Developmental regulation of sterol biosynthesis inCucurbita maxima L

Phytosterol metabolism in plant positive-strand RNA virus replication

The genome of most plant viruses consists of a single positive-strand of RNA (+ ssRNA). Successful replication of these viruses is fully dependent on the endomembrane system of the infected cells, which experiences a massive proliferation and a profound reshaping that enables assembly of the macromolecular complexes where virus genome replication occurs. Assembly of these viral replicase complexes (VRCs) requires a highly orchestrated interplay of multiple virus and co-opted host cell factors to create an optimal microenvironment for efficient assembly and functioning of the virus genome replication machinery. It is now widely accepted that VRC formation involves the recruitment of high levels of sterols, but the specific role of these essential components of cell membranes and the precise molecular mechanisms underlying sterol enrichment at VRCs are still poorly known. In this review, we intend to summarize the most relevant knowledge on the role of sterols in ( +)ssRNA virus replication and discuss the potential of manipulating the plant sterol pathway to help plants fight these infectious agents.

Plant Sterol Diversity in Pollen from Angiosperms

Here we have examined the composition of free sterols and steryl esters of pollen from selected angiosperm species, as a first step towards a comprehensive analysis of sterol biogenesis in the male gametophyte. We detected four major sterol structural groups: cycloartenol derivatives bearing a 9β,19-cyclopropyl group, sterols with a double bond at C-7(8), sterols with a double bond at C-5(6), and stanols. All these groups were unequally distributed among species. However, the distribution of sterols as free sterols or as steryl esters in pollen grains indicated that free sterols were mostly Δ(5)-sterols and that steryl esters were predominantly 9β,19-cyclopropyl sterols. In order to link the sterol composition of a pollen grain at anthesis with the requirement for membrane lipid constituents of the pollen tube, we germinated pollen grains from Nicotiana tabacum, a model plant in reproductive biology. In the presence of radiolabelled mevalonic acid and in a time course series of measurements, we showed that cycloeucalenol was identified as the major neosynthesized sterol. Furthermore, the inhibition of cycloeucalenol neosynthesis by squalestatin was in full agreement with a de novo biogenesis and an apparent truncated pathway in the pollen tube.

Three new triterpene esters from pumpkin (Cucurbita maxima) seeds

Three new multiflorane-type triterpene esters, i.e. 7α-hydroxymultiflor-8-ene-3α,29-diol 3-acetate-29-benzoate (1), 7α-methoxymultiflor-8-ene-3α,29-diol 3,29-dibenzoate (2), and 7β-methoxymultiflor-8-ene-3α,29-diol 3,29-dibenzoate (3), were isolated from seeds of Cucurbita maxima, along with the known compound, multiflora-7,9(11)-diene-3α,29-diol 3,29-dibenzoate (4). Compound 1 exhibited melanogenesis inhibitory activities comparable with those of arbutin. In cytotoxicity assays, compounds 1 and 3 exhibited weak cytotoxicity, with IC₅₀ values of 34.5–93.7 μM against HL-60 and P388 cells.

Three new multiflorane-type triterpenes from pumpkin (Cucurbita maxima) seeds

Three new multiflorane-type triterpenes; 7α-methoxymultiflor-8-ene-3α,29-diol 3-acetate-29-benzoate (1), 7-oxomultiflor-8-ene-3α,29-diol 3-acetate-29-benzoate (2), and multiflora-7,9(11)-diene-3α,29-diol 3-p-hydroxybenzoate-29-benzoate (3), were isolated from seeds of Cucurbita maxima, along with three known compounds. Compound 3 and multiflora-7,9(11)-diene-3α-29-diol 3-benzoate (5) exhibited potent inhibitory effects on melanogenesis, with low cytotoxicities, and 2 exhibited single-digit micromolar cytotoxicity against HL-60 and P388 cells.

Phytosterols, phytostanols, and their conjugates in foods: structural diversity, quantitative analysis, and health-promoting uses

Phytosterols (plant sterols) are triterpenes that are important structural components of plant membranes, and free phytosterols serve to stabilize phospholipid bilayers in plant cell membranes just as cholesterol does in animal cell membranes. Most phytosterols contain 28 or 29 carbons and one or two carbon-carbon double bonds, typically one in the sterol nucleus and sometimes a second in the alkyl side chain. Phytostanols are a fully-saturated subgroup of phytosterols (contain no double bonds). Phytostanols occur in trace levels in many plant species and they occur in high levels in tissues of only in a few cereal species. Phytosterols can be converted to phytostanols by chemical hydrogenation. More than 200 different types of phytosterols have been reported in plant species. In addition to the free form, phytosterols occur as four types of "conjugates," in which the 3beta-OH group is esterified to a fatty acid or a hydroxycinnamic acid, or glycosylated with a hexose (usually glucose) or a 6-fatty-acyl hexose. The most popular methods for phytosterol analysis involve hydrolysis of the esters (and sometimes the glycosides) and capillary GLC of the total phytosterols, either in the free form or as TMS or acetylated derivatives. Several alternative methods have been reported for analysis of free phytosterols and intact phytosteryl conjugates. Phytosterols and phytostanols have received much attention in the last five years because of their cholesterol-lowering properties. Early phytosterol-enriched products contained free phytosterols and relatively large dosages were required to significantly lower serum cholesterol. In the last several years two spreads, one containing phytostanyl fatty-acid esters and the other phytosteryl fatty-acid esters, have been commercialized and were shown to significantly lower serum cholesterol at dosages of 1-3 g per day. The popularity of these products has caused the medical and biochemical community to focus much attention on phytosterols and consequently research activity on phytosterols has increased dramatically.

Regulation of squalene synthase, a key enzyme of sterol biosynthesis, in tobacco

Squalene synthase (SS) represents a putative branch point in the isoprenoid biosynthetic pathway capable of diverting carbon flow specifically to the biosynthesis of sterols and, hence, is considered a potential regulatory point for sterol metabolism. For example, when plant cells grown in suspension culture are challenged with fungal elicitors, suppression of sterol biosynthesis has been correlated with a reduction in SS enzyme activity. The current study sought to correlate changes in SS enzyme activity with changes in the level of the corresponding protein and mRNA. Using an SS-specific antibody, the initial suppression of SS enzyme activity in elicitor-challenged cells was not reflected by changes in the absolute level of the corresponding polypeptide, implicating a post-translational control mechanism for this enzyme activity. In comparison, the absolute level of the SS mRNA did decrease approximately 5-fold in the elicitor-treated cells, which is suggestive of decreased transcription of the SS gene. Study of SS in intact plants was also initiated by measuring the level of SS enzyme activity, the level of the corresponding protein, and the expression of SS gene promoter-reporter gene constructs in transgenic plants. SS enzyme activity, polypeptide level, and gene expression were all localized predominately to the shoot apical meristem, with much lower levels observed in leaves and roots. These later results suggest that sterol biosynthesis is localized to the apical meristems and that apical meristems may be a source of sterols for other plant tissues.

Growth of Cucurbita maxima L. plants in the presence of the cycloartenol synthase inhibitor U18666A

Squash, like other Cucurbitaceae, have unique sterol profiles that offer an excellent opportunity to examine the relationship between sterol biosynthesis and plant growth. To determine the effect of sterol biosynthesis inhibition on squash growth, Cucurbita maxima seedlings with and without cotyledons were subjected to increasing concentrations of the cycloarternol synthase (EC 5.4.99.8) inhibitor 3 beta-(2-diethylaminoethoxy)androstenone (U18666A). Inhibition of shoot growth was concentration-dependent (from 0, 2, 5, 10, and 20 microM); plants with intact cotyledons grew to 26.4, 23.7, 21.6, 20.0, and 15.6 cm, respectively, at the above inhibitor concentrations, compared to 25.5, 19.4, 17.0, 12.0, and 11 cm for plants with severed cotyledons. In plants with severed cotyledons, 10 and 20 microM U18666A caused rapid necrosis of the first two, newly emerged, primary leaves, and halted new leaf formation. Secondary root formation was initially affected at all inhibitor concentrations regardless of whether cotyledons were present or not. Vegetative tissue showed a decrease in the accumulation of the major squash sterol, 7,22-stigmastadienol, accompanied by increased accumulation of minor sterol components. Sterol profiles in cotyledons were unaltered. The data show that sterols are crucial for maintaining plant growth and viability, but do not address the cotyledonary effect on growth with respect to sterol biosynthesis.