The ups and “downs” of conifer defense: Linking aboveground herbivory and belowground induced root defenses

Background/Question/Methods

The application of plant hormones known to elicit systemic changes in defense compounds has been shown to result in higher plant fitness, increased abundance of natural enemies, and decreased herbivore attacks. According to optimal defense theory, defenses against herbivores are costly to plants and hence should be allocated to plant parts that are most valuable to plants and at higher risk of herbivore attacks. While folivory itself may not cause a significant risk of attack to plant roots, induced plant defense responses can play important roles in mediating interactions between above- and belowground organisms. Although effects of herbivory on defense responses of aboveground tissues have been extensively studied, research investigating foliar-feeding influencing the secondary chemistry of belowground tissues is lacking. In this study, we focus on the production of monoterpenes in pinyon pine, a species shown to respond chemically to above-ground herbivory. However, whether these changes in aboveground needle monoterpenes synthesis translate into altered root defenses and affect the rhizosphere, associated fungi, and plant function is unknown. To determine whether aboveground herbivory alters root chemical defenses in pinyon pine, we used various proxies for herbivore damage (MeJA, mechanical damage, MeJA x mechanical damage) on seedlings (n=3). Needles and roots were destructively harvested over the twenty day period and analyzed for monoterpene composition and concentration between treatments and over time.

Results/Conclusions

Our results show that the composition of monoterpenes present in the needles differs from that in the roots, with (-)-α-pinene, making up 20% of the composition in needles and only 6% in the roots. This inverse relationship was also noted with needles having significantly more β-pinene in the needles versus roots while δ-carene made up a significant proportion of total monoterpenes in the roots (27%) and only 3% in the needles. Furthermore, we found that herbivore damage had a significant effect on the concentrations of (+)-α-pinene, β-myrcene, and β-phellandrene in the needles (P<0.05) while simultaneously and significantly altering (-)-α-pinene, β-pinene, and S-(-)-limonene concentrations in the roots (P<0.005), each with different directions of response. These data suggest cross-talk between the biosynthetic pathways producing individual monoterpene compounds in these separate plant organs, but with signal molecules potentially affecting the up-regulation of different monoterpene cyclase enzymes. These results have important implications for assessing monoterpene mediated multi-trophic interactions in this system and susceptibility of pinyon pines to both future herbivory and fungal pathogens following insect feeding events.