Engineering plant cell walls beyond the digestive range of a pest’s bacterial symbiont

About the Project

Most herbivorous insects cannot digest a leaf unaided. Pectin, the polysaccharide that cements plant cell walls together, is a particular obstacle, and many beetles overcome it by carrying bacteria that supply the enzymes they lack. Tortoise beetles are an extreme case. They encode no pectin-degrading enzymes of their own and depend entirely on a single bacterial symbiont, Stammera, which they pass from mother to offspring on the surface of each egg. Deprived of it, larvae cannot process foliage and rarely reach adulthood. Stammera has a drastically reduced genome and supplies only a handful of enzymes, and we have shown that which enzymes it carries determines which plants its host is able to eat. This project asks whether that dependency can be turned into a plant protection strategy, by modifying a plant cell wall so that it falls outside the range of substrates the symbiont's enzymes can process, leaving the insect unable to feed on a plant it would otherwise destroy.

Our earlier work characterised the symbiont's principal enzyme and showed that its activity collapses against pectin that is heavily methylesterified, a single modification that plants make to their own cell walls as a matter of course. The student therefore begins with a defined number to engineer towards. The goal is to reach that point in a living plant, using a diploid wild relative of sweet potato, while developing knowledge transferable to crops where this symbiosis is associated with pests of agricultural importance.

The project will address three key questions:

• Does the enzyme's known limit hold across beetle species, and does the fine structure of pectin matter as much as its overall chemistry?
• Can leaf pectin be pushed past that limit without compromising the plant's own growth?
• Does an insect carrying a fully functional symbiont lose its digestive advantage on such a plant?

The project combines plant genetic engineering, cell wall analytical chemistry, protein biochemistry, microbial genomics and insect experimental biology. Because the enzyme's limit is already established, the plant work begins at the outset. The student will raise the methylesterification of leaf pectin using CRISPR/Cas9 editing and targeted gene overexpression, generating a graded series of lines spanning the known threshold, with every line characterised by cell wall chemistry and antibody labelling to confirm the modification is present in the tissue the insect actually eats. Running alongside this, the symbiont's enzyme will be expressed from several beetle species to test whether the published threshold holds across the subfamily and under gut conditions, and to ask whether the arrangement of chemical groups along the pectin chain matters as much as their number, a question with direct consequences for how far the plant must be pushed. Engineered and control plants will then be fed to beetles reared with and without their symbiont, measuring survivorship, development, feeding rate, gut enzyme activity and symbiont abundance across the series. A final component tests how durable such a trait could be, using an established embryo manipulation protocol to transfer symbionts between beetle species and asking whether a symbiont carrying additional enzymes can restore digestion on engineered plants.

The supervisory team brings together complementary expertise in symbiosis, microbial genomics and plant cell wall biology. The primary supervisor, Dr Hassan Salem (John Innes Centre), works on the evolution and function of digestive symbioses in herbivorous insects. His group established the Stammera system, sequenced the symbiont's genome across the beetle subfamily, and carried out the enzyme characterisation on which this project is founded. The co-supervisor brings an established programme in pectin chemistry and plant cell wall engineering, and will host the plant modification and analytical work.

Interested students are encouraged to get in touch with the supervisory team before the application deadline, directly by email (hassan.salem@jic.ac.uk), to discuss the project and their suitability. Applications should include a CV and a short motivation letter explaining the applicant's interest in the project, how their background and interests align with the research, and why they consider themselves well suited to undertake the PhD. We particularly welcome applications from students who are curious, comfortable working across disciplines, and interested in the possibility that the best way to protect a plant from an insect may be to target the microbe that insect depends on.

Funding Notes

The studentships provide annual stipends above the BBSRC minimum level (£26,212 for 2027/28) plus an annual payment of £5,500 for department laboratory fees, and £700 for books, conferences and travel. Tuition fees are also paid by the Foundation.

The studentship holder will be expected to spend six months during their 3rd or 4th year at another university/institute to gain additional experience, and can claim up to an additional £3,000 for extra expenses. The final award of the grant is conditional on the student obtaining a first or upper second class degree before the start of the Sainsbury PhD Studentship.

References

Salem, H. et al. (2017) Drastic genome reduction in an herbivore's pectinolytic symbiont. Cell 171, 1520–1531.
Salem, H. et al. (2020) Symbiont digestive range reflects host plant breadth in herbivorous beetles. Current Biology 30, 2875–2886.
García-Lozano, M. et al. (2024) Paleocene origin of a streamlined digestive symbiosis in leaf beetles. Current Biology 34, 1621–1634.
Pons, I. et al. (2026) Fidelity in co-diversified symbiosis. Nature Communications 17, 1644.
García-Lozano, M. et al. (2026) Yellow protein co-opted to sustain obligate symbiosis in leaf beetles. Nature Communications 17, 9446.