EFB530 Plant Physiology

Nitrogen assimilation and fixation

Availability of nitrogen to plants is often a limiting factor in growth and productivity

• despite the fact that N₂ is the most abundant gas in the atmosphere

Nitrogen can exist in pools of different chemical forms, nitrogen cycle

• reduction of N₂ to ammonia=nitrogen fixation (bacteria, lightning, industrial)
• ammonia is oxidized to nitrite, then nitrate=nitrification (by some bacteria)
• organics from tissues or biological waste (such as manure) are converted to ammonia=ammonification (by bacteria & fungi)
• nitrate is reduced to N₂, returned to the atmosphere=denitrification (bacteria)

In taking up and utilizing nitrogen, plants incorporate it into carbon-nitrogen compounds

Crop plants take up nitrogen primarily in the form of nitrate (NO₃^-)

• except in conditions where nitrifying bacteria don't grow well (low pH, anaerobic)
• then ammonia (NH₄⁺) will be available for uptake
• many forest species take up primarily NH₄⁺, since forest soils are more acidic (less nitrification)
• NH₄⁺ leaches less than NO₃^-, since NH₄⁺ adheres to soil colloids

NO₃^- is actively transported into and concentrated in the cells by a NO₃^- / H⁺ carrier (symporter)

Assimilation:

NO₃^- is reduced to nitrite (NO₂^-) by nitrate reductase using NADH

• addition of NO₃^- induces the expression of nitrate reductase (regulated at transcription and enzyme activity)
• nitrate reductase has Mo, heme, and FAD cofactors (Mo deficiency can lead to N deficiency)

NO₂^- is reduced to ammonium (NH₄⁺) by nitrite reductase using ferredoxin (Fd)

• nitrite reductase has Fe-S and heme cofactors
• located in plastids

NH₄⁺ is incorporated into carbon compounds: amides (amino acids) and ureides

Incorporation into amino acids is primarily by the GS-GOGAT pathway

GS-GOGAT cycle

2 glutamate + 2 NH_4⁺ + 2 ATP -> 2 glutamine + 2 ADP

• enzyme is glutamine synthetase (GS)

glutamine + alpha ketoglutarate + NAD(P)H -> 2 glutamate + NAD(P)⁺

• enzyme is glutamate synthase (GOGAT)

Direct incorporation of ammonium can also occur:

alpha-ketoglutarate + NH₃ + NADH -> glutamate + NAD⁺

• enzyme is glutamate dehydrogenase

Amino transferase can transfer the amino group to aspartate, which can be converted to asparagine

NH₄⁺ can also be combined with carbon compounds to form ureides

• mainly: allantoin (C₄N₄H₆O₃) or allantoic acid (C₄N₄H₈O₄)

Nitrogen can be transported in the xylem as:

• NO₃^-
• amides - glutamine (C₅N₂H₉O₃), asparagine (C₄N₂H₇O₃)
• ureides (see above)
• amino acids - glutamate (C₅NH₉O₄), asparagine (C₄NH₇O₄)

incorporation of NH₄⁺ into amino acids can occur either in the roots or in the leaves

N₂ fixation

some plants can join in a symbiotic relationship with bacteria that are able to fix N₂

principally the legumes with Rhizobium bacteria (live in the soil), but also Alder (Alnus) with Frankia (an actinomycete)

interaction between plant and bacteria induces the plant to alter its growth pattern

• bacterium produces Nod factors, which are recognized by the root cells
• bacteria penetrate the root through an infection thread (invagination of the root cell plasma membrane)
• bacteria are "packaged" within membrane derived from the root cell plasma membrane=peribacteroid membrane
• they stop dividing and differentiate in bacteroids
• the root cortical cells dedifferentiate, then divide to form a mass of cells=nodule

nitrogen fixation = N₂ + 8 e^- + 8 H⁺ + 16 ATP -> 2 NH₃ + H₂ + 16 ADP

this is catalyzed by a bacterial enzyme = nitrogenase

• 2 components, Fe protein (Fe-S cofactors) and the MoFe protein (Mo-Fe-S)
• both act together to reduce N₂
• VERY sensitive to O₂

oxygen paradox

• nitrogenase is very sensitive to O₂
• but the cell needs alot of ATP and reducing power from respiration
• a plant consumes 12 g of organic carbon per 1 g of N₂ fixed

nodule maintains low O₂ tension

• the nodule has a layer of cells with low permeability to O₂
• the plant produces leghemoglobin-carries O₂ for bacterial respiration

cyanobacteria can also fix N₂

• use specialized cells = heterocysts
• thick cell walls (low O₂ permeability)
• turn off PS II
• active respiration

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