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Steroidal glycoalkaloids (SGAs) are important secondary metabolites in potato which are associated with constitutive host defense mechanism. SGAs like leptines and dehydrocommersonine (DHC) present in wild S. chacoense and S. oplocense, respectively, are known to deter Colorado potato beetle (CPB) feeding. In the current study, LC-MS analysis led to tentative identification of a new SGA with the same molecular mass as α-solanine, solanidenol-chacotriose (SC), which was present in high levels in the CPB susceptible S. tuberosum cv. Shepody, but not in S. oplocense. In a progeny derived from a cross between S. tuberosum, cv. Shepody and S. oplocense derived F1 hybrid, 13213–07, SC was one of the highly variable metabolites along with DHC. Selective genotyping was used for genetic mapping of QTL controlling SC and DHC to chromosome 1. Selective genotyping is dependent on variation between extremes in populations, and it was not effective for QTL mapping of α-solanine and α-chaconine where differences between extremes was low.

Steroidal glycoalkaloids (SGAs) are major secondary metabolites constitutively produced in cultivated potato Solanum tuberosum, and α-solanine and α-chaconine are the most abundant SGAs. SGAs are toxic to humans at high levels but their role in plant protection against pests and pathogens is yet to be established. In this study, levels of SGAs in potato were reduced by RNA interference (RNAi)-mediated silencing of GLYCOALKALOID METABOLISM 4 (GAME4)—a gene encoding cytochrome P450, involved in an oxidation step in the conversion of cholesterol to SGA aglycones. Two GAME4 RNAi lines, T8 and T9, were used to investigate the effects of manipulation of the SGA biosynthetic pathway in potato. Growth and development of an insect pest, Colorado potato beetle (CPB), were affected in these lines. While no effect on CPB leaf consumption or weight gain was observed, early instar larval death and accelerated development of the insect was found while feeding on leaves of GAME4 RNAi lines. Modulation of SGA biosynthetic pathway in GAME4 RNAi plants was associated with a larger alteration to the metabolite profile, including increased levels of one or both the steroidal saponins or phytoecdysteroids, which could affect insect mortality as well as development time. Colonization by Verticillium dahliae on GAME4 RNAi plants was also tested. There were increased pathogen levels in the T8 GAME4 RNAi line but not in the T9. Metabolite differences between T8 and T9 were found and may have contributed to differences in V. dahliae infection. Drought responses created by osmotic stress were not affected by modulation of SGA biosynthetic pathway in potato.

Background Increased atmospheric carbon dioxide (CO2) levels predicted to occur before the end of the century will impact plant metabolism. In addition, nitrate availability will affect metabolism and levels of nitrogen-containing defense compounds, such as glucosinolates (GSLs). We compared Arabidopsis foliar metabolic profile in plants grown under two CO2 regimes (440 vs 880 ppm), nitrate fertilization (1 mM vs 10 mM) and in response to mechanical damage of rosette leaves. Results Constitutive foliar metabolites in nitrate-limited plants show distinct global patterns depending on atmospheric CO2 levels; in contrast, plants grown under higher nitrate fertilization under elevated atmospheric CO2 conditions have a unique metabolite signature. Nitrate fertilization dampens the jasmonate burst in response to wounding in plants grown at elevated CO2 levels. Leaf GSL profile mirrors the jasmonate burst; in particular, indole GSLs increase in response to damage in plants grown at ambient CO2 but only in nitrate-limited plants grown under elevated CO2 conditions. Conclusions This may reflect a reduced capacity of C3 plants grown under enriched CO2 and nitrate levels to signal changes in oxidative stress and has implications for future agricultural management practices. Electronic supplementary material The online version of this article (doi:10.1186/s12870-016-0752-1) contains supplementary material, which is available to authorized users.

Verticillium dahliae Kleb. is a pathogenic fungus causing wilting, chlorosis, and early dying in potato (Solanum tuberosum L.). Genetic mapping of resistance to V. dahliae was done using a diploid population of potato. The major quantitative trait locus (QTL) for Verticillium resistance was found on chromosome 5. The StCDF1 gene, controlling earliness of maturity and tuberization, was mapped within the interval. Another QTL on chromosome 9 co-localized with the Ve2 Verticillium wilt resistance gene marker. Epistasis analysis indicated that the loci on chromosomes 5 and 9 had a highly significant interaction, and that StCDF1 functioned downstream of Ve2. The StCDF1 alleles were sequenced and found to encode StCDF1.1 and StCDF1.3. Interaction between the Ve2 resistance allele and the StCDF1.3 was demonstrated, but not for StCDF1.1. Genome-wide expression QTL (eQTL) analysis was performed and genes with eQTL at the StCDF1 and Ve2 loci were both found to have similar functions involving the chloroplast, including photosynthesis, which declines in both maturity and Verticillium wilt. Among the gene ontology (GO) terms that were specific to genes with eQTL at the Ve2, but not the StCDF1 locus, were those associated with fungal defense. These results suggest that Ve2 controls fungal defense and reduces early dying in Verticillium wilt through affecting genetic pathway controlling tuberization timing.

Plant responses to insect herbivory are regulated through complex, hormone-mediated interactions. Some caterpillar species have evolved strategies to manipulate this system by inducing specific pathways that suppress plant defense responses. Effectors in the labial saliva (LS) secretions of Spodoptera exigua caterpillars are believed to induce the salicylic acid (SA) pathway to interfere with the jasmonic acid (JA) defense pathway; however, the mechanism underlying this subversion is unknown. Since noctuid caterpillar LS contains enzymes that may affect cellular redox balance, this study investigated rapid changes in cellular redox metabolites within 45 min after herbivory. Caterpillar LS is involved in suppressing the increase in oxidative stress that was observed in plants fed upon by caterpillars with impaired LS secretions. To further understand the link between cellular redox balance and plant defense responses, marker genes of SA, JA and ethylene (ET) pathways were compared in wildtype, the glutathione-compromised pad2-1 mutant and the tga2/5/6 triple mutant plants. AtPR1 and AtPDF1.2 showed LS-dependent expression that was alleviated in the pad2-1 and tga2/5/6 triple mutants. In comparison, the ET-dependent genes ERF1 expression showed LS-associated changes in both wildtype and pad2-1 mutant plants and the ORA 59 marker AtHEL had increased expression in response to herbivory, but a LS-dependent difference was not noted. These data support the model that there are SA/NPR1-, glutathione-dependent and ET-, glutathione-independent mechanisms leading to LS-associated suppression of plant induced defenses.