Mutants of Arabidopsis thaliana with a glucosinolate content different from wild type were isolated by screening a mutagenized population of plants. Six mutants were detected out of a population of 1200 screened. One of these mutants, TUI, was analyzed in detail. Leaf and seed tissues of line TUl lack or have reduced amounts of many of the aliphatic glucosinolates found in the wild type due to a recessive allele, gsml, of a single nuclear gene, GSM1. The seed phenotype is inherited as a maternal effect suggesting that the embryo is dependent on the maternal tissue for its glucosinolates. Experiments involving feeding of "4C-labeled intermediates suggested that the gsml allele results in a metabolic block which decreases the availability of several amino acid substrates required for glucosinolate biosynthesis: 2-amino-6-methylthiohexanoic acid, 2-amino-7-methylthioheptanoic acid, and 2-amino-8-methylthiooctanoic acid. The mutation does not result in any obvious changes in morphology or growth rate. A pathway for the biosynthesis of glucosinolates in A. thaliana is proposed.Glucosinolates are anionic thioglucosides (Fig. 1) synthesized by many species of the order Capparales including all the Brassicaceae (for reviews see refs. 6 and 26). The primary biological function of glucosinolates is unknown, although a role in plant defense against bacterial and fungal pathogens and insect predators has been suggested (reviewed by Fenwick et al. [6]).The presence of glucosinolates in crop species has several important consequences. First, dietary problems in livestock can result when fodder with high levels of glucosinolates are consumed. Thus, glucosinolates severely restrict the amount of glucosinolate-containing meal that can be used in animal feed supplements (24). Second, the distinctive flavor associated with Brassicaceae species which serve as vegetable and
Leaf and seed extracts of Arabidopsis thaliana var. Columbia contain a large number of glucosinolates, representing close to 25% of those known to occur in nature. The glucosinolates, in the form of their desulphated analogs, are separated by reversed-phase, high-performance liquid chromatography (HPLC). Seventeen are identified using thermospray liquid chromatography/mass spectrometry (TSP LC/MS). Additional glucosinolates, present in trace amounts, are identified as isothiocyanates by electron impact and chemical ionization gas chromatography/MS (GC/MS). In total, 23 glucosinolates are detected and these include four series of homologs and analogs. Fifteen possess aliphatic side chains, of which six contain ωmethylthioalkyl and six contain ω-methylsulphinylalkyl side chains; eight possess aromatic side chains, of which four constitute an homologous series of benzoic acid esters and three possess 3-indolylmethyl-based structures. Sixteen of the glucosinolates are detected in Arabidopsis thaliana for the first time and three of these, 4-hydroxybutyl glucosinolate, 5-benzoyloxypentyl glucosinolate, and 6-benzoyloxyhexyl glucosinolate, represent novel plant constituents.
Plant chemicals in three cruciferous crop species, Brassica napus L., B. juncea (L.) Czerniak, and Sinapis alba L., that stimulate oviposition in the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae) were investigated in laboratory bioassays. Aerial portions of 4-to 6-week-old plants were extracted and fractionated using ion-exchange liquid chromatography. The oviposition stimulants were identified as glucosinolates, which are found in all Brassicaceae species. Activity of extracts was largely eliminated by treatment with myrosinase or sulphatase, enzymes which degrade glucosinolates. Reference standards of the same glucosinolates and in the same concentrations as in the extracts were equally stimulatory. A test with eight different glucosinolates demonstrated that the moths do not discriminate between glucosinolates with different side-chain structures. However, in tests using allylglucosinolate the oviposition response was dose-dependent. One of the species tested, S. alba, contained a possible oviposition deterrent.
Embryos derived in vitro from isolated microspores of Brassica napus L. were compared with their zygotic counterparts. Parameters investigated included storage-protein accumulation and gene expression, fattyacid composition, storage-lipid biosynthesis, and the appearance of oil-body proteins. The microspore embryos accumulate storage-protein and show increases in levels of their transcripts during the torpedo stage. These embryos were sensitive to abscisic acid (ABA) with respect to accumulation of storage-protein mRNA and oil-body proteins. Post-transcriptional regulation of cruciferin accumulation is indicated by a disparity between ABA-enhanced transcript accumulation and a less marked effect at the level of protein accumulation. To investigate storage-lipid profiles, two cultivars of Brassica napus, Reston and Topas, were used. The former accumulates major quantities of C20 (11.2%) and C22 (39.9%) fatty acids in its seeds, the latter predominantly C18 fatty acids. The higher-molecular-weight fatty acids (>C18) normally occur only in seeds and were used as biochemical markers for seed-specific metabolism in microspore embryos. Microspore embryos from Reston were found to accumulate C20 (10.6%) and C22 (31.2%) fatty acids after 35 d in culture at levels and proportions comparable to those found in seeds. Similarly, microspore embryos of Topas had a fatty-acid profile similar to that of mature Topas seed. Activities of enzymes involved in the accumulation of storage lipids (erucoyl-CoA synthetase [EC 6.2.1.3], erucoyl-CoA thioesterase [EC 3.1.2.2] and erucoyl-CoA acyltransferase [EC 2.3.1.15 or EC 2.3.1.20]) were detected in torpedostage microspore embryos. Their specific activities were higher than have been reported to date for analogous preparations from zygotic embryos of B. napus. The similarities in storage-lipid and protein composition of these embryos to their zygotic counterparts, along with their sensitivity to ABA, indicate that microspore embryos might be exploited to facilitate studies of biochemistry and gene regulation in oilseeds.
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