Germacrene A Synthases for Sesquiterpene Lactone Biosynthesis Are Expressed in Vascular Parenchyma Cells Neighboring Laticifers in Lettuce.

Sesquiterpene lactone (STL) and natural rubber (NR) are characteristic isoprenoids in lettuce (). Both STL and NR co-accumulate in laticifers, pipe-like structures located along the vasculature. NR-biosynthetic genes are exclusively expressed in laticifers, but cell-type specific expression of STL-biosynthetic genes has not been studied. Here, we examined the expression pattern of germacrene A synthase (), which catalyzes the first step in STL biosynthesis in lettuce. Quantitative PCR and Illumina read mapping revealed that the transcripts of two isoforms (/) are expressed two orders of magnitude (~100-200) higher in stems than laticifers. This result implies that the cellular site for / expression is not in laticifers. To gain more insights, promoters of / were cloned and fused to β-glucuronidase (), followed by transformations of lettuce with these promoter- constructs. In in situ GUS assays, the expression driven by the promoters was tightly associated with vascular bundles. High-resolution microsections showed that GUS signals are not present in laticifers but are detected in the vascular parenchyma cells neighboring the laticifers. These results suggest that expression of occurs in the parenchyma cells neighboring laticifers, while the resulting STL metabolites accumulate in laticifers. It can be inferred that active metabolite-trafficking occurs from the parenchyma cells to laticifers in lettuce.