Interactive Effects of Mycorrhizae, Soil Phosphorus, and Light on Growth and Induction and Priming of Defense in Plantago lanceolata

Abstract

Increasing demands to reduce fertilizer and pesticide input in agriculture has triggered interest in arbuscular mycorrhizal fungi (AMF) that can enhance plant growth and confer mycorrhiza-induced resistance (MIR). MIR can be based on a variety of mechanisms, including induction of defense compounds, and sensitization of the plant's immune system (priming) for enhanced defense against later arriving pests or pathogens signaled through jasmonic acid (JA). However, growth and resistance benefits of AMF highly depend on environmental conditions. Low soil P and non-limiting light conditions are expected to enhance MIR, as these conditions favor AMF colonization and because of observed positive cross-talk between the plant's phosphate starvation response (PSR) and JA-dependent immunity. We therefore tested growth and resistance benefits of the AMF Funneliformis mosseae in Plantago lanceolata plants grown under different levels of soil P and light intensity. Resistance benefits were assessed in bioassays with the leaf chewing herbivore Mamestra brassicae. Half of the plants were induced by jasmonic acid prior to the bioassays to specifically test whether AMF primed plants for JA-signaled defense under different abiotic conditions. AMF reduced biomass production but contrary to prediction, this reduction was not strongest under conditions considered least optimal for carbon-for-nutrient trade (low light, high soil P). JA application induced resistance to M. brassicae, but its extent was independent of soil P and light conditions. Strikingly, in younger plants, JA-induced resistance was annulled by AMF under high resource conditions (high soil P, ample light), indicating that AMF did not prime but repressed JA-induced defense responses. In older plants, low soil P and light enhanced susceptibility to M. brassicae due to enhanced leaf nitrogen levels and reduced leaf levels of the defense metabolite catalpol. By contrast, in younger plants, low soil P enhanced resistance. Our results highlight that defense priming by AMF is not ubiquitous and calls for studies revealing the causes of the increasingly observed repression of JA-mediated defense by AMF. Our study further shows that in our system abiotic factors are significant modulators of defense responses, but more strongly so by directly modulating leaf quality than by modulating the effects of beneficial microbes on resistance.

Keywords: Funneliformis mosseae; Mamestra brassicae; defense priming; induced systemic resistance; iridoid glycosides; mycorrhiza-induced resistance, shading; soil phosphorus.

Figure 1

Percent of P. lanceolata roots colonized by the mycorrhizal fungus F. mosseae for plants grown under four combinations of light intensity (L−: low light; L+: high light) and soil phosphorus treatments (P−: low soil P; P+: high soil P). Open bars: 6 weeks after seedling tansplantation; gray bars: 9 weeks after seedling transplantation. Bars that do not share the same letter are significantly different from each other (post hoc tests using LS means, p < 0.05).

Figure 2

(A) Total plant dry weight, and (B) RMF (root biomass/total biomass) of P. lanceolata plants at four time points during growth (panels from left to right: 3, 6, 9, and 12 weeks after seedling transplantation). Each panel displays results for plants grown under four different combinations of light intensity (L-: low light; L+: high light) and soil phosphorus treatment (P-: low soil P; P+: high soil P). Open bars: non-mycorrhizal plants; gray bars: plants inoculated with the AMF F. mosseae. Bars within panels that do not share the same letter are significantly different from each other (post hoc tests using LS means, p < 0.05).

Figure 3

Relative consumption rates (RCRs; amount of leaf mass eaten per unit caterpillar weight per day) of M. brassicae caterpillars feeding on P. lanceolata during two bioassays, performed when plants were (A) 7 weeks old (left panel), and (B) 10 weeks old (right panel). Each panel displays results for plants grown under four combinations of light intensity (L-: low light; L+: high light) and soil phosphorus treatments (P-: low soil P; P+: high soil P). In addition, plants had either be challenged by leaf application of jasmonic acid (JA) 48 h. prior to the bioassay (hatched bars) or not (non-hatched bars) and plants had either been inoculated with the AMF F. mosseae (bars with gray background) or not (white background). Bars within panels that do not share the same letter are significantly different from each other (post hoc tests using LS means, p < 0.05). The top row of letters refers to bars that display treatments without AMF, the second row of letters to those with AMF.

Figure 4

Leaf concentrations of primary and secondary metabolites in P. lanceolata plants at the time of two bioassays, when plants were 7 (left panels) and 10 weeks (right panels) old. (A,D) Leaf phosphorus; (B,E) Leaf nitrogen; (C,F) Leaf catalpol. Each panel displays results for plants grown under four combinations of light intensity (L-: low light; L+: high light) and soil phosphorus treatments (P-: low soil P; P+: high soil P). In addition, plants had either be challenged by leaf application of jasmonic acid (JA) 48 h. prior to the bioassay (hatched bars) or not (non-hatched bars) and plants had either been inoculated with the arbuscular mycorrhizal fungus (AMF) F. mosseae (bars with gray background) or not (white background). Bars within panels that do not share the same letter are significantly different from each other (post hoc tests using LS means, p < 0.05). The top row of letters refers to bars that display treatments without AMF, the second row of letters to those with AMF.

Figure 5

Relationship between leaf traits of P. lanceolata plants and the RCR of caterpillars of M. brassicae feeding on them. (A) Leaf phosphorus concentration at the time of bioassay 1; (B) Leaf catalpol at the time of bioassay 2; and (C) Leaf nitrogen concentration at the time of bioassay 2. Gray dots represent data points for individual plants. Symbols represent treatment means. Closed symbols: plants grown under low light; open symbols: high light. Circles: plants grown under low soil phosphate; squares: high soil phosphate. Symbols with a plus sign: plants inoculated with AMF; no plus sign: without AMF. Thick symbols: plants challenged by jasmonic acid application 48 h. prior to bioassays, thin symbols: no jasmonic acid application. Lines are slopes from linear regressions (all p < 0.05). Note that the values along the leaf catalpol axis are square-root transformed values.