Loline alkaloid

A member of the loline alkaloids was first isolated from the grass Lolium temulentum and its elemental composition determined in 1892. It was initially named temuline and later renamed norloline. (Reviewed by Schardl et al. (2007).) Studies in the 1950s and 1960s by Russian researchers established the name loline and identified the characteristic 2,7 ether bridge in its molecular structure. Lolines are absent in grass plants that do not harbor the epichloae endophytes, and not all epichloae produce the lolines. which suggests that the endophyte is also the producer of the lolines in the grass plant. The lolines have also been reported from some plants in several plant families, suggesting a more widespread occurrence of these compounds in nature. ==Mechanism of action==

Lolines are insecticidal and deterrent to a broad range of insects, including species in the Hemiptera, Coleoptera, Hymenoptera, Lepidoptera, and Blattodea, such as the bird cherry-oat aphid (genus Rhopalosiphum), large milkweed bug (Oncopeltus fasciatus), and American cockroach (Periplaneta americana). LC50 values of N-formylloline or N-acetylloline from grass seed extracts are 1-20 μg/ml for aphids and milkweed bugs and impair insect development and fecundity and cause avoidance of loline-containing grass tissues. However, ergot alkaloids also have repellent and immobilizing effects on P. scribneri, Therefore, the relative importance of the loline and ergot alkaloids to nematode resistance remains unclear. Many epichloae endophytes—including N. coenophialum symbiotic with Lolium arundinaceum (syn. Festuca arundinacea, tall fescue)—also produce ergot alkaloids that are toxic to mammalian herbivores. The ergot alkaloids occur at relatively low concentrations in the plant and are often difficult to detect analytically. By contrast, the lolines frequently accumulate to very high levels in grass tissues, Specifically, the lolines were thought to be responsible for toxic symptoms called fescue toxicosis displayed by livestock grazing on grasses infected by N. coenophialum. and not the lolines which, even at high doses, have only very small physiological effects on mammalians feeders. Another group of alkaloids, the senecio-type alkaloids, are produced by various plants and like the lolines, the senecio alkaloids possess a pyrrolizidine ring structure. Unlike the lolines, however, the senecio alkaloids exhibit strong hepatotoxicity, owing to a double bond between C-1 and C-2 in their ring structure. and to increase resistance of infected grasses against drought, but such effects have not been substantiated under more natural conditions of cultivation or in habitats. ==Production and distribution in the grass plant==

in tall fescue leaf tissue. Lolines commonly accumulate in the N. coenophialum–tall fescue symbiosis, providing protection from insects and other environmental stresses.), exceeding the concentrations of the other endophyte alkaloids by >1000-fold. but loline increases resembling inducible plant defenses have also been reported. Variation of loline concentration with the developmental stage of specific grass tissues The factors that control loline production vary also among endophyte-infected grass tissues: whereas plant-supplied amino acids that are loline precursors limit accumulation of lolines in many grass tissues, ==Biosynthesis==

The lolines are structurally similar to pyrrolizidine alkaloids produced by many plants, notably the necine ring containing a tertiary amine. This led to the early hypothesis that the biosynthesis of the lolines is similar to that of the plant pyrrolizidines, which are synthesized from polyamines. However, feeding studies with carbon isotope–labeled amino acids or related molecules in pure cultures of the loline-producing fungus N. uncinatum recently demonstrated that the loline alkaloid pathway is fundamentally different from that of the plant pyrrolizidines. In the proposed first step in loline biosynthesis, these two amino acids are coupled in a condensation reaction linking the γ-carbon in homoserine to the secondary amine in proline in a PLP–type enzyme–catalyzed reaction to form the loline intermediate, N-(3-amino-3-carboxy)propylproline (NACPP). Further steps in loline biosynthesis are thought to proceed with sequential PLP-enzyme-catalyzed and oxidative decarboxylations of the carboxy groups in the homoserine and proline moieties, respectively, cyclization to form the core loline ring structure, and oxidation of the C-2 and C-7 carbons to give the oxygen bridge spanning the two pyrrolizidine rings. Genetic studies agree with the biosynthetic routes established in the precursor-feeding experiments. The presence of a single locus for loline production was later confirmed by the finding that loline-producing epichloae endophytes contain a gene cluster (LOL cluster) of at least eleven genes. The LOL genes are greatly and coordinately upregulated during loline alkaloid production, These tests and similarities in the peptide sequences of the proteins encoded by these genes to known enzymes indicate that one gene, termed lolC, is likely required for the first step in loline biosynthesis (condensation of L-proline and L-homoserine for NACPP formation), and another gene, LolP —likely encoding a cytochrome P450 monooxygenase—, for oxygenation of one methyl group on the C-1 amine of N-methylloline, which gives the most abundant loline in many grass–endophyte symbionts, N-formylloline. == References ==