Mineral Nutrition in Plants II

OhOh, I get by with a little help from my friends.

- Ringo Starr et al.

Today

• Nitrogen Assimilation;

• Mychorhizzae;

• Questions.

Structural Elements, Covalently Bonded IGroup 1: Nutrients that are parts of carbon compounds...

• N: amino acids, proteins, nucleic acids, co-enzymes, etc.

– forms three covalent bonds,– chains, rings, branches,– one pair unshared electrons for metal coordination,– carbon “substitute”; distorts symmetry in rings and chains,– participates in hydrogen bonding,– metal complexion (see chlorophyll),

• Triple bond is extremely stable.

History

Varro praeceptis adicit equino sit levissium segetes alendi, prata vero graviore quod ex hordeo fiaat multasque gignat herbas…inter omnes autem constat nihil esse utilis lupini segete priusquam siliquetur aratro vel bidentibus versa manipuisve desectae circa radices arborum ac vitium obrutis…

- Pliny the Elder (~A.D. 80)

Haber-Bosch Reaction

N

NN2 gas

…as a reactant, is of limited use to most living organisms,

…some prokaryotes can break the triple bond and form ammonia,

…eukaryotes do not have the enzymes required to perform this task.

Nitrogen Gas

…plants assimilate nitrate and ammonium,

– NO3- and NH4

+ .

Oxidation State

Name

Nitrogen Species

Free-Living Nitrogen Fixation

• Cyanobacteria (blue green algae),

– i.e. Anabaena,

Anabaena

- heterocysts: cells adapted for nitrogen fixation;

• no O2.

• Other bacteria,

Anaerobic: maintain anaerobic lifestyles,Facultative: can switch to anaerobic metabolism,Aerobic: adapt specific cells for “anaerobic” metabolism.

Nitrogen Fixation is Sensitive to O2

N2 8 protons

Ammonia and H2

Output.

O2 is a good electron acceptor.

8 reduced ferredoxins

Sources of Fixed Nitrogen IOxidation of ammonium yields energy for carbon

fixation.

Decomposition of complex organic

molecules.

Nitrogen Assimilation I

• NO3- can be assimilated in either the root or

shoot,

– NO3- is absorbed by the root, stored in the vacuole,

assimilated, or transported to leaves,

– NO3- is reduced to nitrite,

– NO2- is reduced to form ammonium,

• uses light reactions of photosynthesis for reducing power in leaves,

• ferrodoxin reduction by plastid metabolism in roots,

• NH4+ (from the soil matrix, or from NO3

- reduction) is incorporated into amino acids for use, or for N transport.

Nitrate Toxicity• NO3

- is not generally toxic to plants and can be readily stored and transported,

• It is toxic, in high concentrations, to other organisms,

– Methemoglobinemia (“blue baby syndrome”),

• liver reduces nitrate to nitrite (or at higher pH, via bacteria in the gut),• nitrite oxidizes iron in hemoglobin, (Fe2+ ---> Fe3+),• renders hemoglobin unable to bind O2,

– Nitrosamines,

• R2NNO or RNHNO,• Extremely mutagenic and carcinogenic.

Nitrogen Fixation Requires Energy

16 ATPs for hydrolysis

8 reduced ferredoxins

N2 8 protons

Ammonia and H2

Output.

…why would N-fixing bacteria associate with plants?

Plant N2 Symbionts Nitrogen Assimilation II

• Plants supply reducing power,

• Bacteria provide fixed nitrogen,

– Alder (trees), Ceanothus (shrub),

• Frankia (bacteria),

– Sugarcane,

• Nostoc (bacteria),

– Azolla/Fairy Fern (water fern),

• Anaebena, (cyanobacteria)Azolla Anabaena

Ceanothus cordulatus; Mountain Whitethorn

Azolla/Anabaena

Azolla Anabaena

Arrows point to

Anabaena strands.

Azolla cross section

• Anabaena grows in leaf axes and lacuna.

Azolla/Anabaena/Rice/Humans

3. Stomp (or machine smash),

4. Fixed nitrogen is released to soil.1. Spread Azolla, let grow,

2. Break clumps, drain paddy,

More History

shu (soybean)

Chou scholars (~1000 B.C.)

Soy Root Nodules: result from an infection by Rhizobium.

Nitrogen FixationRhizobium symbionts

Rhizobium Infection I

1. Emerging root hair sends chemical attractants (elicitors),

2. Bacteria respond with a recognition signal,

if compatible?

3. Root hair grows and curls around the bacterial colony,

4. Bacteria proliferate within the curl.

Sweet Talk?

1. Plant secretes a specific elicitor

(flavonoid).

2. Bacterial receptor recognizes signal, and transports it across cell

membrane,

3. Flavanoid binds transcription factor (Nod D), complex

bids DNA,

4. nod genes are expressed,

5. Nod factor is produced (a molecule with host specificity),

6. Nod factor is recognized by the host, in turn activating host genes for proper response.

Nod is short for nodulation

Rhizobium Infection II

5. Plant cell wall is degraded,

6. Plant plasma membrane invaginates root hair cell,

7. Infection thread reaches root hair plasma membrane, fuses,

8. Bacteria enter the apoplast.

Rhizobium Infection III

9. New infection threads form,

10. Threads form toward “target cells”,

11. Bacteria “bleb” off of the infection thread, into the cytosol,

12. Bacteria are surrounded by a plant membrane (inside out).

Nodule Formation 12. Bacteria are surrounded by a plant membrane.

- bacterial induce plant cell division in infected and surrounding cells,

- cells in the pericycle begin dividing, (similar to lateral root formation).

13. Affected pericycle and cortical derived cells continue to divide until the regions fuse.

14. Vasculature forms between the nodule and the plant stele,

- nitrogenous compounds are carried to the plant,

- nutrients to the bacteria.

Nitrogen Assimilation II

• Symbiosome,

– plant membrane, surrounding…

– one or more bacteria,

• once inside a symbiosome, bacteria differentiate into bacteroids,

• bacteroids may differentiate,

• Host Cell Synthesizes,

– transport proteins for the symbiosome membrane,

– leghemoglobin, an oxygen binding molecule,

– N assimilation enzymes.

Symbiosomes

Nitrogen Assimilation I vs. II

• Cost of symbiosis (to the plant),

– unless there is a nitrogen deficit, plants without rhizobium infection generally do better than infected plants,

– generally,

• high NO32-, no R (best growth),

• high NO32-, R (good growth),

• low NO32-, R (good growth),

• low NO32-, no R (poor growth).

I II

Mycorrhizae Phosphate Extraction

…phosphate (HPO42-) is readily absorbed by

roots via a H+ / HPO42- symporter,

…however, HPO42- has low solubility and high

sorption capacity in soil,

– low concentration in the soil (1M or less), millimolar in root cells,

... Mycorrhizae increase the root surface area, actively transport HPO4

2-. Eucalyptus Root/Hartig Net

Absorption of Water and Minerals by Roots

Mycorrhizae

ectotropic mycorrhizal fungi

• penetrate the intercellular spaces of the root cortex,

• surround the root to form a dense fungal sheath.

vesicular-arbuscular fungi

• penetrate the intercellular spaces of the root cortex, and penetrate cortical cells,

– do not break the plant plasma membrane,– hyphae/plant structures form that exchange

nutrients.

Focus? Chapter 38?

• Excretion?• Essential Elements?

– Groups 1 - 4,

• Phytochelation and Phytoremediation?• Dose Response curves and Nutrient Deficiencies?• Nitrogen and Nitrogen Assimilation,

– Nitrate and Ammonium Assimilation,– Nitrogen Symbionts,

• Mychorrhizae?

Exam I

• Mean = xx, Median = xx, mode = xx

• 100 - 90: • 89 - 80: • 79 - 70: • 69 - 60: • <59:

Cell walls of plants play an important role in structure and cellular functions, describe the cell walls of cells within the vascular bundle (phloem, xylem, etc) and how each specific type of cell wall benefits the cell for its specific purpose.