The infection process during nodule organogenesis
─ occur simultaneously
Plant: attractants
Rhizobia: Nod factors
Cell wall
degradation
Fuse with the
membrane of
root cell
Infection thread
formation
Penetrate and new
infection thread
formation
Branching and extending
bacteria released into the
cytosol
Bacteroids: a nitrogen-fixing endosymbiotic organelles (p. 301R)
The energetic of nutrient
assimilation
¤ nitrate  ammonium consumes ca. 25% of the total energy
less than 2% of the total dry weight produce
¤ occur in the stroma of the chloroplast
¤ Photoassimilation:
coupling nutrient assimilation to
photosynthetic e- transport
use the surplus reductants of
Calvin cycle
 high light, low [CO2],
photoassimilation proceed
 [CO2] 
inhibit photoassimilation
inhibit photorespiration
C/N assimilation
C4 plants: photoassimilation occur in mesophyll cells, [CO2] low
High [CO2] in bundle sheath cells
 Inhibit photoassimilation
Assimilatory quotient(AQ)  CO2 assimilated / O2 evolved
as a function of light level
Wheat seedling
no photoassimilation
Photoassimilation 
(no photoinhibition)
Photoassimilation:
CO2 fixation may interfere with nitrate photoassimilation
(3)
(1)
(2) stroma
(1) NADH is supported by chloroplast via malate shuttle
(2) The level of reduced ferredoxin
(3) The acidification of stroma dissipate the pH gradient
CO2 itself and ATP regeneration
WebEssay 12.1
The plants receiving NH4+ were
more responsive to CO2 enrichment than those receiving NO3
WebEssay 12.1
Doubling CO2 (enrichment):
short-term:
accelerate carbon fixation in C3 plants by about 30%
long-term (days to weeks):
carbon fixation declines until it stabilizes at a rate that averages
12% above ambient controls
CO2 acclimation
shoot N and proteins contents diminish
Sulfur assimilation
§ Sources:
The weathering of parent rock material
Industrial contamination, the burning of fossil fuels releases sulfur
dioxide
and hydrogen sulfide
§ Absportion pathways:
sulfate: H+– SO42- symporter of the roots from the soil
sulfur dioxide take up from stomata
＞0.3 ppm, 8 h  extensive tissue damage
§ Location
mostly in leaves which can supply reduced ferredoxin and serine (p.
305R)
§ The transported form in the phloem
glutathione (Gly-Cys--Glu), also acts as a signal that coordinates the
absorption of sulfate by the roots and the assimilation of sulfate by the
shoot
gallic acid
glucoside,
glucosinolates,
polysaccharides
In cytosol
2
In plastids
Activated form
(photorespiration)
Cysteine  Methionine
(Web topic 12.3)
S-adenosylmethionine
Sulfur assimilation
§ Functions:
The structural and regulatory roles in proteins, disulfide bridges
Electron transfer through iron-sulfur clusters
Catalytic sites for several enzymes and coenzymes
Secondary metabolites such as Nod factors, antiseptic
alliin in garlic, and anticarcinogen sulforaphane in
broccoli
Oxygen assimilation
¤ Respiration (major)
Photorespiration: rubisco oxygenase activity (major)
oxygen fixation (minor):
oxygen assimilated into organic compounds
¤ the types of oxygenases
dioxygenase:
monooxygenase: mixed-function oxidase
In ER
cell wall protein extensin, posttranslation
hydroxylation
demethylation
A + O2 + BH2  AO + H2O + B
Cation assimilation
¤ K, Mg, Ca, Cu, Fe, Mn, Co, Na, Zn
¤ coordination bonds and electrostatic bonds
p. 306L
neutralization
oxalate