Nitrogen & Nitrogen Fixation

Importance of nitrogen to plant metabolism

• Limiting nutrients in plants (& agriculture)

• Can regulate growth processes, due to integration of N and C metabolism

transcriptional regulation of NR (also NiR, HATS)

• Inducers: NO 3-, light, sucrose, circadian cycle

• repressors: NH 4+, glutamine

tight post-transcriptional regulation of NR

• rapid NR mRNA and protein turnover (short half-life)

• inactivation, phosphorylation are sequential

Key to N metabolism

Aminotransferase

• aspartate aminotransferase

• alanine aminotransferase

Other forms of transport nitrogen

(legumes) – biosynthesis of
ureides: allantoate and allantoic acid

Glutamate: used as a nitrogen donor for molecules….

• all other amino acids

• nitrogenous bases (nucleic acids)

• chlorophyll (glu)

• alkaloids (from trp, tyr)

Symbiotic Nitrogen Fixation (in legumes)

The process by which molecular nitrogen (N 2) is reduced ("fixed") directly to NH 4+

Why is N fixation important?

• net nitrogen input into soil (5-10 % of global N input)

• legume-Rhizobium interaction is a model system for plant-microbe signaling

Nitrogenase

a prokaryotic enzyme found only in some bacteria, but plants often benefit

free-living cyanobacteria

associates with in Azolla water ferns in rice paddies

lichens

Often contain cyanobacteria as symbionts

• net N input into forest ecosystems

Rhizobium

form nodules leguminous plants
of great importance for agriculture

1. The plant provides..

2. The bacteria provides..

1. energy and carbon (sucrose-> malate)

2. NH4+

Features of the Legume-Rhizobium symbiosi

1. Orchestrated formation of nodules, a new organ with distinct development

2. plant & Rhizobium communicate via chemical signals

3. the biochemistry of nitrogen fixation, metabolism and regulation

What is the benefit of this arrangement for Rhizobia?

• some small number of bacteroids will be viable after nodules senesce.

  • Successfully infecting a root thus allows for successful reproduction

Some additional complexities of Rhizobia

• Are some 'parasitic' Rhizobia – they infect, but do not fix nitrogen ("cheaters")
  - some Rhizobia have lost the ability to infect, but they still persist and survive
  (nonsymbionts)