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

Abstract

SUMMARY 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.

Genome-Wide Identification of CYP72A Gene Family and Expression Patterns Related to Jasmonic Acid Treatment and Steroidal Saponin Accumulation in Dioscorea zingiberensis

“…In addition, S. lycopersicum also belongs to dicotyledon, but one SlCYP72A protein has remarkable homology with DzCYP72A proteins. According to previous research, S. lycopersicum contains abundant steroidal glycoalkaloids, which are also biosynthesized from phytosterols [26]. The similar biosynthesis pathway may be connected to the high homology between SlCYP72A and DzCYP72A proteins.…”

Evolutionary origin and functional diversification of aminotransferases

“…ATs can also act as conduits between different metabolic branches, such as Arabidopsis aromatic AT that transfers amino group from methionine to tryptophan, the precursors of phytohormones, ethylene and auxin, respectively ( 19 ). Therefore, AT enzymes participate in various metabolic pathways of amino acids, secondary metabolites, vitamins and cofactors, as well as gluconeogenesis, detoxification, assimilation, and transport of carbon and nitrogen ( 6 , 20 , 21 , 22 ).

Figure 1 Enzymatic and nonenzymatic transamination reactions, and the evolutionary history of the amino acid metabolism, proteinogenesis, and transamination.

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Potatoes dormancy release and sprouting commencement: A review on current and future prospects

“…Solanine accumulates more in potatoes with mechanical injuries and stored in adverse environmental conditions, especially in potato peels, eyes, and buds (Wan et al, 2022). When PGA4, which is an abundant transcript in sprouts, is silenced, the SGAs are decreased in potatoes (Nakayasu et al, 2021).…”

“…Solanine accumulates more in potatoes with mechanical injuries and stored in adverse environmental conditions, especially in potato peels, eyes, and buds (Wan et al., 2022). When PGA4 , which is an abundant transcript in sprouts, is silenced, the SGAs are decreased in potatoes (Nakayasu et al., 2021). Tubers that knock out GLYCOALKALOID BIOSYNTHESIS1 ( PGA1 ) and PGA2 (encoding the SGA biosynthetic gene) do not sprout during storage, which reveals the close relationship between SGA biosynthesis and dormancy: with the end of dormancy comes the accumulation of SGA (Umemoto et al., 2016).…”