Critical aspects of phytoalexins in potato

Abstract

Abstract. Phytoalexins in potato are sesquiterpenoid substances produced in response to infections and are believed to help plants resist attack by pathogens. However, these compounds appear in response to compatible as well as incompatible interactions and only accumulate in the tubers. The amounts of phytoalexins produced depend on the physiological condition of the tubers. Young tubers don't gel easily infected with Phylophlhora infestanseven though they synthesize extremely small amounts of phytoalexins. Furthermore, confusion as to the identity of specific races and the propensity for a given race to produce different effects in the same type of host makes it extremely difficult to predict host-parasite interactions with any acceptable degree of accuracy. It is doubtful that the production of phytoalexins in response to artificial inoculations is representative of that occurring in natural infections. Markedly different types of pathogens induce synthesis of same substances in the host cells. It therefore seems most probable that all the phytoalexins are synthesized in response to stimulation by an endogenous elicitor. Little knowledge is available regarding the biosynthesis of these sesquiterpenes, and many previous determinations have presumably been erroneous. When potato tubers were inoculated with the late blight fungus, secondarily appearing bacteria were not retarded, despite the presence of phytoalexins. There is no generally accepted hypothesis describing the mechanism by which phytoalexins inhibit pathogens and no distinction has been made between the effects on necrotrophs and biotrophs. Adequate bioassays capable of measuring the effects of inhibition have yet to be developed, thus far, no convincing inhibitory effects have been reported. During purification of the phytoalexins there is a high risk for artifact forming, implying that specific compounds cannot be detected with certainty. Moreover, present analytical methods must be improved before we can determine how phytoalexins act in vivo. Probably, phytoalexins are synthesized at a stage in the infection too late to be able to restrict its expansion with the tissues of the host. Phytoalexins are restricted to the attacked parts of the tubers and there is no evidence indicating that these compounds pose any health risks when present in potatoes used fqr consumption.

Antifungal activity toPhytophthora infestans of sesquiterpenoids from infected potato tubers

“…This fungus is the most frequently used microorganism for testing the activity of potato phytoalexins; for instance, tests have been reported on the effect on germination (Ishiaka et al, 1969;Harris & Dennis, 1976), germ tube growth (Ishiaka et al, 1969;Sato et al, 1985), mycelial growth (Beczner & Ersek, 1976;Hohl et al, 1980;St6ssel & Hohl, 1981) ( Hohl et al, 1980). We have concentrated on the activities of the substances on mycelial growth because the production of sesquiterpenoids does not start until the fungus has penetrated the host surface, and therefore it would not be relevant to study spore germination or germ tube growth (Brishammar, 1987;Doke et al, 1979). As a way of making comparisons with previously performed tests we included rishitin and solavetivone, which are well known sesquiterpene phytoalexins of potato tubers.…”

Diversity of Sesquiterpenes in 46 Potato Cultivars and Breeding Selections

“…Much of the research on the biochemistry of fungal disease resistance in potato tubers has focused on the sesquiterpenes, because they are fungitoxic in vitro and they can accumulate to high levels near infection sites. In addition, no harmful effects have been observed following limited testing on mice (Brishammer, 1987) and chicken embryos (Wood, 1976), which suggests that at least some sesquiterpenes may present a low risk of toxicity to humans. More than 20 sesquiterpenes have been isolated and characterized from potatoes (Kuc, 1982); most of these studies have used Kennebec and Russet Burbank, which are widely grown commercial cultivars (Bostock et al, 1981;Rohwer et al, 1987;Zook and Kuc, 1991).…”