Responses to combined abiotic and biotic stress in tomato are governed by stress intensity and resistance mechanism

Abstract

Stress conditions in agricultural ecosystems can occur at variable intensities. Different resistance mechanisms against abiotic stress and pathogens are deployed by plants. Thus, it is important to examine plant responses to stress combinations under different scenarios. Here, we evaluated the effect of different levels of salt stress ranging from mild to severe (50, 100, and 150mM NaCl) on powdery mildew resistance and overall performance of tomato introgression lines with contrasting levels of partial resistance, as well as near-isogenic lines (NILs) carrying the resistance gene Ol-1 (associated with a slow hypersensitivity response; HR), ol-2 (an mlo mutant associated with papilla formation), and Ol-4 (an R gene associated with a fast HR). Powdery mildew resistance was affected by salt stress in a genotype- and stress intensity-dependent manner. In susceptible and partial resistant lines, increased susceptibility was observed under mild salt stress (50mM) which was accompanied by accelerated cell death-like senescence. In contrast, severe salt stress (150mM) reduced disease symptoms. Na(+) and Cl(-) accumulation in the leaves was linearly related to the decreased pathogen symptoms under severe stress. In contrast, complete resistance mediated by ol-2 and Ol-4 was unaffected under all treatment combinations, and was associated with a decreased growth penalty. Increased susceptibility and senescence under combined stress in NIL-Ol-1 was associated with the induction of ethylene and jasmonic acid pathway genes and the cell wall invertase gene LIN6. These results highlight the significance of stress severity and resistance type on the plant's performance under the combination of abiotic and biotic stress.

Keywords: Callose; R-gene resistance; cell death; ethylene; invertase; stress severity..

Fig. 1.

(A) Disease index averaged across the LYC4 ILs and the recurrent parent MM under powdery mildew without salt (0mM NaCl) and in combination with 50, 100, and 150mM NaCl, measured at 10, 15, and 25 days post-inoculation (dpi). (B) Senescence index across the same genotypes and treatments at 15 dpi. Statistically significant differences (P≤0.05) between salinity levels (within each time point of measurement) are designated with different letters.

Fig. 2.

(A) Disease and (B) senescence index of NIL-O-1, NIL-ol-2, and NIL-Ol-4 (written as Ol-1, ol-2, Ol-4 in the figure) and the recurrent parent MM under powdery mildew without salt (0mM NaCl) and in combination with 50, 100, and 150mM NaCl, measured at 15 days post-inoculation. (C) Leaf phenotypes under different powdery mildew and combined stress treatments. Asterisks denote statistically significant pairwise differences (P≤0.05) between powdery mildew without NaCl and each of the combined stress [powdery mildew+NaCl) treatments for each genotype (n=4; error bars represent the SEM)].

Fig. 3.

(A, B) Averaged concentrations of Na⁺ and Cl⁻ across the LYC4 ILs and the recurrent parent MM under powdery mildew without salt and in combination with salt stress. Statistically significant differences (P≤0.05) are designated with different letters. (C, D) Regression analysis between Na⁺ and Cl⁻ concentration and disease index (DI) change across the different salinity treatments in combination with powdery mildew. The dependent variable DI was calculated by subtracting DI under salt stress conditions from that under non-salt stress conditions (only powdery mildew infection). R²=percentage of variance of DI change explained by the Na⁺ and Cl⁻ concentration.

Fig. 4.

Relative Oidium neolycopersici fungal biomass in MM and NIL-O-1, NIL-ol-2, and NIL-Ol-4 (written as Ol-1, ol-2, Ol-4 in the figure) under powdery mildew infection alone and in combination with 50mM and 150mM NaCl. Values are normalized with that of MM under powdery mildew infection (no salt stress). Asterisks denote statistically significant pairwise differences (P≤0.05) between powdery mildew (0mM NaCl+PM) and each of the combined stress treatments (50mM NaCl+PM and 150mM NaCl+PM) for an individual genotype (n=4; error bars represent the SEM).

Fig. 5.

(A) Above-ground biomass (FW) of NIL-O-1, NIL-ol-2, and NIL-Ol-4 (written as Ol-1, ol-2, Ol-4 in the figure) and the recurrent parent MM under salt stress (0, 50, and 150mM NaCl) or powdery mildew alone, and their combination. Level 0 for salinity stress corresponds to stress-free control conditions, while level 0 for powdery mildew-combined stress corresponds to powdery mildew infection alone (no salt stress). (B) Na⁺ and (C) Cl⁻ concentration of Ol-lines and MM under the same treatment scheme Asterisks denote statistically significant differences (P≤0.05) between salinity and powdery mildew-combined stress, within each salt level for an individual genotype (n=4; error bars represent the SEM).

Fig. 6.

(A) Callose deposits in leaves as visualized with UV microscopy after aniline blue staining. (B) Quantification of callose deposition relative to MM under powdery mildew infection (no salt stress). Asterisks denote statistically significant pairwise differences (P≤0.05) between powdery mildew (0mM NaCl+PM) and each of the combined stress treatments (50mM NaCl+PM and 150mM NaCl+PM) for an individual genotype.

Fig. 7.

Expression of marker genes for hormonal, abiotic, and biotic stress signalling pathways in MM, and NIL-Ol-1, NIL-ol-2, and NIL-Ol-4 (written as Ol-1, ol-2, Ol-4 in the figure), relative to EF1a, which was used as a housekeeping gene. The treatment and labelling scheme are the same as in Fig. 5. Asterisks denote statistically significant differences (P≤0.05) between salinity and powdery mildew-combined stress, within each salt level and for an individual genotype (n=4; error bars represent the SEM).