Sciencepic of the day (2): Recognition of a Pathogen effector and plant resistance to pathogens

Molecular Plant Pathologists like to study pathogen proteins and their function(s) in plants. Some of the work is described in post 1 and post 2. However, some of these effector proteins can be recognized by receptors and lead to resistance. 

In the image below, you can see two Nicotiana benthamiana leaves, each infected with a Phytophthora capsici strain that expresses an effector from the potato pathogen P. infestans (PiAVR3a). Expression, secretion, and delivery of this protein normally help P. infestans infection (Bos et al., 2010). However, some potato plants use a receptor protein (called R3A) to recognize this effector and initiate defense responses (Armstrong et al., 2005; Bos et al., 2006). In the figure below, you can see that when P. capsici, expressing this AVR3a protein, infects a leaf that does not have R3a (left leaf labeled 'Wt'), significant lesion growth occurs. When leaves that do express this R3a protein, are infected with the same strain, recognition takes place (Right leaf labeled 'R3a'), restricting P. capsici growth.

Image: Infection assays using a P. capsici strain expressing PiAvr3a on Wildtype (Wt, left) and R3a expressing leaves (R3a, right). Source: Personal collection.

Why this is important

This recognition conditions resistance to P. capsici and prevents growth and reproduction. R-genes such as R3a are currently being identified and deployed in elite crop cultivars to fight disease in the field (Vleeshouwers et al., 2011). Without the availability of this genetically encoded resistance, farmers would have to resort to heavy fungicide use. This would be damaging to the environment as well as economically unsustainable to farmers and consumers (especially those in poorer countries).

REFERENCES

Armstrong, M. R., Whisson, S. C.,  Pritchard, L., Bos, J. I., Venter, E., Avrova, A. O., ... & Hamlin,  N. (2005). An ancestral oomycete locus contains late blight avirulence  gene Avr3a, encoding a protein that is recognized in the host cytoplasm.  Proceedings of the National Academy of Sciences of the United States of America, 102(21), 7766-7771.

Bos, J. I., Kanneganti, T. D., Young,  C., Cakir, C., Huitema, E., Win, J., ... & Kamoun, S. (2006). The  C‐terminal half of Phytophthora infestans RXLR effector AVR3a is  sufficient to trigger R3a‐mediated hypersensitivity and suppress  INF1‐induced cell death in Nicotiana benthamiana. The Plant Journal, 48(2), 165-176.

Bos, J. I., Armstrong, M. R., Gilroy, E. M., Boevink, P. C., Hein, I.,  Taylor, R. M., ... & Dixelius, C. (2010). Phytophthora infestans  effector AVR3a is essential for virulence and manipulates plant immunity  by stabilizing host E3 ligase CMPG1. Proceedings of the National Academy of Sciences, 107(21), 9909-9914.

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