Molecular analysis of alkaloid metabolism in AABB v. aabb genotype Nicotiana tabacum in response to wounding of aerial tissues and methyl jasmonate treatment of cultured roots

Synthesis of the wound-inducible alkaloid, nicotine, in roots of the allotetraploid species Nicotiana tabacum L. is strongly influenced by the presence of two non-allelic genes, A and B. Together, these loci affect baseline transcript levels of genes dedicated to secondary metabolism (e.g. PMT and A622) as well as genes with roles in separate areas of primary metabolism (e.g. ODC, ADC, SAMS — polyamines; QPT — pyridine nucleotide cycle). Experiments comparing high alkaloid variety NC 95 (AABB genotype) and near-isogenic low alkaloid N. tabacum variety LAFC 53 (aabb genotype) indicate that together, mutations in the A and B loci diminish, but do not ablate, the propensity of roots to increase transcript levels of genes involved in alkaloid metabolism after damage to aerial tissues or direct treatment with the wound hormone, methyl jasmonate. Accordingly, roots of aabb genotype can increase their nicotine content somewhat in response to these treatments. Additionally, we show that transcript levels of genes associated with polyamine metabolism (ODC, ADC, SamDC, SAMS and SS) but not alkaloid synthesis (PMT, QPT, A622) are elevated in leaves of N. tabacum in response to wounding. Moreover, respective increases in transcript levels of each gene are similar in wounded leaves of NC 95 and LAFC 53, suggesting that these increases are not controlled by combined action of genes encoded by the A and B loci. Further detailed analysis of wounded leaves of AABB genotype indicates that although transcript levels of these genes of polyamine metabolism and associated enzyme activities for ODC, ADC and SamDC, are markedly increased in leaves in response to wounding, putrescine levels remain unaltered whilst spermidine and spermine levels are reduced to 50–60% of controls levels, when analysed up to 6 h post-wounding. These observations may indicate that any wound-induced increases in polyamine biosynthesis that do occur in leaf cells during this time frame are consumed by metabolic reactions involved in repair and / or strengthening of wounded leaf tissues.

Inducible Flavone in Oats (Avena sativa) Is a Novel Defense Against Plant-Parasitic Nematodes

The induction of defense compounds in oats (Avena sativa) in response to invasion by parasitic nematodes and to application of the wound hormone methyl jasmonate was examined. Oats cv. Quoll seedlings were challenged with Pratylenchus neglectus, Heterodera avenae, and Ditylenchus dipsaci and treated with 1 × 10-4 M methyl jasmonate. Three compounds, isolated in methanolic root and shoot extracts of oats, exhibiting an absorbance spectrum typical of flavone glycosides, were induced by nematode invasion and methyl jasmonate. These were identified as flavone-C-glycosides by mass spectrometry. The effect of the flavone-C-glycosides on the invasion by and development of cereal cyst nematode H. avenae was assessed using methanolic extracts of shoots and roots from methyl jasmonate-treated plants. Both extracts impaired nematode invasion and development. When the extracts were fractionated by high voltage paper electrophoresis, only one flavone-C-glycoside, O-methyl-apigenin-C-deoxyhexoside-O-hexoside, inhibited nematode invasion. The protective effect of the induction of flavone-C-glycosides in oats by methyl jasmonate was evaluated against H. avenae and P. neglectus. Treatment with methyl jasmonate reduced invasion of both nematodes and increased plant mass, compensating for damage caused by the nematodes, and is attributed to the active flavone-C-glycoside. The active compound, O-methyl-apigenin-C-deoxyhexoside-O-hexoside, has not been implicated previously in plant defense against any pest or pathogen, and appears to provide protection against the major cereal nematodes Heterodera and Pratylenchus.