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SummaryTobacco leaf was used to investigate the mechanism of action of auxin-binding protein 1 (ABP1). The distributions of free auxin, ABP1, percentage of leaf nuclei in G2 and the amount of auxin-inducible growth were each determined in control tobacco leaves and leaves over-expressing Arabidopsis ABP1. These parameters were compared with growth of tobacco leaves, measured both spatially and temporally throughout the entire expansion phase. Within a de®ned window of leaf development, juvenile leaf cells that inducibly expressed Arabidopsis ABP1 prematurely advanced nuclei to the G2 phase. The ABP1-induced increase in cell expansion occured before the advance to the G2 phase, indicating that the ABP1-induced G2 phase advance is an indirect effect of cell expansion. The level of ABP1 was highest at the position of maximum cell expansion, maximum auxin-inducible growth and where the free auxin level was the lowest. In contrast, the position of maximum cell division correlated with higher auxin levels and lower ABP1 levels. Consistent with the correlations observed in leaves, tobacco cells (BY-2) in culture displayed two dose-dependent responses to auxin. At a low auxin concentration, cells expanded, while at a relatively higher concentration, cells divided and incorporated [ 3 H]-thymidine. Antisense suppression of ABP1 in these cells dramatically reduced cell expansion with negligible effect on cell division. Taken together, the data suggest that ABP1 acts at a relatively low level of auxin to mediate cell expansion, whereas high auxin levels stimulate cell division via an unidenti®ed receptor.

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Potato tubers naturally contain a number of defense substances, some of which are of major concern for food safety. Among these substances are the glycoalkaloids and calystegines. We have here analyzed levels of glycoalkaloids (α-chaconine and α-solanine) and calystegines (A₃, B₂, and B₄) in potato tubers subjected to mechanical wounding, light exposure, or elevated temperature: stress treatments that are known or anticipated to induce glycoalkaloid levels. Basal glycoalkaloid levels in tubers varied between potato cultivars. Wounding and light exposure, but not heat, increased tuber glycoalkaloid levels, and the relative response differed among the cultivars. Also, calystegine levels varied between cultivars, with calystegine B4 showing the most marked variation. However, the total calystegine level was not affected by wounding or light exposure. The results demonstrate a strong variation among potato cultivars with regard to postharvest glycoalkaloid increases, and they suggest that the biosynthesis of glycoalkaloids and calystegines occurs independently of each other.

Transgenic potato (Solanum tuberosum cv Désirée) plants overexpressing a soybean (Glycine max) type 1 sterol methyltransferase (GmSMT1) cDNA were generated and used to study sterol biosynthesis in relation to the production of toxic glycoalkaloids. Transgenic plants displayed an increased total sterol level in both leaves and tubers, mainly due to increased levels of the 24-ethyl sterols isofucosterol and sitosterol. The higher total sterol level was due to increases in both free and esterified sterols. However, the level of free cholesterol, a nonalkylated sterol, was decreased. Associated with this was a decreased glycoalkaloid level in leaves and tubers, down to 41% and 63% of wild-type levels, respectively. The results show that glycoalkaloid biosynthesis can be down-regulated in transgenic potato plants by reducing the content of free nonalkylated sterols, and they support the view of cholesterol as a precursor in glycoalkaloid biosynthesis.Glycoalkaloids are a family of steroidal toxic secondary metabolites present in plants of the Solanaceae family. In cultivated potato (Solanum tuberosum) the main glycoalkaloids, ␣-chaconine and ␣-solanine, are triglycosylated products of the same aglycone, solanidine, but they differ in their sugar moieties (Friedman and McDonald, 1997). The highest glycoalkaloid level in potato plants is found in flowers and sprouts, followed by the leaves, and the lowest amounts are detected in stems and tubers. The amount of glycoalkaloids increases upon wounding and light exposure, something that may render tubers unsuitable for human consumption. Mild clinical symptoms of glycoalkaloid poisoning include abdominal pain, vomiting, and diarrhea, and an upper safe limit in tubers of 200 mg total glycoalkaloids (TGA) kg Ϫ1 fresh weight has been recommended by leading authorities. However, this upper limit is close to levels found in tubers destined for human consumption, and efforts should be made to keep TGA levels low when introducing new varieties on the market (see Valkonen et al., 1996).The biosynthesis of glycoalkaloids in potato is currently not fully understood. Solanidine has been proposed to be synthesized from the key precursor in plant sterol synthesis, cycloartenol, in a biosynthetic route including cholesterol, a sterol lacking alkylations at the C-24 position in the side chain (Heftmann, 1983; Bergenstråhle et al., 1996;Friedman and McDonald, 1997; Fig. 1). Cholesterol is in most plant species only a minor sterol, but is present at relatively high levels, approximately 15% to 20% of total sterols, in Solanaceous plants such as potato and tobacco (Nicotiana tabacum). One of the final reactions in the synthesis of glycoalkaloids is the glucosylation or galactosylation of solanidine to yield ␥-chaconine or ␥-solanine, respectively. A cDNA encoding the solanidine glucosyltransferase (SGT) enzyme has been cloned (Moehs et al., 1997). The SGT mRNA increased after wounding, in line with previous measurements of wound-induced SGT activity and glycoalkaloid levels. However, the ga...