The understanding of the selection factors
that drive chemical diversification of secondary
metabolites of constitutive defence systems in plants,
such as pyrrolizidine alkaloids (PAs), is still incomplete.
Historically, plants always have been confronted
with microorganisms. Long before herbivores
existed on this planet, plants had to cope with
microbial pathogens. Therefore, plant pathogenic
microorganisms may have played an important role
in the early evolution of the secondary metabolite
diversity. In this review, we discuss the impact that
plant-produced PAs have on plant-associated microorganisms.
The objective of the review is to present
the current knowledge on PAs with respect to antimicrobial
activities, adaptation and detoxification by
microorganisms, pathogenic fungi, root protection
and PA induction. Many in vitro experiments showed
effects of PAs on microorganisms. These results point
to the potential of microorganisms to be important for
the evolution of PAs. However, only a few in vivo
studies have been published and support the results of
the in vitro studies. In conclusion, the topics pointed
out in this review need further exploration by
carrying out ecological experiments and field studies.