Effectors-Role in Host-Pathogen Interaction
Baby Summuna Bhat, Dr. Efath Shahnaz

Effectors are pathogen secreted molecules that manipulate host cell structure and function thereby facilitating infection or triggering defense responses (Kamoun, 2006). These can be toxins or elicitors. This dual activity of effectors has been broadly reported in many plant-microbe pathosystems (Alfano and Collmer, 2004). The term effector is neutral and does not imply a negative or positive impact on the outcome of the disease interaction. Effectors are secreted from pathogens' secretion systems. So far four types of secretion systems (Types I-IV) have been identified. Among them, T3SS (Type III Secretion System) and T4SS (Type IV Secretion System) can cross bacterial cell walls and host eukaryotic cell membranes to deliver effectors into host cells directly without going through extracellular matrix. Those effectors can manipulate host cell functions once entering host cell (Leach, 2003). For a pathogen to survive and multiply, it produces effector molecules to obtain nutrients from its host plant and cultivate the right environment in which to establish infection. Phytopathogenic bacteria use a number of secretion pathways to deliver effector molecules, either into the intercellular spaces or even directly into the host cells. These pathways vary in their complexity for delivery of the effectors (Salmond and Reeves, 1994). The complexity of the pathways is based on the number of proteins involved in the assembly of a channel or pore formed between bacterial inner and outer membranes through which the effectors are transported from the cytosol to the outside of the bacterium.Two classes of effectors target distinct sites in the host plant viz. apoplastic effectors and cytoplasmic effectors. Apoplastic effectors are secreted into the plant extracellular space, whereby they target extracellular targets and surface receptors whereas cytoplasmic effectors are translocated inside the plant cell presumably through specialized structures like infection vesicles and haustoria that invaginate inside living host cells (Kamoun, 2006).Plant pathogenic bacteria and fungi have evolved the capacity to deliver effector proteins inside host cells through a diversity of mechanisms. Gram negative bacteria use specialized secretion systems such as T3SS to deliver proteins inside host cells (Block et al., 2008). Biotrophic fungi have evolved haustoria for this purpose. Haustoria were initially thought to primarily function in nutrient uptake but more recently, evidence emerged that haustoria take part in secretion of particular classes of host-translocated fungal effectors (Whisson et al., 2007). Oomycetes such as Phytophthora infestans, are also known to secrete apoplastic effectors in addition to hosttranslocated (cytoplasmic) effectors (Damasceno et al., 2008). A recent study illustrates the concept that plant pathogenic fungi can evade host immunity by way of effectors that suppress R-gene mediated resistance e.g., the effector Avr1 of Fusarium oxysporum f.sp. lycopersici suppresses the resistance response conferred by the R genes I-2 and I-3 (Saskia et al., 2009). However, various queries regarding effectors still remain unanswered e. g., Are effectors secreted at particular sites at the interface between microbe and plant? Are there waves of effector secretion? Do effectors have distinct functions etc. These are various other issues which need to be investigated in future.

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