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Monday, 21 November 2016

What lies below: how soil bacteria fight off sticky roots

By Nicola Temple

The first horror film I ever watched was Invasion of the Body Snatchers. The film was already dated by about 30 years when I saw it and so aspects seemed silly rather than scary. Yet, those alien plants still managed to evoke nightmares in my pre-teen imagination. Antagonistic plants have cropped up in numerous films over the years - from the musical menace in Little Shop of Horrors to the Devil’s Snare that entangles Harry Potter and his friends. Yet, the cinematic nightmare of being entwined and strangled by the (not so) local flora is based in some truth...if you’re a microbe.

Soil is alive with microbes - on the order of 100 million cells per gram of soil. Some of these are friendly microbes and some are less so. So, as plant roots seek out water and nutrients within the soil they must also be wary of what they encounter. The root tips are sheathed in specialised cells known as root border cells and these are the front line of defence. These cells launch themselves from the root tips and through the release of various chemicals, help to manipulate the environment around the extending root tips. They can attract and stimulate growth of helpful microbes and repel or inhibit the growth of others. 

Earlier this year, researchers at the University of Wisconsin, USA looked closely at how the root border cells of peas and tomatoes interact with the bacteria Ralstonia solanacearum.
R. solanacearum is a pathogen that affects a number of economically important plants, including potatoes, tomatoes, peppers, bananas and tobacco. It follows the chemical signals sent out by plant roots and finds natural openings or wounds within the root in order to invade the plant. Once inside, the bacteria replicate rapidly and take up residency in the xylem of the plant. Eventually, they block this important transport system of the plant and cause it to wilt and die.

A false coloured electron micrograph showing bacteria (blue)
tangled in the DNA-based trap (yellow).
Photo credit: Tran et al.
The Wisconsin researchers found that when the root border cells of both the peas and tomatoes encountered R. solanacearum, they released DNA. Surrounded by stands of sticky DNA, the bacteria become entangled. Unable to move, the bacteria die. It truly is the stuff of horror films.
Other friendlier species of bacteria didn’t induce this projectile DNA trap from the root border cells, which suggests that they are able to recognise the threat of R. solanacearum.

However, as is almost always the case with nature, there is always a counter attack. The researchers discovered that only 25% of the bacteria were dying in the plant’s sticky trap, so how were the rest managing to escape?

The Wisconsin group found that when R. solanacearum encountered the DNA, it triggered a release of an enzyme that cuts DNA. In other words, they were using molecular scissors to cut their way out of the trap.

And so the evolutionary arms race continues. It is those individual bacteria that produce more of the defensive enzyme that will escape the traps and replicate. Perhaps over evolutionary time, those that have limited capacity to produce the enzyme will be weaned out of the population, forcing the root border cells to improve their offensive game.

For scientists, this detailed understanding of how hosts interact with different pathogens could help them to develop disease-resistant plant varieties of these economically important crops.

For me, this insight into the quiet battles being fought below the ground give me an even greater appreciation for the fruits and vegetables I harvest from my small little garden - they have been hard-won!

The paper referred to is:
Tran TM, MacIntyre A, Hawes M, Allen C (2016) Escaping Underground Nets: Extracellular DNases Degrade Plant Extracellular Traps and Contribute to Virulence of the Plant Pathogenic Bacterium Ralstonia solanacearum. PLoS Pathog 12(6): e1005686. doi:10.1371/journal.ppat.1005686

Nicola Temple is a science writer and co-author or the book 'Sorting the Beef from the Bull: The Science of Food Fraud Forensics' . She dabbles in her small veg patch and regularly contributes to the University of Bristol Botanic Garden blog.

Friday, 4 November 2016

Botanical treasures on the beach

By Helen Roberts

Beachcombing is fun no matter what your age. Shells, pieces of driftwood and cloudy coloured glass somehow find a way into pockets, rucksack compartments and lunchboxes. A holiday to Slapton, in the South Hams, over the summer cemented our family’s curiosity in scouring the beaches. But whilst beachcombing, I was also similarly intrigued by the plants growing in this environment.

The coast here has a constantly shifting flora highly specialised for growing in difficult conditions. There is a shingle ridge that runs parallel to the shoreline, dividing the lake (Slapton Ley) from the sea. The plants that I discovered along the ridge path that runs northeast from the village of Torcross looked so intrinsically a part of the place, that to me they amplified the essence of this unique shingle coast.

All of the species on the shingle ridge have evolved ways of coping with difficult coastal conditions. The plants here are frequently subjected to saline spray and blown salts, high winds, exposure to hot summer temperatures and low soil humidity. The physical makeup of the shingle (it is a mix of flint, chert and quartz intermixed with some finer material) means that the substrate is very free draining and this results in the acute leaching of nutrients.

Thick leaves and deep roots

Most species that grow here have developed a cuticle (the protective film covering the leaf) that resists the entry of salt water or the leaves are sclerophyllous (thick, waxy and leathery). Often leaves are succulent and allow more effective retention of water.

Giant leaved sea kale (Crambe maritima)
Photo credit: Peganum [via Flickr CC BY-SA 2.0]
White hairs on the leaf surface, which give a silvery hue, are also adaptive in preventing evaporation as they trap moisture, conserving water within their microclimate. The yellow horned-poppy (Glaucium flavum) has hairy sclerophyllous leaves. This is a toxic but striking perennial, with bright yellow flowers that later produce a stunning display of long horned seedpods. This plant has an extremely deep root system in order to access water deep down, but this also helps with stability during high winds.

Other plants on the shingle bar include the beautifully architectural rock samphire (Crithmum maritimum) and the giant leaved sea kale (Crambe maritima), the latter becoming popularised through its abundance in Derek Jarman’s extraordinary shingle garden near Dungeness. Sea kale taproots reach depths of up to 2 metres, providing them with a significant anchor in coastal winds.

Lying low

Restharrow (Ononis repens) is a common beach dweller.
Photo credit: Matt Lavin [via Flickr CC BY-SA 2.0]
Some plants have adapted a slightly different approach to coastal living. They nestle down as low as they can and spread horizontally to reduce the amount of surface area exposed to the hammering winds. The small fleshy leaves of the sea campion (Silene uniflora) are closely packed and the plant grows in low mats. It is only the flowering branches, with their pretty white flowers and inflated bladders, much like calyx, that dare poke their heads up into the wind. Restharrow (Ononis repens) is an attractive low creeping aromatic herb with hairy stems and small pink pea-like flowers. The evaporation of essential oils released by aromatic plants is thought to cool the environment around the plant and thereby protect it from periods of extreme heat. Restharrow also has an extensive root system. Other species like chicory (Cichorium intybus), viper’s-bugloss (Echium vulgare), thrift (Armeria maritima), wild carrot (Daucus carota) and bird’s-foot-trefoil (Lotus corniculatus) all grow on the shingle ridge and are highly prized by pollinating insects.

These species have adapted to this environment. Although the ridge is only a narrow two metre strip of shingle, it displays a wonderful mosaic of disturbed ruderal* vegetation that forms a linear ribbon of interesting shapes, forms and flowers. Observing this particular plant habitat has helped me in the development of my own gravel garden at home. Some of these species are better suited to a garden environment and so I will begin with these. Although, more often than not it’s a matter of trial and error when trying to take plants outside of the conditions in which they thrive. Beth Chatto’s famous mantra, ‘Right plant, right place’ springs to mind and is always in my thoughts when I am planting my own garden or designing for other people.

*Ruderal is a term in botany that refers to plants that grow on waste ground or among rubbish.

 Helen Roberts is a trained landscape architect with a background in plant sciences. She is a probationary member of the Garden Media Guild and a regular contributor to the University of Bristol Botanic Garden blog.