Eyespot apparatus

The eyespot apparatus (or "stigma") is a photoreceptive organelle found in the flagellate (motile) cells of green algae and other unicellular photosynthetic organisms such as euglenids. It allows the cells to sense light direction and intensity and respond to it by swimming either towards the light (phototaxis) or away from the light ("photoshock" or photophobic response). This helps the cells in finding an environment with optimal light conditions for photosynthesis. Eyespots are the simplest and most common "eyes" found in nature, composed of photoreceptors and a signal transduction system generating a phototactic response.cite journal |author=Hegemann P |title=Vision in microalgae |journal=Planta |volume=203 |issue=3 |pages=265–74 |year=1997 |pmid=9431675 |doi=10.1007/s004250050191]

Microscopic structure

Under the light microscope, eyespots appear as dark, often reddish, spots or "stigmata". They get their color from the chromoproteins they contain, such as chlamyopsin, volvoxopsin or other photopigments.

The eyespot apparatus of "Euglena" comprises the paraflagellar body connecting the eyespot to the flagellum. In electron microscopy, the eyespot apparatus appears as a highly ordered lamellar structure formed by membranous rods in a helical arrangement.cite journal |author=Wolken J |title=Euglena: the photoreceptor system for phototaxis |journal=J Protozool |volume=24 |issue=4 |pages=518–22 |year=1977 |pmid=413913]

In "Chlamydomonas", the eyespot is part of the chloroplast and takes on the appearance of a membranous sandwich structure. It is assembled from chloroplast membranes (outer, inner, and thylakoid membranes) and carotenoid-filled granules overlaid by plasma membrane. It disassembles during cell division and reforms in the daughter cells in an asymmetric fashion in relation to the cytoskeleton.cite journal |author=Dieckmann C |title=Eyespot placement and assembly in the green alga Chlamydomonas |journal=Bioessays |volume=25 |issue=4 |pages=410–6 |year=2003 |pmid=12655648 |doi=10.1002/bies.10259]

Eyespot proteins

The predominant eyespot proteins are the photoreceptor proteins that sense light. The photoreceptors found in unicellular organisms fall into two main groups: flavoproteins and retinylidene proteins (rhodopsins). Flavoproteins are characterized by containing flavin molecules as chromophores, whereas retinylidene proteins contain retinal. The photoreceptor protein in "Euglena" is likely a flavoprotein. In contrast, "Chlamydomonas" phototaxis is mediated by archaeal-type rhodopsins.cite journal |author=Suzuki T, Yamasaki K, Fujita S, Oda K, Iseki M, Yoshida K, Watanabe M, Daiyasu H, Toh H, Asamizu E, Tabata S, Miura K, Fukuzawa H, Nakamura S, Takahashi T |title=Archaeal-type rhodopsins in Chlamydomonas: model structure and intracellular localization |journal=Biochem Biophys Res Commun |volume=301 |issue=3 |pages=711–7 |year=2003 |pmid=12565839 |doi=10.1016/S0006-291X(02)03079-6]

Besides photoreceptor proteins, eyespots contain a large number of structural, metabolic and signaling proteins. The eyespot proteome of "Chlamydomonas" cells consists of roughly 200 different proteins.cite journal |author=Schmidt M, Gessner G, Luff M, Heiland I, Wagner V, Kaminski M, Geimer S, Eitzinger N, Reissenweber T, Voytsekh O, Fiedler M, Mittag M, Kreimer G |title=Proteomic analysis of the eyespot of Chlamydomonas reinhardtii provides novel insights into its components and tactic movements |journal=Plant Cell |volume=18 |issue=8 |pages=1908–30 |year=2006 |pmid=16798888 |doi=10.1105/tpc.106.041749]

Photoreception and signal transduction

The "Euglena" photoreceptor was identified as a blue-light-activated adenylyl cyclase.cite journal |author=Iseki M, Matsunaga S, Murakami A, Ohno K, Shiga K, Yoshida K, Sugai M, Takahashi T, Hori T, Watanabe M |title=A blue-light-activated adenylyl cyclase mediates photoavoidance in Euglena gracilis |journal=Nature |volume=415 |issue=6875 |pages=1047–51 |year=2002 |pmid=11875575 |doi=10.1038/4151047a] Excitation of this receptor protein results in the formation of cyclic adenosine monophosphate (cAMP) as a second messenger. Chemical signal transduction ultimately triggers changes in flagellar beat patterns and cell movement.

The archaeal-type rhodopsins of "Chlamydomonas" contain an all-"trans" retinylidene chromatophore which undergoes photoisomerization to a 13-"cis" isomer. This activates a photoreceptor channel, leading to a change in membrane potential and cellular calcium ion concentration. Photoelectric signal transduction ultimately triggers changes in flagellar strokes and thus cell movement.

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