Activation of three pathogen-inducible promoters in transgeniccitrus (Citrus sinensis Osbeck) after Xanthomonas axonopodis pv.citri infection and wounding

To improve the resistance of citrus to canker
disease caused by Xanthomonas axonopodis pv. citri (Xac),
it is important to identify gene promoters that are specifically induced by pathogen infection. Here, we evaluated
the functionality of PPP1 and hsr203J (Nicotiana tabacum
L.) and gst1 (potato) pathogen-inducible promoters to drive
expression of the b-glucuronidase (GUS) reporter gene in
transgenic citrus. The activities of these promoters in
response to the Xac pathogen and wounding were determined quantitatively and qualitatively by fluorometric and
histochemical GUS assays, and compared with that of the
cauliflower mosaic virus (CaMV) 35S promoter. In citrus,
the hsr203J promoter from tobacco was hardly activated by
the Xac pathogen or wounding, whereas the PPP1 and gst1
promoters were rapidly and efficiently activated by both
inducers. There was very little visible background
expression from the PPP1 promoter, but a high level of
background expression from the gst1 promoter. Because of
its low background expression, the PPP1 promoter was
more responsive to Xac and wounding than was the gst1
promoter. However, the gst1 promoter was more rapidly