Uptake of minerals, Active and passive uptake of minerals, Donnan’s equilibrium, Cytochrome pump mechanism and carrier hypothesis, role of calmodulin
1. Passive Absorption (Passive Uptake):- It does not require expenditure of metabolic energy.
a. Simple Diffusion:- When the concentration of mineral salts is higher in the outer solution than in the cell sap of the root cells, the mineral salts are absorbed according to the concentration gradient by simple process of diffusion.
b. Ion Exchange Mechanism:- The ions adsorbed on the surface of the walls or membranes of root cells may be exchanged with the ions of same sign from external solution. For example, the cation K+ of the external soil solution may be exchanged with H+ ion adsorbed on the surface of the root cells. Similarly, an anion may be exchanged with OH– ion. There are two theories regarding the mechanism of ion exchange:-
i. Carbonic Acid Exchange Theory:- According to this theory, the CO2 released during respiration of root cells combines with water to form carbonic acid (H2CO3). Carbonic acid dissociates into H+ and an anion HCO3– in soil solution. These H+ ions may be exchanged for cations adsorbed on clay particles.
The cations thus released into the soil solution from the clay particles, may be adsorbed on root cells in exchange for H+ ions or as ion pairs with bicarbonate.
ii. Contact Exchange Theory:- According to this theory, the ions adsorbed on the surface of root cells and clay particles (or clay micelles) are not held tightly but oscillate within small volume of space. If the roots and clay particles are in close contact with each other, the ions adsorbed on clay particle may be exchanged with the ions adsorbed on root-surface directly without first being dissolved in soil solution.
c. Donnan Equilibrium Theory:- The membrane is selectively permeable in nature. Some are permeable to ions and some are impermeable.
The ions for which the membrane is permeable enter the cell by diffusion. This disturbs the equilibrium. To re-establish the equilibrium, there is an exchange of cations and anions again.
d. Mass Flow Theory:- Ions are also transferred along with the flow of water. Therefore, the absorption of mineral salts from the soil occurs in a water-soluble state. Higher the rate of transpiration, greater is the absorption of mineral salts.
2. Active Absorption (Active Uptake):-
Ø It has often been observed that the cell sap in plants accumulates large quantities of mineral salts ions
against the concentration gradient. It is an active process which involves the expenditure of metabolic energy through respiration.
Ø Following evidences favor this view:-
i. The factors like low temp., deficiency of O2, metabolic inhibitors etc. which inhibit metabolic activities like respiration in plants also inhibit accumulation of ions.
ii. Rate of respiration is increased when a plant is transferred from water to salt solution. It is called salt respiration.
a. The Carrier Concept:- According to this theory the plasma membrane is impermeable to free ions. But some compounds present in it acts as carrier and combines with ions to form carrier-ion-complex which can move across the membrane. On the inner surface of the membrane this complex breaks releasing ions into the cell while the carrier goes back to the outer surface to pick up fresh ions.
b. Cytochrome Pump Theory:-
Ø Lundegardh and Burstrom (1933) believed that there was a definite correlation between respiration and anion absorption. Thus when a plant is transferred from water to a salt solution the rate of respiration increases. This increase in rate of respiration over the normal respiration has been called as anion respiration or salt respiration.
Ø The inhibition of salt respiration and the accompanying absorption of anions by CO and cyanides (which are known inhibitors of cytochrome oxidase of electron transport chain in mitochondria), later on led Lundegardh (1950, 54) to propose cytochrome pump theory. This is based on the following assumptions:-
i. The mechanism of anion and cation absorption is different.
ii. Anions are absorbed through cytochrome chain by an active process.
iii. Cations are absorbed passively.
Ø According to this theory:-
i. ehydrogenase reactions on inner side of the membrane give rise to protons (H+) and electrons (e–).
ii. The electron travels over the cytochrome chain towards outside the membrane, so that the Fe of the cytochrome becomes reduced (Fe++) on the outer surface and oxidized (Fe+++) on the inner surface.
iii. On the outer surface, the reduced cytochrome is oxidized by oxygen releasing the electron (e–) and taking an anion (A–).
iv. The electron thus released unites with H+ and oxygen to form water.
v. The anion (A–) travels over the cytochrome chain towards inside.
vi. On the inner surface the oxidized cytochrome becomes reduced by taking an electron produced through the dehydrogenase reactions, and the anion (A–) is released.
vii. As a result of anion absorption, a cation (M+) moves passively from outside to inside to balance the anion.
Ø Main defects of the above theory are:-
i. It envisages active absorption of only anions.
ii. It does not explain selective uptake of ions.
iii. It has been found that cations also stimulate respiration.
c. Protein-Lecithin Theory:-
Ø In 1956, Bennet-Clark suggested that because the cell membranes chiefly consist of phospholipids and proteins and certain enzymes seem to be located on them, the carrier could be a protein associated with the phosphatide called as lecithin.
Ø He also assumed the presence of different phosphatides to correspond with the number of known competitive groups of cations and anions (which will be taken inside the cell).
Ø According to this theory:-
i. The phosphate group in the phosphatide is regarded as the active centre binding the cations, and the basic choline group as the anion binding center.
ii. The ions are liberated on the inner surface of the membrane by decomposition of the lecithin by the enzyme lecithinase.
iii. The regeneration of the carrier lecithin from phosphatidic acid and choline takes place in the presence of the enzymes choline acetylase and choline esterase and ATP. The ATP acts as a source of energy.
Pathways of Mineral Ions Movement:-
Once mineral salts reaches inside the epidermal cells of the root, their ionic form move from one cell to another by:-
i) Apoplastic pathway (i.e., through cell walls and intercellular spaces)
ii) Trans membrane pathway (i.e., by crossing the membranes)
iii) Symplastic pathway (i.e., through plasmodesmata)
Ultimately mineral salts reach to xylem vessels and tracheids, from where they are carried to different parts of the shoot along with ascent of sap.
Role of Calmodulin:-
> Calmodulins play an essential role in plant development and adaptation to environmental stimuli.
> Calcium plays a key role in the structural integrity of the cell wall and the membrane system of the cell.
> However, high calcium levels can be toxic to a plant's cellular energy metabolism and, hence, the Ca2+ concentration in the cytosol is maintained at a submicromolar level by removing the cytosolic Ca2+ to either the apoplast or the lumen of the intracellular organelles.
> Ca2+ pulses created due to increased influx and efflux act as cellular signals in response to external stimuli such as hormones, light, gravity, abiotic stress factors and also interactions with pathogens.
a. Nodule formation:-
- Ca2+ plays an important role in nodule formation in legumes.
- Nitrogen is an essential element required in plants and many legumes, unable to fix nitrogen independently, pair symbiotically with nitrogen-fixing bacteria that reduce nitrogen to ammonia.
- This legume-Rhizobium interaction establishment requires the Nod factor that is produced by the Rhizobium bacteria.
- The Nod factor is recognized by the root hair cells that are involved in the nodule formation in legumes.
- Ca2+ responses of varied nature are characterized to be involved in the Nod factor recognition.
- There is a Ca2+ flux at the tip of the root hair initially followed by repetitive oscillation of Ca2+ in the cytosol and also Ca2+ spike occurs around the nucleus.
- DMI3, an essential gene for Nod factor signaling functions downstream of the Ca2+ spiking signature, might be recognizing the Ca2+ signature.
b. Abiotic stress response in plants:-
- Change in intracellular Ca2+ levels is used as a signature for diverse responses towards mechanical stimuli, osmotic and salt treatments, and cold and heat shocks.
- Different root cell types show a different Ca2+ response to osmotic and salt stresses and this implies the cellular specificities of Ca2+ patterns.
- In response to external stress CaM activates glutamate decarboxylase (GAD) that catalyzes the conversion of l-glutamate to GABA. A tight control on the GABA synthesis is important for plant development and, hence, increased GABA levels can essentially affect plant development.
- Therefore, external stress can affect plant growth and development and CaM are involved in that pathway controlling this effect.
c. Pathogen defense:-
- Among the diverse range of defense strategies plants utilize against pathogens, Ca2+ signaling is very common.
- Free Ca2+ levels in the cytoplasm increases in response to a pathogenic infection.
- Ca2+ signatures of this nature usually activate the plant defense system by inducing defense-related genes and the hypersensitive cell death.
- CaMs, CMLs and CaM-binding proteins are some of the recently identified elements of the plant defense signaling pathways.
- Several CML genes in tobacco, bean and tomato are responsive to pathogens.