Disease cycle of potato late blight - MSpace at the University of ...

infestans secretes both cytoplasmic and apoplastic effectors (Kamoun 2005). For
example, serine protease inhibitors EPI1 and EPI10 are apoplastic effectors that are
thought to function in the counter-defense inhibiting PR protein P69B, a subtilisin-like
serine protease of tomato (Tian et al. 2005). In potato, Wang et al. (2004b) reported that
an aggressive P. infestans US8 strain suppresses host defense mechanisms through
transcriptional inhibition of phenylpropanoid (PAL) and isoprenoid (HMGR) pathways
(Wang et al. 2008).
Up-regulation of defense-related genes such as those controlling chitinase, beta-
glucanase, and other pathogenesis-related proteins in both compatible and incompatible
plant-pathogen interactions is well documented (Maleck et al. 2000). However, genes
expressed in an incompatible interaction and represented in a cDNA library do not all
correspond to critical information for future studies on resistance genes, especially when
the same gene is also expressed in the compatible interaction counterpart.
Cases where up-regulation occurs only in the susceptible or the resistant line, and
only in response to either the virulent or avirulent strain of the pathogen, have rarely been
described (Valer et al. 2006). Also, during both incompatible and compatible interactions,
many pathogens are invasive and establish pathogenic structures within the host tissue.
As a result, it is difficult to separate the pathogen tissue from that of the host, and both
are represented in the pool of expressed genes from the interaction.
27