By Freya Varden, John Innes Centre, Norwich
Whether you are a vegan or dedicated carnivore, a gourmet chef or a take-out devotee, plants are directly or indirectly the core of your diet. And in many parts of the world, we take it for granted that the supermarket will be stocked with fresh vegetables, or that the cafe round the corner will have coffee beans available for your flat white. While your parents may have told you ‘Finish everything on your plate – there are starving children in the world, you know’, you probably didn’t pay too much attention (or you may have given the cheeky reply that you’d actually be perfectly happy to put your spare broccoli in the post for the starving children).
But anyway, you don’t need me to tell you that there actually are starving people in the world, and that global poverty and hunger are terrible, shocking issues. We have an increasing number of people in the world, and we will not have enough food for them all; this is a fundamental road block that we share as a planet, and we cannot, and must not, ignore it. And this all comes back to plants: more crops equals more food. But we can’t just stick more seeds in the ground and wait – there are other obstacles to overcome, such as decreasing soil quality and changing climate. One of our major issues in crop production is disease – we compete with insects, fungi and bacteria for our plants, and these hungry pests are quite capable of destroying a whole field of crops before harvest time.
The whole story sounds a bit doom and gloom, but science is providing insights into how pests cause disease, and possibly helping us find solutions for the future. Although it might appear that trees just sit around not doing much until all their leaves fall off in the autumn, nothing could be further from the truth. Plants are able to adapt to changes in their surroundings and, like humans, they have a powerful immune system to help them ward off pests or pathogens.
As well as more obvious defences like spiky branches and nasty-tasting leaves, scientists have uncovered a whole molecular battleground within plant cells; because while some animals might be put off by spiky branches, many of the threats to our crops are diseases caused by bacteria and fungi, and the plant needs a whole new strategy to deal with these. On the surface of plant cells, there are receptors able to recognise possible signs of an imminent invasion, by detecting things like components of the bacterial flagellum (a tail-like structure found on many bacteria) and fungal walls. Recognition of such warning signals leads to a cascade of signalling within the cell, and the plant begins to enhance its defences by building up antimicrobial hormones and reinforcing its cell walls against invasion.
It may sound as though plants are pretty invincible with these kind of immune responses, but sadly bacteria and other plant pathogens are extremely good at adapting and evolving to undermine plant defences. Many bacteria and fungi are able to counter-attack and employ proteins called ‘effectors’, which they can transport into the plant cell. The purpose of these effector proteins is usually to inhibit the plant immune system or alter the normal function of the plant in some way – if successful, the pathogen can now bypass the outer defences and complete its infection (bye bye plant).
But wait, all is not lost! Inside the plant’s cells are yet more receptors that are able to recognise specific effector proteins and, if these receptors can bind successfully, the plant can employ its next layer of immune response. This usually involves the rather drastic measure of killing its own cells where the pathogen has invaded – a partial suicide known as the hypersensitive response. Although it sounds alarming to us, plants can afford to lose a leaf or two, especially when this means that hopefully the pathogen will die too and halt the spread of infection. And so this molecular battle continues – with the pathogen trying to evolve new effector proteins to suppress the plant immune response, and the plant trying to evolve new receptors to prevent it.
Our understanding of these amazing processes is helping science to establish new strategies in the battle against crop diseases. Traditionally, management of such diseases has relied on spraying huge amount of pesticides – a practise that is expensive and likely not sustainable. But research is helping scientists to shift the balance in favour of the crops, building upwards from the molecular level by modifying plant receptors to recognise new pathogen effectors, and boosting immunity in the field. There will inevitably, and rightly so, be debates about how we produce our food – from the way we manage our land, through to the use of chemicals and GM crops. Although there are many questions still to be answered, we are striving towards a world of sustainable agriculture and global food security.
- A more detailed insight into plant immunity: http://www.the-scientist.com/?articles.view/articleNo/45148/title/Holding-Their-Ground/
- The journal Nature recently published an outlook on food security: http://www.nature.com/nature/outlook/food-security/index.html
- In 2016 the UK Government ran a campaign on the theme of ‘Our Food Future’: https://www.food.gov.uk/news-updates/campaigns/ourfoodfuture
I’m a PhD student at the John Innes Centre in Norwich, studying the biochemistry of plant-pathogen interactions. I also enjoy doing outreach and organising fun science events for the public in my voluntary role with the British Science Association. You can find me on Twitter @freyavarden.