Mechanisms of Salinity Tolerance in Barley

1
Mechanisms of Salinity Tolerance in
Barley Zhonghua Chen1, Ian Newman1, Igor
Pottosin2, Sergey Shabala1 1University of
Tasmania and 2Universidad de Colima.
Ian.Newman_at_utas.edu.au
Colima Logo needed here
www.mife.com
Background Crop plant salinity tolerance is a
polygenic trait, generally attained through
maintaining a sufficient ratio of K to Na in
the cell cytoplasm. Three barley varieties
tolerant to salinity and three sensitive were
selected from a range of 70 cultivars, whose
tolerance level was determined from a range of
agronomic measurements. We considered processes
at the root, regardless of any foliar
sequestration. The model here identifies the key
ionic mechanisms, and key transporters,
underlying salinity tolerance in barley.
Conclusions (See Chen et al. Plant Phys.145,
1714) For salinity tolerance, to maintain the
K/Na ratio, the membrane potential Em links
many processes and to maintain its negativity is
crucial. Maintaining Em diminishes K Loss from
the cell and contributes to the ?µH which drives
SOS1like Na/H exchange to remove Na. Higher
intrinsic H-extruding ATPase activity also
assists to maintain ?µH.
Evidence
Root Epidermal cell
Processes 1a. Na influx occurs via Non
Selective Cation Channels (NSCC).This leads to
rising Nai. 1b. The extra charge causes
membrane depolarisation Em rises (less
negative). 2a. Depolarisation stimulates the H
ATPase causing H extrusion and lower Hi. 2b.
Depolarisation activates K outward channels
(KORC) leading to K loss. 3. K loss and H
extrusion both lead to Em recovery. The
recovered Em then limits both KORC opening and H
extrusion. 4a. The lower Hi (from 2a) and the Em
recovery (from 3) both increase the ?µH ( µHo
µHi). 4b. This larger ?µH provides driving
potential for SOS1 Na extrusion. 5. Vacuolar
sequestration plays a minor role in roots.
22Na influx is the same for both tolerant and
sensitive cultivars. Net uptake during 24 h in 80
mM NaCl (with 0.5 mM KCl 0.1 CaCl2) is less for
tolerant than sensitive. µmol g-1 FW Numar(T)
130ZUG293(T) 125Gairdner(S) 170ZUG403(S)
190 Hence tolerant have more effective Na
extrusion to the soil sensitive have more foliar
Na accumulation.
Cytoplasm
Membranepotential Em - Em links all the
processes that contribute to Salinity
tolerance
NSCC Nao Nai
1a
1b
SOS1
5
Vacuole
4b
Ho µHo ? µHi Hi
3
4a
ATPase
Depolarisation caused by Na entry is
consistently larger in sensitive cultivars than
in tolerant ones.
2a
2b
µKo ? µKi
3
Ki
KORC
ATPase is more active in tolerant than in
sensitive cultivars. This results in sensitive
having greater H extrusion and a larger ?µH to
drive the SOS1 Na/H exchanger than sensitive.
K is lost faster by sensitive than by tolerant
cultivars in 0.1 mM CaCl2. The K losses, and the
sensitive/tolerant difference, are much less in
1.0 mM CaCl2.
KORC currents show the same voltage dependence
for both sensitive and tolerant cultivars. Their
different K losses are adequately explained
solely by their different Em depolarisations,
which cause different conductances and different
electrochemical driving forces ?µK.
This ATPase activity for the 6 cultivars is
correlated with the NaCl-induced Em
depolarisation for them (r2 0.8).
Methods Details are given by Chen et al., Plant
Physiol. 145, 1714-1725. Most experiments used
3-d seedlings grown in 0.5 mM KCl, 0.1 mM CaCl2.
Net H and K fluxes from the mature region of
intact roots were measured by the MIFE system.
Membrane potentials of root epidermal cells were
measured by standard microelectrode impalement.
Relevant standard techniques were used for 22Na
tracer influx (into the entire root) and for ATP
content and activity of root tissue. Segments of
mature root were protoplasted for whole cell
patch clamping, selecting those of 20 µm diameter
which indicates epidermal origin.
MIFE Ion Flux Measurement The movement of an ion
in solution can be described in terms of its
electrochemical potential m (chemical and
electrical driving forces), and other parameters
of the ion and solution. It can be shown (see
Newman, 2001, Plant, Cell Environment 24(1),
1-14) that the net flux J of an ion may be found
from a measurement of the change in voltage of an
ion selective microelectrode that is moved
through a small known distance dx in the
solution. The MIFE system, used in this study
for H and K, allows non-invasive measurement of
net ion fluxes with resolution of 10 seconds in
time and 20 ?m in position. A leaflet describing
the commercial MIFE system is available here,
with other information at www.mife.com.
ASPB Mexico June 2008 Poster P10 041