Naila Cannes do Nascimento

Plant Secondary Metabolism and Challenges in Modifying Its Operation: An Overview

Plants have metabolic pathways leading to tens of thousands of secondary products capable of effectively responding to stress situations imposed by biotic and abiotic factors. These pathways, often recruited from essential primary metabolism pathways upon initial gene duplication, are frequently restricted to specific taxonomic groups and play a major role in the plant x environment interaction. A strict spatial and temporal control of gene expression ensures the correct accumulation pattern of various secondary products. The required transport of metabolic intermediates constitutes an additional level of regulation. The induction of secondary metabolism gene expression by wounding, herbivore-derived molecules, pathogen elicitors, and oxidative stress caused by heat, drought, flooding, UV light, or temperature extremes is often mediated by integrating signaling molecules such as jasmonate, salicylic acid, and their derivatives. Ontogeny and circadian clock-controlled gene expression are also important features of plant secondary metabolism, as are master regulatory transcription factors. These regulators are attractive targets for engineering secondary metabolic pathways. In spite of the complexity of secondary metabolism, important advances have been achieved, leading to success stories in engineering this diverse reservoir of useful molecules.

ENVIRONMENTAL AND ONTOGENETIC CONTROL OF ACCUMULATION OF BRACHYCERINE, A BIOACTIVE INDOLE ALKALOID FROM Psychotria brachyceras

Brachycerine is a monoterpenoid indole alkaloid accumulated in Psychotria brachyceras plants (Rubiaceae). To better understand the accumulation patterns of this alkaloid, we investigated its content in different plant organs from field-grown trees, throughout the seasons, during seedling development, and in response to potential biotic factors regulating its biosynthesis. Quantification by RP-HPLC showed that aerial vegetative organs (green stems, young and old leaves) yielded similar amounts of brachycerine [0.1–0.2% dry weight (DW)]. Brachycerine was not detected in roots. In reproductive structures, the highest brachycerine amounts (0.3% DW) were found in inflorescences. Alkaloid concentration decreased in mature fruits (0.045% DW). The lowest concentration in reproductive organs was observed in quiescent seeds (0.004% DW). Apparently, brachycerine content dropped during radicle emission in germinating seeds. During seedling development, an increase in leaf content from 0.02 to 0.1% DW was observed between the stages of 2 and 14 leaves, respectively. Salicylic acid did not affect brachycerine content. A doubling of alkaloid content was observed in wounded plants, and a threefold induction occurred with jasmonic acid treatment, suggesting that brachycerine biosynthesis is regulated by jasmonate production.

Accumulation of brachycerine, an antioxidant glucosidic indole alkaloid, is induced by abscisic acid, heavy metal, and osmotic stress in leaves of Psychotria brachyceras.

Psychotria brachyceras Muell. Arg. produces the antioxidant monoterpene indole alkaloid (MIA) brachycerine, which, besides retaining a glucose residue, has its terpenoid moiety derived not from secologanin, but probably from epiloganin, representing a new subclass of MIAs. In this work we showed that osmotic stress agents, such as sodium chloride, sorbitol and polyethylene glycol (PEG), induced brachycerine accumulation in leaf disks of P. brachyceras. Other oxidative stress inducers, such as exposure to aluminum and silver, also increased brachycerine content. Abscisic acid (ABA) treatment was shown to increase brachycerine yield, suggesting its involvement in brachycerine induction during osmotic stress. Ascorbate peroxidase activity was induced in PEG-treated leaf disks, whereas superoxide dismutase (SOD) activity remained unaltered. Assays with specific inhibitors of the cytosolic mevalonate (MVA) and plastidic 2-C-methyl-D-erythritol 4-phosphate (MEP) pathways showed that the terpenoid moiety of brachycerine derived predominantly from the MEP pathway. These results suggest a potential involvement of brachycerine in plant defense against osmotic/oxidative stress damage, possibly contributing to detoxification of hydroxyl radical and superoxide anion as a SOD-like molecule.