Biosynthetic mechanism and ecological function of chirality in Solanum steroidal alkaloids.

Chirality fundamentally influences the bioactivity and ecological roles of plant natural products. Steroidal glycoalkaloids (SGAs) in tomato and wild relatives exhibit this stereochemical variation. Forward genetics identifies C26 hydroxylase GAME8 as the key enzyme controlling SGA chirality. Cultivated tomato harbors two identical GAME8 copies yielding S-configured intermediates, whereas Solanum pennellii homologs GAME8a and GAME8b dominantly produce R- and S-configured products, respectively. Site-saturation mutagenesis and molecular docking reveal four residues sufficient to switch this chirality. Bioassays demonstrate that R-type SGAs, predominant in wild tomato, offer superior defense against the specialist pest Phthorimaea absoluta, although chirality does not affect resistance to the generalist Spodoptera litura. Collectively, these results elucidate the biosynthetic mechanism and ecological significance of SGA chirality, establishing a basis for tailoring phytochemical stereochemistry to enhance crop protection.