Agriculture faces many challenges, including a warming climate, more frequent occurrence of extreme climate events and increased incidence and severity of crop diseases. Control of plant diseases with major resistance genes has not always proven durable and application of crop protection chemicals is becoming problematic with the development of pathogen insensitivity to the chemicals, as well as increased regulation.
Pathogens and pests, both established and newly emerging, represent major constraints to sustainable crop production. Crop losses due to biotic stresses amount to over 25% globally and present a major barrier for addressing the United Nations SDGs ‘No Poverty’, ‘Zero Hunger’, ‘Good Health and Well-Being’ and ‘Responsible Consumption and Production’.
This project (JHI-B1-1), funded by the Scottish Government through the Rural and Environment Science and Analytical Services (RESAS) Division, brings together scientists at the James Hutton Institute working on plant pathogenic nematodes and oomycetes to develop a deeper understanding of pathogen biology to enable exploitation of potential weaknesses for developing future disease control in the important Scottish crops, potato and soft fruit.
Stage
Work in ProgressDirectory of Expertise
Purpose
Fundamental research into the molecular processes of infections reveals how disease occurs with different pathogens, how plants try to defend themselves, and how pathogens overcome the plants’ immune systems.
We are focussing our efforts on potato late blight caused by Phytophthora infestans, the potato cyst nematodes (PCN) Globodera pallida and G. rostochiensis, and raspberry root rot caused by Phytophthora rubi. This research reveals which proteins are essential for the pathogens to infect and allows us to identify weak spots within the pathogen that we can target with new control measures.
A major focus of this project is on pathogen and pest secreted effector proteins which act to subvert plant immune responses and are essential to drive disease development. Some effectors can also be recognised by resistance proteins in plants which triggers a resistance response and protects the plants.
Work in previous Strategic Research Programmes (SRPs) has developed extensive genetic resources for the pathogens that we study and revealed that each pathogen deploys hundreds of effectors to cause disease.
While many effectors have been identified, there are also many that are yet to be discovered, potentially revealing novel facets of pathogen infection. Furthermore, there are other aspects of pathogen and pest biology that are essential for disease to occur, such as mechanisms by which effectors are delivered to plant cells, which remain to be determined.
In this project we are specifically examining:
- The infection cycle and biology of raspberry root rot, a currently understudied disease causing significant economic losses
- How dormant pathogens like PCN link their life cycle to the presence of a host plant
- Variation within pathogen effectors
- Mechanisms driving pathogen entry into plants
- Identification of effectors that are essential for pathogen infection
Results
Raspberry root rot infection dynamics and effector diversity
Phytophthora rubi is the major cause of root rot in raspberry crops (Figure 1), causing long term contamination of soil, even after infected plants have been removed. The industry has transitioned to over 70% raspberry growth in substrate due to soil contamination with P. rubi rendering fields unsuitable for raspberry cultivation.
Figure 1. Root rot of raspberry caused by Phytophthora rubi. Infected canes have a dark disease lesion (arrow), show stunting and wilting symptoms.
Using fluorescently tagged isolates we have discovered that P. rubi infection of raspberry roots occurs rapidly. Establishment of infection of roots occurs within 7 days, and the next generation of infectious propagules are released within 14 days (Figure 2). Oospores, the survival structures that can cause long-term soil contamination, are formed within a month.
Figure 2: Fluorescently labelled sporangium (infectious propagule; coloured magenta) of Phytophthora rubi emerging from infected raspberry root 14 days after infection. Sporangia can release multiple swimming zoospores that can cause new infections on nearby raspberry plants.
Analysis of P. rubi samples collected from fields since the first UK outbreaks of raspberry root rot, shows that their effector proteins exhibit more diversity between samples than expected. This means that control strategies relying on genetically inherited resistance will have to be carefully designed to avoid driving P. rubi to change and escape deployed resistance in raspberry. This research has helped in the development of new P. rubi-resistant cultivar Glen Mor.
Understanding how potato cyst nematodes hatch
Potato cyst nematodes (PCN) are major pathogens of potato crops (Figure 3).
Figure 3. Potato plants infected with potato cyst nematode. Healthy potato plants can be seen in the background.
PCN are host specific pathogens and therefore need to time their life cycles to coincide with the availability of a potato plant host. They achieve this by entering a dormant juvenile stage inside the egg (Figure 4).
Figure 4: Egg of the potato cyst nematode, Globodera pallida, showing the juvenile nematode coiled inside.
After development is complete, the juvenile does not hatch from the egg until chemical cues from a host plant are detected in their environment. How these chemical cues that are detected by the nematode and translated into a change in nematode biology are not well understood. However, disrupting the process by which the nematode links the life cycle to the host offers an avenue for new control strategies.
We have taken advantage of recent developments in nematode genomics and transcriptomics (funded in part through the RESAS SRP) to identify a modified annexin protein that plays a key role in this process. This modified annexin is only found in species of cyst nematodes that infect potato. The modified annexin protein is localised to the eggshell (Figure 5) and its action is altered in the presence of host root exudates, but not in the presence of root exudates from a non-host plant.
Figure 5: Hatched egg of the potato cyst nematode, Globodera pallida, with the modified annexin detected on the surface by fluorescence microscopy (green staining).
Current work is aimed at identifying compounds that bind to this protein with the aim of investigating whether these can modify the hatching behaviour of the nematode.
Late blight effector delivery and novel functions
To cause disease, Phytophthora pathogens require plant cells to take in some of the effector proteins they secrete. There are two questions that this research is addressing: how Phytophthora secretes these effector proteins, and how they are taken up by host plant cells.
Again, funded in part through the RESAS SRP, we have revealed that one important class of effectors secreted by Phytophthora infestans, the potato late blight agent, are taken up by plant cells via a form of endocytosis. Endocytosis is a cellular process for taking up macromolecules from the extracellular environment. This line of research is being further explored to determine how the effector proteins get from being taken up at the plant cell surface, to the places in the plant cells where they function (Figure 6).
Figure 6: Fluorescently labelled Phythophthora infestans showing the earliest stage of infection inside the first invaded plant cell. Green fluorescence shows the P. infestans hyphae. The finger-like extensions, called haustoria, are the sites of effector protein delivery (shaded magenta).
Other proteins and compounds secreted by pathogens function outside cells, on plant surfaces and in the spaces between the two organisms. These have potential as control targets as they are more accessible than proteins that work inside cells and we have identified novel P. infestans proteins that are essential for causing late blight. This will allow the design of new control measures such as new agrochemicals or biological treatments.
Benefits
Developing new crop varieties with resistance to pathogens, or new agrochemicals, can be a lengthy and costly process. Furthermore, pathogens and pests can evolve to overcome these control measures, signifying that new avenues are constantly required to control crop diseases. Therefore, this project, focussed on developing a greater understanding of the molecules that are critical for disease development, to provide new targets to control crop disease.
Discoveries from this project, when paired with recently developed crop protection strategies such as spray induced gene silencing, have the potential to control disease in a more rapidly adaptable and environmentally benign manner. Furthermore, these new strategies can be combined with traditional approaches including deployment of disease resistances and chemical applications through Integrated Pest Management (IPM) strategies to prolong durability of the control.
Findings from this project will be relevant to diseases caused by pests and pathogens of other Scottish crops and threats to Scotland’s natural biodiversity (e.g. tree diseases). The pests and diseases that are the focus of this work, and related crop pathogens and pests, also cause significant disease burdens to growers in developing nations. Our findings could also help to alleviate the burden of crop disease for those producers.
Location
DefaultRelated Links
Research Papers
- Uptake of oomycete RXLR effectors into host cells by clathrin-mediated endocytosis
- How to convert host plants into nonhosts
- RxLR effectors: master modulators, modifiers and manipulators
- High-efficiency green management of potato late blight by a self-assembled multicomponent nano-bioprotectant
- Spray-induced gene silencing as a potential tool to control potato late blight disease
- An eggshell-localised annexin plays a key role in the coordination of the life cycle of a plant-parasitic nematode with its host
- Potato cyst nematodes Globodera rostochiensis and G. pallida