Abstract
Common mycorrhizal networks (CMNs) can facilitate communication between plants by propagating anti-herbivore defense cues from an herbivore-damaged plant to an undamaged plant. However, whether defense cues can propagate to an undamaged plant connected to a second plant via a CMN is unknown. This study examined whether defense cues propagate through multiple CMN-connected plants and if this form of plant-plant communication could benefit plants receiving those defense cues. Two hypotheses were tested: 1) defense-related genes will be upregulated in multiple undamaged plants connected sequentially to a damaged plant through a CMN; and 2) plants receiving defense cues through a CMN will exhibit increased resistance to herbivore attack compared to plants that did not receive defense cues. To test these hypotheses, tomato plants (Solanum lycopersicum cv. Castlemart) were grown in experimental mesocosms composed of three plants connected in sequence via a CMN: an herbivore-damaged defense cue emitter, an undamaged first receiver of defense cues, and an undamaged second receiver of defense cues. Mesocosms also contained first and second receiver controls that lacked a CMN connection to the emitter plant. Defense cues were induced by damage from herbivorous Manduca sexta larvae. Induction of defense cues was verified by emitter plants that did not receive M. sexta damage. Differential expression of the defense related gene protease inhibitor 2 (PIN2) was used to identify interplant communication and upregulation of defense cues. A feeding trial was conducted in which M. sexta wet mass was compared between first receivers with a CMN connection to damaged emitters and first receivers with severed CMN connections to damaged emitters, and second receivers with a CMN connection to damaged emitters and second receivers with severed CMN to damaged emitters. Increased expression of the defense related gene PIN2 was observed in herbivore damaged emitter plants. Increased PIN2 expression was not observed in first receivers with CMN connection to damaged emitters. Low induction of PIN2 in half of the mesocosms suggests that a CMN may not have established in those replicates. No increase in PIN2 expression was observed in second receivers with a CMN connection. Likewise, there was no increase in PIN2 expression in any receiver plants when connected to undamaged emitter plants. Additionally, there were no significant differences in M. sexta mass among receiver plant treatments. However, these results are likely influenced by heat-stress related mortality of herbivore replicates during the feeding trial. Although the results do not support the hypothesis that anti-herbivore defense cues propagate through multiple CMN-connected plants, they should be interpreted with caution because a CMN connection may not have established in all treatments. Similarly, the results do not support the hypothesis that herbivores would have lower mass when feeding on plants with CMN connection to damaged plants, however, significant differences among receiver plant treatments may not have been apparent due to low replication.