Short signalling distances make plant communication a soliloquy

Abstract

Plants respond to attack by herbivores or pathogens with the release of volatile organic compounds. Neighbouring plants can receive these volatiles and consecutively induce their own defence arsenal. This 'plant communication', however, appears counterintuitive when it benefits independent and genetically unrelated receivers, which may compete with the emitter. As a solution to this problem, a role for volatile compounds in within-plant signalling has been predicted. We used wild-type lima bean (Phaseolus lunatus) to quantify under field conditions the distances over which volatile signals move, and thereby determine whether these cues will mainly trigger resistance in other parts of the same plant or in independent plants. Independent receiver plants exhibited airborne resistance to herbivores or pathogens at maximum distances of 50 cm from a resistance-expressing emitter. In undisturbed clusters of lima bean, over 80 per cent of all leaves that were located around a single leaf at this distance were other leaves of the same plant, whereas this percentage dropped below 50 per cent at larger distances. Under natural conditions, resistance-inducing volatiles of lima bean move over distances at which most leaves that can receive the signal still belong to the same plant.

Figure 1.

Experimental setup. Single shoots of six receiver plants were placed at different distances from induced emitters and then investigated for their level of resistance to herbivores and pathogens.

Figure 2.

Distances in plant–plant communication. (a) EFN secretion was quantified as a measure of indirect resistance to herbivores in milligram soluble solids secreted per gram leaf fresh mass over 6 h. (b) Numbers of CFUs were determined in leaves that had been challenged with Pseudomonas syringae as a measure of resistance to pathogens and are expressed as CFU per mg leaf fresh mass. Bars represent means ±s.e., means marked with different letters were significantly different (p < 0.05 according to LSD post hoc test). Grey bar, day 2; black bar, day 4. (c) Proportions of own (black bar) versus foreign (grey bar) leaves in circles with a radius of the same distances at which receivers had been positioned. 0 cm, leaf in the centre of the circles.