Plant Biotechnology Journal; January 2017
Summary
Natural rubber (NR) is an important raw material for a large number of industrial products. The primary source of NR is the rubber tree Hevea brasiliensis, but increased worldwide demand means that alternative sustainable sources are urgently required. The Russian dandelion (Taraxacum koksaghyz Rodin) is such an alternative because large amounts of NR are produced in its root system. However, rubber biosynthesis must be improved to develop T. koksaghyz into a commercially feasible crop. In addition to NR, T. koksaghyz also produces large amounts of the reserve carbohydrate inulin, which is stored in parenchymal root cell vacuoles near the phloem, adjacent to apoplastically separated laticifers. In contrast to NR, which accumulates throughout the year even during dormancy, inulin is synthesized during the summer and is degraded from the autumn onwards when root tissues undergo a sink-to-source transition. We carried out a comprehensive analysis of inulin and NR metabolism in T. koksaghyz and its close relative T. brevicorniculatum and functionally characterized the key enzyme fructan 1-exohydrolase (1-FEH), which catalyses the degradation of inulin to fructose and sucrose. The constitutive overexpression of Tk1-FEH almost doubled the rubber content in the roots of two dandelion species without any trade-offs in terms of plant fitness. To our knowledge, this is the first study showing that energy supplied by the reserve carbohydrate inulin can be used to promote the synthesis of NR in dandelions, providing a basis for the breeding of rubber-enriched varieties for industrial rubber production.
See: http://onlinelibrary.wiley.com/doi/10.1111/pbi.12672/full
Figure 1: Putative model of the inulin and isoprenoid metabolic network in dandelion roots. Sucrose, either transferred from the apoplast or synthesized from glucose and fructose in the cytosol of the parenchymal root cells, is transported into the vacuole and used as a substrate for inulin biosynthesis. The degradation of inulin by 1-FEH produces free sucrose and fructose. Sucrose is actively transported through the cytoplasm and apoplast into the laticifers and is used as a precursor via the mevalonate pathway for the synthesis of isopentenyl pyrophosphate (IPP), the basic building block of isoprenoids such as triterpenes and poly(cis-1,4-isoprene). F-6-P, fructose-6-phosphate; G-1-P, glucose-1-phosphate; GFF, 1-kestose; MVA, mevalonic acid; DMAPP, dimethylallyl pyrophosphate; FPP, farnesyl pyrophosphate.
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