Animation
Supplementary Material to
P. Gajdanowicz, E. Michard, M. Sandmann, M. Rocha, L.G. Guedes Corrêa, S.J. Ramírez-Aguilar,
J.L. Gomez-Porras, W. Gonzalez, J.-B. Thibaud, J.T. van Dongen, I. Dreyer
K+ gradients serve as a mobile energy
source in plant vascular tissues
[email protected]
The K+ gradient serves as an energy source
for phloem (re)loading
++
H+ HH
SS
S
H++
H+H
S S
K+
1) In source tissues, photoassimilates (S) and K+ ions are transported into the phloem
by consuming chemical energy (ATP):
• H+/S symporters use the energy of the electrochemical H+ gradient established by H+-ATPases
to accumulate S to high concentrations in the phloem
The K+ gradient serves as an energy source
for phloem (re)loading
S
S
S
H+ K+
H+
K+
1) In source tissues, photoassimilates (S) and K+ ions are transported into the phloem
by consuming chemical energy (ATP):
• H+/S symporters use the energy of the electrochemical H+ gradient established by H+-ATPases
to accumulate S to high concentrations in the phloem
• K+-uptake channels use the electrical gradient established by H+-ATPases
The K+ gradient serves as an energy source
for phloem (re)loading
H+ H+
S
S
S
S
S
++
HH
S
K+
S
2) During the transport from source to sink tissues, photoassimilates are “leaking” from
the transport phloem and also undergo metabolic modifcations (B.G. Ayre et al., 2003, Plant Physiol. 131, 1518-1528)
3) Reloading of photoassimilates is (under normal conditions) energized
by ATP-consuming H+-ATPases
The K+ gradient serves as an energy source
for phloem (re)loading
H+
S
S
H+
K+
S
4) A critical parameter in this re-uptake process is the availability of ATP.
ATP has to be produced by local cellular respiration.
5) “Respiration is particularly endangered in transport phloem, which is often deeply embedded
in heterotrophic tissues.” (A.J.E. van Bel, 2003, Plant Physiol. 131, 1509-1510; J.T. van Dongen et al. 2003, Plant Physiol. 131, 1529–1543)
How can the plant bridge temporarily and locally low ATP levels?
The K+ gradient serves as an energy source
for phloem (re)loading
H+
S
S
H+
S
K+
S
4) A critical parameter in this re-uptake process is the availability of ATP.
ATP has to be produced by local cellular respiration.
5) “Respiration is particularly endangered in transport phloem, which is often deeply embedded
in heterotrophic tissues.” (A.J.E. van Bel, 2003, Plant Physiol. 131, 1509-1510; J.T. van Dongen et al. 2003, Plant Physiol. 131, 1529–1543)
How can the plant bridge temporarily and locally low ATP levels?
The K+ gradient serves as an energy source
for phloem (re)loading
H+
S
K+
H+
S
S
K+
+
H
+
K
S
H+
6) The plant can bridge temporarily and locally low ATP levels by using the K+-gradient.
• The AKT2 channel must be converted into a non-rectifying (open) K+ channel.
• The K+-gradient is established by the high K+-concentration in the phloem
AND locally by the surrounding cells that absorb K+ from the apoplast.
Conclusion:
Independent of the local ATP level, there is always a K+ gradient
that can be harvested for phloem reloading by the AKT2 channel.
Some further background:
How can a K+ gradient drive H+/sugar transport?
Step by step
m An open K+ channel allows a K+ ion to permeate
along its electrochemical gradient.
In the present case: From the phloem to the apoplast.
K+
m As a consequence of this charge-transport the
membrane potential gets inside more negative.
-
SE/CC
apoplast
H+
S
m This more negative potential provokes an attractive force
for positive charges in the apoplast,
e.g. for protonated water molecules.
m An active H+/S symporter can now canalize this force
by allowing a coupled H+/S influx into the phloem.
m This charge transport restores the membrane potential.
m In reality, K+ efflux and H+/S influx occur simultaneously.
Thus, this electro-neutral K+ vs. H+/S antiport
does not affect the membrane potential.