A whiter shade of pale – one partner’s reaction to living
together
When
we talk to our students about legume–Rhizobium
symbiosis, we usually emphasize the mutual benefits to the partners – the plant
and the bacterium – in this relationship. However, living together may not be
quite so straightforward, as illustrated by the work of Okazaki et
al. at Sendai, Japan (pp. 55–59). They point
out that the symbiont Bradyrhizobium
elkanii produces a metabolic inhibitor, rhizobitoxine, which is also produced by a plant pathogen Burkholderia andropogonis.
Rhizobitoxine inhibits specifically two enzymes, cystathione-β-lyase, which catalyses a late step in methionine biosynthesis, and ACC synthase,
mediating the penultimate reaction in ethylene biosynthesis. Further, the
biochemist in me notes that methionine is needed to
generate the substrate for ACC synthase, and thus the
two reactions are linked. In terms of evolution of nodulation, the synthesis of
rhizobitoxine ‘makes sense’: ethylene inhibits
nodulation and so inhibition of ethylene biosynthesis encourages nodulation.
However, nodulation is not the only effect seen in soybean (Glycine
max) inoculated with B. elkanii; the plants
also exhibit an obvious chlorosis. The authors have
set out to discover whether this is linked to the production of rhizobitoxine. Firstly, mutants of B. elkanii that do not produce rhizobitoxine
do not cause chlorosis (and are less efficient nodulators than wild-type). Secondly, plants inoculated
with wild-type B. elkanii show reduced levels
of methionine and increased levels of methionine precursors, especially aspartate
and homoserine, consistent with inhibition of cystathione-β-lyase (no data on ACC synthase were reported). Thirdly, supply of methionine, but not of ACC, via the plants’ nutrient
solution relieved the chlorosis. All of these results
are consistent with the view that chlorosis results
from methionine deficiency, which is caused in turn
by rhizobitoxine-induced inhibition of cystathione-β-lyase. Since the chlorosis
is often severe enough to more than counteract the benefits to the plant of
nodulation, the hunt is clearly on for rhizobitoxine-resistant
varieties of soybean.
