Mycorrhiza-Induced Resistance against Foliar Pathogens Is Uncoupled of Nutritional Effects under Different Light Intensities

Abstract

The use of microbial inoculants, particularly arbuscular mycorrhizal fungi, has great potential for sustainable crop management, which aims to reduce the use of chemical fertilizers and pesticides. However, one of the major challenges of their use in agriculture is the variability of the inoculation effects in the field, partly because of the varying environmental conditions. Light intensity and quality affect plant growth and defense, but little is known about their impacts on the benefits of mycorrhizal symbioses. We tested the effects of five different light intensities on plant nutrition and resistance to the necrotrophic foliar pathogen Botrytis cinerea in mycorrhizal and non-mycorrhizal lettuce plants. Our results evidence that mycorrhiza establishment is strongly influenced by light intensity, both regarding the extension of root colonization and the abundance of fungal vesicles within the roots. Light intensity also had significant effects on plant growth, nutrient content, and resistance to the pathogen. The effect of the mycorrhizal symbiosis on plant growth and nutrient content depended on the light intensity, and mycorrhiza efficiently reduced disease incidence and severity under all light intensities. Thus, mycorrhiza-induced resistance can be uncoupled from mycorrhizal effects on plant nutrition. Therefore, mycorrhizal symbioses can be beneficial by providing biotic stress protection even in the absence of nutritional or growth benefits.

Keywords: Botrytis cinerea; arbuscular mycorrhiza; light intensity; mycorrhiza-induced resistance; plant growth; plant nutrition.

Figure 1

Light treatments. Plants grew on 5 different shelves with 2, 3, 4, 5, and 6 LED tubes that refer to treatments I1, I2, I3, I4, and I5, respectively. The resulting light intensity for each treatment is presented, and expressed as photosynthetically active radiation (PAR) (mol µm⁻² s⁻¹).

Figure 2

Percentages of root length colonized by the mycorrhizal fungus Rhizoglomus irregulare in inoculated lettuce plants grown for seven weeks under the different light intensities. I1 to I5 correspond to 28, 44, 63, 81, and 101 PARs (µmol m⁻²s⁻¹), respectively. Data not sharing a letter in common are statistically different according to the LSD test (p < 0.05, n = 6).

Figure 3

Mycorrhizal colonization and proportions of fungal structures in the root systems of plants grown under different light intensities. (a) Representative pictures of fungal colonization within the roots. Arrows illustrate arbuscules (A) and vesicles (V). (b) Quantification of mycorrhizal colonization intensity of the root system (M%) and abundande of vesicles (v%) and abundance (a%) of within the colonized areas,, determined as described by Trouvelot et al. [66]. Values were determined in plants growing under low (I1, 28 µmol m⁻²s⁻¹), medium (63 µmol m⁻²s⁻¹), and high (I5, 101 µmol m⁻²s⁻¹) light intensity. Data not sharing a letter in common are statistically different according to LSD multiple ranged tests (p < 0.05, n = 6).

Figure 3

Mycorrhizal colonization and proportions of fungal structures in the root systems of plants grown under different light intensities. (a) Representative pictures of fungal colonization within the roots. Arrows illustrate arbuscules (A) and vesicles (V). (b) Quantification of mycorrhizal colonization intensity of the root system (M%) and abundande of vesicles (v%) and abundance (a%) of within the colonized areas,, determined as described by Trouvelot et al. [66]. Values were determined in plants growing under low (I1, 28 µmol m⁻²s⁻¹), medium (63 µmol m⁻²s⁻¹), and high (I5, 101 µmol m⁻²s⁻¹) light intensity. Data not sharing a letter in common are statistically different according to LSD multiple ranged tests (p < 0.05, n = 6).

Figure 4

Overview of nutrient organization patterns in lettuce shoots from non-mycorrhizal (Nm) or Rhizoglomus irregulare-colonized plants (Ri) grown under different light intensities. I2 to I5 correspond to 63, 81, and 101 PARs (µmol m⁻²s⁻¹). (A) Principal component analysis (PCA) of nutrients content at different light intensities (I2, I3, I4, I5). (B) PCA comparing Nm and Ri for each light intensity level.

Figure 5

Disease symptoms in mycorrhizal (Ri) or non-mycorrhizal (Nm) lettuce plants grown under different light conditions. Percentage of leaves in the different disease categories for each treatment. Disease index scale: 0, no visible symptoms; 1, necrotic lesions extending below 25% of the leaf surface; 2, necrosis extending more than 50% of the leaf surface; 3: dead leaf.