HDR defense: Alexandre de Saint Germain

The harmonious development of a plant, as well as the successful completion of its life cycle, are made possible by the establishment of communication and signaling mechanisms, notably via chemical compounds known as specialized metabolites. Phytohormones, which are endogenous chemical signals, facilitate communication between tissues and organs and, in particular, enable the plant to coordinate the development of its various parts. Furthermore, the plant’s developmental cycle is controlled by exogenous chemical signals that enable it to optimize its growth in response to the organisms surrounding it; this is known as allelopathy. However, the mechanisms underlying the perception and biosynthesis of these compounds are only partially understood.

Throughout my career, I have focused on unravelling the biosynthesis and perception of strigolactones, a family of molecules that are unique in that they act both as plant hormones – notably by controlling plant branching – and as allelopathic compounds released into the rhizosphere by the roots. The release of these molecules stimulates the germination of parasitic plant seeds (Striga and Orobanche) and also promotes the establishment of arbuscular mycorrhizal symbiosis.

During my PhD and two postdoctoral positions, I focused on the strigolactone signaling pathway in vascular plants such as the pea and Arabidopsis thaliana, as well as in the moss Physcomitrium patens. I was thus able to identify several key players in strigolactones transduction and elucidate the mechanism by which this hormone is perceived in these species. I also investigated the mechanism by which parasitic plants such as Phelipanche ramosa and Orobanche cumana perceive strigolactones, with a view to developing strategies to control these pests.

Following my recruitment as a researcher at the IJPB, I launched a project aimed at identifying new allelopathic compounds exuded by roots, and the genes involved in their biosynthesis. To this end, I use multidisciplinary approaches combining tools from biology/genomics and analytical chemistry. Thanks to work on the model plant Arabidopsis thaliana carried out by a PhD student I supervised; we were able to identify two families of molecules potentially involved in plant-plant interactions. More recently, with a second PhD student, we have developed new genome-editing tools in peas, with a view to identifying the genetic and molecular determinants governing branching in this legume.

My research project is structured around four axes:
(1) the development of genome-editing tools in legumes to
(2) study the mechanism controlling branching in these species. I also aim to
(3) continue characterizing the perception of strigolactones and other allelopathic compounds that stimulate germination, and
(4) understand the molecular and genetic basis of allelopathy using Arabidopsis and services plants. Understanding these mechanisms could be used in agriculture to develop new agroecological approaches, particularly to limit the use of herbicides.

Jury members
> Julia Buitink (Rapportrice) -  INRAE, IRHS Angers, Beaucouzé 
> Patrick Achard (Rapporteur) -  CNRS, IBMP, Strasbourg 
> Sophie Rahuel-Clermont (Examinatrice) -   CNRS, IMoPA Nancy, Vandœuvre-lès-Nancy
> Philippe Delavault (Examinateur) -Nantes Université, US2B, Nantes
> Delphine Moreau (Examinatrice) -  INRAE, AGROECOLOGIE, Dijon
> Erwan Poupon (Examinateur) - Université Paris-Saclay, BioCIS, Orsay

Contact, Alexandre de Saint Germain, "Strigolactones and Allelochemicals Signaling" SAS team
Habilitation to supervise research, Université Paris-Saclay


Research developed at the Institute Jean-Pierre Bourgin for Plant Sciences.