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Exploiting plant pathogen biology for future disease control

Raspberry Breeding Consortium

Potato plants infected with potato cyst nematode

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 Progress

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:

  1. The infection cycle and biology of raspberry root rot, a currently understudied disease causing significant economic losses
  2. How dormant pathogens like PCN link their life cycle to the presence of a host plant
  3. Variation within pathogen effectors
  4. Mechanisms driving pathogen entry into plants
  5. 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).