Saturday, 17 November 2012

Hard science: the future of crop protection

Food security, whilst something of a buzzword, is one of the great challenges facing this generation and every generation for the foreseeable future. A major threat to global security is pestilence. Total yield lost to pathogens and pests is an immensely difficult thing to quantify. It’s subject to a whole host of variables but a decent estimate would be something in the region of 25% globally. Gone are the days when crops could be liberally doused with chemicals designed to kill the disease causing agent as standard procedure, largely because these chemicals have an annoying tendency to kill things we’d really rather keep alive! Not to mention the remarkable speed at which populations of pathogens resistant to our chemical warfare emerge. In the future, a smarter approach is required, one which will probably involve genetic engineering. Those best placed to provide the solutions are molecular biologists working at the interface between pathogens and their hosts.

In the contemporary plant pathology scene, the major paradigm is the study of so-called ‘effectors’. An effector is a small molecule, often a protein, which is secreted into the plant cell by microorganisms with the aim of eliciting some kind of response in the host which will benefit the pathogen. However, the pathogen certainly doesn’t get it all its own way. Plants, unlike animals, don’t have an army of motile immune cells capable of rushing to infected tissue to repel invasion. Plant cells, as a rule, don’t move. Sure, they possess a fully functioning vascular system roughly analogous to an animal circulatory system, which transports water, sugars, hormones and other signalling molecules but it most certainly does not transport immune cells. What they do have is a very efficient surveillance system capable of recognising signals associated with pathogens and then producing an immune response.

The first layer of resistance recognises microbe-associated molecular patterns (MAMPs for short), which are patterns which microorganisms produce on account of being microorganisms. A good example is flagellin, a protein involved in cell motility. MAMP recognition triggers a weak immune response. Why bother with a weak immune response though? If you know you’re under attack surely it’s better to hit the attacker with everything you’ve got? Well, not necessarily. The fact is that all microbes produce MAMPs but not all microbes produce disease. As a ‘strong’ plant immune response tends to involve localised cell death, killing your own cells every time you recognise a microbe would be a terrible strategy in terms of evolutionary fitness. This is where the second layer of resistance comes in, a resistance which concerns the effector proteins I mentioned earlier. One major function of effectors is the subversion of MAMP triggered immunity in order to facilitate infection. However, as well as recognising MAMPs, plants are also capable of recognising effector proteins through the production of recognition proteins from genomic regions termed ‘resistance (R) genes’. Pathogens produce a diverse array of effectors and plants possess an equally diverse set of R genes, the system is a classic example of an evolutionary ‘arms race’, a race which the pathogens most definitely seem to be winning! Recognition of an effector by the product of an R gene often leads to a robust immunity in the form of the hypersensitive response, a localised cell death signal which restricts the growth of pathogens that require living tissue to complete their life cycles. Unfortunately, many effector proteins are not recognised by R gene products and are free to exert their effects on the host.  In order to develop resistance to important crop pathogens we need to fully understand how effectors produce their effects in the host cell and how effectors and R proteins interact. Knowledge of these processes remains thin on the ground so current research is very much focused in their direction.

One fruitful avenue of research is the structural characterisation of the key players involved in this system. It’s parsimonious, then, that the 1000th crystal structure determined at the Diamond Light Source, Oxford, happened to be that of an effector protein secreted by the tomato pathogen Pseudomonas syringae, which was published in the journal PNAS last month. Scientists from the John Innes Centre, using the x-ray diffraction facility at Diamond, were able to figure out the structure of the protein, known as AvrRPS4, by inference from the electron density pattern it produces when x-rays are directed at the protein and diffracted. This structural information was then used to inform the generation of mutant effector proteins which were no longer recognised by the associated R protein and of mutants which were able to interact with the R protein whilst still inducing resistance but without any associated cell death.

The Diamond Light Source, Oxfordshire
The implications of this are pretty cool. Firstly, it tells us which parts of the effector are needed for recognition by the host. Secondly, and more importantly, it adds to an emerging body of evidence suggesting that cell death is not a requirement for resistance. This is a big deal! Resistance without cell death is extremely desirable in terms of food production and this research represents a massive step toward developing it and, ultimately, deploying it in real crop production situations.
As an aside, structural study of effector proteins also allows an incredible snapshot of how protein structure underpins the incredible rate at which these ‘molecular weapons’ evolve. And, trust me, they evolve extremely quickly!

For those interested who have access to PNAS, the reference for the full study is as follows:

Sohn KS, Hughes RK, Piquerez SJ, Jones JDG, Banfield MJ. (2012) Distinct regions of the Psuedomonas syringae coiled-coil effector AvrRps4 are required for activation of immunity. PNAS. 109(40): 16371-16376.

- Ben Hall

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