Figure 1: Resistance elicitors
One of the major problems concerning the production of food crops is the difficulty of controlling plant diseases to maintain the high quality and yield which the producer and consumer expect. For example, many fungal pathogens have developed resistance to the active ingredients of a wide range of fungicides and there is a public perception that ‘pesticides’ are undesirable. One alternative approach to the control of plant diseases is through the induction and enhancement of the plant’s own defence mechanisms which would not involve the application of toxic compounds to plants (see Figure 1, right).

Fundamental studies have helped us to understand many of the biochemical interactions occurring between plants and plant pathogens. Thus we are able to describe resistance reactions as involving not only some preformed components but more importantly, an induced response to infection which includes a ‘cascade’ of induced responses. These include novel antimicrobial compounds (phytoalexins), proteins, and physical barriers to penetration. This cascade of resistance factors is induced when a plant recognises that a potential pathogen is present, and compounds which are capable of triggering such responses are termed elicitors.

Figure 2: Barley leaves sprayed with yeast-derived resistance elicitor before inoculation with mildew

Figure 2: Detached barley leaves cv. Golden Promise sprayed with a yeast-derived resistance elicitor 24 hours before inoculation with mildew. Photograph above shows; (far right) untreated leaves; and (left) three sets of leaves treated with various yeast-derived elicitor formulations. Elicitation of resistance is not associated with any deleterious effects on the plant and the treatment on the far left shows excellent control of mildew.

At the James Hutton Institute we have been studying several naturally occurring resistance elicitors including those obtained from fungi. Importantly, elicitors can be extracted from yeast which is available in large quantities as a low value product from the food and brewing industry. We believe that elicitors from yeast could form the basis of a new non-toxic crop protection system which would be environmentally benign, would enable plants to respond to infection faster than normal and prevent potential pathogens from successfully colonising the host.

Because of the unique mode of action of this control method (through induction of a cascade of resistance responses) it has the added advantage that pathogens should not develop insensitivity as they do to traditional fungicides.

There are many reports in the literature which demonstrate the concept of induced resistance in laboratory or glasshouse experiments, including biotic and abiotic elicitors, either in a pure form or as a crude mixture. Most of these elicitors have not been tested under field conditions and some have been shown to have side-effects such as phytotoxicity.

Our own work, and that of a few other groups, has shown moderate levels of efficacy of disease control in the field (for example, against cereal mildew) demonstrating the principle that induced resistance can work in the field. There have been predictions, by some, that there would be an energetic cost in the induction of resistance, but with yeast-derived elicitors we have not detected any yield loss in replicated field trials, nor have we observed any phytotoxic side effects such as yellowing, necrosis or stunting.

Thus, induced resistance does not necessarily involve a ‘cost’ or loss in yield. The elicitors that are currently available for experimental use in laboratories are less robust than fungicides in that they are far more effective when applied prior to the arrival of the pathogen and will have an inhibitory rather than a curative effect on an existing infection.

Resistance elicitors could have a role in an integrated disease control strategy and permit a reduction in fungicide inputs. For example, they could be combined with reduced dose fungicide sprays in integrated crop management programmes either as a combined tank mix or as alternating sprays to achieve greater disease control and reduce fungicide inputs. Furthermore, because elicitors are not in themselves antimicrobial, it may be possible to integrate them with biocontrol agents thereby increasing the flexibility of disease management.