Melatonin has been demonstrated to play a variety of roles in plants. Of particular importance is its role as a potent antioxidative agent. In the present study, we generated melatonin-deficient tomato plants using virus-induced gene silencing (VIGS) approach and melatonin-rich tomato plants by foliar application of melatonin. These tomato plants were used to assess the effect of melatonin on chilling-induced oxidative stress and chilling-induced photosynthesis inhibition. We found that melatonin deficiency increased accumulation of reactive oxygen species (ROS) and aggravated lipid peroxidation in chilling-stressed tomato leaves, while exogenous application of melatonin had the opposite effect. Under chilling stress, melatonin-deficient tomato plants showed impaired antioxidant capacity as evidenced by lower activities of antioxidant enzymes and decreased rations of reduced glutathione (GSH)/oxidized glutathione (GSSG) and reduced ascorbate (AsA)/oxidized ascorbate (DHA), compared with melatonin-rich tomato plants. Furthermore, suppression of melatonin biosynthesis led to more photosynthesis inhibition under the chilling condition and compromised the capability of subsequent photosynthesis recovery in tomato plants. In addition, melatonin-deficient tomato plants displayed less activity of an important Calvin-Benson cycle enzyme sedoheptulose-1,7-bisphosphatase (SBPase) than melatonin-rich tomato plants under chilling stress. Collectively, our data indicate that melatonin is critical for antioxidant capacity and redox balance and is in favor of photosynthesis in tomato plants under chilling stress.

Both jasmonic acid (JA) and melatonin (MT) have been demonstrated to play positive roles in cold tolerance, however, whether and how they crosstalk in the cold responses in plants remain elusive. Here, we report that JA and MT act synergistically in the cold tolerance in tomato plants (Solanum lycopersicum). It was found that JA and MT were both substantially accumulated in response to cold stress and foliar applications of methyl jasmonate (MeJA) and MT promoted cold tolerance as evidenced by increased Fv/Fm, decreased relative electrolyte leakage (EL) and declined H₂O₂ accumulation in tomato plants. Inhibition of MT biosynthesis attenuated MeJA-induced cold tolerance, while inhibition of JA biosynthesis reduced MT accumulation in tomato plants under cold conditions. Furthermore, qRT-PCR analysis showed that the expressions of two MT biosynthetic genes, SlSNAT and SlAMST, were strongly induced by MeJA, whereas suppression of SlMYC2, a master JA signaling regulator, abated the expressions of SlSNAT and SlAMST under cold stress. Additionally, suppression of SlMYC2 reduced MT accumulation, decreased Fv/Fm and increased EL in cold-stressed tomato plants. Interestingly, exogenous MT promoted JA accumulation, while inhibition of MT biosynthesis significantly reduced JA accumulation in tomato plants under the cold condition. Taken together, these results suggest that JA and MT act cooperatively in cold tolerance and form a positive feedback loop, amplifying the cold responses of tomato plants. Our findings might be translated into the development of cold-resistant tomato cultivars by genetically manipulating JA and MT pathways.

The plant cuticle, composed of cutin and waxes, is a hydrophobic layer coating the aerial organs of terrestrial plants and playing a critical role in limiting water loss. While melatonin has been recently demonstrated to be involved in responses to drought stress in plants, its relationship with cuticle formation is not known. In the present work, we report the effects of melatonin on the formation of cuticle in tomato leaves subjected to water deficit. Preliminary analysis by light microscope showed that tomato leaves pretreated with exogenous melatonin might have thicker cutin than tomato leaves without melatonin pretreatment under water deficit condition. Chemical characterization showed that exogenous application of melatonin increased the level of cuticular waxes in tomato leaves under water deficit. Consistent with the change in cuticular waxes was the increased abundance of wax-associated gene transcripts. Further, assessment of water loss and chlorophyll leaching in tomato leaves revealed the association of cuticle deposition with reduced leaf permeability, which is important in restricting water loss in water deficit-stressed tomato plants. These results suggest a role for melatonin in regulating leaf cuticle formation and non-stomatal water loss in leaves.

As a cold-sensitive species, tomato is frequently challenged by cold stress during vegetative and reproductive growth. Understanding how tomato responds to cold stress is of critical importance for sustainable tomato production. In this work, we demonstrate that jasmonate (JA) plays a crucial role in tomato response to cold stress by promoting abscisic acid (ABA) biosynthesis. It was observed that both JA and ABA levels were substantially increased under cold conditions, whereas the suppression of JA biosynthesis abated ABA accumulation. The ABA biosynthesis gene 9-CIS-EPOXYCAROTENOID DIOXYGENASE2 (NCED2) was subsequently found to be associated with JA-mediated ABA biosynthesis in tomato plants in response to cold stress. NCED2 was rapidly induced by exogenous MeJA and cold treatment. Silencing NCED2 led to a decrease in ABA accumulation that was concurrent with increased cold sensitivity. Moreover, blocking ABA biosynthesis using a chemical inhibitor impaired JA-induced cold tolerance in tomato. Furthermore, MYC2, a core component of the JA signaling pathway, promoted the transcription of NCED2, ABA accumulation and cold tolerance in tomato. Collectively, our results support that JA signaling promotes ABA biosynthesis to confer cold tolerance in tomato.

Temperature is a critical environmental factor that plays a vital role in plant growth and development. Temperatures below or above the optimum ranges lead to cold or heat stress, respectively. Temperature stress retards plant growth and development, and it reduces crop yields. Jasmonates (JAs) are a class of oxylipin phytohormones that play various roles in growth, development, and stress response. In recent years, studies have demonstrated that cold and heat stress affect JA biosynthesis and signaling, and JA plays an important role in the response to temperature stress. Recent studies have provided a large body of information elucidating the mechanisms underlying JA-mediated temperature stress response. In the present review, we present recent advances in understanding the role of JA in the response to cold and heat stress, and how JA interacts with other phytohormones during this process.

Leaf senescence represents the final stage of leaf development and is regulated by diverse internal and environmental factors. Jasmonates (JAs) have been demonstrated to induce leaf senescence in several species; however, the mechanisms of JA-induced leaf senescence remain largely unknown in tomato plants (Solanum lycopersicum). In the present study, we tested the hypothesis that sedoheptulose-1,7-bisphosphatase (SBPase), an enzyme functioning in the photosynthetic carbon fixation in the Calvin–Benson cycle, was involved in methyl jasmonate (MeJA)- and dark-induced leaf senescence in tomato plants. We found that MeJA and dark induced senescence in detached tomato leaves and concomitantly downregulated the expression of SlSBPASE and reduced SBPase activity. Furthermore, CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9)-mediated mutagenesis of SlSBPASE led to senescence-associated characteristics in slsbpase mutant plants, including loss of chlorophyll, repressed photosynthesis, increased membrane ion leakage, and enhanced transcript abundance of senescence-associated genes. Collectively, our data suggest that repression of SBPase by MeJA and dark treatment plays a role in JA- and dark-induced leaf senescence.