Accumulation of the antioxidant alkaloid brachycerine from Psychotria brachyceras Mull. Arg. is increased by heat and contributes to oxidative stress mitigation

Brachycerine is a shoot monoterpene indole alkaloid with antioxidant, UV protectant, and antimutagenic activities present in Psychotria brachyceras. The alkaloid has been shown to be induced by osmotic stress, UV, heavy metals, and wounding. Since brachycerine accumulation is related to redox imbalance, herein we tested the hypothesis that heat induces accumulation of the alkaloid, which helps mitigating oxidative stress. Brachycerine concentration in leaf disks exposed to 40 degrees C for three days, both by abrupt and stepwise temperature raise, increased by 4.5 and 2 fold, respectively, reaching up to 2.0% of the extracted dry weight. Alkaloid concentration was not affected by exposure to 10 degrees C. Lipid peroxidation was reduced in P. brachyceras under acute and stepwise heat stress compared to control condition. Hydrogen peroxide concentration was lower in leaf disks exposed to heat shock (50 degrees C) compared to control. No changes were observed in total chlorophyll under any of the temperature treatments. Leaf disks of the heat-sensitive species Brugmansia suaveolens and Brassica oleracea, which had massive loss of chlorophyll under heat, showed heat shock tolerant phenotype when pre-treated with brachycerine in concentrations equivalent to those found in P. brachyceras. Expression of TRYPTOPHAN DECARBOXYLASE (TDC), encoding an enzyme involved in alkaloid biosynthesis, was repressed in leaf disks exposed to 40 degrees C for 6, 12 and 24 h, suggesting that temperature action may take place at post-transcriptional level. In fact, heat exposed-disks had higher concentration of the alkaloid precursor tryptamine and TDC activity compared to control counterparts. Taken together, data shows that accumulation of brachycerine is induced by heat, probably by a post-transcriptional mechanism, contributing to protection against associated oxidative damage.