The Association With Two Different Arbuscular Mycorrhizal Fungi Differently Affects Water Stress Tolerance in Tomato

Abstract

Arbuscular mycorrhizal (AM) fungi are very widespread, forming symbiotic associations with ∼80% of land plant species, including almost all crop plants. These fungi are considered of great interest for their use as biofertilizer in low-input and organic agriculture. In addition to an improvement in plant nutrition, AM fungi have been reported to enhance plant tolerance to important abiotic and biotic environmental conditions, especially to a reduced availability of resources. These features, to be exploited and applied in the field, require a thorough identification of mechanisms involved in nutrient transfer, metabolic pathways induced by single and multiple stresses, physiological and eco-physiological mechanisms resulting in improved tolerance. However, cooperation between host plants and AM fungi is often related to the specificity of symbiotic partners, the environmental conditions and the availability of resources. In this study, the impact of two AM fungal species (Funneliformis mosseae and Rhizophagus intraradices) on the water stress tolerance of a commercial tomato cultivar (San Marzano nano) has been evaluated in pots. Biometric and eco-physiological parameters have been recorded and gene expression analyses in tomato roots have been focused on plant and fungal genes involved in inorganic phosphate (Pi) uptake and transport. R. intraradices, which resulted to be more efficient than F. mosseae to improve physiological performances, was selected to assess the role of AM symbiosis on tomato plants subjected to combined stresses (moderate water stress and aphid infestation) in controlled conditions. A positive effect on the tomato indirect defense toward aphids in terms of enhanced attraction of their natural enemies was observed, in agreement with the characterization of volatile organic compound (VOC) released. In conclusion, our results offer new insights for understanding the molecular and physiological mechanisms involved in the tolerance toward water deficit as mediated by a specific AM fungus. Moreover, they open new perspectives for the exploitation of AM symbiosis to enhance crop tolerance to abiotic and biotic stresses in a scenario of global change.

Keywords: Solanum lycopersicum; aphid; arbuscular mycorrhizal symbiosis; phosphate transporter; plant tolerance; volatile organic compound; water deficit.

FIGURE 1

Leaf water potential and gas exchanges in AM– and AM+ plants unstressed (NS) and subjected to two different water deficit levels (moderate stress, MS; severe stress, SS), measured at the end of the experiment. (A) Leaf water potential (Ψ_leaf); (B) net photosynthetic rate (A_N); (C) stomatal conductance (g_s); and (D) iWUE levels in AM– (Ctrl) and AM+ (Rin and Fmos) plants. All data are expressed as mean ± SE (n = 5). ns, ^{∗, ∗∗}, and ^∗∗∗: non-significant or significant at P ≤ 0.05, P ≤ 0.01, and P ≤ 0.001, respectively. Different letters above the bars indicate significant differences according to Tukey HSD test (P ≤ 0.05), considering S × M interaction. Analysis of variance on the single variables is reported in Supplementary Table S3.

FIGURE 2

Expression changes of tomato phosphate transporter (PT) genes in roots of AM– and AM+ plants upon irrigation (NS), moderate water stress (MS), and severe water stress (SS) conditions. RT-qPCR analysis of (A) LePT1, (B) LePT2, (C) LePT3, (D) LePT4, and (E) LePT5. Lower letters denotes significant differences attested by Tukey HSD test (P < 0.05). All data are expressed as mean ± SE (n = 3). ns, ^{∗, ∗∗, ∗∗∗}: non-significant or significant at P ≤ 0.05, P ≤ 0.01, and P ≤ 0.001, respectively. Different letters above the bars indicate significant differences according to Tukey HSD test (P ≤ 0.05), considering S × M interaction. Analysis of variance on the single variables is reported in Supplementary Table S3.

FIGURE 3

Expression changes of fungal phosphate transporter (PT) genes in AM+ roots upon irrigation (NS), moderate water stress (MS), and severe water stress (SS) conditions. RT-qPCR analysis of (A) F. mosseae PT (FmPT) and (B) R. intraradices PT (RiPT) genes, respectively. Lower letters denotes significant differences attested by Tukey HSD test (P < 0.05). Data are expressed as mean ± SE (n = 3).

FIGURE 4

Probability of aphid Macrosiphum euphorbiae survival when fed on tomato AM– and AM+ plants (inoculated with R. intraradices), upon irrigation (NS and Rin) and water-stress (WS and WS + Rin) conditions.

FIGURE 5

Flight behavior (oriented flights and landing on source) of Aphidius ervi females (%) toward tomato plants inoculated with R. intraradices, infested by aphids, their combination and relative control. Asterisks assigned by G-test for independence (^∗P < 0.05 and ^∗∗∗P < 0.001).

FIGURE 6

Detrended Correspondence Analysis (DCA) score (A) and loading (B) plots of the first two components based on volatiles emitted tomato plants that were not stressed (NS), aphid-attacked (Aph), water-stressed (WS), and exposed to both water stress and aphid infestation (WS + Aph). (A) Each experiment was carried out using uninoculated (NS, Aph, WS, and WS + Aph) and inoculated with R. intraradices (Rin, Rin + Aph, WS + Rin, and WS + Rin + Aph) plants. In brackets the percentage of variation explained is indicated. Multivariate analysis of variance by permutation was used to test for significant differences between group centroids (see Supplementary Table S4). (B) Loading plot of the DCA first two components, showing the contribution of each VOC (C1–C31, listed in Table 2) to the model.