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Books and chapters

Walter, M. H.; Role of Carotenoid Metabolism in the Arbuscular Mycorrhizal Symbiosis (de Bruijn, F. J., ed.). 513-524, (2013) ISBN: 9781118297674 DOI: 10.1002/9781118297674.ch48

Cleavage products of carotenoids (apocarotenoids) exert a variety of often poorly characterized functions in roots and in rhizospheric interactions of plants with both symbionts and parasites. They are generated by regiospecific cleavage enzymes (CCDs, NCEDs) that act in a single or sequential way on C40 carotenoids. Among such apocarotenoids are one well‐known phytohormone controlling drought stress response networks (abscisic acid, ABA) and a newly discovered class of growth regulators involved in adaptive reactions to nutrient stress (strigolactones, SL). A third class of apocarotenoids consists of derivatives of a cyclic cyclohexenone (CH) and a linear mycorradicin (MR) type. They accumulate abundantly as part of a so‐called yellow pigment complex in roots colonized by arbuscular mycorrhizal (AM) fungi. Mycorrhizal phenotypes of pathway knockdown and loss‐of‐function mutants are reviewed in order to clarify the role of the three apocarotenoid classes for the AM symbiosis. One case of pathway interconnection between SL and CH/MR biogenesis through CCD7 is discussed along with other implications for interplay between pathways. SLs appear to preferentially affect early steps of root colonization by AM fungi and thus colonization levels. In contrast, accumulation of CH/MR derivatives is associated with arbuscule formation. Arbuscules are transient structures, which undergo constant degradation and reformation (turnover). Colocalization of CH/MR derivatives with degrading arbuscules and other observations suggest a phytoalexin‐like function in a plant‐controlled degradation of degenerating or poorly functional arbuscules. A model is presented, which proposes maintenance of high levels of functional arbuscules delivering phosphate through plant management of their rapid turnover.
Books and chapters

Walter, M. H.; Floss, D. S.; Paetzold, H.; Manke, K.; Vollrath, J.; Brandt, W.; Strack, D.; Control of Plastidial Isoprenoid Precursor Supply: Divergent 1-Deoxy-D-Xylulose 5-Phosphate Synthase (DXS) Isogenes Regulate the Allocation to Primary or Secondary Metabolism (Bach, T. J. & Rohmer, M., eds.). 251-270, (2012) ISBN: 978-1-4614-4063-5 DOI: 10.1007/978-1-4614-4063-5_17

Following the description of two separate pathways for isoprenoid precursor biosynthesis in plants, a new level of complexity has been introduced by the discovery of two divergent gene classes encoding the first enzyme of the plastidial methylerythritol phosphate (MEP) pathway. These nonredundant 1-deoxy-d-xylulose 5-phosphate synthase (DXS) isogenes are differentially expressed in such a way that DXS1 appears to serve housekeeping functions, whereas DXS2 is associated with the production of specialized (secondary) isoprenoids involved in ecological functions. Examples of the latter are apocarotenoid formation in roots colonized by arbuscular mycorrhizal fungi and mono- or diterpenoid biosynthesis in trichomes. Knockdown of DXS2 genes can specifically suppress secondary isoprenoid formation without affecting basic plant functions. Analyzing DXS isogenes along the progression of land plant evolution shows separation in structure and complementary expression already at the level of gymnosperms, which is maintained in all angiosperms except Arabidopsis.
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