Rusty Research: Fighting Bread’s Biggest Bad-guy


This has been a bad year for farmers: last year’s wet summer and then the cold winter that just won’t end have scuppered one harvest and probably knocked this year’s right down too. Even when conditions are more ideal than they have been this year, farmers and breeders fight an uphill battle trying to prevent a significant proportion of the crop being lost to various pathogens. When it comes to wheat that means rustblack rust, brown rust and yellow rust. Where it strikes, yield losses are likely to be around 20% in susceptible varieties, and the problem is getting much worse. Most resistance to black rust (Puccinia triticina) is caused by a single gene, which a new resistant kind of rust (Ug99) managed to overcome in much the same way as MRSA became resistant to methicillin in our hospitals.

Close-up of wheat leaf rust (”Puccinia triticinia”) on wheat. Photo by James Kolmer. http://www.ars.usda.gov/is/graphics/photos/jun06/d519-1.htm Image Number D519-1 PD-USGov-USDA-ARS

Now scientists from Norwich, Cambridge and the USA are trying to find out how some kinds of a similar disease, yellow rust, (Puccinia striiformis or PST) are able to overcome the plant’s natural defences and infect. 

Plant disease resistance at its most simple involves the plant detecting the pathogen (disease-vector); and the pathogen trying to disable the detection and reaction symptoms using molecules called effectors. In that respect, it’s not too different to human epidemiology: our white blood cells detect antigens and respond to them, and some diseases try to get around the immune system. In plants the biggest class of disease resistance genes are called R genes. The Ug99 disaster is happening because so many different varieties carried the same resistance: a single R gene that the fungus was bound to eventually overcome. The key to creating durable resistance that is not easily overcome by pathogens is likely to be in fully understanding how they work, not just us

Let’s try to understand one another

The key to overcoming diseases like rust may be in understanding how they infect plants, and how different varieties of rust differ from one another, rather than in looking for a ‘quick fix’. In the same way that we search the genomes of different varieties of wheat in the hope that we might see what makes one more high yielding than another, by comparing different pathogens, we might spot the secret to their virulence. The scientists looked for genes that were highly expressed (testing 22 in particular with qPCR to show how their expression changed during infection), particularly in the haustoria (hyphal tip that invades the plant) and varied between the different isolates of the fungus, which were from both the US and UK. They also paid particular attention to genes coding proteins from known ‘tribes’ that are already known to contain effectors.

We’re all in this together

One interesting discovery they made was that the candidate genes they identified were not absent in any of the lines sequenced: in other pathogens such as Melampsora oryzae  it has been demonstrated that some potential avirulence proteins were absent from virulent varieties, and present in ones that did not cause infection (Yoshida et al, 2009). It seems that the differences in how strains of yellow rust function may be down to them having different alleles of the same genes, rather than having different genes to begin with.

By looking at two UK isolates that vary in pathogenicity (and which are more similar to one another than the UK isolates are to the US isolates) the authors were able to identify 14 of the candidate genes that were most likely to confer virulence. The authors hope to establish the functions of the effector candidates by artificially introducing them into wheat plants, or by using gene-silencing technology to temporarily ‘turn them off’ in the pathogen, as demonstrated by Panwar et al (2012).
______________________________________________________
Cantu, D et al (2013) BMC Genomics 2013, 14:270
doi:10.1186/1471-2164-14-270
Genome analyses of the wheat yellow (stripe) rust pathogen Puccinia striiformis f. sp. tritici reveal polymorphic and haustorial expressed secreted proteins as candidate effectors

Yoshida K et al (2009) Plant Cell Online 21(5):1573–1591.
Association genetics reveals three novel avirulence genes from the rice blast fungal pathogen Magnaporthe oryzae

Panwar V et al (2012) Plant J 2012, 75(3):521–532.
Endogenous silencing of Puccinia triticina pathogenicity genes through in planta-expressed sequences leads to suppression of rust diseases on wheat.

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