Suberin composition of various plants including Arabidopsis (Arabidopsis thaliana) has shown the presence of very long chain fatty acid derivatives C20 in addition to the C16 and C18 series. Phylogenetic studies and plant genome mining have led to the identification of putative aliphatic hydroxylases belonging to the CYP86B subfamily of cytochrome P450 monooxygenases. In Arabidopsis, this subfamily is represented by CYP86B1 and CYP86B2, which share about 45% identity with CYP86A1, a fatty acid v-hydroxylase implicated in root suberin monomer synthesis. Here, we show that CYP86B1 is located to the endoplasmic reticulum and is highly expressed in roots. Indeed, CYP86B1 promoter-driven b-glucuronidase expression indicated strong reporter activities at known sites of suberin production such as the endodermis. These observations, together with the fact that proteins of the CYP86B type are widespread among plant species, suggested a role of CYP86B1 in suberin biogenesis. To investigate the involvement of CYP86B1 in suberin biogenesis, we characterized an allelic series of cyp86B1 mutants of which two strong alleles were knockouts and two weak ones were RNA interference-silenced lines. These root aliphatic plant hydroxylase lines had a root and a seed coat aliphatic polyester composition in which C22-and C24-hydroxyacids and a,vdicarboxylic acids were strongly reduced. However, these changes did not affect seed coat permeability and ion content in leaves. The presumed precursors, C22 and C24 fatty acids, accumulated in the suberin polyester. These results demonstrate that CYP86B1 is a very long chain fatty acid hydroxylase specifically involved in polyester monomer biosynthesis during the course of plant development.

Metabolic plasticity, which largely relies on the creation of new genes, is an essential feature of plant adaptation and speciation and has led to the evolution of large gene families. A typical example is provided by the diversification of the cytochrome P450 enzymes in plants. We describe here a retroposition, neofunctionalization, and duplication sequence that, via selective and local amino acid replacement, led to the evolution of a novel phenolic pathway in Brassicaceae. This pathway involves a cascade of six successive hydroxylations by two partially redundant cytochromes P450, leading to the formation of N1,N5-di(hydroxyferuloyl)-N10-sinapoylspermidine, a major pollen constituent and so-far-overlooked player in phenylpropanoid metabolism. This example shows how positive Darwinian selection can favor structured clusters of nonsynonymous substitutions that are needed for the transition of enzymes to new functions.

Sterols have multiple functions in all eukaryotes. In plants, sterol biosynthesis is initiated by the enzymatic conversion of 2,3-oxidosqualene to cycloartenol. This reaction is catalyzed by cycloartenol synthase 1 (CAS1), which belongs to a family of 13 2,3-oxidosqualene cyclases in Arabidopsis thaliana. To understand the full scope of sterol biological functions in plants, we characterized allelic series of cas1 mutations. Plants carrying the weak mutant allele cas1-1 were viable but developed albino inflorescence shoots because of photooxidation of plastids in stems that contained low amounts of carotenoids and chlorophylls. Consistent with the CAS1 catalyzed reaction, mutant tissues accumulated 2,3-oxidosqualene. This triterpenoid precursor did not increase at the expense of the pathway end products. Two strong mutations, cas1-2 and cas1-3, were not transmissible through the male gametes, suggesting a role for CAS1 in male gametophyte function. To validate these findings, we analyzed a conditional CRE/loxP recombinationdependent cas1-2 mutant allele. The albino phenotype of growing leaf tissues was a typical defect observed shortly after the CRE/ loxP-induced onset of CAS1 loss of function. In the induced cas1-2 seedlings, terminal phenotypes included arrest of meristematic activity, followed by necrotic death. Mutant tissues accumulated 2,3-oxidosqualene and contained low amounts of sterols. The vital role of sterols in membrane functioning most probably explains the requirement of CAS1 for plant cell viability. The observed impact of cas1 mutations on a chloroplastic function implies a previously unrecognized role of sterols or triterpenoid metabolites in plastid biogenesis.albinism ͉ sterols ͉ triterpenoid metabolites ͉ 2,3-oxidosqualene P lant cells use a sterol biosynthetic pathway that is peculiar compared with that of other eukaryotes (1, 2). Animals and fungi cyclize the 30-carbon atom precursor 2,3-oxidosqualene into the tetracyclic triterpene lanosterol, which is metabolized into cholesterol and ergosterol, respectively, whereas plants transform the same precursor into the cyclopropylsterol intermediate cycloartenol, which is converted into sitosterol (Fig. 1). The physiological role of cyclopropylsterol intermediates and the apparent necessity for plants to synthesize sitosterol by the cycloartenol route are not understood. An Arabidopsis thaliana cDNA encoding cycloartenol synthase 1 (CAS1) has been cloned by functional expression in an ergosterol auxotroph of yeast (Saccharomyces cerevisiae) deficient in lanosterol synthesis (3). CAS1 (At2g07050) belongs to a family of 13 triterpene synthases (4), among which At3g45130 has recently been shown to encode a lanosterol synthase 1 (5, 6). Hence, the first committed step in plant sterol biosynthesis may be functionally redundant, as suggested by the possible existence of a lanosterol route besides the major cycloartenol route to sitosterol (7). Other characterized triterpene synthases, such as LUP1 and LUP2 (Fig. 1), catalyze the formation of nonstero...

Genes encoding sterol ester-forming enzymes were recently identified in the Arabidopsis (Arabidopsis thaliana) genome. One belongs to a family of six members presenting homologies with the mammalian Lecithin Cholesterol Acyltransferases. The other one belongs to the superfamily of Membrane-Bound O-Acyltransferases. The physiological functions of these genes, Phospholipid Sterol Acyltransferase1 (PSAT1) and Acyl-CoA Sterol Acyltransferase1 (ASAT1), respectively, were investigated using Arabidopsis mutants. Sterol ester content decreased in leaves of all mutants and was strongly reduced in seeds from plants carrying a PSAT1-deficient mutation. The amount of sterol esters in flowers was very close to that of the wild type for all lines studied. This indicated further functional redundancy of sterol acylation in Arabidopsis. We performed feeding experiments in which we supplied sterol precursors to psat1-1, psat1-2, and asat1-1 mutants. This triggered the accumulation of sterol esters (stored in cytosolic lipid droplets) in the wild type and the asat1-1 lines but not in the psat1-1 and psat1-2 lines, indicating a major contribution of the PSAT1 in maintaining free sterol homeostasis in plant cell membranes. A clear biological effect associated with the lack of sterol ester formation in the psat1-1 and psat1-2 mutants was an early leaf senescence phenotype. Double mutants lacking PSAT1 and ASAT1 had identical phenotypes to psat1 mutants. The results presented here suggest that PSAT1 plays a role in lipid catabolism as part of the intracellular processes at play in the maintenance of leaf viability during developmental aging.

An approach based on an in silico analysis predicted that CYP77A4, a cytochrome P450 that so far has no identified function, might be a fatty acid‐metabolizing enzyme. CYP77A4 was heterologously expressed in a Saccharomyces cerevisiae strain (WAT11) engineered for cytochrome P450 expression. Lauric acid (C12:0) was converted into a mixture of hydroxylauric acids when incubated with microsomes from yeast expressing CYP77A4. A variety of physiological C18 fatty acids were tested as potential substrates. Oleic acid (cis‐Δ9C18:1) was converted into a mixture of ω‐4‐ to ω‐7‐hydroxyoleic acids (75%) and 9,10‐epoxystearic acid (25%). Linoleic acid (cis,cis‐Δ9,Δ12C18:2) was exclusively converted into 12,13‐epoxyoctadeca‐9‐enoic acid, which was then converted into diepoxide after epoxidation of the Δ9 unsaturation. Chiral analysis showed that 9,10‐epoxystearic acid was a mixture of 9S/10R and 9R/10S in the ratio 33 : 77, whereas 12,13‐epoxyoctadeca‐9‐enoic acid presented a strong enantiomeric excess in favor of 12S/13R, which represented 90% of the epoxide. Neither stearic acid (C18:0) nor linolelaidic acid (trans,trans‐Δ9,Δ12C18:2) was metabolized, showing that CYP77A4 requires a double bond, in the cis configuration, to metabolize C18 fatty acids. CYP77A4 was also able to catalyze the in vitro formation of the three mono‐epoxides of α‐linolenic acid (cis,cis,cis‐Δ9,Δ12,Δ15C18:3), previously described as antifungal compounds. Epoxides generated by CYP77A4 are further metabolized to the corresponding diols by epoxide hydrolases located in microsomal and cytosolic subcellular fractions from Arabidopsis thaliana. The concerted action of CYP77A4 with epoxide hydrolases and hydroxylases allows the production of compounds involved in plant–pathogen interactions, suggesting a possible role for CYP77A4 in plant defense.

A fatty‐acid‐metabolizing enzyme from Arabidopsis thaliana, CYP94C1, belonging to the cytochrome P450 family was cloned and characterized. CYP94C1 was heterologously expressed in a Saccharomyces cerevisiae strain (WAT11) engineered for P450 expression. When recombinant yeast microsomes were incubated with lauric acid (C12:0) for 15 min, one major metabolite was formed. The product was purified and identified by GC/MS as 12‐hydroxylauric acid. Longer incubation (40 min) led to the formation of an additional metabolite identified by GC/MS as dodecadioic acid. This diacid was also produced by incubation with 12‐hydroxylauric acid. These compounds were not produced by incubating microsomes from yeast transformed with a void plasmid, demonstrating the involvement of CYP94C1. This new enzyme also metabolized fatty acids of varying aliphatic chain lengths (C12 to C18) and in‐chain modifications, for example, degree of unsaturation or the presence of an epoxide as an additional polar functional group. Transcription of the gene encoding CYP94C1 is enhanced by stress, treatment with the hormone methyl jasmonate and wounding. Treatment with methyl jasmonate also induced lauric acid metabolism in microsomes prepared from Arabidopsis. The induction of hydroxylase activity was dose dependent and increased with exposure time, reaching 16× higher in microsomes from 24‐h treated Arabidopsis compared with control plants. Analysis of the metabolites showed a mixture of 12‐, 11‐ and 10‐hydroxylauric acids, revealing for the first time the presence of fatty acid in‐chain hydroxylase in Arabidopsis.

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.

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