Stress promotes Arabidopsis - Piriformospora indica interaction

Abstract

The endophytic fungus Piriformospora indica colonizes Arabidopsis thaliana roots and promotes plant performance, growth and resistance/tolerance against abiotic and biotic stress. Here we demonstrate that the benefits for the plant increase when the two partners are co-cultivated under stress (limited access to nutrient, exposure to heavy metals and salt, light and osmotic stress, pathogen infection). Moreover, physical contact between P. indica and Arabidopsis roots is necessary for optimal growth promotion, and chemical communication cannot replace the physical contact. Lower nutrient availability down-regulates and higher nutrient availability up-regulates the plant defense system including the expression of pathogenesis-related genes in roots. High light, osmotic and salt stresses support the beneficial interaction between the plant and the fungus. P. indica reduces stomata closure and H2O2 production after Alternaria brassicae infection in leaves and suppresses the defense-related accumulation of the phytohormone jasmonic acid. Thus, shifting the growth conditions toward a stress promotes the mutualistic interaction, while optimal supply with nutrients or low stress diminishes the benefits for the plant in the symbiosis.

Keywords: Biophoton; biotic and abiotic stress; defense; light stress; metal resistance; mutualism; osmotic stress; phytohormones; reactive oxygen species; root architecture; salt stress; stomata.

Figure 1.

A. thaliana seedlings grown under long day light condition (65 µmol m⁻² sec⁻¹) and different access to the nutrients in PNM medium (aquaculture, agar, membrane or cellophane) with or without P. indica. (A) Typical pictures of the seedlings after 30 days of co-cultivation under the four different conditions. (B) Shoot fresh weights of 30 day-old seedlings shown in A. (C) Weight promotion of the shoots by P. indica (in %) relative to the uncolonized control. For statistics, cf. Materials and Methods.

Figure 2.

Effect of P. indica on the root architecture of A. thaliana seedlings grown for 12 days on PNM media under long day light condition (50 ± 15 µmol m⁻² sec⁻¹). (A) Root architecture of Arabidopsis seedlings grown on PNM media with and without cellophane. (B) Microscopical view of the roots' network in control and P. indica-treated seedlings (+ P. indica) grown on PNM plates covered with cellophane. Pictures were from the GiaRoot software. (C) SEM images of control and P. indica-colonized Arabidopsis roots after 10 days of colonization. For statistics, cf. Materials and Methods.

Figure 3.

Effect of P. indica on A. thaliana shoot growth (A), root colonization (B and C), and expression of defense genes (D) in different PNM concentration [7 days co-cultivation under long day light (50 ± 15 µmol m⁻² sec⁻¹)]. (A) Shoot fresh weight (up) and promotion in % relative to the uncolonized control (down). (B) Fluorescent microscopy of Arabidopsis roots stained with trypan blue and fuchsin acid. (C) P. indica ITS mRNA level (relative to plant GAPDH mRNA level) from colonized roots grown on different PNM concentrations. (D) Fold changes of PR genes (+ P. indica / - P. indica) in Arabidopsis shoots grown on different PNM concentrations. For statistics, cf. Materials and Methods.

Figure 4.

Effect of heavy metals and salts on A. thaliana growth in the presence/absence of P. indica under long day light (50 ± 15 µmol m⁻² sec⁻¹). (A) Shoot fresh weights (left) and % weight promotion by P. indica (relative to the uncolonized control) (right) after 10 days of co-cultivation or mock-treatment. 1 mM of different metal salts was used. (B) Shoot fresh weights (left) and % weight promotion by P. indica (right) of seedlings grown for 10 days with/without P. indica on different salt concentrations. (C) Shoot fresh weights (left) and % weight promotion (right) of 10-day old seedlings co-cultivated with/without P. indica under different osmotic and salt conditions. For statistics, cf. Materials and Methods.

Figure 5.

Effect of different light intensities on A. thaliana growth in the presence/absence of P. indica. (A) Shoot fresh weights (left) and % weight promotion by P. indica (right) under continuous light and different light intensities (25–110 µmol m⁻² sec⁻¹). Seedlings were grown for 10 days with/without P. indica. (B) Fluorescence parameters [Y (effective quantum yield of photosystem II), Fs/Fm and NPQ] in Arabidopsis shoots after 10 days co-cultivation with P. indica under continuous light (25 ± 5, 50 ± 15 and 110 ± 10 µmol m⁻² sec⁻¹). All results are based on 3 independent experiments with 10 individual seedlings. Bars represent SE. Asterisks indicate significant differences (relative to its own control) as determined by t-test (* P ≤ 0.05; ** P ≤ 0.01; *** P ≤ 0.001).

Figure 6.

(A) Shoot fresh weights (left) and % weight promotion by P. indica (right) in A. thaliana leaves of seedling with/without P. indica and 10 days after leaf infection with A. brassicae. The seedlings were grown under long day light condition (50 ± 15 µmol m⁻² sec⁻¹). (B) H₂O₂ levels in leaves of Arabidopsis seedlings after infection with A. brassicae in the presence/absence of P. indica after 10 days. (C) Stomata closure after Alternaria leaf infection of colonized and uncolonized seedlings after 10 days. (D) Relative gray values of the images (biophoton records) of Alternaria-infected Arabidopsis leaves treated with/without P. indica. For statistics, cf. Materials and Methods.

Figure 7.

JA, JA-Ile and oxophytodienoic acid (OPDA) levels in the shoots with/without P. indica and 10 days after leaf infection with A. brassicae. The seedlings were grown under long day light condition (50 ± 15 µmol m⁻² sec⁻¹). For statistics, cf. Materials and Methods.