Study system
We conducted this experiment on tomato plants var. Castlemart
(Solanum lycopersicum ). Tomatoes are a valuable field and
greenhouse crop which associate with mycorrhizal fungi and have a range
of chemical defenses against herbivores. Protease inhibitors are common
defenses in tomato leaf tissue (Broadway 1986), which are induced
through the jasmonic acid pathway. Since herbivores are commonly
nitrogen limited, the inability to digest proteins can significantly
retard growth and result in starvation and death.
The efficacy of this defense strategy has been shown in Castlemart
tomatoes (Felton et al. 1989, Farmer and Ryan 1990, Rodriguez-Saona et
al. 2010, Shrivastava et al. 2015). We chose to grow the tomatoes in
intraspecific competition, as tomato plants grown in agricultural
systems will most likely be planted with conspecifics.
Rhizophagus intraradices is a generalist arbuscular mycorrhizal
fungus in the sub-phylum Glomeromycotina. It is commercially available
and used in organic agricultural systems to increase nutrient uptake and
decrease fertilizer use. It has been shown to colonize tomato plants
(Caron et al. 1986, Fierro-Coronado et al. 2013, Shrivastava et al.
2015). The mycorrhizal inoculum containing R. intraradicesspores, and non-mycorrhizal inoculum for controls were obtained from the
International Culture Collection of (Vesicular) Arbuscular Mycorrhizal
Fungi (INVAM) at West Virginia University. Both the mycorrhizal inoculum
and non-mycorrhizal control were produced in leek trap pots in
sand-turface media. The inoculum itself was comprised of sand, turface,
leek root fragments, and in the treatment: mycorrhizal spores and hyphal
fragments. While the control media did not contain mycorrhizal
propagules neither inoculum was sterile.
We used first instar Trichoplusia ni caterpillars in a bioassay
to measure leaf quality. These larvae were obtained from a colony
maintained on artificial diet at Cornell University for many years.Trichoplusia ni are generalist Noctuid caterpillars that feed on
a wide variety of crop plants including Solanaceous and cruciferous
vegetables.