Plastids and Genome Interactions 1.Types of plastids 1)proplastids:most simple and least developed form of plastids. Found in least differentiated tissues such as meristem and reproductive cells. Very small (1 um diameter) and primitive suborganelle structure. TEM picture of A proplastid next to a Mitochondria From bean root cell. The dark dots in the cytoplasm are ribosomes. SO are the dots pointed by a small arrows (the organelle ribosomes).

2) Amyloplast: is also rather simple. Non-colored plastids that accumulate and store starch (thus the name). They are found in storage tissues such as potato tuber etc. Amyloplasts in root cap cells also serves as gravity-sensing statoliths during gravity response. Leucoplasts refer to non-pigmented palstids (proplastids and amyloplasts). TEM picture of a amyloplast from root cap of white clover. This statolith device contain largely starch grains (s) surrounded by the stroma membranes. IN potato tubers, the starch grains fill the entire organelles with even less stroma.

3) Chromoplasts: plastids that are pigmented into red, organge, yellow. Found in fruits, roots/tuber, and flowers. Tomato and pepper, carrots and sweet potatos. Some colorful pigments are also stored in vacuoles such those in flowers. TEM picture of a chromoplast from a cherry fruit cell. The plastid is filled with carotenoids that are formed by enzymes and carotenes associated with the plastid membranes. Depending on the type of carotenoids and other pigments present in the chromoplast, they display different colors, shapes, and sizes.

4) Etioplasts: precursor of chloroplasts when plants were grown in the dark. The etioplasts develop into chloroplasts when light becomes available. This happens a lot during early germination. light Some prolamellar structure getting ready for further development into thylokoid membrane system inside the chloroplast. As the etiolated (dark grown) plants are illuminated, the thylokoid membrane begin to extend and form more extensive membrane networks.

5) Chloroplasts: most important plastid and most complex in suborganelle structure. Green tissues. Stroma and stacked grana Form continuous membrane Network that contain the entire Light reaction machinery to capture light and turn it into chemical energy

2. Development of plastids reflect the differentiation of plant tissues. IN meristem, proplastids dominate and they develop into different types of plastids depending on the type of tissue a meristem cell is differentiated into. In roots, they develop into amyloplast (typical non-color root) or chromoplasts (in carrot). IN shoots, it develops into chloroplast, etioplasts depending on light. An important concept for plastid development is their inter-conversion, ie, mature plastids can be converted into different type of mature plastids under certain conditions. For example, amyloplast in the potato tuber can turn into chloroplast if allowed to stay outside of soil during development (seeing light); chromoplasts can turn into chloroplasts too (green patch on the top of a carrot); chloroplast into chromoplasts (green tomato into red one during ripening); etioplasts certainly turn into chloroplasts when illuminated (yellow plants in the dark—green plants after light)---

3. Plastid replication and cell division Plastids, like many other organelles, cannot be synthesized de novo. They come from mother plastids by replication—division. 1. What types of plastids divide? We know meristem cells, but not differentiated cells, divide. Naturally, proplastids divide. But mature plastids divide too—All types of plastids do! Plastid division keeps pace with cell division in meristem cells, but they also respond to cell enlargement after division. E.g, when a leaf expand, mesophyll cell enlarge, chloroplasts inside the mesophyll cell replicate (otherwise, the larger leaves will look pale due to dilution of chloroplast density).

2) How do they divide? They are thought to evolve from a bacterium, and they indeed divide like one. The division appear to be simple fission—starting with a constriction, deep cleavage, break apart. Recent studies suggest that FtsZ-like molecules are required for the division of chloroplasts. FtsZ is a protein involved in bacterial cell division (formation of septum). This further indicates that division of plastids may have been conserved from bacterial division. An etioplast in bean sprout was caught in division (see the central constriction, arrow)

3) What controls division? a)Cell division and expansion (developmental cues). b)Environmental cues (light is critical for chloroplast division). Mechanism not clear. 4) How does it coordinate with cell division? a)Does not synchronize with cell division although they keep pace roughly. They can amend that later on because all plastids can divide. b)Not necessarily equal distribution of DNA to the daughter plastids. There are usually many copies of genomes in one plastid. They can also replicate their genome after division. Even the DNA synthesis is blocked by inhibitors, division can still occur—in sharp contrast to cell division. c)During cell division, there is no precise control over the number of plastids that are divided into the daughter cells (many other organelles aren’t necessarily evenly distributed either).

4. Plastid inheritance Plastids are not synthesized de novo. They are inherited from parents— they are passed on through egg and sperm. For angiosperms, plastids (and mitochondria) have maternal inheritance in most plants. But for gymnosperms, plastids are often from paternal inheritance. Why and how, details are not clear. 5. The plastid genome 1)Circular DNA of kb encoding some of the proteins involved in plastid function like photosynthesis in chloroplasts. Also encode ribosomal RNA and proteins. 2)DNA replication, Transcription, and translation follow bacterial patterns. 3)Gene expression regulation and chloroplast development: think about how many genes may be involved in chloroplast development from an etioplast or proplastid. All photosynthetic proteins and other metabolic enzymes involved in other chloroplast functions. Developmental cues from nuclear genome and light signal.

6. Nuclear-plastid genome interaction in gene expression and chloroplast development Both nuclear and plastid genome are required for chloroplast development---each encode part of the whole set of proteins in the chloroplast. Studies have shown that the expression of the nuclear and plastids genes are coordinated—making sense as the different subunits of the same enzyme can be encoded by different genome. Classical example: RUBISCO small subunit is encoded by nuclear genes and large subunit is by plastid genome. Fascinating questions: How do the two genome coordinate and what are signals that communicate between the two compartments? Earlier experiments indicated that if chloroplasts do not develop properly (chlorophyll synthesis blocking etc), the nuclear genes for chloroplast development/function will not be expressed even under light. More recent genetic analysis support the idea that chlorophyll metabolism is a key process in inter-genome signaling.

The mutants that uncouple the the two genomes were called “gun” (genome un- couple). These mutants have problems in communicating chloroplast damage to the expression of nuclear genes. Even the chloroplast is bleached, the nuclear genes for chloroplast proteins are still expressed! Several of gun mutants have problem in chlorophyll synthesis. The mutant genes encode enzymes involved in chlorophyll biosynthesis or synthesis of chlorophyll precursors.

Redox signal produced in light reaction is also important for control of plastid and nuclear gene expression