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Betalains replace the anthocyanins in flowers and fruits of plants of most families of the Caryophyllales. Unexpectedly, they were also found in some higher fungi. Whereas the anthocyanin-analogous functions of betalains in flower and fruit colouration are obvious, their role in fungi remains obscure. The nature of newly identified betalains as well as final structure elucidation of earlier putatively described compounds published within the last decade is compiled in this report. Recent advances in research on betalain biosynthesis is also covered, including description of some ‘early’ reactions, i.e. betalain-specific dopa formation in plants and fungi and extradiolic dopa cleavage in fungi. Work on betalain-specific glucosyltransferases (GTs) has given new insights into the evolution of secondary plant enzymes. It is proposed that these GTs are phylogenetically related to flavonoid GTs. It was found that the decisive steps in betalain biosynthesis, i.e. condensation of the betalain chromophore betalamic acid with cyclo-dopa and amino acids or amines in the respective aldimine formation of the red-violet betacyanins and the yellow betaxanthins, are most likely to be non-enzymatic. Betalains have attracted workers in applied fields because of their use for food colouring and their antioxidant and radical scavenging properties for protection against certain oxidative stress-related disorders.This review describes structure elucidation of betalains published within the last decade. Recent advances in betalain biosynthesis are also covered, i.e. enzymatic steps of ‘early’ (dopa formation) and ‘late’ reactions (glucosylation and acylation) as well as non-enzymatic steps (cyclo-dopa and aldimine formation).
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Mycorrhizas are the most important mutualistic symbioses on earth. The most prevalent type are the arbuscular mycorrhizas (AMs) that develop between roots of most terrestrial plants and fungal species of the Zygomycota. The AM fungi are able to grow into the root cortex forming intercellular hyphae from which highly branched structures, arbuscules, originate within cortex cells. The arbuscules are responsible for nutrient exchange between the host and the symbiont, transporting carbohydrates from the plant to the fungus and mineral nutrients, especially phosphate, and water from the fungus to the plant. Plants adapt their phosphate uptake to the interaction with the AM fungus by synthesis of specific phosphate transporters. Colonization of root cells induces dramatic changes in the cytoplasmic organization: vacuole fragmentation, transformation of the plasma membrane to a periarbuscular membrane covering the arbuscule, increase of the cytoplasm volume and numbers of cell organelles, as well as movement of the nucleus into a central position. The plastids form a dense network covering the symbiotic interface. In some of these changes, microtubules are most likely involved. With regard to the molecular crosstalk between the two organisms, a number of phytohormones (cytokinins, abscisic acid, jasmonate) as well as various secondary metabolites have been examined: (i) Jasmonates occur at elevated level, which is accompanied by cell-specific expression of genes involved in jasmonate biosynthesis that might be linked to strong carbohydrate sink function of AM roots and induced defense reactions; (ii) apocarotenoids (derivatives of mycorradicin and glycosylated cyclohexenones) accumulate in most mycorrhizal roots examined so far. Their biosynthesis via the nonmevalonate methylerythritol phosphate (MEP) pathway has been studied resulting in new insights into AM-specific gene expression and biosynthesis of secondary isoprenoids.