| Summary: | Sesquiterpene lactone (STL) and natural rubber (NR) are characteristic isoprenoids in lettuce (<i>Lactuca sativa</i>). 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 (<i>LsGAS</i>), which catalyzes the first step in STL biosynthesis in lettuce. Quantitative PCR and Illumina read mapping revealed that the transcripts of two <i>GAS</i> isoforms (<i>LsGAS1</i>/<i>LsGAS2</i>) are expressed two orders of magnitude (~100–200) higher in stems than laticifers. This result implies that the cellular site for <i>LsGAS1</i>/<i>2</i> expression is not in laticifers. To gain more insights, promoters of <i>LsGAS1</i>/<i>2</i> were cloned and fused to β-glucuronidase (<i>GUS</i>), followed by transformations of lettuce with these promoter-<i>GUS</i> constructs. In in situ GUS assays, the <i>GUS</i> expression driven by the <i>LsGAS1/2</i> 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 <i>LsGAS1/2</i> 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.
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