Letter: Letters : Multiple benefits

From Keith Skene, University of Dundee

It was no surprise to read that mycorrhizal symbiosis brings with it more
than one benefit to the plant (New Scientist, Science, 16 March, p 20).
There is no rule against this. Any adaptation must make sense within the whole
picture.

Unlike a laboratory experiment, life is made up of multiple challenges, none
of which wait in turn. A successful adaptation is likely to be multifunctional.
The difficulty is to discover which of these functions is the most
important.

Just because mycorrhizas occur in soil rich in phosphate, does not mean that
their primary function cannot be related to phosphate acquisition. Over
evolutionary time, this symbiotic system will have evolved in such a way that
the fungus can enter the plant. Thus obtained, the door key may now be used to
allow entrance even when the service is not needed—every double glazing
sales person’s dream.

Furthermore, members of one of the relatively few plant families that do not
form mycorrhizal associations, or that do so weakly, the Proteaceae, employ
another means of improving phosphate acquisition: they possess cluster roots.
Now there is no indication that these are recent adaptations (the Proteaceae are
generally regarded as rather early angiosperms), nor that they provide
protection against pathogenic fungi.

It would seem much simpler to conclude that nutrient acquisition is the name
of the game for mycorrhizas and cluster roots, while other benefits are useful
additions.