Crossing the First Threshold: New Insights into the Influence of the Chemical Structure of Anionic Porphyrins on Plant Cell Wall Interactions and Photodynamic Cell Death Induction

In this study, our fundamental research interest was to understand how negatively charged porphyrins could interact with a plant cell wall and further act inside cells. Thus, three anionic porphyrins differing in their anionic external groups (carboxylates, sulfonates, and phosphonates) were tested. First, the tobacco cell wall was isolated to monitor in vitro its interactions with the three different anionic porphyrins. Unexpectedly, these negatively charged molecules were able to bind to the negatively charged cell wall probably by weak bonds such as hydrogen bonds and/or electrostatic interactions when the tetrapyrrolic core was protonated. Moreover, we showed that at the pH of spent culture medium (4.5), the neutrality of the carboxylated porphyrin (TPPC) facilitated its cell wall crossing while the diffusion of the two other sulfonated (TPPS) or phosphonated (TPPP) porphyrins that remained anionic was delayed. Once inside Tobacco Bright Yellow-2 (TBY-2) cells, TPPC induced higher levels of production of both H2O2 and malondialdehyde compared to TPPS after illumination. That result correlated well with strong cell death induction by photoactivated TPPC. Furthermore, reactive oxygen species-scavenging enzymes such as catalase, peroxidases, and superoxide dismutase were also strongly downmodulated in response to TPPC, while these enzymes were almost unchanged in response to photoactivated TPPS. To the best of our knowledge, this is the first study that took into account the whole story from interactions of porphyrins with a plant cell wall to their photodynamic activity inside the cells.