Phytopathological future for plasma

Loss of crop plants to disease is a major constraint on our ability to provide sufficient food for human consumption and wellbeing. Attempts to control the disease-causing agents, such as bacteria or fungi, or their vectors, e.g. insects, have until now largely been chemical (e.g. fungicides), biological or biotechnological. Now a physical approach has recently been demonstrated by Ochi et al. using plasma.

Plasma is a state of matter additional to the usual trio of solid, liquid, and gas and is described as “a hot ionized gas consisting of approximately equal numbers of positively charged ions and negatively charged electrons”. Although one of the best examples of a plasma is the Sun, plasmas can be made on Earth where they have many applications, such as – and back to plant pathology – irradiating rice seeds to protect them from microbial infections.

Previously, plasmas have been shown to control fungal contamination of harvested plant material. However, Ochi et al. looked at the other end of the plant life cycle – in treating seeds prior to germination and growth where it is important to ensure that the living cells aren’t harmed by the treatment. Plasma was effective at preventing infection of rice by seed surface contamination from the fungus Fusarium fujikuroi* and the bacterium Burkholderia plantarii, which causes bacterial seedling blight. But, lest we get swept along on a tide of unwarranted excessive appreciation for this hi-tech approach, it’s somewhat sobering to know that it wasn’t plasma alone that was used.

In fact, it was a combination of tried-and-tested exposure of the microbe-contaminated seeds to water at 60°C for 10 minutes, followed by the plasma irradiation. This combination was more effective than hot water alone since it is often the case that the water is not hot enough or seeds are not treated for long enough. But, it’s good to see this marriage of the old and the new, and – as important as killing the microbial pathogens – the seeds germinated and grew and the seedlings appeared to have suffered no harm from the plasma treatment.

Quite how the plasma works its ‘magic’ is not yet known, but it is thought that reactive oxygen compounds generated by the irradiation may play a role in the phytopathological protection procedure.

From an external use of plasma to an internal one now with use of such plasmas is to deliver proteins into plant cells. This is an otherwise difficult feat to achieve, but Yuki Yanagawa et al. successfully introduced green fluorescent protein – fused to adenylate cyclase – as a reporter protein into cells of tobacco leaves by treatment with atmospheric nonthermal plasmas. From ‘ectoplasma’ to cytoplasm, what will they think of next?

* For those of us who attempt to teach plant biology to undergraduates F. fujikuroi may be more familiar by its old name of Gibberella fujikuroi, which causes bakanae in rice, a seed-borne disease. The plant hormones known as gibberellins were first identified in this fungus and were named after its generic name. Bakanae is known as ‘foolish seedling’ disease in English because infected plants undergo excessive lengthening compared to non-infected plants. Growth-promotion – such as stem elongation of rice – is one of the effects of gibberellin. Teaching plant biology via plant pathology, my little ‘shout-out’ for the importance of fungi in an attempt not to neglect them.