The
acid test for Al tolerance
Aluminium toxicity is a particular hazard
for organisms in more acid environments and, as discussed by Yang et
al., Hangzhou, China (pp. 579–584), can lead
to significant yield losses in crops growing in acid soils. However, tolerance
to Al has been observed in several species, including a number of crop plants.
Such a trait is clearly useful to plant breeders and it is thus important to
understand the mechanisms underlying the tolerance. One mechanism observed in
several species is the efflux of organic acids that presumably chelate the Al,
reducing its capacity to enter the plant. It is this type of mechanism that is
shown by Vigna umbellata (rice bean),
studied by these authors. Exposure of plants to 50 mm Al led to efflux of citrate from the apical 5 mm of the roots, but
this only occurred after a 3 h lag. Al also induced the accumulation of citrate
in the root. This in fact preceded the efflux but was not confined to the
apical 5 mm. The 3 h lag led the authors to
suggest that protein synthesis may be necessary for citrate efflux, a postulate
that was at least partly supported by the finding that cycloheximide, a protein
synthesis inhibitor, reduced significantly the development of citrate efflux
capability. The question then is which proteins? A partial answer is provided
by the authors’ finding that inhibitors of anion channels and of citrate
carriers both inhibited citrate efflux, with the effects of the two types of
inhibitor being additive. This clearly points to a role for induction of both
types of carrier molecule in the response to Al. Of course, it is possible that
induction or up-regulation of citrate accumulation enzymes is also a key event,
although the authors argue against this, based on evidence that the
accumulation of citrate is separately controlled from the efflux of citrate.
