Plasmodesmata: genetic control of cell-to-cell communication during plant defence

Like animals, plants suffer all manner of diseases caused by various fungal, bacterial and viral pathogens. When a fungal or bacterial pathogen attacks a plant, the battlefront occurs initially at a single cell. One means by which a plant can fight off the pathogen is to initiate a process that results in death of the cell that is being attacked, preventing the pathogen from gaining nutrients from this cell and spreading. This process involves the generation of many toxic molecules and it is crucial that the cells surrounding the attacked cell do not suffer the same fate.

Plant cells are connected to their neighbouring cells by channels called plasmodesmata. These channels are like open doorways into neighbouring cells and small molecules can simply pass from cell to cell. The question then is how a dying cell stops toxic molecules leaking out into its neighbours? Does the cell close the doors into its neighbours? We know that plasmodesmata can open and close to control the movement of signals between cells when a plant is exposed to cold or when the plant undergoes a developmental transition. We want to investigate whether these doors close when a pathogen attacks and how this happens.

Recently, many new proteins that are located at plasmodesmata have been identified. One of these is a receptor for fungal chitin and causes plasmodesmata to close when a fungus is detected. I will examine why plasmodesmata close when a pathogen is perceived by microscopic analysis of living tissue that is under attack from different pathogens. I will also determine how plasmodesmata close by biochemical and genetic identification of proteins that required for this response.

By establishing when and how plasmodesmata open and close when a pathogen attacks we will be able to understand a very important mechanism that a plant uses to fight disease. The last objective of this programme will begin to explore this phenomenon in rice with the aim of understanding how this global food crop uses this mechanism to fight off pathogens. Understanding these mechanisms will enable us to exploit them to help plants fight disease, placing us in a strong position to combat the impact of increased prevalence of plant disease that is likely to arise from global climate changes.

A comprehensive understanding of the complexities of plant-pathogen interactions is crucial to the fight against the increased spread and severity of plant disease posed by a changing global climate. When a pathogen is present, whole tissues and organs must respond to the threat of invasion in a co-ordinated fashion. Most plant cells are connected to their neighbours by dynamic channels called plasmodesmata (PD) which allow and regulate the flux of molecules and information between cells.

I have observed that the pathogen associated molecular patterns (PAMPs) chitin and the flagellin derivative flg22 induce a reduction in molecular flux via PD. Chitin and flg22 also induce dramatic changes in the concentration of small molecules such as reactive oxygen species and calcium ions within the attacked cell and therefore it is possible that an attacked cell isolates itself to concentrate these essential defence molecules at the site of attack. To test this hypothesis I will employ live cell imaging, using a variety of inert and biologically relevant probes, to map the generation of symplastic domains around an infection site.

Chitin-triggered changes to molecular flux are mediated by the plasmodesmata-located protein LYM2. lym2 mutants do not respond to chitin with respect to changes to molecular flux and thus will allow comparative analysis. Further, interactors of LYM2 will be identified to dissect the mechanism by which PD close following chitin perception. For LYM2-interactors, and other PD-located defence associated proteins, overexpressors and genetic knockouts will be analysed with respect to PAMP-triggered PD closure and pathogen susceptibility to determine their role in the regulation of molecular flux. With an outlook to translating this research to agriculturally significant species I will also examine PAMP-induced changes to molecular flux in rice and determine which of the five member rice LYP protein family associates with plasmodesmata.

With the threat of increasing carbon dioxide levels and temperatures across the globe, the relationships between plant pathogens and their hosts are poised to change in ways that will increase the prevalence and severity of plant disease. By impacting agricultural and native species, plant diseases manifest as disasters such as famine and environmental damage. Biotic pathogens cause huge agricultural losses each year and the security of these and related industries is dependent in part on more complete understanding of the interactions that cause disease and the applications of this knowledge.

Agriculture is dependent on the use of chemicals for the management of pathogens. Many chemicals have limited longevity due to the emergence of resistance, and public opposition to chemicals is also increasing. By investigating a poorly described strategy for defence responses, a long term benefit from this research programme will be the novel approaches for the generation of pathogen resistance. This could be through the selection of improved crops or via genetic modification (GM) technologies. Although public opinion relating to this technology is currently divided, it still offers valuable possibilities for generating pathogen resistance, and novel developments allow genetic modifications in the absence of transgenes and selection markers.

This research will accordingly have benefits for a wide cross-section of society. The direct benefits will lie with the agricultural industry: primarily farmers and plant biotechnology companies. The research outputs will also be of benefit to policy makers and the general public, from schools through to general interest groups, given the social relevance of discussion around the optimal paths (GM or selected breeding) for delivering novel traits in crops. The PI has significant experience in delivering scientific output to the general public and will engage with Oxford Brookes University science outreach activities such as the Science Bazaar and the annual Oxfordshire Science festival, as well as with organisations such as Science Oxford and the Science Media Centre.

Results from this project will be first communicated in original research articles, presented at national and international conferences and then summarized in more general publications. The PI and PDRA will submit abstracts for poster or oral presentation at scientific meetings (e.g. International Plasmodesmata Conference 2014, Molecular-Plant Microbe Interactions Congress 2016). The PI and PDRA will also present this research regularly within departmental seminar programmes and research group lab meetings. The PDRA will be involved in all relevant collaborations and be able to meet national and international visitors to the site.

The PI will have overall responsibility for ensuring the delivery of this impact plan. The PDRA will be expected to contribute substantially to these activities and will be given appropriate training.