Systemic resistance is induced by pathogens and confers protection against a broad range of pathogens. Recent studies have indicated that salicylic acid (SA) derivative methyl salicylate (MeSA) serves as a long-distance phloem-mobile systemic resistance signal in tobacco, Arabidopsis, and potato. However, other experiments indicate that jasmonic acid (JA) is a critical mobile signal. Here, we present evidence suggesting both MeSA and methyl jasmonate (MeJA) are essential for systemic resistance against Tobacco mosaic virus (TMV), possibly acting as the initiating signals for systemic resistance. Foliar application of JA followed by SA triggered the strongest systemic resistance against TMV. Furthermore, we use a virus-induced gene-silencing–based genetics approach to investigate the function of JA and SA biosynthesis or signaling genes in systemic response against TMV infection. Silencing of SA or JA biosynthetic and signaling genes in Nicotiana benthamiana plants increased susceptibility to TMV. Genetic experiments also proved the irreplaceable roles of MeSA and MeJA in systemic resistance response. Systemic resistance was compromised when SA methyl transferase or JA carboxyl methyltransferase, which are required for MeSA and MeJA formation, respectively, were silenced. Moreover, high-performance liquid chromatography–mass spectrometry analysis indicated that JA and MeJA accumulated in phloem exudates of leaves at early stages and SA and MeSA accumulated at later stages, after TMV infection. Our data also indicated that JA and MeJA could regulate MeSA and SA production. Taken together, our results demonstrate that (Me)JA and (Me)SA are required for systemic resistance response against TMV.

Summary: Mitochondrial alternative oxidase (AOX) is involved in a large number of plant physiological processes, such as growth, development and stress responses; however, the exact role of AOX in response to drought remains unclear. In our study, we provide solid evidences that the activated AOX capacity positively involved in ethylene‐induced drought tolerance, in tomato (Solanum lycopersicum), accompanied by the changing level of hydrogen peroxide (H₂O₂) and autophagy. In AOX1a‐RNAi plants, the ethylene‐induced drought tolerance was aggravated and associated with decreasing level of autophagy. The H₂O₂ level was relatively higher in AOX1a‐RNAi plants, whereas it was lower in AOX1a‐overexpressing (35S‐AOX1a‐OE) plants after 1‐(aminocarbonyl)‐1‐cyclopropanecarboxylic acid (ACC) pretreatment in the 14th day under drought stress. Interestingly, the accumulation of autophagosome was accompanied by the changing level of reactive oxygen species (ROS) in AOX transgenic tomato under drought stress whether or not pretreated with ACC. Pharmacological scavenging of H₂O₂ accumulation in AOX1a‐RNAi (aox19) stimulated autophagy acceleration under drought stress, and it seems that AOX‐dependent ROS signalling is critical in triggering autophagy. Lower levels of ROS signalling positively induce autophagy activity, whereas higher ROS level would lead to rapid programmed cell death (PCD), especially in ethylene‐mediated drought tolerance. Moreover, ethylene‐induced autophagy during drought stress also can be through ERF5 binding to the promoters of ATG8d and ATG18h. These results demonstrated that AOX plays an essential role in ethylene‐induced drought tolerance and also played important roles in mediating autophagy generation via balancing ROS level.

Significance: Overapplication of nitrogen (N) fertilizer causes delayed flowering and negatively impacts the function and composition of natural ecosystems and climate. In this study, we demonstrate that flowering time variations regulated by altered nitrogen levels are mediated by two key factors: ferredoxin-NADP ⁺ -oxidoreductase (FNR1) and the blue-light receptor cryptochrome 1 (CRY1). Nitrogen regulates FNR1 expression, thereby contributing to changes in NADPH/NADP ⁺ and ATP/AMP ratios, which in turn activates adenosine monophosphate-activated protein kinase to modulate nuclear CRY1 abundance, which further acts in the N signal input pathway to affect central clock function and flowering time. A better understanding of N-regulated floral transition will offer biotechnological solutions to improve sustainable agriculture.

Although the alternative oxidase (AOX) has been proposed to play a role in fruit development, the function of AOX in fruit ripening is unclear. To gain further insight into the role of AOX in tomato fruit ripening, transgenic tomato plants 35S-AOX1a and 35S-AOX-RNAi were generated. Tomato plants with reduced LeAOX levels exhibited retarded ripening; reduced carotenoids, respiration, and ethylene production; and the down-regulation of ripening-associated genes. Moreover, no apparent respiratory climacteric occurred in the AOX-reduced tomato fruit, indicating that AOX might play an important role in climacteric respiration. In contrast, the fruit that overexpressed LeAOX1a accumulated more lycopene, though they displayed a similar pattern of ripening to wild-type fruit. Ethylene application promoted fruit ripening and anticipated ethylene production and respiration, including the alternative pathway respiration. Interestingly, the transgenic plants with reduced LeAOX levels failed to ripen after 1-methylcyclopropene (1-MCP) treatment, while such inhibition was notably less effective in 35S-AOX1a fruit. These findings indicate that AOX is involved in respiratory climacteric and ethylene-mediated fruit ripening of tomato.