Chapter 1 Steroid Alkaloids: The Solanum Group

Transcriptional and proteomic insights into phytotoxic activity of interspecific potato hybrids with low glycoalkaloid contents

BACKGROUND

Glycoalkaloids are bioactive compounds that contribute to the defence response of plants against herbivore attack and during pathogenesis. Solanaceous plants, including cultivated and wild potato species, are sources of steroidal glycoalkaloids. Solanum plants differ in the content and composition of glycoalkaloids in organs. In wild and cultivated potato species, more than 50 steroidal glycoalkaloids were recognized. Steroidal glycoalkaloids are recognized as potential allelopathic/phytotoxic compounds that may modify the growth of target plants. There are limited data on the impact of the composition of glycoalkaloids on their phytotoxic potential.

RESULTS

The presence of α-solasonine and α-solamargine in potato leaf extracts corresponded to the high phytotoxic potential of the extracts. Among the differentially expressed genes between potato leaf bulks with high and low phytotoxic potential, the most upregulated transcripts in sample of high phytotoxic potential were anthocyanin 5-aromatic acyltransferase-like and subtilisin-like protease SBT1.7-transcript variant X2. The most downregulated genes were carbonic anhydrase chloroplastic-like and miraculin-like. An analysis of differentially expressed proteins revealed that the most abundant group of proteins were those related to stress and defence, including glucan endo-1,3-beta-glucosidase acidic isoform, whose expression level was 47.96× higher in potato leaf extract with low phytotoxic.

CONCLUSIONS

The phytotoxic potential of potato leaf extract possessing low glycoalkaloid content is determined by the specific composition of these compounds in leaf extract, where α-solasonine and α-solamargine may play significant roles. Differentially expressed gene and protein profiles did not correspond to the glycoalkaloid biosynthesis pathway in the expression of phytotoxic potential. We cannot exclude the possibility that the phytotoxic potential is influenced by other compounds that act antagonistically or may diminish the glycoalkaloids effect.

Membrane disruption and enzyme inhibition by naturally-occurring and modified chacotriose-containing Solanum steroidal glycoalkaloids

Naturally-occurring 3beta-O-chacotriosides of solasodine (solamargine), of its 22S, 25S isomer tomatidenol (beta-solamarine), and of solanidine (chaconine), as well as ring E- and F-modified derivatives of solamargine were prepared and assayed in order to assess the relevance of aglycone structural features to membrane-disruption and enzyme-inhibitory activities of the related glycoalkaloids. A ring E-opened dihydro-derivative of solasodine (the chacotrioside of dihydrosolasodine A) did not bind to cholesterol, stigmasterol or ergosterol in vitro, disrupt PC/cholesterol liposomes or mammalian erythrocytes. or inhibit acetylcholinesterase in vitro. It did not synergise with the solatrioside of dihydrosolasodine A or solasonine (nor did solamargine with dihydrosolasodine A solatrioside) in haemolysis tests. The ring F modified derivative, N-nitrososolamargine, did not inhibit acetylcholinesterase in vitro, but lysed liposomes at > or = 150 microM and pH 7. Increasing the pH to 8 (but not 9) further enhanced disruption. The combination of N-nitrososolamargine and solasonine did not cause any disruption of liposomes. Beta-solamarine showed no anti-acetylcholinesterase activity in vitro at up to 100 microM, but disrupted liposomes at 75 and 150 microM, although not to the extent caused by solamargine or chaconine. In combination with both the (inactive) solatriosides, solasonine and solanine, 75 microM beta-solamarine produced synergistic effects, with liposome disruption greater than 150 microM beta-solamarine alone. Beta-solamarine, solamargine and chaconine showed similar haemolytic activity. Beta-solamarine synergised with the solatriosides solasonine and solanine in disrupting erythrocytes. Preliminary structure-activity relationships were evaluated for the active chacotriosides in an attempt to define the scope and limitations of this model study.

Steroidal glycoalkaloid content of potato, tomato and their somatic hybrids

Analyses of leaves and 'tubers' from somatic hybrids of potato and tomato ('pomato' with plastids of potato, 'topato' with plastids of tomato) produced by fusion of protoplasts from liquid cultures of dihaploid potato and mesophyll of tomato revealed the presence of the two major potato glycoalkaloids (α-solanine and α-chaconine) as well as the tomato glycoalkaloid (αtomatine). The total alkaloid content of leaves was greater than that of 'tubers' and similar to levels in the foliage of parent plants. However, glycoalkaloids were more abundant in hybrid 'tubers' than in normal potato tubers by a factor of 5-15. In hybrid foliage, approximately 98% of the alkaloid present was of potato origin whereas in 'tubers' the reverse was the case, with tomatine comprising 60-70% of the total alkaloid. The similarities in alkaloid content and ratios between the pomato and the topato lines indicate that plastomes do not influence the biosynthesis and distribution of these alkaloids. The results indicate that major secondary metabolites may prove useful for assessing the hybrid nature of such plants.