Protochlorophylls in Cucurbitaceae - Distribution, biosynthesis and phylogeny

Using high-resolution chromatography we resolved monovinyl (MV)- and divinyl (DV)-protochlorophylls (Pchls) and detected all of their side-chain homologues in the inner seed coat of Cucurbita maxima, C. pepo and their varieties. Furthermore, we analyzed other less common representatives of the Cucurbitaceae family that were found to accumulate mostly MV-Pchls. All these species and varieties showed the characteristic composition of individual Pchls. Additionally, we also detected all of the corresponding protopheophytins, which accounted for between 1.1 and 35.5% of Pchls and are supposed to be degradation products of Pchls, formed during seed storage. A pigment composition analysis of C. maxima seedlings performed during deetiolation revealed that chlorophyll (Chl) a content increased gradually, while the levels of Pchl-GG and Chl-GG, a precursor of Chl a, were low and did not change significantly. However, when the seedlings were incubated with the precursor of tetrapyrrole biosynthesis (δ-aminolevulinic acid) before illumination, the Chl-GG content increased dramatically, while synthesis of Chl a was inhibited. These data indicate that in C. maxima seedlings, Chl a is not synthesized from geranylgeranyl-pyrophoshate via Chl-GG, but rather directly from phytyl-pyrophosphate. Phylogenetic analysis of Chl synthase genes revealed that many species, including those of the Cucurbitaceae family, have two or more Chl synthase genes. This suggests that these additional genes, at least in some species, might encode isoforms involved in Pchl synthesis.