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Alkaloids are one of the largest groups of plant secondary metabolites, being present in several economically relevant plant families. Alkaloids encompass neuroactive molecules, such as caffeine and nicotine, as well as life-saving medicines including emetine used to fight oral intoxication and the antitumorals vincristine and vinblastine. Alkaloids can act as defense compounds in plants, being efficient against pathogens and predators due to their toxicity. Fast perception of aggressors and unfavorable environmental conditions, followed by efficient and specific signal transduction for triggering alkaloid accumulation, are key steps in successful plant protection. Toxic effects, in general, depend on specific dosage, exposure time, and individual characteristics, such as sensitivity, site of action, and developmental stage. At times, toxicity effects can be both harmful and beneficial depending on the ecological or pharmacological context. Different strategies are used to study alkaloid metabolism and accumulation. An efficient approach is to monitor gene expression, enzyme activities, and concentration of precursors and of the alkaloid itself during controlled attacks of pathogens and herbivores or upon the simulation of their presence through physical or chemical stimulation. Detailed understanding of alkaloid biosynthesis and mechanisms of action is essential to improve production of alkaloids of interest, to discover new bioactive molecules, and to sustainably exploit them against targets of interest, such as herbivores, pathogens, cancer cells, or unwanted physiological conditions.

Oleoresin is a key defense strategy of advanced gymnosperms, based on the combination of a complex anatomical structure of resin ducts and elaborate terpene biochemistry. Given the vast array of oleoresin economic applications in the chemical, pharmaceutical, agrochemical, and biofuel industries, translating factors that regulate terpene biosynthesis into higher oleoresin yield is a challenge for the forestry industry. Field tests with approximately 3,500 28-year-old slash pine (Pinus elliottii Engelm. var. elliottii) trees were carried out from 2005 to 2008, under the subtropical climate of Southern Brazil, in order to examine the seasonal profile of oleoresin production stimulation in response to different chemical adjuvants, after mechanical injury. Yields of trees treated with oleoresin-inducing pastes containing alternative adjuvants were compared to the standard commercial one used on an industrial scale (based on the ethylene-releasing compound-2-chloroethylphosphonic acid-CEPA). Significant increases in pine oleoresin yield were observed by modulating its biosynthesis and using chemical stimulants affecting defense responses (benzoic acid, used in addition to CEPA) and biosynthetic enzymes (metal cofactors of terpene synthases, iron or potassium, used as replacements for CEPA). Oleoresin stimulation was consistent over at least four consecutive years. Overall effectiveness of oleoresin yield adjuvant stimulation was higher in the faster growth seasons, although potassium was effective in all of them. Combining metal cofactors did not show synergistic or additive interactions. The results suggest that higher oleoresin yields can be obtained by using individual adjuvants of the same signaling pathway in a season-specific fashion.

A major shoot-specific monoterpene indole alkaloid produced by Psychotria brachyceras, brachycerine, is regulated by either wounding or jasmonate application. Highest concentrations of the alkaloid are found in inflorescences, suggesting a defence role. Brachycerine has antimutagenic and antioxidant properties, capable of quenching singlet oxygen, hydroxyl radical, and superoxide. This study aimed at characterizing the putative role of brachycerine in P. brachyceras responses to wounding and herbivory. Damage to leaves increased the content of brachycerine locally. Wounding did not affect phenolics content in P. brachyceras leaves, and no tannins were detected in the species. In generalist herbivore bioassays, neither brachycerine nor P. brachyceras extracts showed toxic effects. In vivo hydrogen peroxide staining assay showed less wound-generated peroxide accumulation in alkaloid treated tissues. This pattern was confirmed in quantitative assays measuring tissue hydrogen peroxide concentrations. Data indicate that brachycerine is not a herbivore deterrent, but rather an indirect chemical defence, modulating oxidative stress caused by mechanical damage.

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