Independent localization of plasma membrane and chloroplast components during eyespot assembly

Channelrhodopsins: From Phototaxis to Optogenetics

Channelrhodopsins stand out among other retinal proteins because of their capacity to generate passive ionic currents following photoactivation. Owing to that, channelrhodopsins are widely used in neuroscience and cardiology as instruments for optogenetic manipulation of the activity of excitable cells. Photocurrents generated by channelrhodopsins were first discovered in the cells of green algae in the 1970s. In this review we describe this discovery and discuss the current state of research in the field.

Algal Ocelloids and Plant Ocelli

Vision is essential for most organisms, and it is highly variable across kingdoms and domains of life. The most known and understood form is animal and human vision based on eyes. Besides the wide diversity of animal eyes, some animals such as cuttlefish and cephalopods enjoy so-called dermal or skin vision. The most simple and ancient organ of vision is the cell itself and this rudimentary vision evolved in cyanobacteria. More complex are so-called ocelloids of dinoflagellates which are composed of endocellular organelles, acting as lens- and cornea/retina-like components. Although plants have almost never been included into the recent discussions on organismal vision, their plant-specific ocelli had already been proposed by Gottlieb Haberlandt already in 1905. Here, we discuss plant ocelli and their roles in plant-specific vision, both in the shoots and roots of plants. In contrast to leaf epidermis ocelli, which are distributed throughout leaf surface, the root apex ocelli are located at the root apex transition zone and serve the light-guided root navigation. We propose that the plant ocelli evolved from the algal ocelloids, are part of complex plant sensory systems and guide cognition-based plant behavior.

Targeting of Photoreceptor Genes in Chlamydomonas reinhardtii via Zinc-Finger Nucleases and CRISPR/Cas9

The fast-growing biflagellated single-celled chlorophyte Chlamydomonas reinhardtii is the most widely used alga in basic research. The physiological functions of the 18 sensory photoreceptors are of particular interest with respect to Chlamydomonas development and behavior. Despite the demonstration of gene editing in Chlamydomonas in 1995, the isolation of mutants lacking easily ascertained newly acquired phenotypes remains problematic due to low DNA recombination efficiency. We optimized gene-editing protocols for several Chlamydomonas strains (including wild-type CC-125) using zinc-finger nucleases (ZFNs), genetically encoded CRISPR/associated protein 9 (Cas9) from Staphylococcus aureus and Streptococcus pyogenes, and recombinant Cas9 and developed protocols for rapidly isolating nonselectable gene mutants. Using this technique, we disrupted the photoreceptor genes COP1/2, COP3 (encoding channelrhodopsin 1 [ChR1]), COP4 (encoding ChR2), COP5, PHOT, UVR8, VGCC, MAT3, and aCRY and created the chr1 chr2 and uvr8 phot double mutants. Characterization of the chr1, chr2, and mat3 mutants confirmed the value of photoreceptor mutants for physiological studies. Genes of interest were disrupted in 5 to 15% of preselected clones (∼1 out of 4000 initial cells). Using ZFNs, genes were edited in a reliable, predictable manner via homologous recombination, whereas Cas9 primarily caused gene disruption via the insertion of cotransformed DNA. These methods should be widely applicable to research involving green algae.

The Road to Optogenetics: Microbial Rhodopsins

Optogenetics technology (using light-sensitive microbial proteins to control animal cell physiology) is becoming increasingly popular in laboratories around the world. Among these proteins, particularly important are rhodopsins that transport ions across the membrane and are used in optogenetics to regulate membrane potential by light, mostly in neurons. Although rhodopsin ion pumps transport only one charge per captured photon, channelrhodopsins are capable of more efficient passive transport. In this review, we follow the history of channelrhodopsin discovery in flagellate algae and discuss the latest addition to the channelrhodopsin family, channels with anion, rather than cation, selectivity.

MLT1 links cytoskeletal asymmetry to organelle placement in chlamydomonas

Asymmetric placement of the photosensory eyespot organelle in Chlamydomonas is patterned by mother-daughter differences between the two basal bodies, which template the anterior flagella. Each basal body is associated with two bundled microtubule rootlets, one with two microtubules and one with four, forming a cruciate pattern. In wild-type cells, the single eyespot is positioned at the equator in close proximity to the plus end of the daughter rootlet comprising four microtubules, the D4. Here we identify mutations in two linked loci, MLT1 and MLT2, which cause multiple eyespots. Antiserum raised against MLT1 localized the protein along the D4 rootlet microtubules, from the basal bodies to the eyespot. MLT1 associates immediately with the new D4 as it extends during cell division, before microtubule acetylation. MLT1 is a low-complexity protein of over 300,000 Daltons. The expression or stability of MLT1 is dependent on MLT2, predicted to encode a second large, low-complexity protein. MLT1 was not restricted to the D4 rootlet in cells with the vfl2-220 mutation in the gene encoding the basal body-associated protein centrin. The cumulative data highlight the role of mother-daughter basal body differences in establishing asymmetry in associated rootlets, and suggest that eyespot components are directed to the correct location by MLT1 on the D4 microtubules.

A microbial avenue to cell cycle control in the plant superkingdom

Research in yeast and animals has resulted in a well-supported consensus model for eukaryotic cell cycle control. The fit of this model to early diverging eukaryotes, such as the plant kingdom, remains unclear. Using the green alga Chlamydomonas reinhardtii, we developed an efficient pipeline, incorporating robotics, semiautomated image analysis, and deep sequencing, to molecularly identify >50 genes, mostly conserved in higher plants, specifically required for cell division but not cell growth. Mutated genes include the cyclin-dependent kinases CDKA (resembling yeast and animal Cdk1) and the plant-specific CDKB. The Chlamydomonas cell cycle consists of a long G1 during which cells can grow >10-fold, followed by multiple rapid cycles of DNA replication and segregation. CDKA and CDKB execute nonoverlapping functions: CDKA promotes transition between G1 and entry into the division cycle, while CDKB is essential specifically for spindle formation and nuclear division, but not for DNA replication, once CDKA-dependent initiation has occurred. The anaphase-promoting complex is required for similar steps in the Chlamydomonas cell cycle as in Opisthokonts; however, the spindle assembly checkpoint, which targets the APC in Opisthokonts, appears severely attenuated in Chlamydomonas, based on analysis of mutants affecting microtubule function. This approach allows unbiased integration of the consensus cell cycle control model with innovations specific to the plant lineage.