植物生理学
Plant Physiology
Biochemistry & Molecular Biology of Plants
Bob B,Buchanan
Wilhelm Gruissem
Russell L,Jones
植物学报 植物生理学植物生理学通信
Plant Physiology Plant Cell
Plant Science Planta
Annual Review of Plant Physiology and Plant Molecular Biology
Part 1 Photosynthesis
1.1 Overview of Photosynthesis
1.1.1 Photosynthesis is a biological
oxidation-reduction process.
Reaction 1.1,Photosynthesis
CO2+2H2A→(CH 2O)+2A+H2O
ljtang:
Reaction1.2,OxygenicPhototsynthesis
CO2+2H2O→(CH 2O)+O2+H20
Reaction1.3:Photosynthetic sulfur reduction
CO2+2H2A→(CH 2O)+2S+H2O
1.1.2 In photosynthetic eukaryotes,photosynthesis takes place in the
chloroplast,a specialized organelle
Chloroplast,a double-membrane system,consist of an outenvelope
and innerenvelope.
The internal membrane system,known as the thylakoids,contains
granalthylakoids and stroma thylakoids.
The thylakoid membranes are all interconnected and enclose an internal
space,the lumen.
1.1.3 Photosynthesis requires the coordination of two phases,light
reactions and carbon-linked reactions
Light reactions,produce O2,ATP,and NADPH.
Carbon reduction cycle,reduce CO2 to carbohydrate on consuming the
ATP and NADPH.
The conception that water oxidation and CO2 reduction were not
obligately linked was first advanced in 1930s by Robert Hill
.
Reaction 1.4,Hill Reaction
2H2O+2A→O 2+2H2A
1.2 Light Absorption and energy conversion.
1.2.1 Light is absorbed by pigment molecules.
Pigment→Pigment*
number of products formed photochemically
Equation 1.2.1:QuantumYieldФ= -------------------------------------------------------
Number of quanta absorbed
1.2.2 Almost all photosynthetic organisms contain
chlorophyll or a related pigment.
Plants,Algae and Cyanobacteria,chlorophyll a or
chlorophyll b.
Anaerobic photosynthetic bacteria,bacteriochlorophyll a.
Chlorophylls have a porphyrin ring structure that contains a
central Mg atom coordinated to the four modified pyrrole
rings.
Chlorophyll molecules also contain a long hydrocarbon tail
that makes the molecules hydrophobic.
Various chlorophylls differ in their substituents around the ring
structure.
Biosynthesis of chlorophylls and hemes.
1.2.3 Carotenoids and xanthophylls participate in
light absorption and photoprotection.
1.2.4 Absorption spectra of chlorophylls and other
photosynthetic pigments
1.2.5 Red algae and Cyanobacteria contain
accessory pigments that absorb green light.
1.3 The reaction center complex,
1.3.1 Reaction centers are integral membrane protein
complexes involved in conversion of light energy into
chemical products.
1.3.2 Reaction centers contain both special chlorophyll
and electron acceptor molecules involved in energy
conversion.
1.3.3 The structure of a reaction center from a
photosynthetic bacterium has been determined.
1.3.4 The kinetics of primary charge separation events
are understood in great detail,
1.3.5 Oxygenic photosynthetic organisms
contain two photochemical reaction
centers,PSI and PSII
1.3.5.1 Structure model of the PSII
reaction center.
1.3.5.2 Structure model of the PSI reaction
center.
1.4 The Photosystem
1.4.1 A photosystem contains a
photochemical reaction center and multiple
antennae ( auxiliary light-harvesting
pigment-protein complexes).
1.4.2 Most oxygenic photosynthetic
organisms contain chlorophyll a/b proteins
as their principal antennae,
Table 1.4.2 Properties of light-harvesting
chlorophyll protein complexes
Complex Chl a/b ratio Gene Mol,Mass(kDa)
Light-harvesting complexes (LHC) associated with PSⅠ
LHC-Ⅰ a 2.0-3.1 lhca3 20.5
(LHC-Ⅰ 680) lhca2 18
LHC-Ⅰ b 2.2-4.4 lhca1 20
(LHC-Ⅰ 730) lhca4 20
Light-harvesting complexes associated with PSⅡ
LHC-Ⅱ a (CP29) 4.0 lhcb4 29
LHC-Ⅱ b 1.35 lhcb1 27-28
LHC-Ⅱ c (CP26) 2.9 lhcb5 26.5
LHC-Ⅱ d (CP24) 1.51 lhcb6 24
1.4.2 The organization of pigments in PSⅠ and PSⅡ,
1.4.3 The light-harvesting antennae of
organism that contain phycobilins
Reaction 1.4.3 Energy transfer in a
phycobilisome
Phycoerythin(PC) →Phycocyanin(PE)
→Allophycocyanin(APC) →Chl a
1.5 Organization of thylakoid membrane
1.5.1 Protein complexes of the thylakoid membrane
exhibit lateral heterogeneity.
1.5.2 Phosphorylation of LHC-Ⅱ may
influence the distribution of energy
between PSⅠ and PSⅡ
1.6 Electron transport pathways in chloroplast membranes
1.6.1 The chloroplast noncyclic electron transport chain produces
O2,NADPH,and ATP and involves the cooperation of PSⅠ and
PSⅡ
1.6.2 Photosystem stoichiometry variesby species and
is influenced by light environment.
Table 1.6.2 Stoichiometry of the photosystems in different oxygen-
evolving systems and in response to changes in light quality _______
System
PSⅡ /PSⅠ ratio
Cyanobacteria 0.4
Red algae 0.4
Green algae 1.4
Tobacco mutant su/su 2.7
Pea chloroplasts
From plants grown in PSⅡ light (550-660nm) 1.2
From plants grown in sunlight 1.8
From plants grown in PSⅠ light(>660nm) 2.3
1.6.3 PSⅡ functions as a light-dependent water-
plastoquinone oxidoreductase.
Table 1.6.3 Protein subunits of the PSⅡ core
complexe
Hydrophobic subunits
Protein Gene Location of gene Function
D1 psbA Chloroplast Reaction center protein
D2 psbB Chloroplast Reaction center protein
CP47 psbC Chloroplast Antenna binding
CP43 psbD Chloroplast Antenna binding
Cytb559
subunit psbE Chloroplast Unknown
subunit psbF Chloroplast Unknown
PsbH-PsbN psbH-psbN Chloroplast Unknown
22kDa psbS Nucleus Photoprotection
Hydrophilic subunits
33kDa psbO Nucleus Oxygen evolution
23kDa psbP Nucleus Oxygen evolution
17kDa psbQ Nucleus Oxygen evolution
10kDa psbR Nucleus Unknown
1.6.4 Oxidation of water produces O2 and
releases electrons required by PSⅡ
Reaction 1.6.4,Oxidation of water
2H2O→O 2+4H++4e-
The pattern of oxygen evolution is given in
response to a series of short flashes of light
The oxygen evolution apparatus is considered to exist
in five different oxidation states (So through S4)
Structure of the manganese cluster
1.6.5 The cytochrome b6f complex transfers electrons
from reduced plastoquinone to oxidized plastocyanin
Proton translocation via cytochrome b6f is
thought to involve a Q-cycle
Reaction1.6.5,The Q-cycle in the chloroplast
PQH2+2PCox+2H+stroma →PQ +2PC red+4H+lumen
1.6.6 Pastocyanin,a soluble protein,links
cytochrome b6f and PSⅠ
1.6.7 PSⅠ function as a light-dependent
plastocyanin-ferredoxin oxidoreductase
Electrons from PSⅠ are transferred to NADP+
in the stroma in a reaction requiring ferredoxin
and ferredoxin-NADP+ reductase
1.6.8 Specific inhibitors and artificial electron
acceptors have been used to study the
chloroplast electron transport chain
D1
1.6.9 Chloroplasts also contain a cyclic
electron transport chain
1.7 ATP synthesis in chloroplasts
1.7.1 Electron transport and ATP synthesis are
coupled in vivo.
1.7.2 Chloroplasts synthesize ATP by a
chemiosmotic mechanism driven by a proton
gradient.
In the 1960s,Peter Mitchell proposed the chemiosmotic model to explain ATP
synthesis in chloroplasts as well as in mitochondria.,The experimental
verification of this hypothesis in the 1960s and 1970s resulted in Mitchell?s
receiving the 1978 Nobel Prize in chemistry.
1.7.3 Experimental manipulation of lumenal and stromal pH can
promote light-independent ADP phosphorylation in chloroplasts.
Acid-base Phosphorylation
Chloroplasts were first incubated at about pH4,allowing them to
establish this pH internally,The chloroplast suspension was then
rapidly adjusted to pH8,establishing an artificial pH gradient across
the membrane,This artificial pH gradient promoted ATP synthesis
until it was discharged and the pH difference was insufficient to drive
ADP phosphorylation.
Acid-base ATP synthesis required no light and was insensitive to
electron transport inhibitor,such as DCMU,indicating that a pH
gradient alone was sufficient to drive ATP synthesis.
1.7.4 The most widely accepted mechanism of ATP synthesis is
the so-call binding change mechanism proposed originally by Paul
Boyer ( a co-recipient of 1997 Nobel Prize in chemistry)
1.8 Carbon reactions in C3 plants
Most plants produce a three-carbon compound,3-
phosphoglycerate (3-PGA),as the first stable product in
the multistep conversion of CO2 into carbohydrate,This
functionally defined group,which includes most crop
plants,is referred to as C3 plants.
The Calvin cycle is divided into three phases,carboxylation,
reduction,and regeneration.
1.9 C4 plants contain two distinct CO2-fixing enzymes
and have specialized foliar anatomy
1.9.1 The C4 pathway increases the concentration of
CO2 in bundle sheath cell.
The bundle sheath chloroplast lacks stacked thylakoids
and contains little PSⅡ
1.9.2 Variations in C4 photosynthesis
Three variations of C4 photosynthesis are known,differing in the C4
acids transported between mesophyll and bundle sheath cells as well as
in the mechanism of decarboxylation in the bundle sheath cell.
The variations are named on the basis of the decarboxylase enzymes in
the bundle sheath cells.
1.9.4 Ontogeny of C4 patterns
In the development of most C4 leaves,the expression of C4 genes
does not occur until Kranz anatomy has been established.
The appearance of C4 activities has been compared in leaves 1-5 of
maize seedlings,the first leaf initiated (leaf 1) was more C3 in
charater,while the last measured (leaf 5) was fully C4.
Some studies showed that callus cultures derived from C4 plants do
not exhibit a functioning C4 pathway until cells organize into shoots
and vascular development is under way.
Physiological measurements suggest that the observed patterns of
gene expression reflect the function of the C4 carbon fixation
pathway near veins and the C3 pathway at more distant locations
1.9.3 C3-C4 intermediate plants and evolution of
photosynthetic system
Species with anatomical and physiological characteristics intermediate between C3
and C4 exist in many plant families,The characteristics of these intermediate
species represent evolutionary steps between C3 and C4 plants,although all point
out that C3-C4 intermediate physiology need not result from partial function of the
C4 system.
Certain genera ( Flaveria) contain C3,C4,and C3-C4 intermediate species and
therefore provide the opportunity to study the concordance of changes in anatomy
with physiology among related species.
Taxonomical and phylogenetical studies suggest that CAM and C4
plants were derived from C3 plants and the transitions occurred
many times in diverse taxa during the course of evolution,A drastic
decline in atmospheric CO2 level during the late Cretaceous period
(65-85 million years ago),a time of major expansion of the
angiosperms,has been proposed to account for the increase of C4
plants.
1.9.5 The change of photosynthetic pathway
C4 carbon fixation pathway has been found in some C3 plants.
C3 pathway could become a dominant pathway in C4 plants during
the process of senescence.
Many aquatic and?amphibious? C4 species exhibit a remarkable
developmental plasticity that permits modulation of C3 vs C4
fixation along with variations in leaf anatomy depending on
habitat.( Eleocharis )
The submersed monocot Hydrilla verticillata typically exhibits C3
character,but exposure to low ( CO2) induces a C4 system in
which the C4 and Calvin cycle co-exist in the same cell.
1.9.6 Molecular engineering of C4 enzymes in C3
plants
PEPC
Effects of overexpression of PEPC on photosynthesis are
controversial,At temperatures optimal for plant growth,practically
no difference in the rate of CO2 assimilation and CO2 compensation
point was observed in transgenic tobacco expressing the maize
PEPC gene.
In transgenic rice plants expressing the maize PEPC gene,the rate of
CO2 fixation was not altered significantly,but O2 inhibition of net
CO2 assimilation was mitigated with increasing activity of PEPC.
PPDK
Transgenic Arabidopsis,potato,rice expressing the maize C4-
specific PPDK gene,and transgenic tobacco expressing a PPDk
gene from the CAM plant showed no change s in photosynthetic
characteristics.
Future perspectives
Experiments with transgenic plants have reinforced the fact that the
C4 mechanism is a finely tuned metabolic,machine” where both a
high degree of precision in gene expression and structural
morphology work together to concentrate CO2 efficiently at the site
of Rubisco.
1.10 CAM metabolism involves the temporal separation
of CO2 capture and photosynthesis
1.11 Photorespiration
Photorespiration is associated with light-dependent oxygen uptake
and CO2 evolution in green plant tissues.
Oxygenase activity of Rubisco catalyzes the initial step of
photorespiration.
Photorespiratory reactions occur in three organelles,chloroplast,
peroxisome,and mitochondrion.
Production of ammonia during photorespiration requires an ancillary
cycle for its efficient reassimilation.
Photorespiration increases the energy costs associated with
photosynthesis,
Experimental measurements of photorespiratory carbon flux,which
use oxygen isotopes to discriminate between photosynthetic CO2
uptake and photorespiratory CO2 efflux,indicate that the rate of
photorespiratory CO2 release ranges from 18% to27% of the
photosynthetic carbon fixation rate.
Photorespiration may influence response of C3 plants to
future climatic events.
Role of photorespiration in plants
The oxygenase activity of Rubisco is consistent with the
enzyme?s anaerobic origins,Phosphoglycolate formed
during the oxygenase reaction is metabolized through the
photorespiratory carbon cycle to save 75% of the carbon
in the form of phosphoglycerate.
The energy consumption in the photorespiration can
protect the photosystem from the damage of high density
of sun light.
Plant Physiology
Biochemistry & Molecular Biology of Plants
Bob B,Buchanan
Wilhelm Gruissem
Russell L,Jones
植物学报 植物生理学植物生理学通信
Plant Physiology Plant Cell
Plant Science Planta
Annual Review of Plant Physiology and Plant Molecular Biology
Part 1 Photosynthesis
1.1 Overview of Photosynthesis
1.1.1 Photosynthesis is a biological
oxidation-reduction process.
Reaction 1.1,Photosynthesis
CO2+2H2A→(CH 2O)+2A+H2O
ljtang:
Reaction1.2,OxygenicPhototsynthesis
CO2+2H2O→(CH 2O)+O2+H20
Reaction1.3:Photosynthetic sulfur reduction
CO2+2H2A→(CH 2O)+2S+H2O
1.1.2 In photosynthetic eukaryotes,photosynthesis takes place in the
chloroplast,a specialized organelle
Chloroplast,a double-membrane system,consist of an outenvelope
and innerenvelope.
The internal membrane system,known as the thylakoids,contains
granalthylakoids and stroma thylakoids.
The thylakoid membranes are all interconnected and enclose an internal
space,the lumen.
1.1.3 Photosynthesis requires the coordination of two phases,light
reactions and carbon-linked reactions
Light reactions,produce O2,ATP,and NADPH.
Carbon reduction cycle,reduce CO2 to carbohydrate on consuming the
ATP and NADPH.
The conception that water oxidation and CO2 reduction were not
obligately linked was first advanced in 1930s by Robert Hill
.
Reaction 1.4,Hill Reaction
2H2O+2A→O 2+2H2A
1.2 Light Absorption and energy conversion.
1.2.1 Light is absorbed by pigment molecules.
Pigment→Pigment*
number of products formed photochemically
Equation 1.2.1:QuantumYieldФ= -------------------------------------------------------
Number of quanta absorbed
1.2.2 Almost all photosynthetic organisms contain
chlorophyll or a related pigment.
Plants,Algae and Cyanobacteria,chlorophyll a or
chlorophyll b.
Anaerobic photosynthetic bacteria,bacteriochlorophyll a.
Chlorophylls have a porphyrin ring structure that contains a
central Mg atom coordinated to the four modified pyrrole
rings.
Chlorophyll molecules also contain a long hydrocarbon tail
that makes the molecules hydrophobic.
Various chlorophylls differ in their substituents around the ring
structure.
Biosynthesis of chlorophylls and hemes.
1.2.3 Carotenoids and xanthophylls participate in
light absorption and photoprotection.
1.2.4 Absorption spectra of chlorophylls and other
photosynthetic pigments
1.2.5 Red algae and Cyanobacteria contain
accessory pigments that absorb green light.
1.3 The reaction center complex,
1.3.1 Reaction centers are integral membrane protein
complexes involved in conversion of light energy into
chemical products.
1.3.2 Reaction centers contain both special chlorophyll
and electron acceptor molecules involved in energy
conversion.
1.3.3 The structure of a reaction center from a
photosynthetic bacterium has been determined.
1.3.4 The kinetics of primary charge separation events
are understood in great detail,
1.3.5 Oxygenic photosynthetic organisms
contain two photochemical reaction
centers,PSI and PSII
1.3.5.1 Structure model of the PSII
reaction center.
1.3.5.2 Structure model of the PSI reaction
center.
1.4 The Photosystem
1.4.1 A photosystem contains a
photochemical reaction center and multiple
antennae ( auxiliary light-harvesting
pigment-protein complexes).
1.4.2 Most oxygenic photosynthetic
organisms contain chlorophyll a/b proteins
as their principal antennae,
Table 1.4.2 Properties of light-harvesting
chlorophyll protein complexes
Complex Chl a/b ratio Gene Mol,Mass(kDa)
Light-harvesting complexes (LHC) associated with PSⅠ
LHC-Ⅰ a 2.0-3.1 lhca3 20.5
(LHC-Ⅰ 680) lhca2 18
LHC-Ⅰ b 2.2-4.4 lhca1 20
(LHC-Ⅰ 730) lhca4 20
Light-harvesting complexes associated with PSⅡ
LHC-Ⅱ a (CP29) 4.0 lhcb4 29
LHC-Ⅱ b 1.35 lhcb1 27-28
LHC-Ⅱ c (CP26) 2.9 lhcb5 26.5
LHC-Ⅱ d (CP24) 1.51 lhcb6 24
1.4.2 The organization of pigments in PSⅠ and PSⅡ,
1.4.3 The light-harvesting antennae of
organism that contain phycobilins
Reaction 1.4.3 Energy transfer in a
phycobilisome
Phycoerythin(PC) →Phycocyanin(PE)
→Allophycocyanin(APC) →Chl a
1.5 Organization of thylakoid membrane
1.5.1 Protein complexes of the thylakoid membrane
exhibit lateral heterogeneity.
1.5.2 Phosphorylation of LHC-Ⅱ may
influence the distribution of energy
between PSⅠ and PSⅡ
1.6 Electron transport pathways in chloroplast membranes
1.6.1 The chloroplast noncyclic electron transport chain produces
O2,NADPH,and ATP and involves the cooperation of PSⅠ and
PSⅡ
1.6.2 Photosystem stoichiometry variesby species and
is influenced by light environment.
Table 1.6.2 Stoichiometry of the photosystems in different oxygen-
evolving systems and in response to changes in light quality _______
System
PSⅡ /PSⅠ ratio
Cyanobacteria 0.4
Red algae 0.4
Green algae 1.4
Tobacco mutant su/su 2.7
Pea chloroplasts
From plants grown in PSⅡ light (550-660nm) 1.2
From plants grown in sunlight 1.8
From plants grown in PSⅠ light(>660nm) 2.3
1.6.3 PSⅡ functions as a light-dependent water-
plastoquinone oxidoreductase.
Table 1.6.3 Protein subunits of the PSⅡ core
complexe
Hydrophobic subunits
Protein Gene Location of gene Function
D1 psbA Chloroplast Reaction center protein
D2 psbB Chloroplast Reaction center protein
CP47 psbC Chloroplast Antenna binding
CP43 psbD Chloroplast Antenna binding
Cytb559
subunit psbE Chloroplast Unknown
subunit psbF Chloroplast Unknown
PsbH-PsbN psbH-psbN Chloroplast Unknown
22kDa psbS Nucleus Photoprotection
Hydrophilic subunits
33kDa psbO Nucleus Oxygen evolution
23kDa psbP Nucleus Oxygen evolution
17kDa psbQ Nucleus Oxygen evolution
10kDa psbR Nucleus Unknown
1.6.4 Oxidation of water produces O2 and
releases electrons required by PSⅡ
Reaction 1.6.4,Oxidation of water
2H2O→O 2+4H++4e-
The pattern of oxygen evolution is given in
response to a series of short flashes of light
The oxygen evolution apparatus is considered to exist
in five different oxidation states (So through S4)
Structure of the manganese cluster
1.6.5 The cytochrome b6f complex transfers electrons
from reduced plastoquinone to oxidized plastocyanin
Proton translocation via cytochrome b6f is
thought to involve a Q-cycle
Reaction1.6.5,The Q-cycle in the chloroplast
PQH2+2PCox+2H+stroma →PQ +2PC red+4H+lumen
1.6.6 Pastocyanin,a soluble protein,links
cytochrome b6f and PSⅠ
1.6.7 PSⅠ function as a light-dependent
plastocyanin-ferredoxin oxidoreductase
Electrons from PSⅠ are transferred to NADP+
in the stroma in a reaction requiring ferredoxin
and ferredoxin-NADP+ reductase
1.6.8 Specific inhibitors and artificial electron
acceptors have been used to study the
chloroplast electron transport chain
D1
1.6.9 Chloroplasts also contain a cyclic
electron transport chain
1.7 ATP synthesis in chloroplasts
1.7.1 Electron transport and ATP synthesis are
coupled in vivo.
1.7.2 Chloroplasts synthesize ATP by a
chemiosmotic mechanism driven by a proton
gradient.
In the 1960s,Peter Mitchell proposed the chemiosmotic model to explain ATP
synthesis in chloroplasts as well as in mitochondria.,The experimental
verification of this hypothesis in the 1960s and 1970s resulted in Mitchell?s
receiving the 1978 Nobel Prize in chemistry.
1.7.3 Experimental manipulation of lumenal and stromal pH can
promote light-independent ADP phosphorylation in chloroplasts.
Acid-base Phosphorylation
Chloroplasts were first incubated at about pH4,allowing them to
establish this pH internally,The chloroplast suspension was then
rapidly adjusted to pH8,establishing an artificial pH gradient across
the membrane,This artificial pH gradient promoted ATP synthesis
until it was discharged and the pH difference was insufficient to drive
ADP phosphorylation.
Acid-base ATP synthesis required no light and was insensitive to
electron transport inhibitor,such as DCMU,indicating that a pH
gradient alone was sufficient to drive ATP synthesis.
1.7.4 The most widely accepted mechanism of ATP synthesis is
the so-call binding change mechanism proposed originally by Paul
Boyer ( a co-recipient of 1997 Nobel Prize in chemistry)
1.8 Carbon reactions in C3 plants
Most plants produce a three-carbon compound,3-
phosphoglycerate (3-PGA),as the first stable product in
the multistep conversion of CO2 into carbohydrate,This
functionally defined group,which includes most crop
plants,is referred to as C3 plants.
The Calvin cycle is divided into three phases,carboxylation,
reduction,and regeneration.
1.9 C4 plants contain two distinct CO2-fixing enzymes
and have specialized foliar anatomy
1.9.1 The C4 pathway increases the concentration of
CO2 in bundle sheath cell.
The bundle sheath chloroplast lacks stacked thylakoids
and contains little PSⅡ
1.9.2 Variations in C4 photosynthesis
Three variations of C4 photosynthesis are known,differing in the C4
acids transported between mesophyll and bundle sheath cells as well as
in the mechanism of decarboxylation in the bundle sheath cell.
The variations are named on the basis of the decarboxylase enzymes in
the bundle sheath cells.
1.9.4 Ontogeny of C4 patterns
In the development of most C4 leaves,the expression of C4 genes
does not occur until Kranz anatomy has been established.
The appearance of C4 activities has been compared in leaves 1-5 of
maize seedlings,the first leaf initiated (leaf 1) was more C3 in
charater,while the last measured (leaf 5) was fully C4.
Some studies showed that callus cultures derived from C4 plants do
not exhibit a functioning C4 pathway until cells organize into shoots
and vascular development is under way.
Physiological measurements suggest that the observed patterns of
gene expression reflect the function of the C4 carbon fixation
pathway near veins and the C3 pathway at more distant locations
1.9.3 C3-C4 intermediate plants and evolution of
photosynthetic system
Species with anatomical and physiological characteristics intermediate between C3
and C4 exist in many plant families,The characteristics of these intermediate
species represent evolutionary steps between C3 and C4 plants,although all point
out that C3-C4 intermediate physiology need not result from partial function of the
C4 system.
Certain genera ( Flaveria) contain C3,C4,and C3-C4 intermediate species and
therefore provide the opportunity to study the concordance of changes in anatomy
with physiology among related species.
Taxonomical and phylogenetical studies suggest that CAM and C4
plants were derived from C3 plants and the transitions occurred
many times in diverse taxa during the course of evolution,A drastic
decline in atmospheric CO2 level during the late Cretaceous period
(65-85 million years ago),a time of major expansion of the
angiosperms,has been proposed to account for the increase of C4
plants.
1.9.5 The change of photosynthetic pathway
C4 carbon fixation pathway has been found in some C3 plants.
C3 pathway could become a dominant pathway in C4 plants during
the process of senescence.
Many aquatic and?amphibious? C4 species exhibit a remarkable
developmental plasticity that permits modulation of C3 vs C4
fixation along with variations in leaf anatomy depending on
habitat.( Eleocharis )
The submersed monocot Hydrilla verticillata typically exhibits C3
character,but exposure to low ( CO2) induces a C4 system in
which the C4 and Calvin cycle co-exist in the same cell.
1.9.6 Molecular engineering of C4 enzymes in C3
plants
PEPC
Effects of overexpression of PEPC on photosynthesis are
controversial,At temperatures optimal for plant growth,practically
no difference in the rate of CO2 assimilation and CO2 compensation
point was observed in transgenic tobacco expressing the maize
PEPC gene.
In transgenic rice plants expressing the maize PEPC gene,the rate of
CO2 fixation was not altered significantly,but O2 inhibition of net
CO2 assimilation was mitigated with increasing activity of PEPC.
PPDK
Transgenic Arabidopsis,potato,rice expressing the maize C4-
specific PPDK gene,and transgenic tobacco expressing a PPDk
gene from the CAM plant showed no change s in photosynthetic
characteristics.
Future perspectives
Experiments with transgenic plants have reinforced the fact that the
C4 mechanism is a finely tuned metabolic,machine” where both a
high degree of precision in gene expression and structural
morphology work together to concentrate CO2 efficiently at the site
of Rubisco.
1.10 CAM metabolism involves the temporal separation
of CO2 capture and photosynthesis
1.11 Photorespiration
Photorespiration is associated with light-dependent oxygen uptake
and CO2 evolution in green plant tissues.
Oxygenase activity of Rubisco catalyzes the initial step of
photorespiration.
Photorespiratory reactions occur in three organelles,chloroplast,
peroxisome,and mitochondrion.
Production of ammonia during photorespiration requires an ancillary
cycle for its efficient reassimilation.
Photorespiration increases the energy costs associated with
photosynthesis,
Experimental measurements of photorespiratory carbon flux,which
use oxygen isotopes to discriminate between photosynthetic CO2
uptake and photorespiratory CO2 efflux,indicate that the rate of
photorespiratory CO2 release ranges from 18% to27% of the
photosynthetic carbon fixation rate.
Photorespiration may influence response of C3 plants to
future climatic events.
Role of photorespiration in plants
The oxygenase activity of Rubisco is consistent with the
enzyme?s anaerobic origins,Phosphoglycolate formed
during the oxygenase reaction is metabolized through the
photorespiratory carbon cycle to save 75% of the carbon
in the form of phosphoglycerate.
The energy consumption in the photorespiration can
protect the photosystem from the damage of high density
of sun light.
