Soil/Rhizosphere Microorganisms are responsible for most nutrient - - PowerPoint PPT Presentation

Soil/Rhizosphere

• Microorganisms are responsible for most nutrient

transformation in soil.

• 109 cells and 106 distinct taxa per g of soil.
• Different plants have different root exudates (up to 20% of

photosynthate).

• Arabidopsis does not grow in g-irradiated soil

From the first land plants to cereals and legumes

• First land plant used AM fungi as rooting systems
• Legume Family (Leguminosae) 3rd largest family of

plants 19,400 species

• Legumes have symbiosis with bacteria (the rhizobia)

From the first land plants to cereals and legumes

• First land plant used AM fungi as rooting systems
• Legume Family (Leguminosae) 3rd largest family of

plants 19,400 species

• Legumes have symbiosis with bacteria (the rhizobia)

The Players

Soybean

Soybeans twice protein per acre as any other major vegetable or grain crop, 5 to 10 times than land used for dairy, and 15 times land for meat production.

Bradyrhizobium japonicum

+ =

N2 fixation in nodules

Nitrogen cycle

• Nitrogen cycles between inert N2 and biologically

available NH3

• Nitrogen main limiter of biological production
• 50-60% of biospheres nitrogen comes bacterial

N2 fixation in legume nodules

Pisum sativum

• 7 chromosomes diploid 5 Gbp (human 3.2 Gbp)
• Mendel examined 7 traits on 4 chromosomes
• Bateson championed Mendel’s work and coined Genetics

European civilisation and legumes

• Umberto Eco considers the bean saved Western

civilisation (New York Times)

• From 1000-1500AD Europe's population tripled because
• f bean cultivation (Animal manure not sufficient)

Oil of the 19th century

• Chincha Islands Guano
• War of the Pacific 1879-1883

Greatest Scientific development in last 200 years?

“It is the chemist who must come to the rescue of the threatened communities“ Sir William Crookes 1898 to BAAS

N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92.4 kJmol-1

CH4 + H2O → CO + 3H2

• Haber discovered 1909 Nobel prize 1918
• Bosch scaled up and won Nobel in 1931

Nitrogen and population

• Addition of nitrogen and

phosphorous: Haber Bosch process 1909

• Dwarf varieties of wheat

and rice (short stem, large grain): Green revolution 1960’s

Nitrogen cascade

• Massive application of Haber-Bosch Nitrogen for agriculture (120 Mt year)
• Reactive Nitrogen in biosphere now 2X pre-industrial level
• Nitrate leaches to ground water
• Denitrifiers convert excess soil ammonia to N2O (nitrous oxide)
• N2O 300X more potent greenhouse gas than CO2

Rockstrom et al. Nature 2009: 473

Global grain yields

Nutrient limitation

Mueller et al. Nature 490, 254 (2012)

A=maize B=wheat C=rice

We have a problem

Nitrogen cycle

• Nitrogen cycles between inert N2 and biologically

available NH3

• 50-60% of biospheres nitrogen comes bacterial

N2 fixation in legume nodules

• Nitrogen main limiter of biological production

Gage and Margolin Current Opinions in Microbiology 2000, 3: 613-617

Getting inside a nodule

Ca++ spiking CCaMK TFs Nod Gene induction

Nod Factor

Receptors LRR-kinase Cation channel Nucleoporins

Nodulation signalling pathway (shared with mycorrihzal fungi)

Myc Factor TFs Myc Gene induction

The exchange that limits life

Udvardi and Poole, Downie Annu Rev Plant Biology (2013) H+ H+ H+ H+ H+ ATP ADP+ Pi

Ile Leu Val

Aap Bra DctA Dct?

Malate

Pyr Acetyl- CoA Ala

TCA

cycle PH B

OAA

Aap Bra SO4

2-

Sst1 ? MoO4

2

• ?

Mol Fe

2+

Dmt1 ? Mg

2 +

? MgtE

Symbiotic plant cell

NCR golgi ER nucleus Bac A SPC

Oxaloacetate PEP Hexoses

CO2

Homocitrate Acetyl-CoA α-ketoglutarate Fen1 ? ?

N2 NH3

e-& ATP

NH4

+

Gln Glu

GS AS Nitogenase complex

Asn

GS/ GOGAT

Amino acid pool

ENGINEERING SYNTHETIC SYMBIOSES

• Engineer tailored nitrogen-fixing plant microbe

associations using established endophytic plant microbes

• Engineer nodules onto cereals
• Express nitrogenase in plant plastids

Natural solutions

• Eat legumes not meat and dairy
• Improve use and efficiency of legumes

Beans in Africa

Problem with beans

• Beans in Africa get nodulated by rhizobia that fix little nitrogen
• How can we identify the good nitrogen fixing bacteria?

Glowing bacteria light up nodules

• nifH::luxCDABE fusion in pea/bean nodulating bacteria
• Bacteria glow when they fix N2, i.e. report how effective

Acknowledgements

• Vinoy Ramachandran
• Alison East
• Izzy Webb
• Jason Terpollili
• Kushboo Boorah
• Rachel Wheatley
• Barney Geddes
• Amelie Joffrin
• Ponraj Paramasivan (Cambridge)
• Nick Kruger
• George Ratcliffe
• Stuart Conway
• Giles Oldroyd (Cambridge)