Agriculture Reference
In-Depth Information
5.4 The Role of Aquaporins
These discoveries emphasised the importance of boron efflux transporters as a ma-
jor tolerance mechanism across a diversity of plant types. The ability of efflux-
ers to maintain reduced intracellular boron concentrations is also dependent on the
rate at which boron can enter the cell across the plasma membrane. Fitzpatrick
and Reid (
2009
) provided evidence that possibly 50 % of boron influx was medi-
ated by aquaglyceroporins, the remainder presumably entering via direct diffusion
through the lipid bilayer. Theoretically, closure of these channels under boron tox-
icity conditions could greatly increase the effectiveness of the efflux transporters.
However, although they were able to show that two PIP1 type aquaporins were
capable of transporting boron in yeast, Fitzpatrick and Reid (
2009
) could not detect
any change in expression of the genes for these transporters between high and low
boron conditions in barley. Schnurbusch et al. (
2010
) showed that boron could also
enter cells through NIP2, 1, an aquaglyeroporin from the nodulin-26-like intrinsic
protein (NIP) subfamily, and that the expression of the gene for this transporter was
lower in a tolerant barley cultivar compared to a sensitive cultivar. From this, they
proposed that this could confer an extra level of tolerance to that provided by the
boron efflux transporters.
5.5 Leaf Tolerance
Reduced boron accumulation via efflux pumping from the root back into the ex-
ternal medium is easy to comprehend. Less intuitive is tolerance conferred by the
same transporters operating in the shoot. By careful dissection of necrotic lesions
on leaves of barley and wheat, Reid and Fitzpatrick (
2009
) were able to show that in
tolerant cultivars, death of leaf cells occurred at higher tissue boron concentrations
than in sensitive cultivars. To explain the higher expression of
Bor2
genes in leaves
of tolerant cultivars, they proposed that toxicity was reduced by pumping of boron
from the sensitive cytoplasmic compartment into the cell walls where it was much
less toxic. Thus for the same total leaf concentration, much less of the boron would
be exposed to metabolic processes within the cell in tolerant cultivars. Reid and
Fitzpatrick (
2009
) provided evidence in support of this hypothesis by showing that
boron was much more rapidly eluted from leaves of tolerant cultivars, consistent
with a larger fraction of boron being located outside of the cell.
The leaf elution experiments conducted by Reid and Fitzpatrick (
2009
) high-
lighted the ease with which tissue boron could be solubilised, and caused them to
revisit the observations by Nable et al. (
1990b
) on the lack of consistency of boron
concentrations required to cause toxicity in the field compared to the glasshouse.
Nable et al. (
1990b
) suggested that the lower concentration observed in the field
was due to leaching of boron from leaves by rain, but were not able to demonstrate
this under controlled conditions. Reid and Fitzpatrick (
2009
) simulated seasonal
