For plants, a soft touch can induce strong disease resistance

Plants can be exposed to a lot of stress; stress from the environment, such as exposure to wind, rain, and cold, and especially stress from pathogens. Although pathogenic agents like fungi and bacteria can pack a punch when it comes to infection, plants have mechanisms in place to combat this, mobilising defence signals to prime themselves for a future fight. Research published in BMC Plant Biology by Jean-Pierre Métraux of the University of Fribourg, Switzerland and colleagues now shows that it only takes a gentle hand to enable a short-term, localised, and highly effective defence against infection.

Previous work from Métraux’s lab, has shown that aggressive leaf-wounding of the model cress species Arabidopsis thaliana is enough to mount a strong defence against the pathogenic fungus Botrytis cinerea – the causative agent of grey mould – by releasing protective compounds called Reactive Oxygen Species (ROS) in a process independent of the plant’s usual defensive pathways. They questioned, how much of a mechanical insult is needed to induce this protective response that primed the immune system to an attack?

Rather than be forceful with their forceps, the group instead gently rubbed individual leaves between finger and thumb, in what they term soft mechanical stress (SMS). Again, ROS were released in a rapid burst immediately following the treatment, in a coordinated process accompanied by a peak in intracellular calcium levels and activation of genes associated with the touch response. This response also translated into powerful resistance to infection.

Following inoculation of the leaves with spores of B.cinerea, SMS treatment reduced the lesion size of infection in a manner that was strongly dose-dependent – the more times the leaves were rubbed, the greater the reduction in lesion size. However, this effect was transient. After only a few hours, half of this induced resistance was lost, with full susceptibility to infection regained as little as 24 hours after treatment. Could this transient resistance be because the plants sensed the cellular damage, as seen in the Métreaux’s previous work using wounded leaves? Examination of the leaf surface under a powerful electron microscope suggests not.

Although some minor damage was observable to the tiny protrusions on the leaf surface, known as trichomes, very little overt cellular damage was observed under SMS treatment. Indeed, when the researchers replicated these experiments using a mutant Arabidopsis that lacked trichomes, no cellular damage was seen – yet resistance remained. They therefore looked a little closer.

Examining the cellular cuticle for clues, they found that the outer layer of the cell was markedly more permeable than those of untreated leaves, and that this permeability was in turn associated with the leakage of bioactive compounds onto the leaf surface. They speculate that it is these bioactive compounds that may have a key role in preventing the development, and therefore pathogenesis, of mould.

Previous work on wounding in plants has identified several signalling pathways involved in the defence and immune responses – most notably those involving salicylic acid, ethylene, jasmonate, and other hormones. This new research highlights how the wounding response is far more sensitive than previously thought, adding a new layer of complexity to the sensory world of plants and their fight against infection.