Mineral Nutrient Absorption and Assimilation HORT 301 – Plant Physiology October 12, 2007 Taiz and Zeiger, Chapter 6 (p ) and Chapter 12 Class Handout Mineral nutrient (ion) uptake into roots, xylem loading and transport into shoots Mineral nutrient assimilation – incorporation of mineral elements into organic molecules

Mineral nutrient (ion) uptake into roots, xylem loading and movement to shoots – absorption by roots, radial movement to the xylem, uptake to shoots in the transpiration stream (movement of water) Movement of ions through the soil is due primarily to pressure- driven bulk flow, with water Ion uptake from soil into roots occurs predominantly in the root hair zone (extension of the epidermis) rather than the meristematic and elongation zones, primary and secondary roots

Ion absorption by the root – apoplastic, transmembrane (sequential transport into and out of cells), or symplastic (intracellular via plasmodesmata) Plasmodesmata – pores in the membrane that join cells

Radial transport – through the apoplast or symplast of the root hair, epidermis and cortex At the endodermis, ions must enter the symplast of endodermal cells because the suberized Casparian strip restricts apoplast movement

Xylem loading – movement from the endodermis to the tracheary elements (tracheids or vessel elements), uptake into cells is by diffusion that is driven by electrochemical potential Xylem parenchyma cells - directly connected to the endodermis and tracheary elements (tracheids or vessel elements) Xylem parenchyma cells “load” or restrict ion movement into the xylem Transport proteins regulate ion transport into and out of the xylem Ion movement from root to shoot is primarily in the transpiration stream, pressure-driven bulk flow

The transpirational sink size - concentrates ions in leaves Some mineral nutrients are remobilized from older to younger tissues, via phloem

Mineral nutrient assimilation – incorporation of mineral nutrients into organic molecules Assimilation - requires substantial energy, e.g. 25% of the plant energy budget is consumed for N assimilation Assimilated mineral nutrients - N either NH 4 + or NO 3 -, SO 4 2 -, and H 2 PO 4 2 -

Nitrogen – biogeochemical cycling of nitrogen N 2 (N≡N) - 78% of the atmospheric volume N 2 – fixed biologically, physical natural processes or by the Haber- Bosch process into NH 4 +, oxidized to NO 3 -

N 2 (N≡N) fixation symbiosis - primarily legumes by bacterial symbionts (Rhizobia) into ammonium (NH 3 ) (nitrogenase), which at physiological pH is converted to NH 4 + Otherwise, nitrogen absorbed into roots as NO 3 - (NO 3 -  H + symporter) or NH 4 + (uniporter) NO 3 - is reduced to NH 4 + (nitrate and nitrite reductases w/ferrodoxin as the electron donor) NH 4 + is assimilated into glutamine and then glutamate (glutamine synthase, glutamate synthase), sometimes ureides (legumes), 12 ATP/N assimilated is required

Sulfur – SO 4 2- is absorbed by roots (SO H + symporter) and translocated SO 4 2- a product of soil weathering APS is then reduced to produce SO 3 2- (APS reductase), SO 3 2- is reduced to sulfide (S 2-, sulfite reductase, ferrodixin), which condenses with O- acetylserine (OAS) (S 2- + OAS → cysteine) to form cysteine (then methionine) Assimilation occurs primarily in leaves, photosynthesis produces reduced ferrodoxin and photorespiration generates serine, 14 ATP consumed per S assimilated SO 4 2- is assimilated into 5’-adenylsulfate/adenosine-5’-phosphosulfate (SO ATP → APS + PPi, reaction catalyzed by ATP sulfurylase

Phosphorous – HPO 4 2-, uptake (PO H + symporter) and translocated form Assimilated into ATP (ATP synthase), photosynthesis, oxidative phosphorylation (respiration) Cation mineral nutrients (K, Ca, Mg, Fe, Mn Cu, Co, Na, Zn) – function as ions or exist in complexes with organic molecules via noncovalent bonds, metals facilitate redox reactions Coordination bonds (several oxygen or nitrogen atoms share electrons) to form a bond with a cation nutrient, chlorophyll a

Electrostatic interactions – charge group attraction, e.g., Ca 2+ for carboxylate groups in pectin (Ca 2+ -pectate) Class change to Figure 12.17!!!