Especially stilbenes and AQs are very simple compounds
that nevertheless serve complex ecological roles and are
the result of unusual biosyntheses. Whereas stilbenes are
typical plant metabolites (e.g. resveratrol in grapes and
red wine) Photorhabdus is the only stilbene producer
outside the plant kingdom. Moreover, in Photorhabdus
stilbenes are derived from a head-to-head condensation of
two b-ketoacyl intermediates instead of the consecutive
elongation of cinnamoyl-CoA by malonyl-CoA in the
plant pathway (Figure 2) [33]. They show antibiotic
activity against Gram-positive bacteria and fungi [37],
inhibit the phenol oxidase that is part of the insect
immune system [37] and are required as a signal molecule
for proper nematode development [33]. Stilbenes are
truly multifunctional compounds that have been identified
in all Photorhabdus strains analyzed so far. Thus they
can be regarded as the ‘swiss army knife’ of Photorhabdus
as they are involved in almost all different parts of the
complex tritrophic relationship between bacteria, nematodes,
and insects. In contrast to other biosynthesis gene
clusters in Photorhabdus, the genes involved in the stilbene
biosynthesis are not clustered pointing to a complex
regulation of this important compound.
AQs are common metabolites of plants, fungi, and bacteria.
They are also postulated to serve an important
ecological function in Photorhabdus as this class of compounds
in general is known as ant and bird deterrent [38]
that would ensure that infected larvae are avoided from
potential