IJPB Master 2 proposals

Two subjects in the team

> Climate Change: A Challenge for Our Understanding of Plant Growth

Supervisor: Alexis Peaucelle, contact

We might ask ourselves if we have enough knowledge about how plant cells grow to face the challenge of climate change. Indeed, the physiological analysis of plant growth has rarely considered the effect of temperature. The reference model for plant growth since the 1940s proposes that they grow by extending the pectocellulosic wall under the action of turgor pressure. The control of the rate and direction of growth would depend on the mechanical and rheological characteristics of the wall. However, alternative models have been proposed in the past decade, notably giving the wall an active role in the process. It is time to explore how exactly temperature influences growth in plants.
The growth rate of plants is very sensitive to temperature. The growth rate can double when the temperature increases from 19.5 to 21 degrees for the hypocotyl growing in the dark. A classical explanation links the growth rate to the overall metabolism of plants. However, the differential growth of tissues following thermal stimuli (thermo-nasty), as observed in flower petals (tulip model), suggests that the effect of temperature on growth is complex and deserves thorough study. Our team specializes in studying the physiological events involved in plant growth. We develop tools to dissect all stages of growth and observe in real-time the chemical changes correlated with growth. Our tools have been developed using the model of the semi-isolated cell: the root hair.
Thus, we are able to determine the direct and indirect effects of a stimulus such as temperature on growth. In this research project, we propose to study the temperature-sensitive growth events in plants, using the model of the root hair of Arabidopsis thaliana, as well as in the context of thermo-nasty in tulip petals. For each effect, we will seek to establish whether there is a correlation between the temperature change and the physiological modifications.

> The role of pectin métabolism in plant cell wall assembly and expansion

Supervisors: Kalina Haas, contact & Herman Höfte, contact

This project is part of a program in the host laboratory to use Arabidopsis root hairs to understand the role of pectin metabolism in the control of cell expansion. Pectins are abundant charged multiblock polymers in the plant cell wall characterized by the presence of D-galacturonic acid (D-GalA). Homogalacturonan (HG) is a linear a-1,4-linked D-GalA polymer, which is synthesized in a methylesterified form and demethylesterified in the cell wall by pectin methylesterases (PME). The latter exposes negative charges, which dramatically change the physicochemical properties of the polymer and the cell wall. Our team has observed that HG demethylesterification promotes cell expansion and organ growth in a number of cell types and contexts ^{(1, 2)}. How this affects growth is not exactly understood. In theory this change can lead to an increased hydration of the cell wall, which may drive expansion directly or indirectly by favoring the mobility of wall loosening proteins. In addition, HG demethylesterification may promote HG turnover since the plant-derived HG-degrading enzymes endo-polygalacturonases (EPG) and pectate lyases (PL) require demethylesterified substrates, which somehow may increase the extensibility of the wall. Finally, the effect on growth can also be indirect for instance through the release of biologically active oligogalacturonides that may act on cellular growth-controlling processes. The host team also observed in root hairs that demethylesterified HG forms a network in the cell wall through its binding to a complex consisting of a cationic peptide (RALF22) and the cell wall protein Leucin Rich repeat eXtensin (LRX)1, which promotes the condensation of the pectin. In a loss of function mutant for the RALF22 peptide, no LRX1-RALF22-pectin network is formed, and pectin levels in the cell wall are greatly reduced. This suggests reduced synthesis and/or increased turnover of pectins that escape incorporation into the LRX1-RALF22-pectin network ^(3,4).
The candidate will study the role of the 2 classes of cell wall pectin-metabolizing enzymes in root hair growth. CrispR-Cas9 mutants for root hair-specific PMEs and PLs will be analyzed in detail for root hair growth characteristics, cell wall composition and signaling. In addition, the biochemical activity of recombinantly produced enzymes will be analyzed in vitro.

References
1. Haas, K.T., Wightman, R., Meyerowitz, E. M. and Peaucelle, A. Science (1979). 367, 1003–1007 (2020)
2. Haas, K.T., Wightman, R., Peaucelle, A. and Höfte, H. The Cell Surface, 100054 (2021)
3. Moussu, S. et al. Science 382, 719-725 (2023)
4. Schoenaers, S. et al., Nature Plants 10, 494-511 (2024)


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