1. The Assimilation of Sulfur
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What is the effect of sulfur deficiency?
- sulfur deficiency: sulfur is an essential macronutrient for plant growth and plants show yellowing when it is deficient
- crop yields: reduces crop yields
How do plants acquire sulfur?
- from the soil as sulfate through the roots
- sulfate is transported through plants
- absorb sulfur dioxide from the atmosphere by absorption through the leaf surface
What is the role of air pollution in sulfur nutrition?
- accumulation of sulfur from air pollution
- burning of fossil fuels
- sulfate fertilization
Roles of cysteine and methionine
- sulfur-containing amino acids: sulfur is used to synthesize cys and met
- animals and humans are unable to reduce sulfur - require from diet
- sulfur proteins: cys is essential for met synthesis, critical components of proteins
- - play critical roles in structural and catalytic functions of proteins
cysteine is essential for forming thiol compounds
- glutathione and phytochelatin
- important for forming disulfide bonds
- formation: by oxidizing the thiol groups of two cys residues, oxidation by NADP+
- form covalent linkages involved in establishing tertiary and quaternary protein structures
structure and functions of glutathione
- major end product - present in mM
- structure: glutamate-cysteine-glycine
- protects cells from oxidative stress
- glutathione reductase - for every one mole of oxidized glutathione (GSSG), one mole of NADPH is required to reduce GSSG -> GSH
- redox control
- transport and storage
- response to environmental stresses
- glutathione peptides - form thiolate coordination complexes with heavy metals
- low molecular weight phytochelatin-Cd complexes - detoxify toxic metals by binding the metal ions using the thiol groups as the ligand, transported into the vacuole by an ABC transporter
- high molecular weight phytochelatin-Cd complexes - withing the vacuoule, includes the low molcular weight complex and Cd (brought in by antiporter) and sulfide
- Cd sulfide crystallite - phytochelatins hold Cd in an ordered structure
In what other ways is S important?
- food flavor
- pathogen defense
How is S important in food flavor?
- allicin - sulfur compounds add flavors to food products, garlic, onions
- alliinase enzyme in the vacuole - acts on alliin in the cytosol
- crushing produces flavor - mixing the enzyme with the substrate produces allicin
Which S compounds are involved in pathogen defense?
- allicin - deters herbivores, antimicrobial agent
- phtyalexins - defend plants against pathogen attacks, especially fungi like camalexin
- glucosinolates - plant glycosides that break down to release volatile defense substances isothiocyanates and nitriles
other roles for glucosinolates
- antioxidant properties of some vegetables like broccoli
- taste in condiments like mustard, horseradish
- protects against cancer
In what ways is DMSP important?
- marine algae
- DMSP roles:
- 1. osmoprotectant - helps against osmotic stress
- 2. cryoprotectant - protects from freezing damage
- 3. climate-regulating mechanism
- 4. repellent against plankton herbivores
How is DMSP involved in climate regulation?
- Phytoplankton produces DMSP: phytoplankton may regulate climate, DMSP is broken down to DMS by bacteria
- DMS moves into atmosphere and is oxidized to sulfate, leads to cloud formation
- clouds lead to cooling, sulfate is dissolved in rain, returns to ocean
- cloud cover reduces growth of phytoplankton
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