Phytoalexins, carrot

The dramatic increase in 6-MM content--90 to 270 fold in two samples--was unexpected. 6-MM is a potent antifungal agent (4) and one of the most important carrot phytoalexins. Usually 6-MM, one of the components that contributes to the bitterness of stored carrots ( O), is not detected in fresh carrots, but develops during storage. Biosynthetic studies indicate that 6-MM is synthesized via the acetate pathway and its production is stimulated by ethylene (. Thus, UV light may trigger ethylene production in carrots which in turn leads to 6-MM accumulation. 

Amin, M., Kurosaki, F. andNishi, A. (1998) Carrot phytoalexin alters the membrane permeability of Candida albicans and multilamellar liposomes. J. Gen. Microbiol. 134, 241-246 Apel, K. and Hirt, H. (2004) Reactive oxygen species metabolism, oxidative stress and signal transduction. Ann. Rev. Plant Biol. 55, 373-399 Ameson, P.A. and Durbin, R.D. (1968) The sensitivity of fungi to a-tomatine. Phytopathology 58,536-537... 

Myristicin has not been reported to possess antifungal activity, and therefore is not a phytoalexin according to the standard interpretation of this term (1 ). It does, however, potentiate the activity of the insecticide, paraoxon, in flies by inhibiting its degradation (2), and may in similar manner potentiate the action of phytoalexins of carrot root (falcarinol, falcarindiol,... 

In the following experiments carrot roots were exposed to various sources of ultraviolet light in the laboratory and set aside to allow time for enzyme synthesis. Following this period, changes in myristicin and phytoalexin levels were measured. All of these components of carrot root are measured in one assay. Myristicin and 6-methoxymellein concentrations increased in some samples after irradiation with ultraviolet light falcarinol and falcarindiol concentration changes did not appear to be related to the ultraviolet light used in this study. 

Falcarinol and falcarindiol concentration changes are small compared to those in myristicin and 6-MM content, and do not appear related to radiation (Table II). These polyacetylenes are present in fresh carrots. Immediately upon wounding, they are translocated to the surface through oil ducts (3 ) > and, therefore, their function as phytoalexins apparently does not depend upon novo synthesis. 

DC 193 Kurosaki, E., and A. Nishi. Isolation and antimicrobial activity of the phytoalexin 6-methoxymellein from cul- DC205 tured carrot cells. Phytochemistry 1983 22(3) 669-672. 

INDUCTION AND REGULATION OF BIOSYNTHETIC ACTIVITY OF PHYTOALEXIN IN CARROT CELLS... 

Further support for the hypothesis that Ca2+ plays a central role in regulating phytoalexin accumulation is provided by experiments in which the turnover of phosphatidylinositol was measured in the plasma membrane of elicitor-treated carrot cells [17]. The carrot cells were first labelled with [3H]myo-inositol and, after the addition of elicitors, acid extracts of the cells were analyzed chromatographically for the production of inositol trisphosphate (IP3). In cells treated with elicitor, the release of radioactive IP3 increased with time and attained a maximum at 3 - 5 min after treatment. Phospholipase activity responsible for the degradation of phosphorylated phosphatidylinositol increased correspondingly. Several reports have shown that IP3 induces rapid release of Ca2+ from intracellular stores in animal cells [18, 19]. Studies on plant cells have also demonstrated that exogenous IP3 releases Ca2+ from microsomal preparations at micromolar concentrations, although only limited... 

Phan, C.T. Phenolics and polyketides in carrots. In Mycotoxins and Phytoalexins. Sharma R.P., Salunkhe D.K., eds., CRC Press, Boca Raton, USA 1991. 

In carrot, Daucus carota (Umbelliferae), 6-methoxymellein was identified as a phytoalexin (Figure 29). It is biosynthesized by 6-hydroxymellein-O-methyltransferase from 6-hydroxymellein, which is biosynthesized from 1 mol of acetyl coenzyme A (CoA) and 4 mol of malonyl-CoA by a polyketide biosynthetic enzyme, 6-hydroxymellein hydroxylase.2 ... 

Minimal processing of carrots induces phenylalanine ammonia lyase (PAL) activity and phenolics accumulation. In shredded carrots, chlorogenic acid, which is rapidly accumulated, represents 60% of the total phenolics. In addition traces of 3 -caffeoylquinic and 4 -caffeoylquinic acids are biosynthesised, and 3, 4 -dicaffeoylquinic and 3, 5 -dicaffeoyl quinic acids also accumulate [44]. p-Hydroxybenzoic acid derivatives are also biosynthesised but more slowly, and are related to defence against microbial attack (phytoalexin response), the degree and speed of which depends on the cultivar [45],... 

It has been suggested (46) that phytoalexins can be used in some cases as taxonomic markers. That application may indeed be appropriate in the case of carrots. 

One of the earliest reported phytoalexins was 6-me-thoxymellein (57) (Fig. 5.28) produced by carrot root slices after inoculation with Ceratocystis fimbriata. This fungus, which causes black rot disease in sweet potatoes (Ipomoea batatas, Convolvulaceae), is not pathogenic in carrots Dau-... 

Kurosaki, F. (1996) Effect of NADPH associated keto-reducing domain on substrate entry into 6-hydroxymellein synthase, a multifunctional polyketide synthetic enzyme involved in phytoalexin biosynthesis in carrot Arch. Biochem. Biophys. 328, 213-217... 

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