TOR signaling downregulation increases resistance to the cereal killer Fusarium graminearum

Abstract

TOR is the master regulator of growth and development that senses energy availability. Biotic stress perturbs metabolic and energy homeostasis, making TOR a good candidate to participate in the plant response. Fusarium graminearum (Fusarium) produces important losses in many crops all over the world. To date, the role of TOR in Fusarium infection has remained unexplored. Here, we show that the resistance to the pathogen increases in different Arabidopsis mutants impaired in TOR complex or in wild-type plants treated with a TOR inhibitor. We conclude that TOR signaling is involved in plant defense against Fusarium.

Keywords: Arabidopsis; Fusarium Head Blight; Fusarium graminearum; TOR; biotic stress.

Figure 1.

Arabidopsis thaliana TOR complex mutants are more resistant to Fusarium graminearum infection. (A) Disease reaction of Arabidopsis TOR complex mutants. Lesion development in WT and mutants leaves (3-weeks-old seedlings) at 7 dpi with Fusarium (5 µl of 1–5 × 10⁵ conidia/ml suspension) (I) compared with their respective control (C). For induction of tor-RNAi of tor-es mutant, 10 µM of β-estradiol was added 24 h previous to infection. Representative images are shown. Scale bar = 2 cm. (B) Fusarium mycelial growth and plant cell death. The same leaves of (A) were stained with trypan blue reagent and visualized under stereoscope (20x). Representative images are shown. The red arrowheads indicate cell death and infectious hyphae on inoculated leaves. Scale bars = 200 µm. (C) Assessment of resistance to Fusarium infection of Arabidopsis WT and TOR complex mutants. Quantification of development disease in leaves of WT and TOR mutants plants at 7 dpi with Fusarium. Standard error bars (SE) are shown. Significant statistically differences (Tukey´s test) are indicated with asterisks (P < 0.05). (D) Lesion size measurement. Expanding lesions were measured on excised leaves at 7 dpi with Fusarium. SE are indicated. Significant statistically differences (Tukey´s test) are indicated with asterisks (p < 0.05). (E) Fusarium infection on Arabidopsis whole plants. Symptoms in Arabidopsis WT and raptor3g mutant after infection with 20 ml of Fusarium suspension (5 µl of 1–5 × 10⁵ conidia/ml suspension) at 14 dpi. Representative images are shown. (F) Fusarium mycelial growth and plant cell death. The same leaves of (E) was stained with trypan blue reagent and visualized under stereoscope (20x). Representative images are shown. Scale bar = 200 µm. (G) Whole plant decay of (E) was measured each day of the disease assay. SE are shown.

Figure 2.

Arabidopsis WT plants with pharmacological inhibition of TOR signaling are more resistant to Fusarium infection. (A) Disease reaction of WT plants impaired in TOR signaling. Lesion development in Arabidopsis WT or WT leaves treated with the TOR inhibitor PP242 (2 µM) at 7 dpi with Fusarium (5 µl of 1–5 × 10⁵ conidia/ml suspension) (I) compared with their respective control (C). Scale bar = 2 cm. Representative images are shown. (B) Fusarium mycelial growth and plant cell death. The same leaves of (A) were stained with trypan blue reagent and visualized under stereoscope (20x). The red arrowheads indicate cell death and infectious hyphae on inoculated leaves. Scale bars = 200 µm. (C) Assessment of resistance to Fusarium infection in pharmacological TOR-inhibited Arabidopsis WT leaves. Quantification of disease development of WT or PP242-treated WT leaves (2 µM) at 7 dpi with Fusarium. Standard error bars (SE) are indicated.