Autofluorescence: Biological functions and technical applications.

Fluorescence is a physico-chemical energy exchange where shorter-wavelength photons are absorbed by a molecule and are re-emitted as longer-wavelength photons. It has been suggested a means of communication in several taxa including flowers, pitcher plants, corals, algae, worms, squid, spiders, stomatopods, fish, reptiles, parrots and humans. The surface or object that the pigment molecule is part of appears to glow due to its setting rather than an actual production of light, and this may enhance both signals and, in some cases, camouflage. This review examines some known uses of fluorescence, mainly in the context of visual communication in animals, the challenge being to distinguish when fluorescence is a functional feature of biological coloration or when it is a by-product of a pigment or other molecule. In general, we conclude that most observations of fluorescence lack enough evidence to suggest they are used in visually driven behaviours.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.2016.0335

Fluorescence as a means of colour signal enhancement.

In situ photo-irradiation of trans-2-(4-fluorostyryl)benzo[d]oxazole (t-FSBO) and trans-2-(2,4-difluorostyryl)benzo[d]oxazole (t-2FSBO) with aggregation-caused quenching (ACQ) characteristics in the solid-state afforded unprecedented aggregation-induced emission (AIE) luminogens through [2+2] cycloaddition. Although the photoreaction products were weakly conjugated, they were emissive. Interestingly, the product cisoid-2,2′-(3,4-bis(2,4-fluorophenyl)cyclobutene-1,2-diyl)bis(benzo[d]oxazole) (c-2FPCBO) could form blue and yellow emissive crystals due to the different packing modes. Crystallographic results and computational calculation suggested that this anomalous phenomenon was attributed to intramolecular and intermolecular through-space conjugation. To the best of our knowledge, this is the first reported in situ ACQ-to-AIE transformation achieved by UV irradiation, deepening the understanding that non-conjugated molecules can be emissive. In addition to this novel and accessible ACQ-to-AIE transformation, the photosalient effect was also observed during the reactions. Notably, a macroscopic actuator was achieved by simply coating t-FSBO on a nitrile butadiene glove, which could bend over 70° towards UV light source. This system thus offers a new class of AIE luminogens with a “living” character.

“Living” luminogens: light driven ACQ-to-AIE transformation accompanied with solid-state actuation

Although proteins in the green fluorescent protein family (GFPs) have been discovered in a wide array of taxa, their ecological functions in these organisms remain unclear. Many hypothesized roles are related to modifying bioluminescence spectra or modulating the lightregimeforalgalsymbionts,butthesedonotexplainthepresence of GFPs in animals that are non-luminous and non-symbiotic. Other hypothesized functions are unrelated to the visual signals themselves, including stress responses and antioxidant roles, but these cannot explain the localization of fluorescence in particular structures on the animals. Here we tested the hypothesis that fluorescence might serve to attract prey. In laboratory experiments, the predator was the hydromedusa Olindias formosus (previously known as O. formosa), which has fluorescent and pigmented patches on the tips of its tentacles. The prey, juvenile rockfishes in the genus Sebastes, were significantly more attracted (P<1×10 −5 ) to the medusa’s tentacles under lighting conditions where fluorescence was excited and tentacle tips were visible above the background. The fish did not respond significantly when treatments did not include fluorescentstructuresortookplaceunderyelloworwhite lights,which did not generate fluorescence visible above the ambient light. Furthermore, underwater observations of the behavior of fishes when presented with a brightly illuminated point showed a strong attraction to this visual stimulus. In situ observations also provided evidence for fluorescent lures as supernormal stimuli in several other marine animals, includingthe siphonophoreRhizophysaeysenhardti. Our results support the idea that fluorescent structures can serve as prey attractants, thus providing a potential function for GFPs and other fluorescent proteins in a diverse range of organisms.

/pdf/fluorescent-proteins-function-as-a-prey-attractant-4cc8ang0s2.pdf

Fluorescent proteins function as a prey attractant: experimental evidence from the hydromedusa Olindias formosus and other marine organisms.

Chlorophyll breakdown is a visual phenomenon of leaf senescence and fruit ripening. It leads to the formation of colorless chlorophyll catabolites, a group of (chlorophyll-derived bilin-type) linear tetrapyrroles. Here, analysis and structure elucidation of the chlorophyll breakdown products in leaves of banana (Musa acuminata) is reported. In senescent leaves of this monocot all chlorophyll catabolites identified were hypermodified fluorescent chlorophyll catabolites (hmFCCs). Surprisingly, nonfluorescent chlorophyll catabolites (NCCs) were not found, the often abundant and apparently typical final chlorophyll breakdown products in senescent leaves. As a rule, FCCs exist only fleetingly, and they isomerize rapidly to NCCs in the senescent plant cell. Amazingly, in the leaves of banana plants, persistent hmFCCs were identified that accounted for about 80 % of the chlorophyll broken down, and yellow leaves of M. acuminata display a strong blue luminescence. The structures of eight hmFCCs from banana leaves were analyzed by spectroscopic means. The massive accumulation of the hmFCCs in banana leaves, and their functional group characteristics, indicate a chlorophyll breakdown path, the downstream transformations of which are entirely reprogrammed towards the generation of persistent and blue fluorescent FCCs. As expressed earlier in related studies, the present findings call for attention, as to still elusive biological roles of these linear tetrapyrroles.

/pdf/chlorophyll-breakdown-in-senescent-banana-leaves-catabolism-owb448tqo2.pdf

Chlorophyll breakdown in senescent banana leaves: catabolism reprogrammed for biosynthesis of persistent blue fluorescent tetrapyrroles.

Carnivorous plants acquire most of their nutrients by capturing ants, insects and other arthropods through their leaf-evolved biological traps. So far, the best-known attractants in carnivorous prey traps are nectar, colour and olfactory cues. Here, fresh prey traps of 14 Nepenthes, five Sarracenia, five Drosera, two Pinguicula species/hybrids, Dionaea muscipula and Utricularia stellaris were scanned at UV 366 nm. Fluorescence emissions of major isolates of fresh Nepenthes khasiana pitcher peristomes were recorded at an excitation wavelength of 366 nm. N. khasiana field pitcher peristomes were masked by its slippery zone extract, and prey capture rates were compared with control pitchers. We found the existence of distinct blue fluorescence emissions at the capture spots of Nepenthes, Sarracenia and Dionaea prey traps at UV 366 nm. These alluring blue emissions gradually developed with the growth of the prey traps and diminished towards their death. On excitation at 366 nm, N. khasiana peristome 3:1 CHCl3–MeOH extract and its two major blue bands showed strong fluorescence emissions at 430–480 nm. Masking of blue emissions on peristomes drastically reduced prey capture in N. khasiana pitchers. We propose these molecular emissions as a critical factor attracting arthropods and other visitors to these carnivorous traps. Drosera, Pinguicula and Utricularia prey traps showed only red chlorophyll emissions at 366 nm.

S Sankar

Papers

Fluorescent prey traps in carnivorous plants