigated and compared with that in domesticated potato (Solanum tuberosum) tubers. The results indicated that the increase in the concentration of solanidine-based glycoalkaloids, alpha-solanine and alpha-chaconine was broadly similar in both species. However, in the S. phureja tubers, light exposure also induced the synthesis of tomatidenol-based glycoalkaloids. These have been identified as alpha- and beta-solamarine. These glycoalkaloids were not detected in tubers continually stored in darkness.