Abstract
The plant sterol pathway exhibits a major biosynthetic difference as compared with that of metazoans. The committed sterol precursor is the pentacyclic cycloartenol (9β,19-cyclolanost-24-en-3β-ol) and not lanosterol (lanosta-8,24-dien-3β-ol), as it was shown in the late sixties. However, plant genome mining over the last years revealed the general presence of lanosterol synthases encoding sequences (LAS1) in the oxidosqualene cyclase repertoire, in addition to cycloartenol synthases (CAS1) and to non-steroidal triterpene synthases that contribute to the metabolic diversity of C30H50O compounds on earth. Furthermore, plant LAS1 proteins have been unambiguously identified by peptidic signatures and by their capacity to complement the yeast lanosterol synthase deficiency. A dual pathway for the synthesis of sterols through lanosterol and cycloartenol was reported in the model Arabidopsis thaliana, though the contribution of a lanosterol pathway to the production of 24-alkyl-Δ5-sterols was quite marginal (Ohyama et al. (2009) PNAS 106, 725). To investigate further the physiological relevance of CAS1 and LAS1 genes in plants, we have silenced their expression in Nicotiana benthamiana. We used virus induced gene silencing (VIGS) based on gene specific sequences from a Nicotiana tabacum CAS1 or derived from the solgenomics initiative (http://solgenomics.net/) to challenge the respective roles of CAS1 and LAS1. In this report, we show a CAS1-specific functional sterol pathway in engineered yeast, and a strict dependence on CAS1 of tobacco sterol biosynthesis.
Highlights
Plants, algae and some protists synthesize their sterols through a biosynthetic route that contains a pentacyclic steroidal cyclization product of 2,3-oxidosqualene, namely, cycloartenol (9b,19-cyclolanost-24-en-3b-ol, Fig. 1) the product of the cycloartenol synthase (CAS1, EC 5.4.99.8) [1,2,3,4,5]
We identified in the genome of Nicotiana benthamiana two genes whose products had 70% and 58% identity with AtCAS1 and AtLAS1, respectively
It is worth noting that probing the genome of Nicotiana benthamiana with the functionally identified tomato triterpene synthases [29] resulted in the identification of a predicted protein (NbS00041716g0004.1) that exhibited 85% identity with the tomato b-amyrin synthase and 77% with the Arabidopsis thaliana b-amyrin synthase (Table S3)
Summary
Algae and some protists synthesize their sterols through a biosynthetic route that contains a pentacyclic steroidal cyclization product of 2,3-oxidosqualene, namely, cycloartenol (9b,19-cyclolanost-24-en-3b-ol, Fig. 1) the product of the cycloartenol synthase (CAS1, EC 5.4.99.8) [1,2,3,4,5]. The nearly ubiquitous presence of b-amyrin synthases and lupeol synthases that form one large group indicates that some triterpenes of the oleanane or lupane series may have a general physiological significance beyond the species-specific well-known accumulation of triterpene derivatives, such as saponins in Gypsophila trichotoma [12]. This has been described recently in a series of studies describing the effect of triterpene lipids on the structure of cuticular lipophilic barriers in Arabidopsis thaliana [13], or the role of these triterpenes as epicuticular crystals in the mediation of plant-insect interactions in a species-specific manner [14]. The presence of a lanosterol metabolism in plants, possibly related to phytosterol biosynthesis, deserves further investigations
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