Physiology and biochemistry of the softening mechanism in ripening fruits: fundamental and commercial approaches

Rationale. This project comprises novel research that is of both fundamental and industrial interest. The softening of ripening fruit involves dramatic changes in cell-wall structure. In many species, softening is important because it makes fruit pleasant to eat. On the other hand, excessive softening is detrimental, causing wastage on the supermarket shelf and making crops more vulnerable to pathogens. Slowing the softening process, e.g. in raspberries, would cause less waste when UK crops peak; imported crops, e.g. from Southern Europe and Morocco in winter, could go further; and opportunities would be raised for summer UK exports. Slow-softening fruits would bring less disease and longer picking intervals.

Preliminary data from the applicants’ laboratories. Professor Fry’s laboratory in Edinburgh is elucidating novel mechanisms by which ripening fruits soften. This work draws on many years’ experience of cell-wall biochemistry. Collaborators at Edward Vinson Ltd. (Kent) are commercial fruit breeders, and have a molecular breeding pipeline, e.g. in strawberry, raspberry and blackberry. We aim to link the genetic markers (Edward Vinson) to physiological/biochemical processes (Edinburgh), benefiting future breeding efforts. Edward Vinson Ltd. is developing a molecular map for raspberry.

Proposed new research
Available genotypes differ in firmness, e.g. recently developed commercial genotypes versus ‘heritage’ varieties. We will explore the relationship between their genetics and their cell-wall biochemistry. In Fry’s laboratory, the student will develop convenient new tests for cell-wall ‘softening markers’, including:
• vitamin C (ascorbate) secretion and turnover;
• hydroxyl radicals non-enzymically attacking cell-wall polysaccharides in vivo;
• solubilisation and partial degradation of pectic polysaccharides;
• specific pectic enzyme activity and/or action in living fruit—
o pectin methylesterase
o pectate lyase and rhamnogalacturonan lyase (recent evidence suggests these enzymes participate in fruit ripening)
o endopolygalacturonase
o xyloglucan endotransglucosylase and endo-hydrolase (XTH)

Student’s role: initially working under close guidance. The student will explore the physiological/biochemical processes occurring in softening fruit cell walls. He/she will develop novel, simplified assays for these processes suitable for routine use by plant breeders. This study will employ chromatography and electrophoresis, with detailed practical guidance. Training/experience will also be provided in preparing seminars, posters, reports and manuscripts; and undergraduate demonstrating.

Student’s role: subsequent opportunities for independent exploration. Later, depending on initial findings and personal interests, the student will have the opportunity to devise the best ways forward to fully characterise the processes. This would include in-vivo radiolabelling in Fry’s laboratory and NMR spectroscopy and mass spectrometry under Professor Uhrín’s supervision.

Industrial liaison. The student will be encouraged to put into practice the novel assays he/she has developed, applying them in the fruit breeding laboratory at Edward Vinson Ltd.; he/she will work closely with the molecular breeder and use his laboratory on-site. The student will also look at the novel softening markers in breeding material in the field, potentially linking field measurements of firmness with shelf-life. The period(s) of industrial liaison will give the student insight into the opportunities and practicalities of commercial applied biology.

The student will be mainly based in the Edinburgh Cell Wall Group (ECWG); please see http://fry.bio.ed.ac.uk// Currently the ECWG comprises Professor Stephen C. Fry (Principal Investigator), 3 BBSRC-funded postdocs, 5 PhD students (all within years 1–4; funded by various sources, including self-funding, University of Edinburgh/CelluComp co-funding, a Commonwealth Scholarship, the Omani Ministry of Manpower, and Mars Chocolate Ltd.), and 1 BBSRC-funded technician. Their research projects focus on plant cell-wall biochemistry/physiology and are thus of direct relevance to the present project.
The student will also have the benefit of a second academic supervisor, Professor Dušan Uhrín (Department of Chemistry, The University of Edinburgh), who will collaborate on nuclear magnetic resonance (NMR) spectroscopic analysis of interesting chemical components of fruit relevant to the softening process. This will therefore be an interdisciplinary project.
In addition, the student will spend at least 3 months at Edward Vinson Ltd., in Kent, supervised by the industrial collaborator (Dr Graham J.J. Clarkson).

If your PhD will be self-funded, or you’re a strong candidate for a Scholarship [e.g. Darwin Trust, Commonwealth Scholarship, or a stipend from your Government], you are welcome to apply for this project. In such cases, the project would receive partial funding for laboratory consumables from Edward Vinson Ltd., who would also pay the student’s additional expenses during his/her time in Kent.