Biosynthesis, regulation, and domestication of bitterness in cucumber

Abstract

Cucurbitacins are triterpenoids that confer a bitter taste in cucurbits such as cucumber, melon, watermelon, squash, and pumpkin. These compounds discourage most pests on the plant and have also been shown to have antitumor properties. With genomics and biochemistry, we identified nine cucumber genes in the pathway for biosynthesis of cucurbitacin C and elucidated four catalytic steps. We discovered transcription factors Bl (Bitter leaf) and Bt (Bitter fruit) that regulate this pathway in leaves and fruits, respectively. Traces in genomic signatures indicated that selection imposed on Bt during domestication led to derivation of nonbitter cucurbits from their bitter ancestors.

The biosynthetic pathway of the nonsugar, high-intensity sweetener mogroside V from Siraitia grosvenorii

“…S4). Homologous CDS genes from various Cucurbitaceae have been functionally implicated in the synthesis of cucurbitadienol since its initial identification in Cucurbita (19,(32)(33)(34), but 24,25-monoepoxycucurbitadienol, derived from the cyclization of the 2,3;22,23 di-oxidosqualene, has not previously been reported as a product. Modeling of the SgCDS protein indicated that both 2,3 mono-oxidosqualene and 2,3;22,23 di-oxidosqualene can be cyclized and that the additional epoxy group at the 22,23 position does not interfere with the docking (Fig.…”

“…In fact, in striking contrast to the hypothesis of novel mogroside metabolism clustering in Siraitia, the genomic organization of the identified mogroside pathway genes is syntenously preserved by their closest homologs in genomes of Cucurbitaceae that do not accumulate mogrosides, i.e., melon, cucumber, and watermelon (SI Appendix, Table S10). For example, the cluster harboring the CDS gene, described by Shang et al (19), is syntenously preserved in Siraitia, melon, watermelon, and cucumber (SI Appendix, Table S11), as is the cluster surrounding the CYP performing the C11 hydroxylase reaction (SI Appendix, Table S12). Similarly, there is no indication of novel tandem duplications that can account for the pathway genes.…”

“…4B). The cucumber ortholog of the latter, Csa3G903540 (58% identity), was recently reported to also carry out the C19 hydroxylation in cucumber cucurbitacin biosynthesis (19). In the same study of cucumber cucurbitacin metabolism, a CYP450 that carried out C25 hydroxylation was functionally identified (19) (Csa6G088160), but the closest Siraitia ortholog to this gene (contig 24468, listed as 9654_1, CYP81Q58, 74% identity) is not expressed in the developing Siraitia fruit but only in the root and thus cannot be responsible for mogroside C25 hydroxylation.…”

“…A paradigm breakthrough of the past decade in the area of novel secondary plant metabolism has been the discovery of "operon-like" or regulon gene clustering (17,18), reported so far in over a dozen biosynthetic pathways including those encoding, for example, cucumber bitter triterpenoid cucurbitacins (19) and tomato tomatine glycoalkaloids (20). Wada et al (21) estimated about 100 such gene clusters for secondary metabolism in the Arabidopsis genome, and other plant species are likely not to be different.…”

Methyl jasmonate-induced accumulation of metabolites and transcriptional responses involved in triterpene biosynthesis in Siraitia grosvenorii fruit at different growing stages

“…Cucurbitadienol can form a variety of cucurbitanetype triterpenoid glycosides which occur mainly in the family Cucurbitaceae. These include mogrosides, as in Siraitia grosvenorii, and cucurbitacins in Trichosanthes kirilowii [16], Cucumis sativus [17], and Cucurbita pepo [18]. It has been known as the basic backbone of cucurbitane-type triterpenes.…”