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 120-160 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 uncouple). 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