Elucidating pathogen interactions in Tanacetum cinerariifolium (pyrethrum) using fluorescently labelled Didymella tanaceti and Stagonosporopsis tanaceti

This study reports the first successful development of fluorescently labelled strains of the pyrethrum pathogens *Didymella tanaceti* and *Stagonosporopsis tanaceti* via *Agrobacterium*-mediated transformation, enabling the first detailed visualisation of *D. tanaceti*'s infection biology and its co-infection dynamics with *S. tanaceti* within host tissues.

The Gist

Imagine a high-stakes battle in a microscopic world. The battlefield is a pyrethrum plant (the source of natural insecticides), and the invaders are two sneaky fungal enemies: Didymella tanaceti and Stagonosporopsis tanaceti.

For years, scientists knew a lot about one of these invaders (Stagonosporopsis), but the other one (Didymella) was like a ghost. They knew it was there causing damage, but they didn't know how it attacked, when it struck, or how it interacted with its partner in crime.

This paper is the story of how scientists decided to give these invisible ghosts glow-in-the-dark suits so they could finally watch the battle unfold in real-time.

The Problem: The Invisible Enemy

Think of the pyrethrum plant as a city. The two fungi are burglars trying to break in.

• The Known Burglar (Stagonosporopsis): We have blueprints of how this one breaks in. We know their tools and their schedule.
• The Unknown Burglar (Didymella): We see the damage (holes in the roof, broken windows), but we can't see the burglar. We don't know if they sneak in through the front door, the back window, or if they dig a tunnel. Because we can't see them, we can't stop them effectively.

The Solution: The "Glow-Up"

To solve this, the scientists used a molecular "paintbrush" called Agrobacterium-mediated transformation.

Imagine Agrobacterium as a tiny, helpful delivery truck. The scientists loaded this truck with two special packages:

1. A package containing a gene for Green Glow (mNeonGreen).
2. A package containing a gene for Red Glow (tdTomato).

They sent these trucks to deliver the packages to the fungi. Once inside the fungi's DNA, the fungi started producing their own glow.

• The Didymella fungi started glowing bright green.
• The Stagonosporopsis fungi started glowing bright red.

The Experiment: Watching the Heist

Now that the burglars were wearing neon suits, the scientists could watch them invade the plant leaves under a special microscope.

1. Did the glow change them?
First, they had to make sure the glowing suits didn't make the burglars clumsy. They checked if the glowing fungi grew at the same speed and looked the same as the non-glowing ones.

• Result: Perfect! The glowing fungi were just as strong and fast as the wild ones. The "suits" didn't slow them down.

2. How does the unknown burglar attack?
With the green-glowing Didymella, the scientists finally saw the attack plan.

• The Break-in: Instead of waiting for an open door (like a plant's breathing pores), the fungus smashed right through the leaf's outer skin (the epidermis). It's like a burglar kicking down the front door rather than picking the lock.
• The Spread: Once inside, they moved through the leaf's "living room" (the mesophyll tissue), eating away at the plant cells.

3. The Two-Burglar Dance (Co-infection)
The coolest part? They put both the Green Burglar and the Red Burglar on the same leaf at the same time.

• Because one glowed green and the other red, the scientists could see exactly where they were.
• They found that both fungi were invading the same areas of the leaf, fighting for the same space and food. It was like watching two gangs occupy the same neighborhood, circling the same house.

Why This Matters

Before this study, managing these diseases was like trying to catch a thief in the dark. You knew a crime happened, but you didn't know the method.

Now, scientists have a flashlight.

• They can see exactly when the infection starts.
• They can see how the fungi move.
• They can see if the two fungi are helping each other or fighting each other.

This "glow-in-the-dark" technology gives farmers and scientists the intelligence they need to develop better sprays and strategies to protect the pyrethrum crop, ensuring we keep having those natural insecticides to keep our homes bug-free.

In short: Scientists gave invisible plant-killers neon jackets so they could finally watch the crime in progress and figure out how to stop it.

Technical Summary

1. Problem Statement

Pyrethrum (Tanacetum cinerariifolium) is a critical global crop for natural insecticide production, with Australia being the largest producer. The industry faces significant yield losses due to two primary fungal pathogens:

• Stagonosporopsis tanaceti: Causes "ray blight." Its infection biology and epidemiology are well-characterized, allowing for targeted management.
• Didymella tanaceti: Causes "tan spot." Despite being the most prevalent foliar pathogen, its infection biology, early infection stages, and interactions with S. tanaceti remain poorly understood.

The Gap: Traditional histological staining techniques lack the specificity to distinguish between multiple pathogen species during mixed infections. This hinders the development of management strategies for D. tanaceti and prevents the study of pathogen-pathogen interactions (co-infection dynamics) in natural settings.

2. Methodology

The study employed Agrobacterium tumefaciens-mediated transformation (ATMT) to generate stable, fluorescently labeled strains of both pathogens.

• Strain Selection: Wild-type strains D. tanaceti (BRIP 61988) and S. tanaceti (UOM ST2) were used.
• Vector Construction:
  • pMAI31: Carries the mNeonGreen gene (driven by the act1 promoter) for D. tanaceti.
  • pMAI32: Carries the tdTomato gene (driven by the act1 promoter) for S. tanaceti.
  • Both vectors included a hygromycin resistance gene (hygR) for selection.
• Transformation Process:
  • Spore suspensions of wild-type fungi were co-cultured with A. tumefaciens carrying the respective plasmids on induction media with acetosyringone.
  • Transformants were selected on media containing hygromycin and cefotaxime.
  • Pure cultures were established via single-spore isolation (for D. tanaceti) or hyphal tipping (for S. tanaceti).
• Characterization:
  • Fluorescence: Confirmed via compound microscopy (YFP filter for mNeonGreen, DsRed filter for tdTomato) and quantified using Corrected Total Cell Fluorescence (CTCF) values.
  • Genomics: Whole-genome sequencing (Illumina HiSeq) was performed on selected transformants to determine T-DNA copy number, insertion sites, and genomic rearrangements.
  • Phenotyping: Growth rates, colony morphology, and spore dimensions were compared against wild types.
• In Planta Assays:
  • Detached Leaf Assays: Leaves were inoculated with wild-type and transformed strains to assess disease progression and pathogenicity.
  • Co-infection Assays: Leaves were simultaneously inoculated with both fluorescent strains to visualize spatial interactions.

3. Key Contributions

• First Transformation: This study reports the first successful genetic transformation of D. tanaceti and S. tanaceti.
• Novel Tool Development: It provides the first fluorescently labeled strains of these specific pathogens, enabling live tracking of infection processes.
• Infection Biology Discovery: It offers the first detailed description of the early infection stages of D. tanaceti in pyrethrum leaves.
• Co-infection Visualization: It demonstrates the utility of dual-color fluorescence to distinguish and track two distinct pathogens within the same host tissue simultaneously.

4. Key Results

A. Transformation and Stability

• Five D. tanaceti and four S. tanaceti transformants were recovered.
• All strains retained stable fluorescence after subculturing.
• Fluorescence Intensity: S. tanaceti strains showed consistent red-orange fluorescence. D. tanaceti strains showed variable green fluorescence, with strain UOM DT5 exhibiting the highest intensity.
• Morphology: Most transformants retained wild-type colony morphology and growth rates. One D. tanaceti strain (UOM DT4) showed reduced growth and was excluded from further analysis.

B. Genomic Characterization

• Single-Copy Insertions: Whole-genome sequencing confirmed single-copy T-DNA insertions in all selected transformants.
• Insertion Sites & Rearrangements:
  • D. tanaceti UOM DT1: Insertion downstream of a rad5C (DNA repair) gene; 16 bp deletion.
  • D. tanaceti UOM DT5: Insertion within a GMC oxidoreductase gene; complex rearrangements including deletions and inversions (11 bp upstream, 657 bp downstream).
  • S. tanaceti UOM STC1R2: Insertion upstream of a RhebGTPase gene; 539 bp deletion.
  • S. tanaceti UOM STC4R2: Insertion in a hypothetical protein region; no deletions/inversions.
• Pathogenicity: Detached leaf assays confirmed that transformed strains (specifically UOM DT5 and UOM STC1R2) caused disease symptoms indistinguishable from wild types in terms of timing and lesion severity.

C. Infection Biology Insights

• D. tanaceti: Spores germinated within 24 hours. The pathogen penetrates the leaf via direct epidermal penetration (not stomatal entry) and colonizes the mesophyll through the middle lamella by 72 hours.
• S. tanaceti: Confirmed previous findings; mesophyll invasion occurs by ~54 hours.
• Co-infection: In dual-inoculation assays, both pathogens were successfully visualized occupying similar spatial niches within the leaf mesophyll by 7 days. While they co-existed, no direct antagonistic interaction was observed at this stage.

D. Technical Challenges

• The study addressed issues with plant autofluorescence (chlorophyll, glandular trichomes). The emission peaks of mNeonGreen (517 nm) and tdTomato (581 nm) were sufficiently distinct from plant background signals to allow clear differentiation.

5. Significance

This research provides a foundational toolkit for the pyrethrum industry and plant pathology community:

1. Management Strategies: By elucidating the previously unknown infection cycle of D. tanaceti, the study enables the development of targeted fungicide applications and cultural practices to disrupt specific infection windows.
2. Ecological Understanding: The ability to visualize co-infection dynamics allows researchers to study competition, synergism, or antagonism between pathogens in a way that was previously impossible with non-fluorescent methods.
3. Methodological Advancement: The successful use of ATMT with mNeonGreen and tdTomato in these specific ascomycetes validates these vectors for future functional genomics studies in pyrethrum pathogens.
4. Future Applications: These strains can now be used to screen for resistant germplasm, evaluate new fungicides, and investigate systemic movement of pathogens within the plant.