Week 5 Flashcards
1
Q
What are macronutrients used by plants and mammals?
A
C, H, O, N, K, Ca, Mg, P, S
2
Q
What are micronutrients used by plants and mammals?
A
Cl, Fe, Mn, Zn, Cu, Mo, Ni
3
Q
What are elements used by just plants?
A
Boron (B, in cell walls)
Silicon (Si, e.g. in grasses)
4
Q
What are elements used by just mammals?
A
Fluorine (F, in teeth)
Cobalt (Co, in vitamin B12)
Selenium (Se, as selenocysteine in 4 enzymes)
Iodine (I, in thyroid hormone)
5
Q
What are examples of mineral deficiencies in humans?
A
Iron deficiency: fatigue, anaemia ~ 2 billion people
B12 deficiency: anaemia (in vegans and vegetarians)
Zinc deficiency: skin rash
Iodine deficiency: Goitre
6
Q
Why are iron, zinc and other metals so important?
A
17% of all known proteins bind Fe or Zn
Without the bound metal, the proteins (e.g. enzymes) are not functional
7
Q
What are the paradoxes of iron?
A
- Iron is one of the most abundant elements in the earth’s crust and yet it is often a limited nutrient due to insolubility of Fe3+
- Iron is essential for all living organisms and yet it is toxic
“Iron is expensive to acquire and costly to handle”
8
Q
What are the properties of iron in biology?
A
Iron mostly functions in electron transfer: Fe3+ + e- <–> Fe2+
Fe2+ and Fe3+ can also generate hydroxyl radicals (Fenton Reaction), which damage DNA, proteins and lipids
9
Q
What is an overview of Fe(III) concentration?
A
The availability of iron in the soil depends upon the pH: the
The amount of iron required for the normal growth of plants is 10-9 to 10-4 M
Fe(III) equilibrium concentration in neutral soils is ~10-17M, more in acidic soils, less in alkaline soils (e.g. lime)
10
Q
What is an overview of the photosynthetic protein complexes that contain most of the iron?
A
PSII - Fe Mn4Ca
Cytb6f - 2 Fe (haem) [2Fe-2S]
PsaC in PS1 - [2Fe-2S]
11
Q
What is an overview of iron deficiency symptoms in plants?
A
When iron is limiting, the plant economizes on photosynthetic complexes, and chlorophyll biosynthesis is down-regulated. This causes the pale appearance (lack of green Mg-chlorophyll), except for the veins
12
Q
What is an overview of zinc?
A
Relatively abundant in most soils, present as Zn2+ (free or chelated)
Zn deficiency in crops is not very common, but if it occurs it presents as necrotic tissue; Zn toxicity can also occur, e.g. near zinc mines
13
Q
What is an overview of zinc in plants?
A
Some plant species can tolerate zinc-rich soils by hyper-accumulating the metal in cell walls
Used in Zn-finger transcription factors and a range of enzymes (e.g. alcohol dehydrogenase)
14
Q
What is the estimated number of genes for handling iron and zinc?
A
Storage of minerals in seeds for the seedling upon germination - ~10 genes
Transport - 50 transporters and 5 for chelators
Use of metals for enzyme catalysis (Fe, Zn) or protein structure (Zn) - 30 for inserting metals
Active mobilization of minerals from the soil - 5 for uptake and 10 for chelators
15
Q
What are the 4 basic parts of iron homeostasis?
A
High-affinity iron transport
Deposition of intracellular iron stores
Control of iron consumption
Iron-responsive regulatory system
16
Q
What is an overview of high-affinity iron transport?
A
High-affinity iron transport enabling iron to scavenge, in
various forms, from the surroundings
17
Q
What is an overview of intracellular iron stores?
A
Deposition of intracellular iron stores to provide a source of iron that can be drawn upon when external supplies are limited
18
Q
What is an overview of control of iron consumption?
A
Control of iron consumption by down-regulating the expression of iron-containing proteins under iron-restricted conditions
19
Q
What is an overview of iron-responsive regulatory system?
A
An iron-responsive regulatory system that coordinates the expression of the above iron homeostatic machinery according to iron availability
20
Q
What is an overview of homeostasis?
A
The maintenance of a stable equilibrium, especially through physiological processes
21
Q
What are the two mechanisms for Fe soil uptake?
A
Production of small organic molecules
Acidification of the soil
22
Q
What is an overview of the production of small organic molecule production for Fe soil uptake
A
Production of small organic molecules that are secreted into the soil and chelate iron. The iron chelates (called phytosiderophores) are taken up by specific transporter proteins
23
Q
What is an overview of the acification of soil for Fe uptake?
A
Acidification of the soil to solubilize iron hydroxides and iron chelates, reduction of Fe3+ to Fe2+, then uptake of Fe2+ via a divalent metal transporter
24
Q
What is an overview of the genes for Fe uptake mechanisms under Fe deficiency?
A
Genes involved in these pathways are upregulated under Fe deficiency
25
What is an overview of the production of phytosiderophores?
Methionine --> Nicotianamine --> Phytosiderophore (PS)
26
What is an overview of the uptake of phytosiderophores?
Phytosiderophores (PS) secreted to rhizosphere
PS binds with iron to form Fe (III)-PS
Fe (III) - PS uptaken and Fe is released with PS repeating system
27
What is an overview of phytosiderophores dereived from methionine?
Mugineic acid (MA) is secreted to help take up of Fe and Zn in grasses
Nicotianamine (NA) helps to transport iron inside all plants
28
How is nicotianamine (NA) produced?
L-methionine to S-adenosylmethionine by SAM synthase
S-adenosylmethionine to nicotianamine by nicotanamine synthase (unknown mechanism)
29
How is nicotianamine converted to mugineic acid?
Nicotianamine to 3*-oxo form by nicotianamine aminotransferase
3*-oxo form by unknown reductase to mugineic acid
30
What happens if there is a mutant knock out of nicotianamine synthase?
Pronouced Chlorosis caused by mineral deficiency
31
What is an overview of AHA2 for acidification of soil for Fe2+ uptake?
ATP --> ADP + Pi
AHA transports proton across cell membrane
32
How does bts-1 impact proton releasing?
bts-1 mutants cant regulate iron levels so constantly think they need more so release protons
33
What is an overview of Ferric chelate reductase?
Ferric chelate reductase (FRO) uses the reducing power of NADPH to “extrude” electrons
NADPH loses 2 electrons which convert Fe3+ to Fe2+ costing energy
34
What is an overview of IRT1?
IRT1 is the founding member of a family of metal transporters called ZIPs
IRT1 expression is strongly induced in the roots of iron-deficient plants
IRT1 can transports other divalent metals (Zn2+, Mn2+, Cd2+), especially when Fe is scarce
35
What is an overview of irt1 mutant plants?
WT - grown
Irt1-1 - lack of growth
Heterozygous - growth
Irt1-1 + fe - growth (shows its iron)
36
What is an overview of irt1 mutant plants uptake of 55Fe?
WT when grown in high iron environments = low uptake
WT when grown in low iron environments = high uptake
irt1-1 when grown in high iron environments = low uptake
irt1-1 when grown in low iron environments = low uptake
37
What is the mechanism to prevent the overabsorbtion of metals?
IRT1 protein is rapidly internalized, regulated by ubiquitinylation, in response to increased intracellular levels of Zn2+ and Mn2+
38
What is the dogma of iron transporters?
Dogma (since 1984):
Grasses and cereals take up Fe3+-phytosiderophores
Non-grasses take up Fe2+ using AHA / FRO2 / IRT1
39
Ddoes wheat have functional IRTs?
Six IRT1 homologues in wheat;
Typical features of IRT1, including His-rich cytosolic loop;
Two IRT1 genes are regulated by Fe
These two genes can rescue an Arabidopsis irt1 mutant
40
How does age impact plant nutrients mobalisation?
During senescence of the plant N, S, P, Mg, Fe, Zn (etc) are released from degrading proteins typically from leaves, then remobilized to the seeds
41
What is an overview of iron and zinc remobalisation?
For Fe and Zn, the mobilization requires the chelator nicotianamine and various transporters.
42
What is an overview of iron and zinc storage?
Fe and Zn are stored in vacuoles (in Arabidopsis; cereals).
Fe can also be stored in ferritin
43
What is an overview of the distrubution of Fe, Mn and Ca in Arabidopsis seeds in vacuoles?
Fe and Mn elements has a individual place not overlapping
Iron - endodermous
Manganese - Some in epidermis and propholem bundles
Calcium - Everywhere
44
What is an overview of ferratin storage?
Uses intrinsic ferroxidase activity and can store 4000 to 4500 iron atoms
45
What is an overview of wheat storge of iron?
High levels of ferratin and vacuolar iron transporter in aleurones
46
What is an overview of iron deficiency anaemia?
IRON DEFICIENCY IS THE MOST WIDESPREAD NUTRITIONAL PROBLEM IN THE WORLD
Debilitated health of 500 million women
>60,000 deaths/y during childbirth
Lost productivity of up to 2% of GDP in the worst affected countries
Particular in LIC
47
What is an overview of Iron nutrition?
RDA for women: 14.8 mg / day and for men: 7.8 mg / day
48
What is an overview of dietary iron?
Haem iron (meat products) absorption is 25-30%
Non-haem iron (plant foods) absorption is 0-15%
Vitamin C and meat increase iron absorption
Phytate (in cereals) and tannins (in some vegetables and tea) decrease absorption
49
How did they test for the bioavaliability of iron in different vegetables?
Food is simulated invitro digestion
Caco-2 cells (though chemical induction act like epithelial cells)
Ferretin production is used to show iron absorption
50
What were the results of testing the bioavaliability of iron in different vegetables?
FeSO4 without vitamin C - 15 ng / mg protein of ferritin
FeSO4 with vitamin C - 55 ng / mg protein of ferritin
Peas without vitamin C - 5 ng / mg protein of ferritin
Peas with vitamin C - 5 ng / mg protein of ferritin
Spinach with vitamin C - 5 ng / mg protein of ferritin
Broccoli with vitamin C - 30 ng / mg protein of ferritin
Cabbage with vitamin C - 45 ng / mg protein of ferritin
51
What is an overview of dietary Zn deficiency?
75%+ Very high in saharan africa and indian subcontinent
60% in subsaharan africa
45% South America
>5% Western Europe and Nouth America
52
What is an overview of Zn nutrition?
<1-5mg/d Zn is required to replace daily losses
Depending on body weight, total Zn intake and dietary phytate
53
What are good sources of Zn?
Adequate Zn intake: high Zn foods include
- Oysters (6 will provide 16 mg Zn)
- Red meat and poultry
- Seafood
- Pulses, nuts
- Whole grains
54
What is an overview of natural variation in iron and zinc levels?
Lentils 111 39 (Fe:Zn mg/kg)
Beans (blackeye) 59 32
Spinach 21 7
Broccoli 17 6
Rice (polished) 4 30
Potatoes 3 2
55
Why isnt Fe Zn deficieny a problem in the west?
With a varied diet, including vegetable sources with good Fe / Zn bioavailability, mineral deficiency should not be a problem. However, lower income parts of the world consume mostly staple crops, without vegetables or meat
56
What are the current approaches to reduce micronutrients?
Micronutrient supplements for children
Micronutrient fortification
Biofortification
57
What if an overview of micronutrient supplements for children?
Vitamin A and zinc
58
What if an overview of micronutrient fortification?
In the UK: iron, calcium, niacin (B3), thiamin (B1) in wheat flour. This is mandatory.
Recent recommendation to add folate.
Voluntary: Vit D and A in spreads
Other countries: iodine (in salt or in milk)
59
What are the methods for biofortification?
Breeding programs
Nutrient fertilization (Zn, but not Fe)
Gene transfer (GM)
Gene editing
60
How can genetics boost Fe levels?
Mutant studies in Arabidopsis and rice
Find genes for Fe homeostasis
Experiment on genes: overexpression, ectopic expression or loss-of function
Find genes in seeds with increased Fe
61
How can breeding boost Fe levels?
Old landraces and wildrealives with high seed Fe levels
QTL mapping: GWAS
Validation of candidate gene
Gene for Fe homeostasis
62
What is an overview of harvest plus?
HarvestPlus-funded programme focussed on beans, Phaseolus
Important staple food in East Africa and Latin America
Breeding programmes increased Fe: ~55 to > 94 μg/g Fe
But.. phytate and polyphenols may inhibit iron uptake
63
What is an overview of the results of iron levels in maize, wheat and rice from breeding?
Conventional breeding will only be able to deliver 10-20% of target iron levels
Maize - target 32ppm got 26 ppm
Wheat - target 40ppm got 33ppm
Rice - target 19ppm got 7ppm
Transgenic breeding could deliver 100% of target level
64
Why is rice, wheat and maize important to study?
Rice, wheat & maize are the most important crops consumed by the world’s population and particularly (disproportionately) for the poor
65
What is an overview of the genes targeted in iron uptake?
TOM1 and YSL2: transporters of phytosiderophores (in cereals)
IRT1: Fe2+ transporter in root epidermis, mostly in non-cereals
Combination strategies (are most successful!)
66
What is an overview of the genes targeted in iron transport?
NAS, NAAT: synthesis of nicotianamine and mugineic acid,
Fe and Zn chelators involved in inter- and intracellular transport
Combination strategies (are most successful!)
67
What is an overview of the genes targeted in iron storage?
FER = ferritin, an Fe-storage protein
VIT = vacuolar iron transporter
Combination strategies (are most successful!)
68
What are examples of genes and their increase in iron and zinc in rice?
AtNAS1, Pvferritin and Afphytase in Japonica Taipei increase by ~6 times Fe or 1.6 times Zn
SoyFer in Indica cv IR68144 increase by ~4 times Fe or 1.4 times Zn
OsYSL2, SoyFerH2+ and HvNAS1 in Japonica cv Tsukinohikari ~6 times Fe or 1.6 Zn
69
What is an overview of the overexpression of rice NAS (Nicotianamine Synthase) genes in rice?
Rice has 3 NAS genes, with different (but over-lapping) patterns of expression
Each gene was constitutively over-expressed in rice
More than 20 independent lines were analysed
Correlation between Fe and Zn accumulation
70
What is an overview of Fe and Zn location in rice?
Most of the iron accumulates in the seed coat and is removed by polishing, but zinc is sufficiently increased in white rice
71
What is an overview of the concentrations of Fe (ug^-1) and Zn (ug^-1) in unpolished compared to polished grain of WT and transgenic rice?
WT- Fe- 22 to 4.5 Zn- 42 to 34
OE-OsNAS1S - Fe- 47 to 9.7 Zn- 63 to 48
OE-OsNAS2B - Fe- 64 to 19 Zn- 91 to 76
OE -OsNAS3B - Fe- 51 to 9.9 Zn- 65 to 49
72
What is an overview of cassava biofortification?
Overexpression of Arabidopsis IRT1 and Ferritin
They also tried a mutant version of IRT1, in which the mechanism for internalization is disrupted
Tubers have 7–18 times more Fe and 3–10 times more Zn
73
Why bread wheat is difficult to biofortify with iron?
Plants have tight iron homeostasis mechanisms
Bread wheat has hexaploid genome – this buffers genetic variation in mineral traits for conventional breeding
After milling, most grain iron is in the bran fraction, not the white flour fraction.
Natural variation of iron concentrations is low, due to wheat having only been around for ~10,000 years and selection for high yield.
74
What is an overview of the iron differences between wholemeal and white bread flower lines?
Wholemeal - 26-69 ppm difference of 2.4x
White - 5-16 ppm difference of 3.2x
75
How do they find iron and zinc transport in wheat?
Browse the genome for homologs to metal transporters in rice and the model plant Arabidopsis
76
What is an overview of the number of vacular iron transporters in wheat?
The bread wheat (Ta) genome has 6 genes encoding Vacuolar Iron Transporters (VIT)
Across 2 branches(not sure why 2 branches)
77
What is an overview of the expression of Vacuolar Iron Transporters?
Vacuolar Iron Transporters is low in the endosperm
Endosperm - 10 TPM (trancription per million) for both TaVIT1 and TaVIT2
Roots - 25 TPM for TaVIT1 and 105 TPM for TaVIT1
78
What are the outcomes for vacuolar iron transporters in yeast on different media with/without plasmid reintroduction?
WT + empty plasmid
On plan media and media with iron both grow well
ccc1 mutation + empty plasmid
On plan media grow but on media with iron dead
ccc1 mutation + CCC1 plasmid
On plan media and media with iron both grow well
79
Why does yeast with ccc1 knockout and no CCC1 plasmid kill itself when exposed to media + Fe?
Vaculoe keeps iron storage safe but cant be transported in without CCC1 so oxidative damage from iron kills yeast
80
What is the different in the results with TaVIT1 and TaVIT2 with CCC1 knockout?
TaVIT2 rescues ccc1 mutants
TaVIT1 localized predominantly to the cell membrane
TaVIT2 also transports manganese, but not zinc
81
What is an overview of TaVIT2 promter for wheat?
To select for transformants:
CaMV 35S promoter
Hygromycin resistance gene
Terminator
To express TaVIT2 in endosperm
Wheat endosperm promoter (HMW-GLU)
TaVIT2cDNA
Terminator
82
What is the TaVIT2 lines after handmilling and sieving compared to fortificaiton requirement?
Contol - wholemeal (25 mg/kg) above fortification line (16.5 mg/kg) but white (10 mg/kg) is below
For both single and 20 copies of TaVIT2 both above (WM 27 mg/kg and W 20 mg/kg) fortificaiton line for whote and wholemeal bread. Though 20 copies are greater than 1 for both (WM 40 mg/kg and W 30 mg/kg)
83
What is the distribution of iron and phosphorus (~phytate) and zinc in roller-milled flour?
Iron - 1st break through to 2nd reduction TaVIT2 gene much higher than control wheat but both same for control
1st break - con - 2 up/g Ta - 8 up/g 2nd red - con - 10up/g Ta - 20 up/g
Zinc and phosphorus both control and wheat equal across all groups
84
What are the results of the field trial for TaVIT wheat iron levels?
TaVIT wheat had high levels of iron
1st break - con - 2 up/g Ta - 18 up/g
2nd reduction - con - 42 up/g Ta - 62 up/g
85
What is an overview of the results of the field trials wheat?
~10% decrease in thousand grain weight.
Partially compensated for by a trend for more grains per spike
86
What is an overview of nicotianamine?
Nicotianamine (NA) helps mobilise iron, zinc and other Me2+ around the plant and into grain.
NA also helps increase iron and zinc bioavailability with Caco-2 and chicken studies of OsNAS2-expressing wheat
87
What is an overview of Iron concentrations in VIT2-NAS2 lines grown in the field (2022)?
Higher concentrations of Iron in wholemeal and white flour
White flower (23 mg/kg) above legal requirement (16.5 mg/kg) but control (12 mg/kg) was below and wholemeal was above for both (con - 39 mg/kg GM - 62 mg/kg)
88
What is an overview of zinc concentrations in VIT2-NAS2 lines grown in the field (2022)?
Higher concentrations of zinc in wholemeal and white flour
White flower (60mg/kg) above legal requirement (32 mg/kg) but control (8mg/kg) was below wholemeal was above for both
89
What is an overview of Iron and zinc levels in roller-milled fraction of VIT2-NAS2 wheat vs control?
Roller-milled fractions show dramatically increased iron in white flour, and higher zinc in bran fractions (but also some increase in white flour).
90
What is an overview of nicotianamine (NA) concentration in mature grain and mineral solubility in invitro digestion?
B71 - 35 nmol/g
uM of iron released from control - 6
uM of iron released from B71 - 20
uM of zinc released from control - 5
uM of zinc released from B71 - 8
91
What is an overview of photosynthesis?
Photosynthesis harnesses light energy from the sun to create chemical energy in the form of reduced carbon.
Source of all food and almost all of our current energy resources.
Stromatolites are nearly 3 billion years old and may have been formed by oxygenproducing cyanobacteria
92
What is the evolutionary history of chloroplasts?
Chloroplasts arose from a single primary endosymbiotic event between a photosynthetic cyanobacterium and a eukaryotic phagotroph about 1.5 billion years ago
93
What is an overview of secondary endosymbiosis of photosynthetic organisms?
Secondary endosymbiosis events created more photosynthetic organisms, like brown algae, diatoms, dinoflagellates, euglenoids
94
What is an overview of photosynthesis?
The overall process can be summarised as:
6 CO2 + 6 H2O --> C6H12O6 + 6 O2
Photosynthesis involves the light and light-independent reactions, which are linked via pools of ATP and NADPH
95
What is an overview of light-dependant photosynthesis?
The first step is the capture of light energy as ATP and reducing power, NADPH
Light reactions take place on thylakoid membranes
96
What is an overview of light-independant photosynthesis?
The second step is the transfer of energy and reducing power from ATP and NADPH to CO2, to produce high-energy, reduced sugars
97
What are the main photosynthetic pigments?
The light that hits a leaf is mainly in the visible spectrum.
Chlorophyll a and Chlorophyll b are abundant in plants.
98
What are accesory pigments that absorb light energy?
Accessory pigments also absorb light energy and transfer it to chlorophyll during photosynthesis.
Carotenoids (like β-carotene) are in all photosynthetic organisms.
Phycoerythrin are found in cyanobacteria and non-green algae.
Phycocyanin in cyanobacteria
99
What are the four large protein complexes involved in light dependant photosynthesis?
Photosystems I and II
Cytochrome b6f (Cyt b6f)
ATP Synthase
100
What is an overview of photosystems I and II?
Photosystems I and II – Photosystem I (PSI) and Photosystem II
(PSII) that harvest light. Light harvesting activates linear electron transport. NADPH production is linked to PSI.
These photosystems contain light harvesting complexes that contain chlorophyll + other pigments
101
What is an overview of plastoquinone?
Plastoquinone (PQ) is a small molecule and mobile electron carrier
102
What is an overview of plastocyanin?
Plastocyanin (PC) is a small protein and mobile electron carrier
103
What happens during light dependant photosynthesis?
PSII absorbs light and passes excited electron to plastoquinone (the electrons are replaced by splitting of water releasing oxygen, protons and electrons)
Plastoquinonone passes it onto to cytochrome b6f then onto plastocyanin
Plastocyanin passes the electrons onto PSI
104
What occurs at PSI?
The electron recieves further energy from light
Has enough energy to convert NADP+ to NADPH a reductive molecule
105
How does light dependant photosynthesis produce ATP?
As electron moves across Cyt b6f this pumps protons into lumen by greating a proton motive force
In addition to protons formed from the splitting of water creates a proton gradient
The protons diffuse across ATP synthase converting ADP + Pi into ATP
106
What is an overview of cytochrome b6f and ATP synthase?
Cytochrome b6f (Cyt b6f) that builds a proton gradient.
ATP Synthase that uses the proton gradient to make ATP.
107
What is an overview of the calvin-benson reaction?
ATP and NADPH produced in the light reactions are used to run the CalvinBenson cycle.
Rubisco is the enzyme involved.
Cycle occurs in three phases.
Also called C3 cycle or dark reaction
108
What is an overview of phase 1 of the calvin-benson reaction?
Carbon fixation
RuBP + CO2 --> 3-PGA
Uses rubisco
109
What is an overview of phase 2 of the calvin-benson reaction?
Reduction
3-PGA + ATP + NADPH + H+ --> Triose-P
110
What is an overview of phase 3 of the calvin-benson reaction?
Regeneration of CO2 acceptor
A series of chemical reaction to convert triose-P into RuBP
Pathway releases starch from F6P and sugars releases
111
What is an overview of rubisco?
Ribulose 1,5-bisphosphate carboxylase / oxygenase (Rubisco) catalyses the reaction between Ribulose 1,5-bisphosphate (RuBP) and CO2 or O2
112
What is the main problem with rubisco and how have plants overcome it?
Rubisco is a slow catalyst that poorly distinguishes between CO2 and O2
In C3 plants, <50% of soluble leaf protein can be Rubisco
113
What happens when rubisco works well?
CO2 fixation by Rubisco results in two 3-PGA molecules to run the Calvin cycle
114
What happens when rubisco fixes oxygen?
Makes both a 2-phosphoglycolate and 2-phosphoglycerate
2-phosphoglycolate cant be used in calvin-benson cycle so has to be converted into 3-PGA by phosphorespiration
115
What is an overview of photorespiration?
Photorespiration recovers carbon from 2-PG in a multi-organellar process.
Makes 2-PG into 3-PGA for re-entry into the Calvin-Benson cycle.
Releases CO2! Also releases a NH3.
Uses ATP and NAD(P)H
116
What is an overview of C4 and CAM photosynthesis?
Both C4 and CAM types of photosynthesis are based around PEP
Carboxylase (PEPC) that ‘pre-fixes’ bicarbonate into a C4 acid.
Carboxylases are spatially separated in C4 photosynthesis. In CAM, Carboxylases are temporally separated
117
What is an overview of C4 photosynthesis?
Pyruvate is transported to mesophyll cell form bundle sheath cell
Pyruvate is converted to PEP by ATP
PEP is converted to oxaloacetate (OAA) with HCO3- (fixing CO2) by PEPC forming C4 acids
OAA is reduced to malate by NADPH
Malate is transported into bundle sheath
Malate decarboxylation creating pyruvate and CO2
CO2 is put into the C3 cycle
Pyruvate repeats cycle
118
What is the advanatage of C4 pathway?
The advantage of the C4 pathway is that it avoids
photorespiration
119
What happens during the CAM photosynthesis at night?
Sugars turned into C3 acids
C3 acids are converted into C4 acids by carbon fixation of HCO3-
The C4 acids are stored into C4 acids
120
What happens during the CAM photosynthesis during the day?
C4 acids exported from vacuoles
C4 acid decarboxylase is converted into C3 acid and CO2
CO2 undergoes into C3 cycle
C3 acids converted into sugar
121
What are the advanatges of CAM?
CAM conserves water by opening stomata at night, and also avoids photorespiration
122
What are examples of important C4 photosynthetic crops?
Corn (maize, Zea mays) is the most economically important C4 plant and the 4th most economically important crop plant
Sugar cane (Saccharum spp.) is the second most economically important C4 plant and the 6th most economically important crop plant
123
What is an overview of C4 plants?
Kranz anatomy
Mesophyll and bundle sheath cells must be organised to exchange metabolites during C4 photosynthesis.
In plants with Kranz anatomy, bundle sheath cells form a ring around the vascular tissue, and mesophyll cells form a ring around them
124
What is an overview of C4 photosynthesis evolution?
C4 photosynthesis evolved more than 60 times during plant diversification
Mainly arose in hot, dry climates
125
Why has C4 photosynthesis evolved multiple times?
All enzymes required for the C4 pathway are present in C3 plants! It has been relatively easy for plants to acquire the capability for C4 photosynthesis from ancestral C3 state
126
What are the advantages of C4 photosynthesis in high temperature?
High temperature: Rubisco selectivity decreases and photorespiration increases with temperature in C3 plants. Relative solubility of CO2 to O2 also decreases with temperature.
127
What are the advantages of C4 photosynthesis in dry climate?
Dry climates: Plants close stomata when water is limiting, letting less CO2 into the leaf. PEPC fixes CO2 even when levels are low
128
What are the advantages of C4 photosynthesis with light intensity?
Light availability: Because carbon-fixation in C4 plants is not carbon-limited, they are able to take advantage of high light intensities
129
What are the downsides of C4 photosynthesis?
C4 is expensive! (additional carboxylation steps require energy)
At low temperature higher irradiance favours C3 photosynthesis
At high temperature higher irradiance favours C4 photosynthesis
130
How can photosystem 1 potentially be improved for improving the light reaction?
A hypothetical system in which PSI is replaced by a bacterial photosystem that uses longer wavelength light.
(The system would have to be extensively engineered to bridge the gap between water oxidation and NADPH reduction in the absence of PSI.)
131
How can the spectrum of light absorbtion be improved?
Another suggestion is to increase green-light absorption by introducing phycoerythrin from red algae
132
What is an overview of the typical canopy of leaves?
In a typical canopy, upper leaves intercept too much light and must dissipate some energy as heat, meanwhile shading lower leaves
133
What is an overview of smart canopy?
A “smart canopy”, upper leaves would have a more vertical orientation, permitting more light to reach lower leaves and greater overall efficiency
134
Is faster rubisco possible?
Rubisco from different organisms shows variation in specificity and speed.
Trade-off between speed and specificity: the more specific the Rubisco, the slower it is
135
What are examples of different rubisco speeds in organsisms?
Rubisco in red algae is more specific but slower than that in plants
Rubisco in cyanobacteria algae is faster but less specific than that in plants
In many organisms, the enzyme cannot be both fast and specific
136
What is an overview of C4 rice?
Engineering C4 into rice would boost yields and improve water use efficiency. An international multi-institutional team is working towards this goal (since 1995).
Not only a biochemical problem! Need to substantially change leaf anatomy and cell biology.
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What are the benefirs of C4 rice?
Introduction of ‘C4’ traits into rice is predicted to increase photosynthetic efficiency by 50%, improve nitrogen use efficiency and double water use efficiency
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What is an overview of cyanobacterial carboxysomes?
Cyanobacteria concentrate CO2 and Rubisco in a special body called a carboxysome. They also actively pump CO2 and bicarbonate into their cells
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Can we engineer such a CCM in plant chloroplasts?
A functional cyanobacterial Rubisco could be expressed in
tobacco plants
Carboxysome components could also partially assemble into bodies in tobacco chloroplasts
Problem the carboxysome dont fully work as they lack the pumps required to actively transport CO2 inside them
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What is an overview of Algal pyrenoids?
Starch sheath with thylakoids running through them allowing the rubisco condensate and CO2 to be actively pumped in
Many algae also concentrate CO2 and Rubisco in a structure within the chloroplast called a pyrenoid
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What is the current state of using algal pyrenoids?
Expression of algal Rubisco subunits and pyrenoid proteins results in condensation of Rubisco into ‘proto-pyrenoids’ in Arabidopsis chloroplast
We can now engineer the “starch sheath” around the proto pyrenoid by additionally expressing an algal starch-binding protein
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What is an overview of reducing the cost of photorespiration?
Alternative pathways of glycolate metabolism (using enzymes borrowed from other species) introduced into the chloroplasts of tobacco plants
20-24% increase in biomass in the field
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What is an overview of alternative synthetic carbon fixation?
Completely re-imagined carbon fixation cycles designed with computational and synthetic biology approaches
Works in vitro, but engineering into a photosynthetic organism is a challenge.
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