It’s not in Australia yet and that’s exactly how we want it to stay!
We’re talking about ToxB, a fungal effector that has a known role in causing wheat yellow (or tan) spot disease in many other countries around the globe.
As toxins produced by plant pathogens to allow infection to take hold, effectors often play key roles in crop disease, so the last thing we want is for ToxB and our Australian wheat varieties to meet – this is one of those toxic relationships that’s best stopped before it begins.
Yellow spot isolates that produce ToxB are already a problem for wheat growers in countries including Canada, the United States, Azerbaijan, Algeria, Syria and Turkey.
So far Australia has been off-limits, and that’s how our CCDM researchers want it to stay, so rather than watch from afar and hope for the best they’re on the front foot, investigating the impact this effector could have if it ever paid us a visit.
Best to be prepared!
ToxB is one of three known effectors produced by the fungus Pyrenophora tritici-repentis (Ptr), the causal agent of yellow spot disease.
Another, ToxA, the major gene within yellow spot that causes necrosis (cell death) is already here in Australian isolates and a third, ToxC, is a yet-to-be characterised effector that CCDM researchers continue to investigate.
The potential biosecurity risk if isolates of yellow spot containing ToxB ever made their way to Australia is something CCDM researchers (pictured above) are actively addressing, carrying out lab tests on Australian wheat varieties to determine how they would cope if it did arrive on our shores and impact our domestic cereal varieties.
“It’s important we have as much advance information as possible on ToxB, should it ever be detected in Australia,” said CCDM Cereal Diseases Theme Leader, Dr Caroline Moffat.
“Understanding the interaction between effectors and the host plant is key for researchers who are working to develop tools to help improve varietal resistance.”
Taking the lead on ToxB
Conducting a series of tests, CCDM’s researchers were able to evaluate the sensitivity of over 100 different varieties of Australian wheat to ToxB, providing some valuable insight into its potential impact.
The first step was to come up with a sophisticated and effective method to produce ToxA and ToxB in the lab so they could compare the impact of both.
“The method we used involved cloning the ToxA and ToxB genes and using an engineered E. coli strain called SHuffle to produce large quantities of effectors,” said CCDM researcher Dr Pao Theen See.
“We then tested these effectors on a collection of Australian cereals, including 122 varieties of bread wheat, as well as 16 durum and 20 triticale varieties, to determine their sensitivity levels.”
According to Caroline Moffat the study not only provides a useful snapshot of wheat variety sensitivities to both ToxA and ToxB in the lab, but it’s important information that could also translate to the paddock.
“We’re hopeful that by better understanding these two effectors – one that already impacts Australian grains and one that we hope never will – we can offer breeders some additional insight into selecting germplasm that preserves good traits, but breeds out effector sensitivity,” said Dr Moffat.
Down the track, our researchers also hope to use the same technique to screen commercial wheat varieties against effectors that are newly discovered.
Research findings in detail
The following points provide a breakdown of key outcomes from the study:
- Australian durum varieties are highly sensitive to ToxB but sensitivity in Australian bread wheats was rare (only 4%).
- Of the 16 durum wheat varieties evaluated, six were ToxA sensitive and seven were ToxB sensitive.
- ToxB-induced chlorosis in the 16 durum wheat varieties was notably stronger than that observed in the bread wheats. Effector sensitivity included moderately resistant durum varieties to yellow spot disease.
- 55% of the Australian bread wheats and 38% of the durum wheats were sensitive to ToxA.
- All of the bread wheat varieties rated in the most resistant categories (MR and MRMS) were found to be ToxA insensitive.
- 20 varieties of wheat-rye hybrid triticale were screened for effector sensitivity. Seven exhibited sensitivity to ToxA and eight to ToxB.
- ToxB-induced chlorosis was also detected on barley.
The full results are in our team’s research paper recently published by the journal Frontiers in Microbiology: “Heterologous expression of the Pyrenophora tritici-repentis effector proteins ToxA and ToxB, and the prevalence of effector sensitivity in Australian cereal crops”. Table 2 in the paper provides a list of Australian wheat varieties and their sensitivities to ToxA and ToxB.