Root nodule symbioses may have evolved from a pre-existing
pathway for the formation of arbuscular mycorrhiza
There are parallels between the establishing of arbuscular mycorrhiza
and of root nodule symbiosis. In both cases, receptor-like kinases (RLK,)
appear to be involved, linked to signal
cascades, which induce the synthesis of the proteins required for the
controlled infection. These sig-nal cascades probably involve G-proteins,
MAP-kinases, and Ca++ ions as messengers . For several legume
species, mutants are known that have lost the ability to establish both root
nodule symbiosis and arbus-cular mycorrhiza. One of the genes that causes such
a defect in different legume species has been identified as encoding an RLK,
indicating that this RLK has an essential function in the formation of both
arbuscular mycor-rhiza and root nodule symbiosis. Fungi and bacteria, despite
their different natures, apparently induce similar genetic programs upon
infection.
Molecular phylogenetic studies have shown that all plants with the
abil-ity to enter root nodule symbiosis, rhizobial or actinorhizal, belong to a
single clade ( =branch of phylogenetic tree, named Eurosid I). This implies that these species go back to a common
ancestor, although not all descend-ants of this ancestor are symbiotic.
Obviously, this ancestor has acquired a property on the basis of which a
bacterial symbiosis could develop. Based on this property, root nodule
symbiosis evolved about 50 million years ago, not as a single evolutionary
event, but reoccurred about eight times. In order to transfer the ability to
enter a root nodule symbiosis to agricul-turally important monocots, such as
rice, maize, and wheat by genetic engi-neering, it will be necessary to find
out which properties of the Eurosid I clade plants allowed the evolution of
such symbiosis.
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