Part 2 Long-Distance Transport
2.1 Overview of diffusive and convective transport in plants
2.1.1 Diffusion and convection are the basis for solute transport.
Diffusion,An independent movement of molecules driven by a
concentration gradient.
Convective transport,Bulk flow of solution driven by a pressure
gradient.
2.1.2 Water and solutes can move from cell to cell
along either of two parallel aqueous pathways.
2.1.3 The concept of water potential provides a
thermodynamic basis for the study of water movement.
Equation 2.1.3
ψw=ψp+ψs+ψg+ψm
ψw,water potential
Ψp,pressure potential
ψs,solute (osmotic) potential
ψg,gravitational potential
ψm,matrix potential
2.1.4 Concentration gradients over short distances tend to drive
both diffusion and osmotically generated pressure flow (OGPF).
2.2 Comparison of xylem and phloem transport
2.2.1 Some generalizations and useful comparisons can be
made about the composition of xylem and phloem exudates.
Xylem and phloem transport streams can be sampled
by several methods
Table 2.2 General characteristics of xylem and phloem
exudates
Phloem exudates Xylem exudates
Sugars 100-300gl-1 0gl-1
Amino acids 5-40gl-1 0.1-2gl-1
Inorganics 1-5gl-1 0.2-4gl-1
Total solutes 250-1200mmol kg-1 10-100mmol kg-1
pH 7.3-8.0 5.0-6.5
2.2.2 Xylem-borne solutes follow the transpiration
stream from roots to mature leaves,whereas phloem
transports solutes from sources to sinks.
2.2.3 Solute transfer between the xylem and the phloem plays
an important role in nutrient partitioning between organs
2.3 Transpirational water movement in the xylem
2.3.1 The cohesion-tension
mechanism is the leading
theory for transpirational water
movement.
2.3.2 Cavitation is the Achilles’ heel of the cohesion-
tension mechanism.
Cavitation is a fairly regular occurrence in the xylem,which
becomes progressively less conductive as the number of
embolized vessels increase.
2.3.3 Embolized xylem can be repaired
Several mechanisms can raise the pressure within gas-filled
xylem vessels to values ranging from slightly negative to
positive:
The increased pressure augmented by the additional pressure
exerted by surface tension in small bubbles can be sufficient to
drive the gas back into solution.
In herbaceous species,nightly root pressure can play an essential
role in reversing daily cavitation.
Curiously,embolized xylem can also become recharged during
periods of substantial xylem tension,The mechanism involved is
unclear,although most hypotheses envision an active role for the
xylem parenchyma in solute secretion and water movement into
embolized vessels.
Xylem tension can be measured with the pressure bomb.
2.4 Water transport through aquaporins
2.4.1 Membrane permeability to water can be defined with either
an osmotic coefficient (Pf) or a diffusional coefficient (Pd).
The osmotic coefficient for water permeability (Pf) of
plant membranes can be determined.
2.4.2 The nonequivalence of Pf and Pd provides
evidence for water channels.
For example,in Chara internodal cells,Pf has a value of
about 250μm·s -1,whereas Pd is only 8μm?s-1
2.4.3 Aquaporins are members of the major intrinsic
protein family,which can form water channels when
expressed in heterologous system
In plants,aquaporins have been identified in both the
vacuolar and plasma membranes known as
tonoplast intrinsic protein (TIP)
And plasma membrane intrinsic protein (PIP).
2.4.4 Aquqporin activity is regulated transcriptionally
and posttranscriptionally
Arabidopsis thaliana possess many aquaporin isoforms in both the TIP and PIP
families,Each isoform has a tissue-specific distribution,Some are up-regulated
in response to certain environmental stimuli such as blue light,ABA,and
gibberellic acid,
Aquaporin activity can be regulated by phosphorylation,For both the TIP and
PIP families,phosphorylation is catalyzed by a Ca2+-dependent protein kinase
that may form a component of a signaling pathway linking water stress to
channel activity.
2.5 Symplasmic transport via plasmodesmata
2.5.1 In vascular plants,the basic plasmodesmal structure
is a tube of plasma membrane surrounding a strand of
modified endoplasmic reticulum,with particulate material
between them.
Plasmodesmal morphology takes on several forms,but
the functional significance of this variation is unknown.
2.5.2 Plasmodesmal function is usually evaluated by
electrical coupling or by cell-to-cell movement of
fluorescent tracers (dye coupling)
Measurements of electrical coupling in the Azolla
rhizoid demonstrate a strong correlation between
electrical coupling and the number of plasmodesmata
connecting the cells
Mobile and immobile fluorescent probes in the Abutilon
nectary trichome.
2.5.3 Work is ongoing to characterize the molecular
architecture of plasmdesmata.
2.6 Phloem transport
2.6.1 Controversy over the structure of functioning sieve
elements slowed acceptance of OGPT as the mechanism of
phloem transport.
2.6.2 Physiological evidence strongly supports OGPF as the
mechanism of phloem transport.
Movement along the path is passive; energy is required only to maintain
structural integrity,Cold treatment is particularly suited for this purpose.
Phloem exudate from trees typically decreases in
concentration from the canopy to ground level.
2.6.3 Depending on the species,phloem loading in
leaves may occur by transmembrane movement
(apoplastic) or via plasmodesmata (symplasmic loading)
When the minor vein SE/CC complex is symplasmically
isolated (closed configuration),it absorbs solutes from the
apoplast by membrane transport.
Symplasmic continuity from the mesphyll to the SE/CC
complex allows assimilates to move directly into the
translocation stream.
Supplying sucrose via the apoplast causes a sharp depolarization of sieve
tube membrane potential,suggesting a H+-sucrose cotransport mechanism.
2.6.4 In most cases,solutes leave the SE/CC complex
by bulk flow through plasmodesmata,which serve as
high-resistance leaks from the low-resistance
conducting pathway,
Plasmodesmata involved in sieve element unloading and
postphloem transport appear to be substantially more
conductive than most plasmodesmata.
After unloading from the SE/CC complex via
plasmodesmata,assimilates follow a symplasmic
pathway in most sinks,although a later apoplastic step
may intervene.
Discotinuities in the symplasm or apoplast have
important consequences for water and solute
movement within a sink.
In the absence of a strong xylem connection,the water
potential of a sink can be relatively independent of
changes in plant water potential.
2.6.5 Control of assimilate import by a sink must
coordinate sieve element unloading with the
characteristics of the particular sink type
Given the diversity of sink types and the present state of
uncertainty about possible linkages between transport and
solute utilization within a sink,few generalizations are
possible about the control of assimilate.
Continual monitoring of apple fruit growth shows that the rate
of phloem import was unaffected by changes in the relative
water status of the fruit and tree,G— growth; P— phloem
import; X— xylem flow; T— transpiration.
2.7 Intercellular transport of endogenous
macromolecules
2.7.1 The ongoing presence of soluble proteins in the sieve
tube requires continual entry of proteins in the source and
their removal at the sink; additional turnover occurs by
SE/CC exchange along the path.
Two-dimensional separation of soluble proteins in aphid
stylet exudate collected from wheat plants.
2.7.2 Some non-STEP(sieve tube exudate proteins)
proteins can also be unloaded from sieve tubes and
move along the postphloem pathway.
Figure 2.7
Green fluorescent protein (GFP) expressed in mature
Arabidopsis leaves under a phloem-specific promoter is
exported to sinks,where it is unloaded from the sieve tubes
and moves along the postphloem pathway in young leaves
and root tips.
The protein composition of phloem exudates from the
same plant may differ substantially,depending on the
collection method.
2.7.3 Some nonviral RNAs are phloem-mobile.
Using PCR and primers based on sequences for three
proteins present in rice phloem exudate,investigators
demonstrated the presence of mRNAs for these proteins in
the exudate,Ribonucleases were undetectable,indicating
that free RNAs should be sufficiently stable to move long
distances in the phloem.
The long-distance movement of proteins and its mRNA within the
phloem was demonstrated by grafting cucumber onto pumpkin and
collecting exudate from the cucumber phloem.
2.7.4 Many aspects of viral movement have
implications for possible underlying mechanisms of
endogenuos macromolecule trafficking,including RNA
movement.
Model Ⅰ,Movement protein (MP) binds to vRNA,and
the MP-vRNA complex moves through the plasmodesma.
Model Ⅱ,In addition to MP,this model postulates the
involvement of endogenous plant cell proteins in vRNA
trafficking,
MP— Movement protein;BP— Binding protein; Re—
receptor protein;D— a stationary docking protein;CW—
cell wall.
A conceptual model for cell-to-cell trafficking of specific
proteins of a size larger than the passive SEL(size
exclusion limit),
2.1 Overview of diffusive and convective transport in plants
2.1.1 Diffusion and convection are the basis for solute transport.
Diffusion,An independent movement of molecules driven by a
concentration gradient.
Convective transport,Bulk flow of solution driven by a pressure
gradient.
2.1.2 Water and solutes can move from cell to cell
along either of two parallel aqueous pathways.
2.1.3 The concept of water potential provides a
thermodynamic basis for the study of water movement.
Equation 2.1.3
ψw=ψp+ψs+ψg+ψm
ψw,water potential
Ψp,pressure potential
ψs,solute (osmotic) potential
ψg,gravitational potential
ψm,matrix potential
2.1.4 Concentration gradients over short distances tend to drive
both diffusion and osmotically generated pressure flow (OGPF).
2.2 Comparison of xylem and phloem transport
2.2.1 Some generalizations and useful comparisons can be
made about the composition of xylem and phloem exudates.
Xylem and phloem transport streams can be sampled
by several methods
Table 2.2 General characteristics of xylem and phloem
exudates
Phloem exudates Xylem exudates
Sugars 100-300gl-1 0gl-1
Amino acids 5-40gl-1 0.1-2gl-1
Inorganics 1-5gl-1 0.2-4gl-1
Total solutes 250-1200mmol kg-1 10-100mmol kg-1
pH 7.3-8.0 5.0-6.5
2.2.2 Xylem-borne solutes follow the transpiration
stream from roots to mature leaves,whereas phloem
transports solutes from sources to sinks.
2.2.3 Solute transfer between the xylem and the phloem plays
an important role in nutrient partitioning between organs
2.3 Transpirational water movement in the xylem
2.3.1 The cohesion-tension
mechanism is the leading
theory for transpirational water
movement.
2.3.2 Cavitation is the Achilles’ heel of the cohesion-
tension mechanism.
Cavitation is a fairly regular occurrence in the xylem,which
becomes progressively less conductive as the number of
embolized vessels increase.
2.3.3 Embolized xylem can be repaired
Several mechanisms can raise the pressure within gas-filled
xylem vessels to values ranging from slightly negative to
positive:
The increased pressure augmented by the additional pressure
exerted by surface tension in small bubbles can be sufficient to
drive the gas back into solution.
In herbaceous species,nightly root pressure can play an essential
role in reversing daily cavitation.
Curiously,embolized xylem can also become recharged during
periods of substantial xylem tension,The mechanism involved is
unclear,although most hypotheses envision an active role for the
xylem parenchyma in solute secretion and water movement into
embolized vessels.
Xylem tension can be measured with the pressure bomb.
2.4 Water transport through aquaporins
2.4.1 Membrane permeability to water can be defined with either
an osmotic coefficient (Pf) or a diffusional coefficient (Pd).
The osmotic coefficient for water permeability (Pf) of
plant membranes can be determined.
2.4.2 The nonequivalence of Pf and Pd provides
evidence for water channels.
For example,in Chara internodal cells,Pf has a value of
about 250μm·s -1,whereas Pd is only 8μm?s-1
2.4.3 Aquaporins are members of the major intrinsic
protein family,which can form water channels when
expressed in heterologous system
In plants,aquaporins have been identified in both the
vacuolar and plasma membranes known as
tonoplast intrinsic protein (TIP)
And plasma membrane intrinsic protein (PIP).
2.4.4 Aquqporin activity is regulated transcriptionally
and posttranscriptionally
Arabidopsis thaliana possess many aquaporin isoforms in both the TIP and PIP
families,Each isoform has a tissue-specific distribution,Some are up-regulated
in response to certain environmental stimuli such as blue light,ABA,and
gibberellic acid,
Aquaporin activity can be regulated by phosphorylation,For both the TIP and
PIP families,phosphorylation is catalyzed by a Ca2+-dependent protein kinase
that may form a component of a signaling pathway linking water stress to
channel activity.
2.5 Symplasmic transport via plasmodesmata
2.5.1 In vascular plants,the basic plasmodesmal structure
is a tube of plasma membrane surrounding a strand of
modified endoplasmic reticulum,with particulate material
between them.
Plasmodesmal morphology takes on several forms,but
the functional significance of this variation is unknown.
2.5.2 Plasmodesmal function is usually evaluated by
electrical coupling or by cell-to-cell movement of
fluorescent tracers (dye coupling)
Measurements of electrical coupling in the Azolla
rhizoid demonstrate a strong correlation between
electrical coupling and the number of plasmodesmata
connecting the cells
Mobile and immobile fluorescent probes in the Abutilon
nectary trichome.
2.5.3 Work is ongoing to characterize the molecular
architecture of plasmdesmata.
2.6 Phloem transport
2.6.1 Controversy over the structure of functioning sieve
elements slowed acceptance of OGPT as the mechanism of
phloem transport.
2.6.2 Physiological evidence strongly supports OGPF as the
mechanism of phloem transport.
Movement along the path is passive; energy is required only to maintain
structural integrity,Cold treatment is particularly suited for this purpose.
Phloem exudate from trees typically decreases in
concentration from the canopy to ground level.
2.6.3 Depending on the species,phloem loading in
leaves may occur by transmembrane movement
(apoplastic) or via plasmodesmata (symplasmic loading)
When the minor vein SE/CC complex is symplasmically
isolated (closed configuration),it absorbs solutes from the
apoplast by membrane transport.
Symplasmic continuity from the mesphyll to the SE/CC
complex allows assimilates to move directly into the
translocation stream.
Supplying sucrose via the apoplast causes a sharp depolarization of sieve
tube membrane potential,suggesting a H+-sucrose cotransport mechanism.
2.6.4 In most cases,solutes leave the SE/CC complex
by bulk flow through plasmodesmata,which serve as
high-resistance leaks from the low-resistance
conducting pathway,
Plasmodesmata involved in sieve element unloading and
postphloem transport appear to be substantially more
conductive than most plasmodesmata.
After unloading from the SE/CC complex via
plasmodesmata,assimilates follow a symplasmic
pathway in most sinks,although a later apoplastic step
may intervene.
Discotinuities in the symplasm or apoplast have
important consequences for water and solute
movement within a sink.
In the absence of a strong xylem connection,the water
potential of a sink can be relatively independent of
changes in plant water potential.
2.6.5 Control of assimilate import by a sink must
coordinate sieve element unloading with the
characteristics of the particular sink type
Given the diversity of sink types and the present state of
uncertainty about possible linkages between transport and
solute utilization within a sink,few generalizations are
possible about the control of assimilate.
Continual monitoring of apple fruit growth shows that the rate
of phloem import was unaffected by changes in the relative
water status of the fruit and tree,G— growth; P— phloem
import; X— xylem flow; T— transpiration.
2.7 Intercellular transport of endogenous
macromolecules
2.7.1 The ongoing presence of soluble proteins in the sieve
tube requires continual entry of proteins in the source and
their removal at the sink; additional turnover occurs by
SE/CC exchange along the path.
Two-dimensional separation of soluble proteins in aphid
stylet exudate collected from wheat plants.
2.7.2 Some non-STEP(sieve tube exudate proteins)
proteins can also be unloaded from sieve tubes and
move along the postphloem pathway.
Figure 2.7
Green fluorescent protein (GFP) expressed in mature
Arabidopsis leaves under a phloem-specific promoter is
exported to sinks,where it is unloaded from the sieve tubes
and moves along the postphloem pathway in young leaves
and root tips.
The protein composition of phloem exudates from the
same plant may differ substantially,depending on the
collection method.
2.7.3 Some nonviral RNAs are phloem-mobile.
Using PCR and primers based on sequences for three
proteins present in rice phloem exudate,investigators
demonstrated the presence of mRNAs for these proteins in
the exudate,Ribonucleases were undetectable,indicating
that free RNAs should be sufficiently stable to move long
distances in the phloem.
The long-distance movement of proteins and its mRNA within the
phloem was demonstrated by grafting cucumber onto pumpkin and
collecting exudate from the cucumber phloem.
2.7.4 Many aspects of viral movement have
implications for possible underlying mechanisms of
endogenuos macromolecule trafficking,including RNA
movement.
Model Ⅰ,Movement protein (MP) binds to vRNA,and
the MP-vRNA complex moves through the plasmodesma.
Model Ⅱ,In addition to MP,this model postulates the
involvement of endogenous plant cell proteins in vRNA
trafficking,
MP— Movement protein;BP— Binding protein; Re—
receptor protein;D— a stationary docking protein;CW—
cell wall.
A conceptual model for cell-to-cell trafficking of specific
proteins of a size larger than the passive SEL(size
exclusion limit),
