The fate of methyl salicylate in the environment and its role as signal in multitrophic interactions

Methyltransferase TaSAMT1 mediates wheat freezing tolerance by integrating brassinosteroid and salicylic acid signaling

Cold injury is a major environmental stress affecting the growth and yield of crops. Brassinosteroids (BRs) and salicylic acid (SA) play important roles in plant cold tolerance. However, whether or how BR signaling interacts with the SA signaling pathway in response to cold stress is still unknown. Here, we identified an SA methyltransferase, TaSAMT1 that converts SA to methyl SA (MeSA) and confers freezing tolerance in wheat (Triticum aestivum). TaSAMT1 overexpression greatly enhanced wheat freezing tolerance, with plants accumulating more MeSA and less SA, whereas Tasamt1 knockout lines were sensitive to freezing stress and accumulated less MeSA and more SA. Spraying plants with MeSA conferred freezing tolerance to Tasamt1 mutants, but SA did not. We revealed that BRASSINAZOLE-RESISTANT 1 (TaBZR1) directly binds to the TaSAMT1 promoter and induces its transcription. Moreover, TaBZR1 interacts with the histone acetyltransferase TaHAG1, which potentiates TaSAMT1 expression via increased histone acetylation and modulates the SA pathway during freezing stress. Additionally, overexpression of TaBZR1 or TaHAG1 altered TaSAMT1 expression and improved freezing tolerance. Our results demonstrate a key regulatory node that connects the BR and SA pathways in the plant cold stress response. The regulatory factors or genes identified could be effective targets for the genetic improvement of freezing tolerance in crops.

Aerosol Alteration of Behavioral Response to Pheromone in Bombyx mori

Because of the complexity to study them, aerosols have been neglected in nearly all studies on olfaction, especially studies dealing with odor capture. However, aerosols are present in large quantities in the atmosphere and have the physico-chemical ability to interact with odor molecules, in particular the many pheromones with low volatility. We submitted male moths of Bombyx mori to bombykol puffs, the main fatty alcohol component of its sex pheromone, depending on whether the air is free of aerosols, charged with ambient concentration aerosols or supplemented with aqueous aerosols and recorded their arousal behavior. Aerosols and pheromone do interact consistently over all experiments and moths react better in low aerosol-concentration conditions. We propose four hypotheses for explaining this impediment, the two most likely resorting to competition between odor molecules and aerosols for the olfactory pores and postulate a reversal to a positive impact of aerosols on communication, depending on the particular physico-chemical properties of the multiphasic interaction. Studying the partitioning between gas and particulate phases in the transport and reception of odors is key for advancing the chemico-physical understanding of olfaction.

Arbuscular Mycorrhizal Fungus Alters Alfalfa (Medicago sativa) Defense Enzyme Activities and Volatile Organic Compound Contents in Response to Pea Aphid (Acyrthosiphon pisum) Infestation

Pea aphid (Acyrthosiphon pisum) infestation leads to withering, reduced yield, and lower quality of the host plant. Arbuscular mycorrhizal (AM) fungi have been found to enhance their host plants’ nutrient uptake, growth, and resistance to biotic stresses, including pathogen infection and insect pest infestation. Therefore, we evaluated the effects of AM fungus Rhizophagus intraradices on alfalfa defense responses to pea aphid infestation. Aphid infestation did not affect the colonization of AM fungus. The inoculation of AM fungus, on average, enhanced alfalfa catalase and the contents of salicylic acid and trypsin inhibitor by 101, 9.05, and 7.89% compared with non-mycorrhizal alfalfa, respectively. In addition, polyphenol oxidase activities significantly increased by six-fold after aphid infestation in mycorrhizal alfalfa. Moreover, the fungus significantly (p < 0.05) improved alfalfa shoot N content, net photosynthetic and transpiration rates, and shoot dry weight in aphid infected treatment. The aphid infestation changed the total volatile organic compounds (VOCs) in alfalfa, while AM fungus enhanced the contents of methyl salicylate (MeSA). The co-expression network analysis of differentially expressed genes (DEGs) and differentially expressed VOCs analysis showed that three DEGs, namely MS.gene23894, MS.gene003889, and MS.gene012415, positively correlated with MeSA both in aphid and AM fungus groups. In conclusion, AM fungus increased alfalfa’s growth, defense enzyme activities, hormones, and VOCs content and up-regulated VOC-related genes to enhance the alfalfa’s resistance following aphid infestation.

Individual adsorption of low volatility pheromones: Amphiphilic molecules on a clean water-air interface

Environmental conditions can alter olfactory scent and chemical communication among biological species. In particular, odorant molecules interact with aerosols. Thermodynamics variables governing the adsorption from air to water surface of bombykol, the most studied pheromone, and of three derivative molecules, bombykal, bombykoic acid, and bombykyle acetate, are computed by steered and un-biased molecular dynamics in order to compare the role of their polar head group on adsorption on aqueous aerosols. When adsorbed, the molecule center of mass stands at about 1.2 Å from the interface and oscillates on the same length scale, trapped in an energy well. Gibbs energy of adsorption and desorption time of bombykol are found to be 9.2 k_BT and 59 µs, respectively. The following ordering between the molecules is observed, reading from the more to the least adsorbed: bombykoic acid > bombykol > bombykoic acetate > bombykal. It originates from a complex interplay of entropy and enthalpy. The entropy and enthalpy of adsorption are discussed in the light of structural arrangement, H-bonding, and hydrophilic tail positioning of the molecules at the interface. Our results show that, when dispersed in the air, pheromones adsorb on aqueous aerosols. However, the individual residence time is quite short on pure water surfaces. Aerosols can, therefore, only have a decisive influence on chemical communication through collective effects or through their chemical composition that is generally more complex than that of a pure water surface.

Methyl salicylate is the most effective natural salicylic acid ester to close stomata while raising reactive oxygen species and nitric oxide in Arabidopsis guard cells

Modulation by salicylic acid (SA) and its six esters of stomatal closure was evaluated in Arabidopsis thaliana. The seven compounds tested are salicylic acid (SA), acetylsalicylate (ASA), methyl salicylate (MeSA), propyl salicylate (PrSA), amyl salicylate, benzyl salicylate, and salicin. Among these, MeSA was the most effective to induce stomatal closure, followed by salicin and SA, while ASA was the least effective. Since SA, ASA, and MeSA could modulate plant function, the effects of these three compounds on the levels of reactive oxygen species (ROS) or nitric oxide (NO) in guard cells were studied. MeSA and SA raised the content of ROS or NO in as with ABA. The extent of ROS/NO production in response to ASA was the lowest. Reversal by cPTIO or catalase of stomatal closure by MeSA indicated the essentiality of NO and ROS for stomatal closure. Further studies revealed peroxidase as the ROS source during stomatal closure by MeSA, unlike the dominant role of NADPH oxidase in ROS production induced by ABA. The rise in NO production by ABA or MeSA was dependent on nitrate reductase and NO synthase-like enzyme. Given its most effective nature, MeSA can be an excellent tool to examine the signaling components in guard cells and other plant tissues. The ability of MeSA to induce stomatal closure is physiologically relevant because of its volatile nature, stability, and systemic action.

How Adsorption of Pheromones on Aerosols Controls Their Transport

We propose a general transport theory for pheromone molecules in an atmosphere containing aerosols. Many pheromones are hydrophobic molecules containing polar groups. They are low volatile and have some properties similar to those of hydrotropes. They therefore form a nonsoluble film at the water-air interface of aerosols. The fate of a small pheromone puff in air is computed through reaction-diffusion equations. Partitioning of pheromones between the gas and the aerosol surface over time is studied for various climate conditions (available aerosol surface) and adsorption affinities (energy of adsorption). We show that, for adsorption energy above 30 k B T per molecule, transport of pheromones on aerosols dominates over molecular transport typically 10 s after pheromone emission, even when few adsorbing aerosols are present. This new communication path for airborne chemicals leads to distinctive features including enhanced signal sensibility and increased persistence of pheromone concentration in the air due to slow diffusion of aerosols. Each aerosol droplet has the ability to adsorb thousands of pheromones to the surface, keeping a "history" of the atmospheric content between emission and reception. This new mechanism of pheromone transport leads to dramatic consequences on insect sensing revisiting the way we figure the capture of chemical signals.