Seminar and PhD defense

> Dr Hugues Renault, IBMP, Strasbourg, seminar 10 am
A glimpse of plant adaptation to land through the biopolymer lens
Upon transition from water to land half a billion years ago, pioneer plants were exposed to the challenging terrestrial conditions. The ability to build extracellular protective barriers has most likely been a critical adaptation of land plants, as they shield cells from damaging environmental insults and allow the formation of specialized structures required for water management (e.g., cuticle). In vascular plants (i.e., tracheophytes), these barriers are essentially comprised of four hydrophobic biopolymers – cutin, suberin, sporopollenin and lignin – that reinforce and waterproof the cell wall. Based on angiosperm models, the understanding of genetic, biochemical and cellular determinants of biopolymer-mediated apoplastic barrier formation has made tremendous progress over the last decade. However, the evolutionary mechanisms that led to the emergence of land plant biopolymers are still unknown. To fill this gap, we implement multidisciplinary research in bryophytes, a group of non-vascular land plants reportedly able to produce only two biopolymers - cutin and sporopollenin. The presentation shall illustrate the power of this approach to elucidate the origin and early functions of biopolymers in the context of plant adaptation to land.

Invited by Catherine Rameau contact to attend to the seminar

> Florent Cornet, SAS team, PhD defense, 1:30 pm
Identification and characterization of cytochromes P450 involved in the structural diversification of strigolactones
Strigolactones (SL) are plant hormones that repress shoot branching. They are also known for their activity in the rhizosphere, in particular to stimulate symbiosis with endomycorrhizal fungi. More than thirty natural SL with diverse structures have been characterized and plants can synthesize a cocktail of many SL, sometimes specific to plant families or species. The biosynthetic pathways of SL begin with the successive conversion of trans-ß-carotenes via three enzymes. These steps, apparently common to all vascular plants, constitute what is known as the "CORE PATHWAY". The rest of the biosynthetic pathway, on the other hand, diversify and seems to vary greatly between species. However, current knowledge of the biosynthetic pathways does not explain the full structural diversity of SL. How can plants produce such structural diversity in SL? Which enzymes are responsible for this diversification?
My thesis project aimed to better understand the genetic basis responsible for this structural diversity in SL. To do this, I used transcriptomic results to identify candidate genes that are repressed by SL treatment. I tested and invalidated the hypothesis of the involvement in the SL biosynthetic pathway of the CYP450, KLUH. This study highlighted the impact of SL on the modulation of plastochron via the transcriptional regulation of KLUH in Arabidopsis. The second part of my thesis focuses on the two MAX1 homologs in pea. We have obtained several mutants named Psmax1s by TILLING approaches. We have performed in vitro enzymatic tests and in planta SL quantifications to investigate the biochemical and biological functions of the two pea MAX1 enzymes. Unexpectedly, single and double mutants do not show the high branching phenotype characteristic of SL mutants. However, these Psmax1s are deficient in the known canonical SL present in pea. Our results suggest that the branching inhibitory signal is not one of the SL already described so far in pea and that the real branching inhibitor signal remains to be discovered