Biochemical responses in leaf tissues of alkaloid producing Psychotria brachyceras under multiple stresses.

Under natural conditions plants are generally subjected to complex scenarios of combined or sequential environmental stresses. Among the various components of plant biochemistry modulated by abiotic variables, a pivotal role is played by antioxidant systems, including specialized metabolites and their interaction with central pathways. To help address this knowledge gap, a comparative analysis of metabolic changes in leaf tissues of the alkaloid accumulating plant Psychotria brachyceras Müll Arg. under individual, sequential, and combined stress conditions was carried out. Osmotic and heat stresses were evaluated. Protective systems (accumulation of the major antioxidant alkaloid brachycerine, proline, carotenoids, total soluble protein, and activity of the enzymes ascorbate peroxidase and superoxide dismutase) were measured in conjunction with stress indicators (total chlorophyll, ChA/ChB ratio, lipid peroxidation, H2O2 content and electrolyte leakage). Metabolic responses had a complex profile in sequential and combined stresses compared to single ones, being also modified over time. Different stress application schemes affected alkaloid accumulation in distinct ways, exhibiting similar profile to proline and carotenoids, constituting a complementary triad of antioxidants. These complementary non-enzymatic antioxidant systems appeared to be essential for mitigating stress damage and re-establishing cellular homeostasis. The data herein provides clues that may aid the development of a key component framework of stress responses and their appropriate balance to modulate tolerance and yield of target specialized metabolites.

Leaf nodule endosymbiotic Burkholderia confer targeted allelopathy to their Psychotria hosts.

After a century of investigations, the function of the obligate betaproteobacterial endosymbionts accommodated in leaf nodules of tropical Rubiaceae remained enigmatic. We report that the α-D-glucose analogue (+)-streptol, systemically supplied by mature Ca. Burkholderia kirkii nodules to their Psychotria hosts, exhibits potent and selective root growth inhibiting activity. We provide compelling evidence that (+)-streptol specifically affects meristematic root cells transitioning to anisotropic elongation by disrupting cell wall organization in a mechanism of action that is distinct from canonical cellulose biosynthesis inhibitors. We observed no inhibitory or cytotoxic effects on organisms other than seed plants, further suggesting (+)-streptol as a bona fide allelochemical. We propose that the suppression of growth of plant competitors is a major driver of the formation and maintenance of the Psychotria-Burkholderia association. In addition to potential agricultural applications as a herbicidal agent, (+)-streptol might also prove useful to dissect plant cell and organ growth processes.

Shoot accumulation kinetics and effects on herbivores of the wound-induced antioxidant indole alkaloid brachycerine of Psychotria brachyceras.

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.

Metabolic engineering of cell cultures versus whole plant complexity in production of bioactive monoterpene indole alkaloids: recent progress related to old dilemma.

Monoterpene indole alkaloids (MIAs) are a large class of plant alkaloids with significant pharmacological interest. The sustained production of MIAs at high yields is an important goal in biotechnology. Intensive effort has been expended toward the isolation, cloning, characterization and transgenic modulation of genes involved in MIA biosynthesis and in the control of the expression of these biosynthesis-related genes. At the same time, considerable progress has been made in the detailed description of the subcellular-, cellular-, tissue- and organ-specific expressions of portions of the biosynthetic pathways leading to the production of MIAs, revealing a complex picture of the transport of biosynthetic intermediates among membrane compartments, cells and tissues. The identification of the particular environmental and ontogenetic requirements for maximum alkaloid yield in MIA-producing plants has been useful in improving the supply of bioactive molecules. The search for new bioactive MIAs, particularly in tropical and subtropical regions, is continuously increasing the arsenal for therapeutic, industrially and agriculturally useful molecules. In this review we focus on recent progress in the production of MIAs in transgenic cell cultures and organs (with emphasis on Catharanthus roseus and Rauvolfia serpentina alkaloids), advances in the understanding of in planta spatial-temporal expression of MIA metabolic pathways, and on the identification of factors capable of modulating bioactive alkaloid accumulation in nontransgenic differentiated cultures and plants (with emphasis on new MIAs from Psychotria species). The combined use of metabolic engineering and physiological modulation in transgenic and wild-type plants, although not fully exploited to date, is likely to provide the sustainable and rational supply of bioactive MIAs needed for human well being.