Upgrading root physiology for stress tolerance by ectomycorrhizas: insights from metabolite and transcriptional profiling into reprogramming for stress anticipation

Abstract

Ectomycorrhizas (EMs) alleviate stress tolerance of host plants, but the underlying molecular mechanisms are unknown. To elucidate the basis of EM-induced physiological changes and their involvement in stress adaptation, we investigated metabolic and transcriptional profiles in EM and non-EM roots of gray poplar (Populus x canescens) in the presence and absence of osmotic stress imposed by excess salinity. Colonization with the ectomycorrhizal fungus Paxillus involutus increased root cell volumes, a response associated with carbohydrate accumulation. The stress-related hormones abscisic acid and salicylic acid were increased, whereas jasmonic acid and auxin were decreased in EM compared with non-EM roots. Auxin-responsive reporter plants showed that auxin decreased in the vascular system. The phytohormone changes in EMs are in contrast to those in arbuscular mycorrhizas, suggesting that EMs and arbuscular mycorrhizas recruit different signaling pathways to influence plant stress responses. Transcriptome analyses on a whole genome poplar microarray revealed activation of genes related to abiotic and biotic stress responses as well as of genes involved in vesicle trafficking and suppression of auxin-related pathways. Comparative transcriptome analysis indicated EM-related genes whose transcript abundances were independent of salt stress and a set of salt stress-related genes that were common to EM non-salt-stressed and non-EM salt-stressed plants. Salt-exposed EM roots showed stronger accumulation of myoinositol, abscisic acid, and salicylic acid and higher K(+)-to-Na(+) ratio than stressed non-EM roots. In conclusion, EMs activated stress-related genes and signaling pathways, apparently leading to priming of pathways conferring abiotic stress tolerance.

Figure 1.

Anatomical characteristics of nonmycorrhizal (A) and ectomycorrhizal (D) root tips of wild-type poplar (P. × canescens) and of poplar line 51 transformed with the auxin-responsive GH3∷GUS reporter (cross sections [B and E] and longitudinal sections [C and F]) of nonmycorrhizal (B and C) and mycorrhizal (E and F) root tips. EC, Epidermal cell; CC, cortical cell; HM, hyphal mantle; HN, Hartig net. Representative micrographs are shown. Blue staining indicates GUS activity. Line 31 yielded the same staining pattern. [See online article for color version of this figure.]

Figure 2.

Soluble carbohydrates (CHO) of major pathways (A; Suc, black; Glc, red; Fru, green) and minor pathways (B; myoinositol, black; Gal, red; mannitol, green; sorbitol, blue; trehalose, cyan), amino compounds (C; Pro, black; Ser, red; Ala, green; Asn, blue; Asp, cyan; Gln, pink; Glu, yellow; γ-aminobutyric acid, burgundy; NH₄⁺, gray), and cations (D; potassium, black; sodium, red; calcium, green; magnesium, blue; iron, cyan; manganese, pink) in mycorrhizal (M) or nonmycorrhizal (N) fine roots of P. × canescens under control conditions (C) or after exposure to salt stress (S). Data indicate means ± se (n = 4). Different letters indicate significant difference for the sum of each metabolite class between the treatments at P ≤ 0.05. D.wt., Dry weight.

Figure 3.

Genes with significantly increased (red) or decreased (blue) transcript levels in mycorrhizal compared with nonmycorrhizal fine roots of P. × canescens. AGI annotations and putative functions were assigned to poplar gene models and grouped according to MapMan categories. The scale indicates increasing (R-fold) or decreasing (−1/R-fold) response factors for mycorrhizal compared with nonmycorrhizal roots without salt stress (MC/NC = PiRG), overlapping responses for EM-associated genes in mycorrhizal and nonmycorrhizal roots with salt stress (MS/NS = EMAG), and stress-associated genes in nonmycorrhizal roots (NS/NC = STAG) and in mycorrhizal roots (MS/MC = MSAG). ns, Not significantly differentially affected.

Figure 4.

Transverse sections of nonmycorrhizal (A and B) and ectomycorrhizal (C and D) root tips of poplar (P. × canescens) grown under control conditions (A and C) or in the presence of salt stress (B and D). Sections were viewed with an epifluorescence microscope. Bars = 50 μm.

Figure 5.

A, Correlation between the relative transcript abundance of P. involutus mycorrhiza-responsive genes under control conditions (PiRGs = MC/NC) and mycorrhiza-responsive genes under salt stress (EMAGs = MS/NS = 0.589x, r = 0.919, P ≤ 0.001) or with salt-responsive genes in nonmycorrhizal roots (STAGs = NS/NC = 0.768x, r = 0.571, P = 0.009). B, Correlation of salt-responsive genes in nonmycorrhizal (STAG) and mycorrhizal roots (MSAG = MS/MC = 0.241x, r = 0.729, P = 0.008). Data were obtained from Supplemental Tables S2 and S3.