Discussion
Our results demonstrate that an intraspecific competitive environment alters the effect of the ubiquitous plant-fungi mutualism on plant-herbivore interactions. Arbuscular mycorrhizae decreased plant resistance to herbivores when the plants were grown without competition, but not when they were grown in competition. We found that arbuscular mycorrhizae decreased plant nutritional quality when grown in competition and induced with jasmonic and this correlated with increased resistance to herbivores. While most studies on mycorrhizae find positive or neutral effects on plant quality, mycorrhizae conferred benefits are highly conditional. Our results are in agreement with the work of Gange and West (1998) and Wurst et al. (2004) who found that, under certain conditions, mycorrhizae can reduce plant resistance to herbivores. Furthermore, we found that mycorrhizae suppress induction of jasmonic acid pathway based defensive protease inhibitors. This supports a number of studies which have found that mycorrhizal do not always benefit plant growth (Ryan and Angus 2003; Bennett and Bever 2007) but can still have effects on plant resistance to herbivores.
Our results support the vast body of literature ((Zimdahl 1980; Weiner 1990; Casper and Jackson 1997; Schwinning and Weiner 1998; Getman-Pickering et al. 2018 among many) showing that competition reduces plant growth. However, our results show no effect of mycorrhizae on size dimorphism in competing plants contradicting the findings by Weremijewicz and Janos (2012), and Ayers, Gange and Aplin who found that mycorrhizae increased size dimorphism in Andropogon gerardii and decreased size dimorphism in Plantago lanceolater espectively. However, both studies do not use entangled root systems, which may account for the difference.
Mycorrhizae suppressed induction of defensive protease inhibitors. These results support the growing body of work that shows that the relationship between arbuscular mycorrhizae and plants can be antagonistic, especially in tri-trophic frameworks. Arbuscular mycorrhizae have been shown to suppress defensive compounds and increase susceptibility of their hosts to a variety of insect herbivores, including garden tiger moths, chrysanthemum leaf-miners and garden buckeyes (Gernns et al. 2001; Gange et al. 2003; Bennett and Bever 2007; Hartley and Gange 2009; Gehring and Bennett 2009). Similar effects have been shown in mycorrhizae-plant-pathogen studies (Volpin et al. 1995; Shaul et al. 1999; Gernns et al. 2001). In our study and others, a suppression of defenses did not necessarily mean a decrease in overall growth. The decrease in induction and the fact that mycorrhizae did not increase growth suggest a more parasitic relationship between the plant and fungus in this experiment.
The fact that arbuscular mycorrhizae did not alter the plant’s constitutive defenses but did suppress induced defenses supports previous research that finds that the formation of the association between plants and arbuscular mycorrhizae alters defensive hormone signaling in the plant. As protease inhibitors are known to be regulated by the jasmonate pathway in tomato plants, it is not surprising that protease inhibitor induction was altered by arbuscular mycorrhizae. However, while overall trends in protease inhibitor activity matched the treatment effects on cabbage looper caterpillars, protease inhibitor activity in each individual plant was not correlated with caterpillar weight or the leaf area they consumed. This result implies that while arbuscular mycorrhizae are altering expression of the jasmonic acid pathway, the protease inhibitors are not the primary chemicals responsible for the effect on cabbage loopers.
The presence of mycorrhizae and competition alter the way that plants induce changes in nitrogen levels and subsequently the C/N ratio. While carbon levels independently didn’t impact herbivory, high ratios of carbon to nitrogen decreased cabbage looper feeding. As herbivores are frequently nitrogen limited, increased C/N ratios in foliage can be an effective defense strategy for deterring herbivory. Plants associated with mycorrhizae may have lower nutritional levels due to competition between the two organisms for limited nutrients in the soil media (Kaye and Hart 1997). This novel demonstration that mycorrhizae can suppress leaf nutrient content following herbivory provides a new mechanism for the effects of mycorrhizae on plant nutrition and resistance. It is particularly interesting that mycorrhizae can affect constitutive and induced defenses quite differently.
In our study, competition had no effect on induction of protease inhibitors or leaf area consumed. This supports the growing body of literature that fails to find support for the competition-defense tradeoff (Viola et al. 2010). While our study only addressed intraspecific competition, chosen because it is common in agricultural settings, weeds and intercropping systems can result in interspecific competition for the target crop. We predict that in interspecific competitive situations, the effects of arbuscular mycorrhizae on herbivory might not be suppressed for both partners as we found in this study. Rather, the stronger partner might continue to see a strong effect of arbuscular mycorrhizae on herbivory, while the weaker partner will not, depending on the strength of the interaction as mycorrhizae often preferentially give nutrients to one partner over another (Marler et al. 1999b).
Our results, along with the many papers cited above, show that tri-trophic effects of arbuscular mycorrhizae are context dependent, and while previous studies have shown that the species of insect and arbuscular mycorrhizae can alter the outcome of interactions, our work shows that the presence of competition can change the direction of the interaction between plant and mutualist. Due to the challenges of working with mycorrhizae, the vast majority of studies on mycorrhizae and herbivory have been conducted in greenhouse experiments where the plants are grown individually (Riedel et al. 2008; Bennett et al. 2009; Tomczak et al. 2016), although a few have studied this phenomenon in field settings (Gehring & Whitham, 1991; Gange & West, 1994; Gangeet al. , 2005). As plants mostly exist in competition, either with conspecifics or other species, it is important that future tests of the effect of mycorrhizae-conferred resistance to herbivores account for the role of competition.