Agriculture Reference
In-Depth Information
binding to ribose for which boron has a high affinity. The latter hypothesis is sup-
ported by observations of a reduction in the mitotic index in
Viciafaba
root tips in
the range of 1-10 mM B (Liu et al.
2000
). DNA contains deoxyribose which lacks
the necessary cis-diol groups to bind boron, so inhibition of DNA replication does
not seem likely. Although RNA does contain ribose, one of the hydroxyl groups is
involved in linking the nucleotide bases so that it no longer presents a cis-diol for
complexation. However, both of these hydroxyl groups of ribose are exposed at the
3' end of RNA molecules, and potentially more frequently during the processing of
mRNA. In plants and animals, RNA undergoes extensive splicing, during which the
3'ribose would be briefly exposed to boron. Shomron and Ast (
2003
) have dem-
onstrated in vitro that B can indeed inhibit one of the steps in splicing of mRNA.
Perhaps more importantly, both hydroxyl groups are also exposed at the 3' end of
tRNA molecules which could potentially interfere with translation of proteins be-
cause one of these hydroxyls is the target for attachment of the amino acid by amino
acyl tRNAsynthetases.
5 Boron Tolerance
5.1 Early Observations
It is well known that different species and different cultivars of the same species
have different abilities to grow on soil high in boron. Screening studies such as those
conducted in southern Australia by Nable, Paull and their colleagues (Nable
1988
;
Paull et al.
1988
) identified cultivars of wheat and barley with significant tolerance
to boron. Similar studies by Kaur et al. (
2006
) subsequently identified tolerant culti-
vars of
Brassica rapa
. The common feature of tolerant cultivars was that the boron
concentrations in their tissues were lower than in sensitive cultivars. From this, it
was hypothesised that the tolerance trait was associated with an ability to restrict
boron uptake from the soil into the roots, thereby reducing transfer to the shoot.
Further research using the tolerant and sensitive cultivars led to the identifica-
tion of chromosome regions in wheat, barley and rape seed that were associated
with tolerance to high boron concentrations (Jefferies et al.
1999
,
2000
; Kaur et al.
2006
,
2008
).
5.2 Tolerance Mechanisms
The observation that boron tolerant cultivars had lower concentrations of the chemi-
cal in their roots and shoots implied some form of control over net boron uptake
from the soil. This could arise by two separate mechanisms; either by restricting
entry into the root or by efflux of boron from the root. A problem in deciding
