Towards a photo-herbicide: synthesis and studies of water-soluble fluorescent tagged porphyrins, from solution to plant cells – PorphyPlant

Two families of molecules were synthesized and studied during the project. For each of them, checking their structures and purity were carried out mainly using mass spectrometry and nuclear magnetic resonance. Their photo-physical properties arising from their interaction with light has been precisely established, as well as their ability to produce reactive oxygen species, toxic species for plants, by spectroscopic studies. On the 1st Family (non-water soluble tagged porphyrins), these studies were supported by a theoretical analysis of their structure and optical properties. For the 2nd family (water soluble untagged porphyrins substituted by charged groups), their behavior relying on the acidity of the medium and their stability over time were studied in order to mimic experimental conditions required for TBY-2 tobacco cells. Their effects on plant organisms (cells and seedlings) were assessed by viability studies and enzymatic assays. Their visualization in the cells has also been performed by confocal microscopy.

These studies have established a relationship between structure and activity. Although the role of charges (positive / negative) and their efficiency depending on the biological model (seedling / cell) is not fully understood yet, it represents a major scientific issue. As this is a fundamental project, some applicative scientific issues have to be overcome (formulation, systems’ fate in the ground...) before emerging in agronomy. A strong and complementary collaboration between Chemistry-Plant Biology have also been initiated and must be pursued.

The expectations defined at the beginning of the project were almost fully achieved, except for the in situ and in real-time visualization of singlet oxygen production.
Concerning the charged porphyrins, the investigations went beyond expectations and a first structure-activity relationship was established. However, water solubilization remains a major drawback and prevents the study of tagged systems in plant cells. In this context, water solubilization has been envisaged through encapsulation.
Moreover, encouraged by first results on tomato plantlets, the project will be pursued for a new application in Antimicrobial Photodynamic treatment for agronomic purpose.
This project has been a fascinating adventure, which has linked a fundamental understanding to real-world applications. Indeed, the comprehensive understanding of the optical properties of the new synthetized dyads under various environmental conditions has appeared crucial to allow its usage in plant cells. From this solid knowledge, we can now better guide syntheses to develop novel efficient (porphyrin-based) photoherbicide.

Three peer-reviewed articles have been accepted during the project, concerning its different parts: studies of the effects of charged porphyrins on tobacco TBY-2 cells (C. Riou et al., Photochem Photobiol Sci, 2014 621-625), the seedlings (Guillaumot D. et al., J. Photochem. Photobiol. B Biology, 2016, 156, 69-78) and tagged porphyrins (Rezazgui O. et al., New J. Chem, 2016, DOI: 10.1039/C5NJ02901E). This work has also been the subject of six oral communications and two posters in national and international conferences.