Forty-five years studying plant silicon and phytoliths: a personal retrospective

When Martin J. Hodson walked through the door of Dafydd Wynn Parry’s office at Bangor University in Wales on 20 October 1980, he had no idea that the encounter would define the next four and a half decades of his scientific life. Forty-five years later, Hodson, now of Oxford Brookes University, has published a sweeping personal review of plant silicon and phytolith research in the journal Plant and Soil, charting how a once-fringe discipline of a few isolated correspondents grew into a global enterprise with thousands of papers, nineteen books, and international conferences on nearly every continent. The review, published as an open-access Lambers Opinion Paper, blends historical analysis with memoir, offering a rare insider’s view of how an entire field of science assembled itself.

The subject of the review is, at first glance, deceptively narrow: silicon in plants. Silicon is the second most abundant element in the Earth’s crust, yet for most of the twentieth century it was regarded as biologically incidental. Plants absorb soluble silica, orthosilicic acid, from soil solution through their roots, transport it in the transpiration stream, and deposit it as amorphous silica in cell lumina, cell walls and intercellular spaces. These microscopic silica bodies, known as phytoliths, are remarkably durable, surviving in soils and sediments for hundreds or thousands of years after the plant dies. That durability has made phytoliths invaluable to archaeologists and palaeoecologists as proxies for past vegetation, agriculture and climate, while the physiology of silicon uptake has become central to agronomy, plant pathology and ecosystem science. Hodson’s review insists that these are two distinct sub-fields, one populated largely by plant scientists and the other by archaeologists, with only a small overlap zone that includes phytolith taxonomy, phytolith chemistry and the dissolution of silica in soils.

The early history Hodson recounts is one of remarkable isolation. From the 1950s through the 1970s, only a handful of researchers worldwide worked on plant silicon, often as the sole specialist in their country, communicating by post. Frank Smithson, a sedimentary petrologist working in Bangor, related opaline phytoliths in Anglesey soils to those in local grasses in 1956, a novel linkage at the time, and later introduced Dafydd Wynn Parry to the field. In France, François Bartoli demonstrated in the early 1980s that the biogeochemical cycling of silicon in temperate forests was driven not only by mineral weathering but substantially by plant uptake and phytolith dissolution, a conceptual shift that reshaped how scientists view the silicon cycle at the soil-plant scale. In Japan, Eiichi Takahashi dominated the field for four decades, showing that silicon was “agronomically essential” for rice and among the first to recognize silicon’s role in resistance to both biotic and abiotic stresses.

A particularly productive group formed at CSIRO in Australia, where George Baker, recruited into phytolith research by Smithson himself, drew in LHP Jones, Angela Milne and Kevin Handreck. Their 1967 review “Silica in soils, plants, and animals” in Advances in Agronomy has accumulated more than 1,000 citations, a striking figure for a plant silicon paper of that era. In the United States, Peter Kaufman’s group at the University of Michigan pioneered electron microscopy and x-ray microanalysis to map silica deposition in oat internodes, while archaeologist Irwin Rovner of North Carolina State University recognized in a seminal 1971 paper that phytoliths could be used in palaeoecological reconstruction much as pollen had been, effectively founding modern phytolith archaeology.

The technical heart of Hodson’s own early career lay in cryo-techniques and x-ray microanalysis. Working with Allan Sangster, first in Bangor and later in Toronto, Hodson used freeze substitution, cryo-scanning electron microscopy and x-ray microanalysis to localize silicon in wheat roots. Their 1989 paper in Protoplasma identified soluble silicon in the large central metaxylem lumina and in adjacent pericycle and parenchyma cells bridging toward the endodermis, and Hodson believes it remains the only study to have detected soluble silicon in plant material by x-ray microanalysis, since the approach typically captures only deposited, insoluble silica. Their earliest joint work with Parry, on silica in the inflorescence bracts of foxtail millet, touched on an unexpected medical question: whether sharp plant silica particles contaminating flour might contribute to oesophageal cancer in high-incidence regions. The idea faded after its leading proponent retired, but Chinese researchers have recently revived it, detecting plant-derived silica contamination in wheat flour as a first step toward assessing dietary exposure.

If the 1980s were a decade of careful anatomy, the 1990s brought intellectual synthesis and institutional structure. Emanuel Epstein, one of the towering plant scientists of the twentieth century, turned to silicon after his nominal retirement and produced the two most cited publications in the field’s history: “The anomaly of silicon in plant biology” in Proceedings of the National Academy of Sciences in 1994 and a comprehensive Annual Review article in 1999 that tops the citation list with 2,677 citations. Epstein, Hodson writes, had the rare gift of synthesizing sprawling, disparate literature into a coherent argument, and his handwritten note to Hodson shortly before his death at age 106 declared that “Si has a resurrection which it deserves.” The decade also saw the first dedicated international meetings: the 1st International Meeting on Phytolith Research in Madrid in 1996 and the 1st International Conference on Silicon in Agriculture in Florida in 1999, gatherings that transformed a scattered community into a self-aware discipline.

By Hodson’s assessment, the 2000s were the most consequential decade of all. The International Code for Phytolith Nomenclature, published in 2005 and updated as ICPN 2.0 in 2019, standardized the confusing patchwork of terminology that had plagued phytolith studies. In the same year, Parr and Sullivan proposed that carbon trapped within phytoliths could be harnessed for long-term soil carbon sequestration, an idea that launched a still-controversial research program on climate mitigation. But the decade’s landmark came in 2006, when Jian Feng Ma and colleagues published “A silicon transporter in rice” in Nature, identifying the first molecular silicon transporter, Lsi1, and ending decades of speculation about how silicon entered plant roots. Hodson calls it the most important experimental paper of the past 45 years; it is also the most cited, and subsequent work has revealed a family of influx and efflux transporters governing silicon uptake, xylem loading and distribution. Fittingly, Hodson’s own most cited paper, a 2005 phylogenetic analysis of silicon composition across plants published in Annals of Botany, appeared just one year before the transporter discovery and has passed 1,000 citations, its timing, he admits, accidentally perfect.

The 2010s deepened the molecular story. Research groups led by Rivka Elbaum and Alexander Lux clarified the two fundamental modes of silicification: deposition within cell lumina and deposition on carbohydrate matrices within cell walls. Work on sorghum showed that silica aggregates in root endodermis are predetermined by cell wall architecture, and in 2020 the protein Siliplant1 was shown to actively precipitate silica in sorghum silica cells. The decade also revived an idea John Raven had floated in 1983: that silicon might substitute for carbon in structural tissues, a cheaper mechanical strategy. The resulting literature on silicon-carbon trade-offs has proven fertile but contentious, with Hodson and colleagues arguing that whole-organ analyses obscure what happens at the cellular level, where silicon is distributed with striking heterogeneity. Meanwhile, the proposed “apoplastic obstruction hypothesis,” framing silicon as an extracellular prophylactic agent rather than an active intracellular one, reframed debates over how silicon protects plants from pathogens and stress, and Caroline Strömberg pushed phytolith analysis into palaeontology, extracting and classifying silica bodies from fossil deposits.

Looking at the present decade, Hodson identifies the field’s open frontiers. Carbon sequestration in phytoliths remains unresolved and, in his view, urgent: recent high-resolution microanalytical work suggests cell wall phytoliths may trap far more carbon than acid-digestion methods indicated, and some cell wall phytoliths persist in soils for centuries, but the true magnitude of this carbon sink is unknown. New transporters linking silicon delivery to deposition sites are only beginning to be identified, and machine-learning approaches to three-dimensional phytolith morphometrics are starting to identify cereal species from a handful of microscopic remains. The review also honors longevity, cataloguing 36 researchers with at least 30 years in the field, and highlights prize-winning early-career scientists from the 2025 conferences in Barcelona and Belgrade who will carry the work forward. For a field that began with isolated scientists exchanging reprints by mail, the trajectory Hodson documents is a quiet testament to how persistence, standardization and a single transformative discovery can turn a scientific backwater into a discipline with global reach, and with big questions still unanswered, not least why some plants silicify at all, and why so many do not.