Cardenolide Increase in Foxglove During Stresses Corresponds to Cholesterol and Phytosterol Biosynthesis

Cardenolides are a group of steroidal metabolites found in Digitalis lanata with potent cardioactive effects on animals. In plants, cardenolides are involved in various stress responses. However, the molecular mechanism of cardenolide increase during stresses is mostly unknown. Here we show that cardenolides were increased upon methyl jasmonate (MJ), sorbitol, potassium (KCl) chloride, and salicylic acid analog (BTH: 2,1,3-benzothiadiazole) treatments. However, the expression of three known genes for cardenolide biosynthesis did not correlate well with these increases. Specifically, the expression of progesterone-5{beta}-reductases (P5{beta}R and P5{beta}R2) did not correlate with cardenolide increase. The expression of 3{beta}-hydroxysteroid dehydrogenase (3{beta}HSD) correlated with cardenolide levels only during the BTH treatment. Mining the D. lanata transcriptome identified genes involved in cholesterol and phytosterol biosynthesis: SSR1 (sterol sidechain reductase), SMO1, and SMO3 (sterol methyl oxidase). Surprisingly, the expression of all three genes correlated well with the cardenolide increase after BTH treatment. Phylogenetic analysis showed that SSR1 is likely involved in both cholesterol and phytosterol biosynthesis. In addition, SMO1 is likely specific to phytosterol biosynthesis, and SMO3 is specific to cholesterol biosynthesis. These results suggest that both cholesterol and phytosterol are involved in cardenolide biosynthesis. In summary, this work shows that cardenolides are important for stress responses in D. lanata and revealed a novel relationship between phytosterol and cardenolide biosynthesis.