Exogenous spermidine, arsenic and beta-aminobutyric acid modulate tobacco resistance to tobacco mosaic virus, and affect local and systemic glucosylsalicylic acid levels and arginine decarboxylase gene expression in tobacco leaves.

The polyamine spermidine and the metalloid arsenic increased resistance responses in the well-known pathosystem NN tobacco/tobacco mosaic virus (TMV). Both the hypersensitive response to TMV in a leaf disk model system (inoculated disks floating in the 0.1mM treatments) and systemic acquired resistance (SAR) in whole plants were significantly affected. In the latter case, 1mM foliar sprays of spermidine and arsenic were as effective as TMV and dl-beta-aminobutyric acid (BABA), both taken as positive controls, in improving the plant's response to subsequent challenge inoculation with TMV. Moreover, this phenotypic response was correlated with changes in the endogenous concentration of the SAR-related molecule salicylic acid and in transcript levels of some pathogenesis/stress-related genes (pathogenesis-related proteins PR-1a and PR-2 and arginine decarboxylase (ADC)). Concentrations of free salicylic acid and of 2-O-beta-d-glucosylsalicylic acid and mRNA amount of PR-1a, PR-2 and ADC were analyzed in plants treated with either spermidine or arsenic, and compared with those from untreated plants and from positive (TMV-inoculated or BABA-treated) controls. Conjugated salicylic acid content and ADC transcripts were found to significantly increase, at both the local and systemic levels, relative to untreated controls.

Induction of flavonoid production by UV-B radiation in Passiflora quadrangularis callus cultures.

Callus cultures from several species of Passiflora were initiated in vitro, and their capacity to produce four glycosyl flavonoids (orientin, isoorientin, vitexin, isovitexin) was analysed. The aim of the present work was to examine the possible role of UV-B irradiation and elicitation with methyl jasmonate (MJ) on the production of these compounds in callus cultures. All the species tested (P. incarnata, P. quadrangularis, P. edulis) formed friable callus from leaf explants after 4 weeks on medium supplemented with kinetin and 2,4-dichlorophenoxyacetic acid. Among them, P. quadrangularis turned out to have a faster growth rate and a more friable texture, and was therefore chosen for experiments with elicitors. In callus cultures only small amounts of isoorientin were found, while the concentration of the other flavonoids was below the detection limit. UV-B irradiation of calluses was able to increase the production of all four glycosyl flavonoids. After a 7-day exposure of cultures to UV-B light, the production of isoorientin reached concentrations similar to those found in fresh leaves from glasshouse-grown plants. Elicitation with methyl jasmonate also enhanced orientin, vitexin and isovitexin concentrations, even though the stimulation was about 6-fold weaker for orientin and vitexin and about 40-fold for isovitexin, than that exerted by UV-B treatment. Callus cultures treated with the UV-B dose which most enhanced flavonoid production showed a higher antioxidant activity compared to untreated calluses, with an increase ranging from 28% to 76%. Results show that the secondary metabolite biosynthetic capacity of Passiflora tissue cultures can be enhanced by appropriate forms of elicitation.