Constitutive Steroidal Glycoalkaloid Biosynthesis in Tomato is Regulated by the Clade IIIe Basic Helix-Loop-Helix Transcription Factors MYC1 and MYC2

Specialized metabolites are produced by plants to fend off biotic enemies. Across the plant kingdom, the biosynthesis of these defense compounds is promoted by jasmonate signaling in which clade IIIe basic helix-loop-helix (bHLH) transcription factors take on a central role. Tomato (Solanum lycopersicum) produces cholesterol-derived steroidal glycoalkaloids (SGAs) that act as phytoanticipins against a broad variety of herbivores and pathogens. The biosynthesis of SGAs from cholesterol occurs constitutively in tomato plants and can be further stimulated by jasmonates. Here, we demonstrate that the two tomato clade IIIe bHLH transcription factors, MYC1 and MYC2, redundantly and specifically control the constitutive biosynthesis of SGAs. Double myc1 myc2 loss-of-function tomato hairy roots displayed suppressed constitutive expression of cholesterol and SGA biosynthesis genes, and consequently severely reduced levels of the main tomato SGAs -tomatine and dehydrotomatine. In contrast, basal expression of genes involved in canonical jasmonate signaling or in the biosynthesis of highly jasmonate-inducible phenylpropanoid-polyamine conjugates was not affected. Furthermore, CRISPR-Cas9(VQR)-mediated genome editing of a specific cis-regulatory element, targeted by MYC1/2, in the promoter of a cholesterol biosynthesis gene led to decreased constitutive expression of this gene, but did not affect its jasmonate inducibility. Our results demonstrate that clade IIIe bHLH transcriptional regulators might have evolved to regulate the biosynthesis of specific constitutively accumulating specialized metabolites independent of jasmonate signaling. One sentence summaryThe clade IIIe basic helix-loop-helix transcription factors MYC1 and MYC2 control the constitutive biosynthesis of tomato steroidal glycoalkaloids and might do so independently of jasmonate signaling.