CHLOROPLAST CELL ORGANELLE PLASTID.pdf

CHLOROPLAST

1. CHLOROPLASTS AND
OTHER PLASTIDS

2. •Responsible for the
photosynthetic conversion of
CO
2
to carbohydrates


•Synthesize amino acids, fatty
acids, and the lipid
components of their own
membranes

3. The Structure and Function of Chloroplasts
•Large organelles (5 to 10 μm long)
that, like mitochondria, are bounded
by a double membrane called the
chloroplast envelope
•A third internal membrane system,
called the thylakoid membrane
•The thylakoid membrane forms a
network of flattened discs called
thylakoids, which are frequently
arranged in stacks called grana

5. •Three membranes divide chloroplasts
into three distinct internal
compartments:
•(1) the intermembrane space
between the two membranes of the
chloroplast envelope
•(2) the stroma, which lies inside the
envelope but outside the thylakoid
membrane
•(3) the thylakoid lumen

7. •The outer membrane of the
chloroplast envelope contains
porins and freely permeable to
small molecules
•The inner membrane is
impermeable to ions and
metabolites, which are therefore
able to enter chloroplasts only
via specific membrane
transporters

8. •Stroma contains the
chloroplast genetic system
and a variety of metabolic
enzymes, including those
responsible for the critical
conversion of CO
2
to
carbohydrates during
photosynthesis

9. •The thylakoid
membrane plays a role
in electron transport
and the chemiosmotic
generation of ATP
•Protons are pumped
across this membrane
from the stroma to the
thylakoid lumen
•The resulting
electrochemical
gradient then drives
ATP synthesis as
protons cross back into
the stroma

10. Import of proteins into the thylakoid lumen
•Proteins are imported
into the thylakoid
lumen in two steps
• The first step is
import into the
chloroplast stroma
• Cleavage of the
transit peptide then
exposes a second
hydrophobic signal
sequence, which
directs protein
translocation across
the thylakoid
membrane.

11. genomes
•The chloroplast genome consists of
homogeneous circular double
stranded DNA molecules and devoid
of histones and other proteins
•The chloroplast genomes of land
plants and green algae contain about
110 different genes, which can be
classified into two main groups:
genes involved in gene expression
and those related to photosynthesis.

12. •they comprise a single circular
molecule with a quadripartite
structure that includes two identical
fragments called inverted repeats
(IR) that separate large and small
single-copy (LSC and SSC) regions
(Mower and Vickrey 2018)
•Among different plant species, the IR
regions are highly conserved and
have a length ranging from 20,000 to
25,000 bp (Morley et al. 2019a)

13. •Recent studies have identified
considerable diversity within
non-coding intergenic spacer
regions, which often include
important regulatory sequences
• to the IR regions the chloroplast
intron sequences can also be
considered as highly conserved
(Daniell et al. 2016)

14. • Exceptions from this rule are a few
plant species, such as barley (Hordeum
vulgare), bamboo (Bamboo sp.),
Cassava (Manihot esculenta) or
chickpeas (Cicer arietinum) in which a
loss of introns is observed in genes
encoding some proteins
• Like bacteria, many chloroplast genes
are organized into operons whose
transcription is carried out using one or
more promoters (Börner et al. 2015).

15. •Chloroplast genes contain at
least three structurally distinct
promoters and transcribe two
or more classes of RNA
polymerase
• Two chloroplast genes, rps12
of land plants and psaA of
Chlamydomonas, are divided
into two to three pieces and
scattered over the genome

16. •each portion is transcribed separately,
and two to three separate transcripts are
joined together to yield a functional
mRNA by trans-splicing.
• RNA editing (C to U base changes)
occurs in some of the chloroplast
transcripts.
• Most edited codons are functionally
significant, creating start and stop
codons and changing codons to retain
conserved amino acids.

17. •In many cases, the GC content
of cpDNA differs from that of
nuclear DNA and
mitochondrial DNA
•Complete cpDNA sequences
have been determined in
tobacco (155, 844 bp) and rice
(135, 42 bp).

19. •Multiple copies of cpDNA are
present in the nucleoid region of
each chloroplast.
•In the green alga
Chlamydomonas, one chloroplast
contains 500 to 1500 cpDNA
molecules. Chloroplasts divide
by growing and then dividing into
two daughter chloroplasts.

20. •The proportion of introns in
chloroplast DNA could be high,
38% in Euglena. Among the
expressed genes in chloroplast
genome, 70 to 90% of the genes
encode proteins including those
involved in photosynthesis, four
genes code for rRNAs (one each
for 16S, 23S, 4.5S and 5S), and
about 30 genes encode tRNAs.

21. •Chloroplast protein synthesis
uses organelle-specific 70S
ribosomes consisting of 50S
and 30S subunits.
• The 50S subunit contains one
copy each of 23S, 5S and 4.5S
rRNAs, while the 30S subunit
contains one copy of a 16S
rRNA.

22. Molecular mechanisms of chloroplast
biogenesis
•The process of chloroplast
biogenesis is highly complex and the
molecular intricacies have not been
fully characterised.
•The complexity of this process is not
surprising in light of its ancestry,
having originated through
endosymbiosis with species of
cyanobacteria

23. •Generally, the chloroplast
develops from undeveloped
proplastids, which contain
vesicles but no
differentiated structures.
• During this differentiation
thylakoids are formed and
stacked into defined grana.

24. •The thylakoids are the internal
lipid membranes interlaced
with protein complexes,
which provide the platform for
the light reactions of
photosynthesis and thus
could be considered as one of
the most important structures
in the chloroplast

25. •the thylakoid itself is an
intriguing and
complicated structure,
and the mechanism for its
formation is not fully
characterised, but several
hypotheses are presented
in a recent review

26. •Under specific circumstances,
the dark-intermediate etioplast
develops from the proplastid,
which is defined by the
prominent prolamelar body
(PLB); a lattice-like membranous
structure and a few metabolites
and proteins required for
photosynthesis.

28. •From this lattice-like structure
prothylakoids emanate into the
plastid stroma and the PLB
disassembles and reforms into
thylakoids upon exposure to
light
• In some cases, chloroplasts can
also develop from other plastids
such as chromoplasts.

30. •The development of flowering plant
chloroplasts occurs in the light, and
involves the expression of 1000s of
genes encoded in the nucleus, the
import of their products into
developing plastids, as well as the
expression of ca. 120 protein and
RNA-encoding genes by the genome
of the chloroplast itself (Waters and
Langdale, 2009; Jarvis and
López-Juez, 2013)

31. •Light is a key inductive signal
for the expression of genes
involved in the assembly of a
photosynthetically competent
chloroplast, the so-called
photosynthesis associated
nuclear genes (PhANGs).

32. • In seedlings germinated in the
absence of light leaf development is
repressed and plastids in the
cotyledons develop as etioplasts,
containing partially developed
internal membranes and a
chlorophyll precursor,
protochlorophyllide, associated to a
light-requiring protochlorophyllide
oxido-reductase (Reinbothe et al.,
1996)

33. •This renders seedlings
photosynthetically
incompetent
• At the same time the
expression of PhANGs in
cells developing chloroplasts
is closely coordinated with
the functional state of the
plastid.

34. •If ongoing plastid biogenesis is impaired,
by failure to safely complete chlorophyll
biosynthesis (because of photooxidative
damage to membrane complexes, caused
by carotenoid synthesis mutations or
chemical inhibitors like norflurazon), or to
express the chloroplast genome (because
of organelle translation mutations or
inhibitors), or to import nuclear-encoded
proteins, PhANG expression is also
down-regulated (Inaba et al., 2011; Chi et
al., 2013)

35. •This reveals the existence of
plastid-to-nucleus communication, also
called plastid retrograde signaling, more
specifically plastid biogenic signaling.
•The term “biogenic” is used to
distinguish it from operational or
environmental signaling, which refers to
the later influence of functional but
stressed chloroplasts on nuclear genes
when subjected to environmental
challenges (Woodson and Chory, 2012;
Pogson et al., 2015)

36. •Light control of PhANG
expression is part of a broader
program of control by light of
development overall, the
so-called photomorphogenesis
program, initiated by the
activation of phytochrome and
cryptochrome photoreceptors

37. •This program contrasts with
that of development in the
dark, skotomorphogenesis,
which instead ensures
investment into elongating
organs and prevents the
development of
photosynthesising leaves
(Arsovski et al., 2012)

38. •the presence of
phytochrome and
cryptochrome
photoreceptors has been
shown in all major groups of
land plants (Sharrock and
Mathews, 2006) but the
nature of photomorphogenic
responses varies.

39. •Many gymnosperm seedlings grow
partially skotomorphogenically but
green in the dark (Alosi et al., 1990;
Yamamoto et al., 1991; Burgin et al.,
1999).
• This stems from the presence of a
light-independent
protochlorophyllide oxido-reductase
(Forreiter and Apel, 1993), and from
the expression of PhANGs in the
dark (Yamamoto et al., 1991; Peer et
al., 1996).

40. •Prominent prolamelar body
•These plastids are formed in
light-deprived tissues of angiosperm
plants that would become
chlorenchyma in the light.
• Etioplasts have a unique inner
membrane consisting of highly
regular, paracrystalline prolamellar
bodies (PLBs) and of lamellar
prothylakoids (PTs).

41. •Etioplasts, which are the
chloroplast counterparts in
darkness, can be formed in
nature during the first phase of
plantlet growth, before the
emergence from soil.
•Chloroplast Development and
Expression of Photosynthetic
Genes.

42. •In the light, etioplasts
differentiate into
chloroplasts, chlorophyll is
formed from
protochlorophyllide, and
photosynthetic genes (e.g.,
CAB, RBCS) are expressed.

44. •While the exact function of the
PLB remains obscure, it
probably represents a ‘holding
pattern’ for the large amount
of membrane and protein that
will ultimately be utilized in
formation of the thylakoid
system

45. •In the light, etioplasts differentiate
into chloroplasts, chlorophyll is
formed from protochlorophyllide,
and photosynthetic genes (e.g., CAB,
RBCS) are expressed
•Etioplasts are the plastids that form
when leaves and other organs grow
in darkness. Etioplasts are not
photosynthetic organelles, but rather
a stage in the differentiation of
chloroplasts.

46. The proplastid is the progenitor plastid, point of
differentiation, developing into either etioplast or
chloroplast depending on exposure to darkness
or light,

47. •Proplastids are also able to develop into
the intermediate plastid, the leucoplasts
that may further differentiate into either
amylo-, proteino-or elaioplasts
accumulating starch, proteins or oils,
respectively.

48. •Chloroplasts can also further
differentiate depending on
environmental stimuli and plant cell
types into chromoplasts,
phenyloplasts and the dying
gerontoplast, in which resources are
recycled and redistributed.
• Chromoplast and phenyloplast
accumulate phytochemicals such as
carotenoids and phenylpropanoids,
respectively.

49. Other Plastids
• The different types of plastids are
frequently classified according to
the kinds of pigments they contain
• Chromoplasts lack chlorophyll
but contain carotenoids ,
responsible for the yellow, orange,
and red colors of some flowers
and fruits, although their precise
function in cell metabolism is not
clear

50. • Leucoplasts are
nonpigmented plastids,
which store a variety of
energy sources in
nonphotosynthetic tissues
• Amyloplasts and
elaioplasts are examples of
leucoplasts that store starch
and lipids, respectively