A natural indole alkaloid, norharmane, affects PIN expression patterns and compromises root growth in Arabidopsis thaliana.

Norharmane is an indole alkaloid that can be found in several terrestrial plants, as well as in some dinoflagellates and cyanobacteria. The aim of this study was to focus on the way this metabolite impacts the plant metabolism of the model species Arabidopsis thaliana. This metabolite caused increase of secondary and adventitious roots, as well as torsion, toxic effects, and a decrease in root length. Moreover, norharmane altered the cellular arrangement, resulting in unfinished cell walls, decreased auxin content and inhibited PIN proteins activity. All the alterations suggest that norharmane alters polar auxin transport by inhibiting PIN2, PIN3 and PIN7 transport proteins, thus causing a significant inhibitory effect on the growth of A. thaliana seedlings.

Rosmarinic acid induces programmed cell death in Arabidopsis seedlings through reactive oxygen species and mitochondrial dysfunction.

Phytotoxic potential of rosmarinic acid (RA), a caffeic acid ester largely found in aromatic species, was evaluated on Arabidopsis through metabolomic and microscopic approaches. In-vitro bioassays pointed out that RA affected root growth and morphology, causing ROS burst, ROS scavengers activity inhibition and consequently, an alteration on cells organization and ultrastructure. In particular, RA-treatment (175 µM) caused strong vacuolization, alteration of mitochondria structure and function and a consistent ROS-induced reduction of their transmembrane potential (ΔΨm). These data suggested a cell energy deficit also confirmed by the metabolomic analysis, which highlighted a strong alteration of both TCA cycle and amino acids metabolism. Moreover, the increase in H2O2 and O2- contents suggested that RA-treated meristems underwent oxidative stress, resulting in apoptotic bodies and necrotic cells. Taken together, these results suggest that RA inhibits two of the main ROS scavengers causing high ROS accumulation, responsible of the alterations on mitochondrial ultrastructure and activity through ΔΨm dissipation, TCA-cycle alteration, cell starvation and consequently cell death on Arabidopsis seedlings. All these effects resulted in a strong inhibition on root growth and development, which convert RA in a promising molecule to be explored for further use in weed management.

Auxin-like effects of the natural coumarin scopoletin on Arabidopsis cell structure and morphology.

The mode of action and phytotoxic potential of scopoletin, a natural compound belonging to the group of coumarins, has been evaluated in detail. Analysis conducted by light and electron transmission microscopy showed strong cell and tissue abnormalities on treated roots, such as cell wall malformations, multi-nucleated cells, abnormal nuclei and tissue disorganization. Scopoletin compromised root development by inducing wrong microtubule assembling, mitochondrial membrane depolarization and ultimate cell death, in a way similar to auxin herbicides. The structural similarities of the natural compound scopoletin and the auxin herbicide 2,4-D, as well as the ability of scopoletin to fit into the auxin-binding site TIR1, were analyzed, suggesting that the phytotoxic activity of scopoletin matches with that exhibited by auxinic herbicides.