Nomilin, biosynthesis

Stems are the major site of nomilin biosynthesis from acetate in citrus (26). Analysis of the phloem, the cortex and the inner core regions of the stem showed that the phloem region is the site of nomilin biosynthesis from acetate (27). Root tissues also have this capacity. Leaves, fruits and seeds are either incapable of biosynthesizing limonoids from acetate or have a very low capacity. However, these tissues are capable of biosynthesizing limonoids from nomilin. Nomilin is translocating from the stem to other locations, where it is further biosynthesized to other limonoids (26). 

Inhibition of Limonoid Biosynthesis by Auxins. Auxins are potent inhibitors of nomilin biosynthesis in citrus seedlings (31). For instance, up to 91% inhibition was observed when 10 ppm of indoleacetic acid was fed to the stem of a lemeon seedling two days prior to and two days following feeding of 25 pCi of 14-C acetate (Table 1). Other auxins tested include 1-naphthaleneacetic acid (NAA), indolepropionic acid, indolebutyric acid, 3-indole acetonitrile, ethyl indole-3-acetate, 3-indoleacrylic acid, 3-(2-hydroxyethyl)indole, indole-2-carboxylic acid and 2,3,4-trichlorophenoxyacetic acid. They were all very effective. 

Table 1. Inhibition of Nomilin Biosynthesis by Indoleacetic Acid in Young Lemon Seedlings...

Hasegawa, S., V. P. Maier, Z. Herman, and P. Ou, Phytohormone bioregulation of nomilin biosynthesis in Citrus limon seedlings. Phytochemistry, 25, 1323-1325 (1986). 

Thus, nomilin acetyl-lyase appears to play a key role in the regulatory system which controls the biosynthesis and accumulation of limonoids in citrus. 

Limonin and nomilin cause bitter flavor in citrus products. The bitterness is a major problem for the industry. Hasegawa et al. (this volume) have identified three target enzymes involved in limonoid biosynthesis for development of transgenic citrus free of limonin and nomilin bitterness. They are linoleate dehydrogenase, UDP glucose transferase and nomilin deacetylesterase. The isolation of the genes for these enzymes is currently being conducted in order to eventually insert them into cultured citrus cells where they will convert the bitter compounds to non-bitter derivatives. From the cultured cells mature citrus plants will be produced and these plants should produce fhiit free of bitter flavor. 

Radioactive tracer work has shown that nomilin (3) is biosynthesized by the terpenoid pathway from acetate in the phloem region of stems and translocated to other parts of plants such as leaves, fruit tissues and seeds (13,14). At those locations, nomilin is further biosynthesized to other limonoids. Limonoid biosynthesis occurs at each location independently, thus the composition of limonoids in fruit tissues, seeds and leaves are different from each of the others. Limonin is biosynthesized from nomilin via obacunone (4), obacunoate (5) and ichangin (6) (15-17) (Fig. 2)... 

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