Far-red light promotes Botrytis cinerea disease development in tomato leaves via jasmonate-dependent modulation of soluble sugars

Abstract

Plants experience a decrease in the red:far-red light ratio (R:FR) when grown at high planting density. In addition to eliciting the shade avoidance response, low R:FR also enhances plant susceptibility to pathogens via modulation of defense hormone-mediated responses. However, other mechanisms, also affected by low R:FR, have not been considered as potential components in FR-induced susceptibility. Here, we identify FR-induced accumulation of leaf soluble sugars as a novel component of FR-induced susceptibility. We observed that phytochrome inactivation by FR or phytochrome B mutation was associated with elevated leaf glucose and fructose levels and enhanced disease severity caused by Botrytis cinerea. By experimentally manipulating internal leaf sugar levels, we found that the FR-induced susceptibility in tomato was partly sugar-dependent. Further analysis revealed that the observed sugar accumulation in supplemental FR occurred in a jasmonic acid (JA)-dependent manner, and the JA biosynthesis mutant def1 also displayed elevated soluble sugar levels, which was rescued by exogenous methyl jasmonate (MeJA) application. We propose that the reduced JA responsiveness under low R:FR promotes disease symptoms not only via dampened induction of defense responses, but also via increased levels of soluble sugars that supports pathogen growth in tomato leaves.

Keywords: Botrytis; defense; jasmonate; phytochromes; soluble sugars; tomato.

FIGURE 1

phyB1phyB2 double mutants display increased susceptibility to Botrytis cinerea associated with increased soluble sugar levels in leaves. (a) Relative stem elongation over 5 days for 3‐week‐old Moneymaker and phyB1phyB2 mutants. Data correspond to day₅–day₀ measured at ZT = 3. (b) Soluble sugars (glucose, fructose and sucrose) quantified at midday (ZT = 8) for 3‐week‐old WL‐grown Moneymaker and phyB1phyB2. (c) Lesion area induced by B. cinerea spores on Moneymaker and phyB1phyB2 detached leaflets measured at 3 dpi. Data correspond to the mean ± SEM, asterisks represent significant differences based on Student's t‐test between genotypes (p < .05). n = 5–10 plants per treatment

FIGURE 2

Far‐red light enrichment leads to a gradual increase in soluble sugars and susceptibility to Botrytis cinerea. Percentage increase in (a) total soluble sugars, (b) glucose, (c) fructose and (d) sucrose levels measured in WL + FR‐treated Moneymaker plants compared to WL for each timepoint. Light treatment started at 10 a.m. on Day 1 (ZT = 3) and soluble sugar measurements and bioassays were performed a 3 p.m. (ZT = 8). Data correspond to mean ± SEM, n = 4–6 plants per treatment. The percentage increase in total soluble sugar levels was. (e) Disease caused by B. cinerea inoculated at different days after the start of WL or WL + FR treatment of 3‐week‐old tomato leaflets. Lesion area was measured at 3 dpi. Data correspond to mean ± SEM, n = 8 plants per treatment. Asterisks represent significant differences between WL and WL + FR treatment according to Student's t‐test (p < .05)

FIGURE 3

Far‐red light enrichment leads to increased disease severity possibly in a sugar‐dependent manner. (a–d) Disease rating of pathogens on 3‐week‐old tomato plants (Moneymaker) pretreated in WL or WL + FR for 5 days. Pretreated leaflets were inoculated with (a) Botrytis cinerea (Bc 05.10 and (b) another strain isolated from pepper), (c) the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 and (d) the oomycete Phytophthora infestans strain 88069. (e–h) Pathogen growth measurements on media supplemented with increasing concentrations (0, 10, 20 or 50 g L⁻¹) of glucose (Control, Gluc_10, Gluc_20 and Gluc_50). (e and f) Fungal growth measurement on half strength PDA‐based media for B. cinerea strains 3 days after depositing a 5 μl droplet of a 1.5 × 10⁶ spores ml⁻¹ solution in the centre of the plates. (g) Bacterial growth measurements in liquid KB medium for P. syringae by measuring the OD₆₆₀ every hour for 3 hr compared to timepoint 0. (h) Mycelial growth measurements for P. infestans on half strength V8‐based media 7 days after placing a mycelium plug in the centre of the plate (ø 0.6 cm). n = 3–10. Data represent mean ± SEM. Asterisks represent significant differences according to Student's t‐test and different letters according to ANOVA with Tukey's post‐hoc test (p < .05), respectively

FIGURE 4

Internal soluble sugar levels in tomato leaflets dictate the severity of the symptoms induced by Botrytis cinerea. (a) Setup used to modulate soluble sugar levels in tomato detached leaflets. (b and d) Quantifications of total soluble sugars composed of glucose (pink), fructose (yellow) and sucrose (blue) fractions after 5 days in WL or WL + FR conditions, including a 24 hr treatment with either 0.5 M glucose, 100 μM DCMU or a mock solution on detached leaflets. (c and e) Disease rating on tomato leaflets treated with glucose (c) or DCMU (e) and drop‐inoculated by B. cinerea spores. Lesion areas were measured at 3 days post inoculation. Data show mean ± SEM, letters represent significant differences according to ANOVA with Tukey's post‐hoc test (p < .05) with capital letters representing significance for the total soluble sugar levels (glucose + fructose + sucrose), n = 7–8 plants per treatment

FIGURE 5

Local far‐red illumination on Leaf 4 increases soluble sugar content and lesion development in both Leaf 4 and Leaf 3. Schematic overview of the local FR illumination procedure on Leaf 3 (L3 in a) and on Leaf 4 (L4 in d). Experiments were carried out on the illuminated leaflet (local) or a leaflet located on another leaf (distal). Three‐week‐old tomato plants were illuminated for 5 days with local FR (FR) or exposed to white light (WL) as a control. Soluble sugar quantifications including the glucose, fructose and sucrose fractions (b and e) as well as lesion area measurement on L3 and L4 after inoculation with Botrytis cinerea spores were performed at midday on Day 5 (c and f). Data show mean ± SEM and different letters represent significant differences (ANOVA with Tukey's post‐hoc test, p < .05), n = 7–14 plants per treatment

FIGURE 6

Jasmonic acid signalling inhibits sugar accumulation in tomato associated with increased lesion development. (a–c) Leaf total soluble sugar levels including glucose (pink), fructose (yellow) and sucrose (blue) fractions as well as (d–f) lesion area induced by B. cinerea on detached leaflets at 3 dpi in (a and d) Moneymaker plants exposed for 5 days to WL or WL + FR conditions and treated daily with 50 μM MeJA or a mock solution, (b and e) in tomato cv. Castlemart plants and the JA biosynthesis mutants def1 (defenseless 1) exposed to WL or WL + FR conditions for 5 days or (c and f) in the def1 mutant treated for 5 days in WL and WL + FR and treated daily with a 50 μM MeJA or a mock solution. Different letters represent significant differences according to ANOVA, Tukey's post‐hoc test (p < .05). Data represent mean ± SEM, n = 3–8 plants per treatment