High-molecular-mass natural rubber is a valuable plant-derived poly(cis-1,4-isoprene) with many industrial and medical applications. It is synthesized by a rubber cis-prenyltransferase (CPT) complex on the surface of rubber particles in specialized latex-producing cells known as laticifers. Here we show that Taraxacum brevicorniculatum rubber transferase activator (TbRTA), a dandelion homologue of the human Nogo-B receptor, is an essential component of the rubber transferase complex which interacts with rubber CPTs on the surface of rubber particles. The knockdown of TbRTA by RNA interference eliminated rubber biosynthesis, without affecting dolichol accumulation or protein glycosylation in the latex. We also found that TbRTA is localized on the endoplasmic reticulum membrane, supporting the current favoured model of rubber particle biogenesis. We therefore propose that TbRTA acts as a rubber CPT-binding protein that is necessary for the formation of an active rubber transferase complex
SummaryNatural 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.
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