Fig 18.71 Some of the known interactions in the plant cell signal transduction network.

6. Calcium-permeable channels in the plasma membrane provide potential routes for entry of Ca2+ to the cytosol during signal trans

duction.

1) Transient Ca2+ increase is central to signal transduction. 2) Ca2+ permeable channels : activation of increase in cytosolic Ca2+, upstream elements in Ca2+ - based signal transduction pathway,

3) Voltage-gated Ca2+_ permeable channels are activated by membrane depolarization.

Calcium-based signal transduction in a typical plant cell.

Activation of a wheat root plasma membrane Ca2+ channel by voltage.

Activation of plasma membrane Ca2+ channel by stretch.

may be responsible for the signaling during the early stages of mechano-sensory transduction. (touch, wind )

7. Calcium-permeable channels in endomembranes are activated by both voltage and ligands

(A) Diagram illustrating channel activities at the guard cell vacuolar membrane during stomatal closure

(B) During plasma membrane-based signal transduction

Activity of the SV channel increases with increasing cytosolic concentration of Ca2+ (A) Slow activation of the SV channel in barley aleurone vacuoles in response to positive voltages(B) Ca2+-dependence of whole-vacuole channel activity. Increasing free calcium above approximately 1μM increasing the activity of the channel. Ca2+ is thought to interact with calmodulin associated with the channel or a channel regulatory protein

8. Plasma membrane anion channels facilitate salt release during turgor adjustment and elicit membrane depolarization after stimulus perception

Anion channels in guard cell(A) Current-voltage relationship for rapidly activating (R-type) anion channels.(B) Current-voltage relationship for slowly activating (S-type) channels

Function of Anion channels

1. Controling salt release during turgor adjustment stimulus loss of Cl- depolarization open outward K+ channel l

oss of turgor pressure 2. To depolarize the plasma membrane leads the activation of voltage-gated Ca2+ channels

3. Are activated by extreme membrane hyperpolarization

9. Vacuolar malate channels participate in malate sequestration

Current-voltage relationship for vacuolar uptake of malate through time-dependent anion channel in the tonoplast.Malate uptake by anion channel is strongly promoted by negative membrane otentials and increases with cytosolic malate concentration. In this figure, cytosolic malate concentration were 10mM(filled squares), 20mM(open squares), 50mM(open circles), and 100mM(filled triangles)- all with a vacuolar malate concentration of 10mM. Malate uptake with equal concentration of malate(50mM) presente on both side of the membrane is indicated by stars.

Accumulation of malate in the root of CAM plants. Malate2- is thought to enter the vacuole though malate-selective channels.These channel are strongly inward rectifying and do not allow substantial malate2- efflux. Once inside the vacuole,malate2- is protonated to H.malate and H2.malate0. This maintains the effective concentration difference for malate2- across the membrane.

10. Integrated channel activity at the vacuolar and plasma membranes yields sophisticated signaling systems

Ca2+ signaling coordinates the activities of multiple ion channels and H+-pumps during stomatal closure. In this model, perception of ABA by a receptor(R) results in an increase in cytosolic free Ca2+ through Ca2+ influx or Ca2+ release from internal stores. Increased cytosolic Ca2+ promotes opening of plasma membrane anion and K+out channels and inhibits opening of K+in channels. As more ions leave the cell than enter it, water follows, turgor is lost, and the stomatal pore is closed

3.7.

Water transport through aquaporins

1. Directionality of water flow is determined by osmotic and hydraulic forces

2. Membrane permeability to water can be defined with either an osmotic coefficient (Pf) or a diffusional coefficient (Pd)

Transcellular osmotic Pressure probe

3. The nonequivalence of Pf and Pd provides evidence for water channels

• Pf involves net flow of water. Each water molecule entering the channel form the left will knock out one molecule on the right.

• In the diffusion flow case, a molecule of labeled water entering the channel from the left can diffuse back into the solution on the left.

Model for water flow through a single-file, multiple occupancy aquaporin

Water movement across biological membranes occurs through both the lipid bilayer and the pores formed by water channels.

4. Aquaporins are members of the major intrinsic protein family, which can form water channels when expressed in heterologous systems

characterized by the highly conserved NPA (Asn-Pro-Ala) residue in the N and C terminal.

plasma membrane intrinsic protein, PIP in plasma membrane

tonoplant intrinsic protein, TIP in vacuole

Structure of an aquaporin showing the six transmembrane helices and two conserved NPA (Asn-Pro-Ala) residue

Aquaporin function can be confirmed by expression of the cDNA in Xenopus oocytes.

cDNA expression

Hypoosmotic shock

Faster swelling (inhibited by Hg2+)

Three-dementional structure of aquaporin-1 from human erythrocytes.

Extracellular view of eight asymmetrical subunits that form two tetramers.

One of the monomers of the central tetramer is colored gold.

H2O

Aquaporin

Transcription Posttranslation

Environmental stimuli (blue light, ABA, GA, cold & drought)

Phosphorylation by Ca2+ dependent protein kinase

5. Aquaporin activity is regulated transcriptionally and posttranslationally

- Each isoform has a tissue specific distribution

- There is evidence that some are up-regulated in response to certain environmental stimuli such as blue light, ABA, GA.

- Aquaporin activity can be regulated by phosphorylation (CDPK).

Figure . Schematic representation of putative mechanisms involved in plant aquaporin regulation.(a) Control of transcription and protein abundance. Drought and salinity, as other environmental stimuli, are known to act on aquaporin gene transcription and possibly interfere with aquaporin translation and degradation, thereby determining protein abundance.

(b) Sub-cellular relocalization. The redistribution of a TIP aquaporin, from the tonoplast (TP) to small intracellular vesicles, was demonstrated in Mesembryanthemum crystallinum suspension cells exposed to a hyperosmotic treatment (Vera-Estrella et al. 2004). The occurrence of a similar relocalization mechanism for PIP aquaporins is shown but remains hypothetical.

6. Plasma membrane aquaporins may play a role in facilitating transcellular water flow

- in water absorption in root - in water transpiration in leaf

7. Differential water permeabilities of the vacuolar and plasma membranes can prevent large changes in cytoplasmic volume during water stress

water permeability of the vacuolar membrane

water permeability of the plasma membrane

(100-fold)

Normal stressed

Requirement for maintenance of cytosolic volume during osmotic stress