School of Engineering and Science - Jacobs University

Discussion
stationary phase transition (Galperin, 2006; Romling & Amikam, 2006). It is
possible that this signaling pathway is involved in the differentiated regulation
of siderophore biosynthesis in Pss22d.
The obvious differences in siderophore production observed during growth of
Psg1a and Pss22d in vitro can not directly be associated to the wellinvestigated
iron-mediated regulation of siderophore biosynthesis like the Fur
signaling. Although iron availability did not differ between the two growth curve
experiments depicted in Fig.22, the oxygen saturation of the culture medium
must have varied between the two settings. Possible oxygen stress is the most
likely explanation for the reduced growth of Pss22d shown in Fig.22B. It is
interesting to note that Pss22d reacts to this stress by an up-regulation of
synthesis of the achromobactin-like siderophore. Achromobactin is supposed
to protect D. chrysanthemi against oxidative stress (Expert, 1999). It could be
possible that a similar protection mechanism occurs in Pss22d. However, the
pathogenic Psg1a demonstrated an opposite regulation pattern. Pyoverdin
production was up-regulated under high oxygen conditions. However, while the
growth rate of Pss22d was reduced under high oxygen conditions, Psg1a
growth was not effected. This result could indicate that in Psg1a other means
are effective to avoid oxygen stress like for example in increased production of
superoxid dismutase. Up to now there is no experimental prove for this
assumption.
6.3 Influence of siderophore production on epiphytic fitness of Pss22d
We could clearly rule out the hypothesis, that siderophore production directly
influences biological control of Psg1a by Pss22d via competition for iron.
However, it was also essential to investigate the influence of siderophore
production on the overall epiphytic fitness of the antagonist in planta. One of
the most basic requirements for a versatile biological control agent is its ability
to survive in the respective habitat. It can only suppress pathogen development
sufficiently if it is able to compete with other epiphytic microorganisms. The
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