Theeyespot apparatus (orstigma) is a photoreceptiveorganelle found in the flagellate or (motile) cells ofgreen algae and otherunicellularphotosynthetic organisms such aseuglenids. It allows the cells to senselight direction and intensity and respond to it, prompting the organism to either swim towards the light (positivephototaxis), or away from it (negative phototaxis). A related response ("photoshock" or photophobic response) occurs when cells are briefly exposed to high light intensity, causing the cell to stop, briefly swim backwards, then change swimming direction. Eyespot-mediated light perception 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 ofphotoreceptors and areas of bright orange-red red pigment granules.[1] Signals relayed from the eyespot photoreceptors result in alteration of the beating pattern of the flagella, generating a phototactic response.[2]
Under thelight microscope, eyespots appear as dark, orange-reddish spots orstigmata. They get their color fromcarotenoid pigments contained in bodies called pigment granules. The photoreceptors are found in theplasma membrane overlaying the pigmented bodies.
The eyespot apparatus ofEuglena comprises the paraflagellar body connecting the eyespot to theflagellum. Inelectron microscopy, the eyespot apparatus appears as a highly ordered lamellar structure formed by membranous rods in a helical arrangement.[3]
InChlamydomonas, the eyespot is part of thechloroplast and takes on the appearance of a membranous sandwich structure. It is assembled fromchloroplast membranes (outer, inner, and thylakoid membranes) andcarotenoid-filled granules overlaid byplasma membrane. The stacks of granules act as aquarter-wave plate, reflecting incoming photons back to the overlying photoreceptors, while shielding the photoreceptors from light coming from other directions. It disassembles duringcell division and reforms in the daughter cells in an asymmetric fashion in relation to thecytoskeleton. This asymmetric positioning of the eyespot in the cell is essential for proper phototaxis.[4]
The most critical eyespot proteins are thephotoreceptor proteins that sense light. The photoreceptors found in unicellular organisms fall into two main groups:flavoproteins andretinylidene proteins (rhodopsins). Flavoproteins are characterized by containingflavin molecules aschromophores, whereas retinylidene proteins containretinal. The photoreceptor protein inEuglena is likely a flavoprotein.[3] In contrast,Chlamydomonas phototaxis is mediated by archaeal-type rhodopsins.[5]
Besides photoreceptor proteins, eyespots contain a large number of structural, metabolic and signaling proteins. The eyespotproteome ofChlamydomonas cells consists of roughly 200 different proteins.[6]
TheEuglena photoreceptor was identified as a blue-light-activatedadenylyl cyclase.[7] Excitation of this receptor protein results in the formation ofcyclic adenosine monophosphate (cAMP) as asecond messenger. Chemicalsignal transduction ultimately triggers changes in flagellar beat patterns and cell movement.
The archaeal-type rhodopsins ofChlamydomonas contain an all-trans retinylidene chromatophore which undergoesphotoisomerization to a 13-cis isomer. This activates a photoreceptor channel, leading to a change inmembrane potential and cellular calcium ion concentration.[5] Photoelectric signal transduction ultimately triggers changes in flagellar strokes and thus cell movement.[2]
