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Bananas Glow Blue! Yes, They Do!

Dr. Axel Tiessen, Head of the Laboratory of Metabolomics and Molecular Plant Physiology, CINVESTAV, and Head of the National PlanTECC Laboratory, Irapuato, Guanajuato, Mexico


Mexico is among the most biodiverse nations in the world, so it stands to reason that top research institutes, such as the Center for Research and Advanced Studies of the National Polytechnic Institute (CINVESTAV) devote special attention towards understanding this biological and genetic diversity to foster renewable sources of energy, nutrition, therapeutics and more. Dr. Axel Tiessen is Head of the Laboratory of Metabolomics and Molecular Plant Physiology at CINVESTAV’s Irapuato, Guanajuato, Mexico campus, and also Head of the National PlanTECC Laboratory. An important aim of his research is to understand major signals that regulate primary metabolism in plants, including responses to environmental factors and physiological inputs.

Dr. Tiessen’s research team performs research on several crop plants such as maize, amaranth, tomato and potato. They have developed a genetically improved variety of tropical corn called "Vitamaize" that contains antioxidants and vitamins. Another plant that Dr. Tiessen’s lab studies is the banana, whose delicious fruit is popularly found on breakfast tables and as a portable, self-contained snack. In the ripening process, chlorophyll, carotenoids, and polyphenols are respectively responsible for the visual appearance of bananas as they morph from green to yellow and finally to black. Additionally, bananas emit a blue glow under UV light, yet the exact reason for this phenomenon remains unclear. Previous research postulated that chlorophylls fade to give rise to fluorescent chlorophyll catabolites (FCCs) in senescent banana leaves and in ripening banana peels. FCCs were thought to be responsible for the appearance of bright blue luminescent rings in banana peels, and the reason that yellow bananas glow blue under UV light; and it was also implicitly assumed that green bananas lack FCC and therefore do not emit the same glow under identical conditions.

Professional and amateur researchers alike tried to replicate the findings, but many, including Dr. Tiessen’s lab, were unable to do so. Using BioTek’s Cytation™ 5 Cell Imaging Multi-Mode Reader to image bananas with varying degrees of ripeness, they found that green, yellow and brown/black bananas all emitted some level of blue glow when exposed to UV light. Their observations, along with others, opened the door to a number of unanswered questions that Dr. Tiessen was determined to address using images and data from Cytation 5. The lab used Cytation 5 for enzymatic coupled assays, colorimetric assays and for phenotyping whole plant leaves using hyperspectral scanning and fluorescence measurements. “Cytation 5 is radically different from a traditional fluorescent microscope in that we spend less time manually looking at the samples; it’s less taxing on our eyes, and it’s nice to look at a large display instead of peering into an objective pressed up against the face,” Dr. Tiessen says. “The autofocus and autoexposure features add convenience, and we can also perform kinetic imaging over large areas.” He also relied on integrated Gen5™ Microplate Reader and Imager Software to control the instrument and create publication-ready images.

Dr. Tiessen published his findings in Plant Science with a paper entitled, “The fluorescent blue glow of banana fruits is not due to symplasmic plastidial catabolism but arises from insoluble phenols estherified to the cell wall.” (doi. 10.1016/j.plantsci2018.07.006). Among many findings, he shows that blue fluorescence in banana peels emerges from the cell wall, not the symplasm (Figure 1). Analyzing several botanical species, he found that blue fluorescence is not an intrinsic property of the cell wall of all samples (Figures 2-3). The cell wall of dicot plants glows weaker blue than monocot species. In Hibiscus rosa-sinensis L. leaves, blue fluorescence is prominent in the vacuole (Figure 2) while in arabidopsis leaves, it emerges from the trichomes (Figure 3). Strong apoplastic blue fluorescence is present in the banana peel as well as the chlorophyll-lacking fruit pulp (Figure 1). Dr. Tiessen concluded that the blue glow in bananas is not primarily caused by soluble chlorophyll catabolites in the vacuoles, but rather it is caused by insoluble ferulic acid bound to the cell wall. Green chlorophyll is a pigment that acts as a light trap generating black holes of blue fluorescence, making green bananas appear to glow less than yellow ones.

Cytation 5 will continue to be an invaluable addition – and in fact, a “perfect fit” – to Dr. Tiessen’s research, and additional filter sets will enable further, deeper exploration of other plant metabolites. He notes, “With BioTek’s Cytation 5, I feel that I can compete with the research done in other international labs and make a tangible difference in the world.”

Fluorescence images, 20x objective

Figure 1. Fluorescence images, 20x objective, of a yellow banana fruit (inside fruit pulp and outside peel) taken using Cytation 5 with DAPI and GFP filter sets; no added fluorescent dyes. Note that both blue and green fluorescence emerge from the cell wall and not from inside the cell.


Fluorescence images, 20x objective, of a green Hibiscus rosa-sinensis L. leaf

Figure 2. Fluorescence images, 20x objective, of a green Hibiscus rosa-sinensis L. leaf (dicot plant) taken using Cytation 5 with DAPI, GFP and CHL filter sets; no added fluorescent dyes. Living tissue was observed intact without microtome cutting. Note that blue fluorescence emerges from inside the cell and not the cell wall. Note that green and red fluorescence emerge from the plastids.

Fluorescence images, 4x objective

Figure 3. Fluorescence images, 4x objective, of a green arabidopsis leaf taken using Cytation 5 with DAPI, GFP and CHL filter sets; no added fluorescent dyes. Living tissue was observed intact without microtome cutting. Note that blue fluorescence emerges mostly from the trichome cells on the epidermis. Green fluorescence emerges from the cells and most strongly from the basis of the trichome cells, while red fluorescence emerges from inside the plastids only.


Thanks to Dr. Axel Tiessen for sharing his BioTek experience.

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