Topic 1.9 Directed Organelle Movement and Cytoplasmic Streaming
Cytoplasmic streaming is a basic feature of most higher plant cells. There are some higher plant cells that do not stream, such as the cells in dormant seed tissues. In fact, streaming rates are slower in smaller, nonvacuolate, meristematic cells than in larger cells. Although streaming is a basic feature of most plant cells, its function and mechanism are still active areas of research. It is clear, however, that most streaming occurs in association with the actin filament network as shown in Movie 1. The actin network pervades the cytoplasm of the plant cell, and movement along it is driven by one of the plant myosins, myosin XI.
Spheroid organelle streaming: Oil bodies, Golgi, peroxisomes, mitochondria in Nicotiana tabacum and Arabidopsis thaliana
Oil bodies and actin: Oil bodies are small and spherical. Because they are made of oil, they bend light much more than water (i.e., they have a higher refractive index, and consequently, can be recognized with regular microscopy). In those forms of microscopy which are based on the different refractive index of organelles, such as phase contrast microscopy, they are very easily seen. They move along cytoplasmic strands inside the cell and can sometimes be seen passing each other in opposite directions, making it clear that they are not being carried along by bulk flow of the cytoplasm, but are independently motorized, probably by myosin. In Movie 1, you can see oil bodies (stained with the vital dye nile red) that are tracking along the filamentous actin inside the cell.
Golgi: As shown in Web Topic 1.7 on the Golgi and the ER exit sites, the Golgi stream at variable rates, with a maximum of about 2 micrometers/second, while they are connected to the ER. This can be seen in movie 2, which is a time lapse of Golgi and ER labeled with green fluorescence protein (GFP) labeled ERD2, the HDEL recycling receptor between the ER and the Golgi. This is from tobacco leaves that are transiently-expressing the GFP-ERD2. Note that some Golgi remain stationary. The function of moving versus stationary Golgi is unclear, but one hypothesis is that as the Golgi stream, they release vesicles like a bubble wand. The vesicles then move independently to their site of fusion with the plasma membrane.
The Golgi also has a slightly different appearance depending on whether it is labeled with a fluorescent fusion protein that is targeted to the cis side or trans side. Movies 3 and 4 are from stably transformed lines of Arabidopsis thaliana that are available through the Arabidopsis Biological Resource Center. Movie 3 shows the streaming of Golgi labeled with YFP fused to Memb12, a cis-side protein. Movie 4 shows the streaming of Golgi labeled with YFP fused with Got1, a trans-side protein. The Golgi-Got1 movie shows the 3D distribution of the streaming Golgi.
Peroxisomes: Peroxisomes stream at about the same rate as Golgi, and are shown in Movie 5. The movement of peroxisomes in a variety of tissues is shown: the petiole, the root, and the root hair. For more movies and analysis of movement of peroxisomes, see Movie 9 (chloroplasts). Peroxisomes sometimes produce long tubules called peroxules that often “lead” the streaming particle, as shown in Movie 6. The function of peroxules is unknown. As with Golgi, peroxisomes can also be stationary. Sometimes stationary peroxisomes are associated with the chloroplast, where they are probably closely linked in order to receive the products of photo-oxidation.
Mitochondria: Mitochondria also stream at about the same rate as Golgi and peroxisomes—averaging about 2 micrometers/second. Again, there is quite a bit of variation in their movement in epidermal cells from hypocotyls, Movie 7. The 3D distribution of non-streaming mitochondria in an epidermal cell is also shown. Depending on the tissue, mitochondria also show a lot of branching, fusion, and fission. As you can see in second part of Movie 7, the branched mitochondria divide—at multiple fission sites. Some of this occurs when they stop, and perhaps this tethering helps them divide, as it does in yeast cells.
As shown in Movie 8, and in the ERD2 movies and the chloroplast movie, the ER streams rapidly, remodeling and reshaping itself while streaming. The ER is labeled here with GFP that has an ER retention signal, KDEL, on its carboxy terminus. In Arabidopsis, hypocotyl ER also has fusiform “ER bodies” associated with it (but not most other plants), which may be part of the lytic compartment of the plant cell. The ER mesh is fairly dense in epidermal trichomes and in the guard cells of stomata. The ER forms a remodeling network around the chloroplasts in the mesophyll cells of transgenic Nicotiana benthamiana cells. Because the ER is co-extensive with the nuclear envelope (the NE is actually a subdomain of the ER), the nuclear envelope is easily seen in 3D reconstructions of cells that express luminal ER markers (see the epidermal trichome and guard cell reconstructions).
Chloroplasts stream in the water plant Elodea, as shown in the first part of Movie 9. Because the chloroplasts are large and visible, this is often the only streaming that students of biology get to see. However, in other plants and tissues, the chloroplasts do not move much and many appear tethered, while other organelles, such as ER stream around them. Others can stream, but appear to move separately from organelles, such as peroxisomes. This movie compares the streaming rate of peroxisomes with chloroplasts in the hypocotyls of Arabidopsis.
There are also particular environmental responses of plant cells to light that include the movement of chloroplasts to the sides of epidermal cells to avoid high light levels, as shown in Movie 10.