The extracellular matrix (ECM) of Volvox is modified during development or
in response to external stimuli, like the sex-inducing pheromone. It has recently
been demonstrated that a number of genes triggered by the sex-inducing pheromone
are also inducible by wounding. By differential screening of a cDNA library,
a novel gene was identified that is transcribed in response to the pheromone.
Its gene product was characterized as an ECM glycoprotein with a striking feature:
it exhibits a hydroxyproline content of 68% and therefore is an extreme member
of the family of hydroxyproline-rich glycoproteins (HRGPs). HRGPs are known
as constituents of higher plant ECMs and seem to function as structural barriers
in defense responses. The Volvox HRGP is also found to be inducible by wounding.
This indicates that the wound response scenarios of higher plants and multicellular
green algae may be evolutionary related.