LC-MS/MS based comparative proteomics of floral nectars reveal different mechanisms involved in floral defense of Nicotiana spp., Petunia hybrida and Datura stramonium

Ornamental tobacco floral nectar is a rich source of antimicrobial peptides

Although floral nectar is a rich source of nutrients, it is rarely infected by microorganisms. Defense molecules such as proteins have been identified in this fluid, but defense peptides have been largely overlooked. Thus, the aim of this study was to perform an extensive peptidomic analysis of the ornamental tobacco floral nectar to seek peptides involved in nectar defense. Using LC-MS/MS, 793 peptides were sequenced and characterized. After extensive bioinformatics analysis, six peptides were selected for further characterization, synthesis, and evaluation of their antimicrobial properties against phytopathogenic fungi and bacteria. All six peptides had antimicrobial activity to some extent. However, the activity varied by peptide concentration and microorganism tested. An analysis of the action mechanism revealed damage in the cell membrane induced by peptides. The results show that floral nectar is rich in peptides and that, together with proteins and hydrogen peroxide, they contribute to plant defense against microorganisms during pollination.

Identification and quantitation of the novel insecticide sulfoxaflor and its metabolites in floral nectar from Salvia splendens Ker Gawl. (Lamiaceae)

Sulfoxaflor is a new systemic insecticide developed as a replacement for older neonicotinoids which are known to be toxic to pollinators. However, its metabolism in nectar and effect on nectar biosynthesis have not been investigated. After soil and foliar application, sulfoxaflor and its main metabolites in soil, leaf and Salvia splendens nectar, were measured by liquid chromatography coupled with triple quadrupole mass spectrometer (LC-MS/MS). The chemical composition between the clean and sulfoxaflor spiked nectar were also compared. The activities of two possible sulfoxaflor detoxifying enzymes in S. splendens nectar, nitrile hydratase and glutathione-s-transferase, were measured by LC-MS and spectrophotometry. S. splendens nectar proteome was investigated by high-resolution orbitrap-based MS/MS to screen for sulfoxaflor detoxifying relevant proteins. S. splendens could absorb sulfoxaflor through root or leaf surface and secrete a proportion of sulfoxaflor along with its metabolites into the nectar. After soil application, sulfoxaflor's low toxic metabolite X11719474 was dominant in the nectar and reached an average concentration of 8905 ppb. However, after foliar application, sulfoxaflor was dominant over its metabolites in the nectar. S. splendens nectar has no nitrile hydratase and glutathione-s-transferase activity and none of the 106 proteins identified in the nectar were predicted to function in detoxifying sulfoxaflor. Soil and foliar sulfoxaflor application can result in different profiles of sulfoxaflor and its metabolites presented in the nectar. However, sulfoxaflor had no effects on S. splendens nectar secretion and chemical composition and cannot be directly detoxified by components in the nectar.

Comparing the contents, functions and neonicotinoid take-up between floral and extrafloral nectar within a single species (Hemerocallis citrina Baroni)

BACKGROUND AND AIMS

Many angiosperms can secrete both floral (FN) and extrafloral (EFN) nectar. However, much remains unclear about how EFN and FN differ in secretion, composition and ecological function, especially when both FN and EFN are secreted on flowers of the same species.

METHODS

Hemerocallis citrina flowers secrete both FN and EFN. The FN and EFN traits including volume, presentation pattern and temporal rhythms of secretion were compared by field observation. Sugar and amino acid contents were analysed using regular biochemical methods, whereas the proteome was investigated by combined gel-based and gel-free approaches. Animal feeders on FN and EFN were investigated by field observation. Hemerocallis citrina plants were exposed by soil drenching to two systemic insecticides, acetamiprid and imidacloprid, and the concentration of these in FN and EFN was measured by ultra-high performance liquid chromatography coupled with mass spectrometry.

KEY RESULTS

Hemerocallis citrina FN was concentrated and sucrose dominant, secreted in the mature flower tube and served as a reward for pollinators. Conversely, EFN was hexose rich, more dilute and less rich in sugar and amino acids. EFN was secreted on the outside of developing floral buds, and was likely to attract predatory animals for defence. EFN had fewer phenolics, but more pathogenesis-related components, such as chitinase and glucanase. A significantly different proteomic profile and enzymatic activities between FN and EFN suggest that they had different biosynthesis mechanisms. Both neonicotinoid insecticides examined became present in both nectar types soon after application, but in greater concentration within EFN; EFN also attracted a wider range of insect species than FN.

CONCLUSIONS

Hemerocallis citrina FN and EFN differed in production, composition and ecological function. The EFN pathway could be a significant way for neonicotinoids to enter the wild food chain, and must be considered when evaluating the risks to the environment of other systemic insecticides.

Floral organ-specific proteome profiling of the floral ornamental orchid (Cymbidium goeringii) reveals candidate proteins related to floral organ development

BACKGROUND

Cymbidium goeringii, belonging to the Orchidaceae family, is an important ornamental plant with striking petals and lips. Extremely diversified floral patterns and morphologies make C. goeringii good research material to examine floral development of orchids. However, no floral organ-specific protein has been identified yet. To screen floral development associated proteins, four proteomes from petal (PE), lip (LI), gynostemium (GY), and sepal (SE) were analyzed using Tandem Mass Tag-based proteomic analysis.

RESULTS

A total of 6626 unique peptides encoding 2331 proteins were identified in our study. Proteins in several primary metabolic pathways, including amino acid metabolism, energy metabolism, and lipid metabolism, were identified as differentially expressed proteins. Interestingly, most of the energy metabolism-related proteins highly expressed in SE, indicating that SE is an important photosynthetic organ of C. goeringii flower. Furthermore, a number of phytohormone-related proteins and transcription factors (TFs) were identified in C. goeringii flowers. Expression analysis showed that 1-aminocyclopropane-1-carboxylate oxidase highly expressed in GY, IAA-amino acid hydrolase ILR1-like 4 and gibberellin receptor 1 C greatly expressed in LI, and auxin-binding protein ABP20 significantly expressed in SE, suggesting a significant role of hormones in the regulation of flower morphogenesis and development. For TFs, GY-highly expressed bHLH13, PE-highly expressed WRKY33, and GY-highly expressed VIP1, were identified.

CONCLUSIONS

Mining of floral organ differential expressed enzymes and TFs helps us to excavate candidate proteins related to floral organ development and to accelerate the breeding of Cymbidium plants.

Nectar antimicrobial compounds and their potential effects on pollinators

Nectar is a sugary, aqueous solution that plants offer as a reward to animal mutualists for visitation. Since nectars are so nutrient-rich, they often harbor significant microbial communities, which can be pathogenic, benign, or even sometimes beneficial to plant fitness. Through recent advances, it is now clear that these microbes alter nectar chemistry, which in turn influences mutualist behavior (e.g. pollinator visitation). To counteract unwanted microbial growth, nectars often contain antimicrobial compounds, especially in the form of proteins, specialized (secondary) metabolites, and metals. This review covers our current understanding of nectar antimicrobials, as well as their interplay with both microbes and insect visitors.

A single honey proteome dataset for identifying adulteration by foreign amylases and mining various protein markers natural to honey

Honey adulteration is a common practice that deceives consumers and devalues the unique curative and food properties of honey. For marketing, each honey must satisfy an internationally valid Codex standard. One of the quality parameters is diastase/amylase activity, which, if lowered, may be compensated for by the addition of foreign amylases. However, the estimation of enzyme activity does not enable identification of artificially added amylases. 45 honey samples were analyzed using label-free nanoLC-MS/MS proteomics. Four honeys were found to contain the foreign amylases from Aspergillus niger, Bacillus amyloliquefaciens and/or Bacillus licheniformis. This result was confirmed via proof of specificity at multiple levels. Furthermore, we identified a series of plant-related protein groups. Despite plant-related proteins constituting a significant portion of honey proteins, they were minor components compared to the major honey bee-derived proteins. Bioinformatic analysis also provided evidence for aphid and catalase proteins in honey, but the limited specificity of the MS/MS identified peptides must be considered. Overall, we demonstrate a proteomics approach employing LC-MS/MS that is useful for proving adulteration and assessing honey quality. As an resource useful for reference, we provide curated sequence databases. In addition, we provide many markers that are naturally found in honey for future studies. SIGNIFICANCE: Honey is unique natural product used since ancient times as a food and natural medicine. Humans strive to understand honey components because they can characterize different types of honey and be used for authentication and origin assessment. One of the important honey components are proteins. The proteins present in honey can naturally occur in honey, but some of them can be used to mask deficiencies in some honey quality properties. Diastases/amylases are such proteins, and their activity, a measure of honey freshness, can decrease in time or due to processing. To our knowledge, we for the first time specifically identify foreign amylases in honey. However, this study provided new information on other non-honey bee proteins in honey. Thus, this study is also of importance due to its identification of plant and aphid proteins and catalase-related proteins. This study provides a clue explaining the controversial presence of catalase in honey, since catalases can be identified and their origin determined via proteomics.