PhD offer (M/F) - Why do plants make rock? Ecological drivers of silica accumulation in Mediterranean herbaceous species

Job Description

Thesis Subject

Why do plants make rock? Ecological drivers of silica accumulation in Mediterranean herbaceous plants

The project
Plants exhibit a range of growth, survival, and reproductive strategies that allow them to optimize their fitness. Among these strategies, biomineralization both fascinates and puzzles scientists (He et al., 2014; de Tombeur et al., 2023). This process—encompassing all the ways plants form mineral structures within their tissues—is particularly widespread in the case of silica (SiO₂) deposits.
These deposits of amorphous silica in cell walls, comparable to sand, have multiple effects on plant physiological functioning while also influencing numerous biogeochemical cycles. Among the functions attributed to silicification, the mitigation of various biotic stresses (e.g., herbivory) and abiotic stresses (e.g., water stress) is frequently reported (Cooke & Leishman, 2016; de Tombeur et al., 2023; Johnson et al., 2024).
Over the past 20 years, active research on silicon (Si) in plant physiology has improved our understanding of variation in silicification among plants, as well as the potential functions associated with this process (Coskun et al., 2019; Quigley et al., 2020; de Tombeur et al., 2023; Pang et al., 2025). However, many questions remain. In particular, there has been a rise in hypotheses regarding ecological factors that influence Si concentrations—both between and within species—(herbivory, water stress, temperature, etc.) without a general framework emerging (de Tombeur et al., 2023).
The main objective of this thesis is to better understand the ecological determinants of silica accumulation in herbaceous plants. To address this broad question, the thesis will be structured around two main axes.
1 — Determine how disturbance and resource levels affect silicification in herbaceous species of Mediterranean grasslands.
This main objective will be addressed through a long-term monitoring study located on the limestone plateau of the Causse du Larzac: the La Fage Experimental Unit (INRAE). At this long-term study site, fertilization and sheep exclusion (exclosures) experiments were established in 1978 and updated in 2012. This makes it possible to evaluate the influence of resource availability and the presence of primary consumers on community structure and ecosystem functioning over time. To date, more than 45 years of data have been collected at this site.
Testing how these treatments have affected Si concentrations in plants—both at inter- and intraspecific levels—and examining potential links with other functional traits involved in growth and defense will be one of the first steps of this thesis. Fine-scale measurements of the local environment (soil, microclimate, etc.) will also be considered.
2 — Determine the costs and benefits of silicification in herbaceous species of Mediterranean grasslands.
The still limited evaluation of the costs and benefits associated with silicon (Si) is one of the main obstacles to understanding its variability and role in plants (de Tombeur et al., 2023). In a second axis, we will refine these costs and benefits through a multifactorial experiment under controlled conditions (Si fertilization, NPK fertilization, presence/absence of herbivores, etc.).
This experiment will be conducted using a comparative approach across different Mediterranean species. However, the recent identification of strong intraspecific variability in Si content in Brachypodium distachyon suggests the possibility of using this model species to test certain hypotheses, depending on the interests of the future PhD student. Working at the intraspecific level would allow deeper exploration of potential eco-evolutionary mechanisms linked to Si accumulation.
3 — Determine the mechanisms controlling intraspecific variability of silicon (Si) in plants.
The final component will assess whether silicification constitutes an adaptive trait in specific environments (e.g., stressful conditions). To do so, we will test whether ecotypes originating from contrasting environments accumulate different amounts of Si, both in the field and under controlled conditions. The choice of species remains open for discussion.
In addition to “classical” analyses of Si concentrations in plant tissues, we propose using advanced microscopic imaging techniques (X-ray microtopography) across the different components to analyze variations in cellular allocation of silica depending on species, treatments, and environments of origin. A meta-analysis of the literature may also be considered.
Overall, by combining different tools, concepts, and scales, this thesis will help overcome key barriers in our understanding of silica accumulation in plants. The project will enable the future student to gain experience with a range of tools, scales, and concepts in functional ecology and ecophysiology.
Keywords: biomineralization; plant growth–defense trade-offs; biodiversity; ecological strat-egies; environmental disturbances; eco-evolutionary mechanisms

Further reading
de Tombeur et al. 2023 (Trends in Ecology and Evolution) | Quigley et al. 2020 (Ecology) | Cooke et al. 2011 (Trends in Plant Science) | Coskun et al. 2019 (New Phytologist) | Johnson et al. 2024 (Ecology Letters) | de Tombeur et al. 2021 (Ecology Letters)

References
Cooke J, Leishman MR. 2016. Consistent alleviation of abiotic stress with silicon addition: a meta-analysis. Functional Ecology 30: 1340–1357.
Coskun D, Deshmukh R, Sonah H, Menzies JG, Reynolds O, Ma JF, Kronzucker HJ, Bélanger RR. 2019. The controversies of silicon's role in plant biology. New Phytologist 221: 67–85.
He H, Veneklaas EJ, Kuo J, Lambers H. 2014. Physiological and ecological significance of biomineralization in plants. Trends in Plant Science 19: 166–174.
Johnson SN, Waterman JM, Lagisz M, Nakagawa S. 2024. Plant Silicon Defences Suppress Herbivore Performance , but Mode of Feeding Is Key. Ecology Letters: 1–10.
Pang Z, Tombeur F de, Hartley SE, Zohner CM, Nikolic M, Violle C, Mo L, Crowther TW, Guan D-X, Luo Z, et al. 2025. Convergent evidence for the temperature-dependent emergence of silicification in terrestrial plants. Nature Communications 16: 1155.
Quigley KM, Griffith DM, Donati GL, Anderson TM. 2020. Soil nutrients and precipitation are major drivers of global patterns of grass leaf silicification. Ecology 101: 1–10.
de Tombeur F, Raven JA, Toussaint A, Lambers H, Cooke J, Hartley SE, Johnson SN, Coq S, Katz O, Schaller J, et al. 2023. Why do plants silicify? Trends in Ecology and Evolution 38: 275–288.

Your Work Environment

Candidate profile
We encourage applications from candidates holding a Master's degree or an engineering de-gree in plant sciences, agronomy, ecology, environmental sciences, or related fields.
The candidate should be comfortable with data analysis in R and interested in working at the interface between functional ecology, biodiversity science, and plant physiology. The PhD includes fieldwork—particularly for the first research axis—as well as controlled experiments and laboratory work. Depending on the candidate's background and interests, it will also be possible to incorporate a biogeochemical perspective into the project.
Good organizational skills, scientific curiosity, and the ability to work both independently and collaboratively are essential. A strong command of written and spoken English is required.
What we offer
The successful candidate will join the research unit Centre d'Écologie Fonctionnelle et Évolu-tive (CEFE) in Montpellier, France. They will be enrolled in the Gaïa Doctoral School and employed by the CNRS for a duration of 36 months, starting on October 1, 2026. The CEFE provides a stimulating research environment and access to the technical infrastructure required for the project.
The PhD will be supervised by Félix de Tombeur, Sylvain Coq, and Cyrille Violle at CEFE, where most of the work will take place. The project is funded by the French National Re-search Agency (ANR) as part of the PhytoStone project—“Understanding the ecological mechanisms underlying variation in silicon in plants”—awarded to F. de Tombeur as a junior researcher.
The project will involve close collaboration with a postdoctoral researcher, a laboratory techni-cian, and an international network of grassland observation sites (NutNet). Operating funds are also included.

Please submit a resume, a cover letter describing your interest in and relevant experience with the subject, your available college transcripts, and the contact information for two references who can provide letters of recommendation. You must combine these documents into two PDF files, as the portal only allows you to upload two attachments.

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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