COMT1 Silencing Aggravates Heat Stress-Induced Reduction in Photosynthesis by Decreasing Chlorophyll Content, Photosystem II Activity, and Electron Transport Efficiency in Tomato

Abstract

Despite a range of initiatives to reduce global carbon emission, the mean global temperature is increasing due to climate change. Since rising temperatures pose a serious threat of food insecurity, it is important to further explore important biological molecules that can confer thermotolerance to plants. Recently, melatonin has emerged as a universal abiotic stress regulator that can enhance plant tolerance to high temperature. Nonetheless, such regulatory roles of melatonin were unraveled mainly by assessing the effect of exogenous melatonin on plant tolerance to abiotic stress. Here, we generated melatonin deficient tomato plants by silencing of a melatonin biosynthetic gene, CAFFEIC ACID O-METHYLTRANSFERASE 1 (COMT1), to unveil the role of endogenous melatonin in photosynthesis under heat stress. We examined photosynthetic pigment content, leaf gas exchange, and a range of chlorophyll fluorescence parameters. The results showed that silencing of COMT1 aggravated heat stress by inhibiting both the light reactions and the carbon fixation reactions of photosynthesis. The photosynthetic pigment content, light absorption flux, trapped energy flux, energy dissipation, density of active reaction center per photosystem II (PSII) cross-section, the photosynthetic electron transport rate, the maximum photochemical efficiency of PSII photochemistry, and the rate of CO₂ assimilation all decreased in COMT1-silenced plants compared with that of non-silenced plants particularly under heat stress. However, exogenous melatonin alleviated heat-induced photosynthetic inhibition in both genotypes, indicating that melatonin is essential for maintaining photosynthetic capacity under stressful conditions. These findings provide genetic evidence on the vital role of melatonin in photosynthesis and thus may have useful implication in horticultural crop management in the face of climate change.

Keywords: JIP test; chlorophyll fluorescence; heat stress; melatonin; photosynthesis; tomato.

FIGURE 1

Effects of COMT1 silencing and heat stress on plant phenotype and photosynthetic pigment concentration. (A) Plant phenotype, (B) chlorophyll a content, (C) chlorophyll b content, and (D) carotenoids content in third fully expanded leaves. Tomato seedlings at five-leaf stage were exposed to 40°C temperature for 9 h before analysis of the phenotype and pigment contents. The results are expressed as the mean values ± SE, n = 6. The mean values denoted by the same letter do not significantly differ at a P < 0.05 according to the Tukey’s test.

FIGURE 2

Changes in gas exchange parameters as influenced by COMT1 silencing and heat stress in tomato leaves. (A) The light-saturated rate of CO₂ assimilation (A_sat), (B) stomatal conductance (G_s), (C) intercellular CO₂ concentration (C_i), and (D) transpiration rate (T_r). Tomato seedlings at five-leaf stage were exposed to 40°C temperature for 9 h before analysis of gas exchange parameters. Data are presented as the mean of six replicates (±SE). Different letters indicate significant differences (P < 0.05) according to the Tukey’s test.

FIGURE 3

Energy pipeline leaf models of phenomenological fluxes (per cross-section, CS) in the third fully expanded leaf in tomato as influenced by COMT1 silencing and heat stress. The results are expressed as the mean values ± SE, n = 6. Each relative value is drawn by the width of the corresponding arrow, standing for a parameter. Empty and full black circles indicate the percentage of active (Q_A reducing) and non-active (non-Q_A reducing) reaction centers of photosystem II (PSII), respectively; ABS/CSm, photon flux absorbed by the antenna pigments per CS; TR/CSm, trapped energy flux per CS; ET/CSm, electron transport flux per CS; D/CSm, non-photochemical quenching. Different letters indicate significant differences (P < 0.05) according to the Tukey’s test.

FIGURE 4

Effect of COMT1 silencing and heat stress on quantum yield and electron transport rate in PSI and PSII in tomato leaves. Tomato seedlings at five-leaf stage were exposed to 40°C temperature for 9 h before analysis of chlorophyll fluorescence parameters. The bars (means ± SE, n = 6) labeled with different letters are significantly different at P < 0.05 according to Tukey’s test. Y(I), effective quantum yield of PSI; Y(II), effective quantum yield of PSII; ETR(II), electron transport rate of PSII; ETR(I), electron transport rate of PSI.

FIGURE 5

Effects of COMT1 silencing, exogenous melatonin and heat stress either alone or combined on the maximum photochemical efficiency of photosystem II (F_v/F_m) and CO₂ assimilation rate. (A) F_v/F_m shown in pseudo color images, the false color code depicted at the bottom of the image ranges from 0 (black) to 1 (purple); (B) F_v/F_m values; and (C) the light-saturated rate of CO₂ assimilation (A_sat). Tomato seedlings at five-leaf stage were exposed to 40°C temperature for 9 h before analysis. Plants were pretreated with 10 μmol L⁻¹ melatonin on leaves at 8 h prior to imposition of the heat stress. F_v/F_m and A_sat both were measured on the third fully expanded leaves; however, plants were dark adapted for 30 min before recording the F_v/F_m by using an imaging pulse amplitude modulated (PAM) fluorimeter (IMAG-MAXI; Heinz Walz, Effeltrich, Germany). The results are expressed as the mean values ± SE, n = 6. Different letters indicate significant differences (P < 0.05) according to the Tukey’s test.