An ‘arms race’ raging beneath our plants

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Bacteria (blue) ensnared in a DNA-based trap (yellow). So why does the trap only manage to hold on to a quarter of prisoners it takes?
Credit: Tran et al.

There’s an arms race raging underground well, between microbes and plants anyway.

When bacteria attack crop roots, plants fight back by snaring the pathogens in a sticky trap made from their own DNA secretions. But a new study shows how the bacteria bust out, using a set of enzymes that act as molecular scissors, splitting the DNA like bubble wrap.

Researchers at the University of Wisconsin, US, led by Caitlyn Allen published their findings in the journal PLOS Pathogens.

Plant roots have their own defence mechanisms, such as border cells, which work to fight off microbes in their search for water and nutrients in soil.

One of these microbes, a bacterium called Ralstonia solanacearum, kills valuable crops such as potato, tomato and banana by squeezing through openings in plant root tips and multiplying. This eventually blocks the plant’s water flow, causing it to wilt and die.

Plants have a defense mechanism against this disease – their sticky DNA traps, which kill around a quarter of the invading bacteria. But why doesn’t the method kill all of the invaders?

Allen and colleagues used pea and tomato plants as model root systems to better understand the battle and why the majority of bacteria emerge victorious.

The DNA secretion works like a spider web, entrapping the bacteria in its sticky tendrils.

When plants were exposed to more benign forms of bacteria, the roots didn’t activate their DNA secretion, suggesting a specific response to some aspect of the pathogen.

The plants also failed to secrete DNA in response to a mutation of the bacteria that affected its shape – specifically, its propeller-like tail – suggesting the microbe’s structure was a key component in eliciting the defensive response.

So the researchers used mutated forms of R. solanacearum to identify which genes in particular set off the sticky-trap response.

The entrapment process, the researchers identified, was kicked into gear by a DNA-associated protein called histone H4, which is present in both plant and animal cells.

Interestingly, the researchers found that bacteria that managed to escape the sticky traps in pea plants contained two genes that work like molecular scissors, cutting into the defensive DNA molecules and breaking free.

A mutation of the bacterium that lacked one or both of those genes remained motionless when trapped, and was also less able to invade a tomato plant’s roots.

Once the “scissors” – called nucleases – were added to the mutant bacterium, it was once again able to struggle free of the sticky roots.

The researchers hope that understanding the role of nucleases in the battle of plant vs bacteria will contribute to developing disease-resistant plants, eventually producing more hardy crops.

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