To fight the enemy, you must first detect the enemy, a truism that applies equally to war and disease resistance. In the latter, the enemy (or pathogen) generally conducts multiple activities that can betray its presence. For example, pathogens can disrupt cellular structures, producing pathogen-associated molecular patterns. Also, many pathogens specifically inject effector proteins that subvert the plant’s defense responses (reviewed in Deslandes and Rivas, 2012). Different bacterial, fungal, or oomycete pathogens carry diverse sets of effectors with varying molecular functions, but these effectors generally target similar defense pathways (reviewed in Dou and Zhou, 2012). Undetected, effectors sabotage the plant defense response, allowing the pathogen to grow and reproduce unimpeded; different effector types facilitate different pathogen growth strategies, which require dead or living plant cells. Detected, effectors trigger the plant’s defenses, including gene activation, production of antimicrobial compounds, and possibly programmed cell death.
Given the broad diversity of effectors, detection poses a substantial problem for the plant. In contrast to the combinatorial diversity of mammalian antibodies, plant genomes generally contain only a few hundred resistance (R) genes, which encode recognition proteins. To examine recognition, Césari et al. (pages 1463–1481) characterize the interaction between rice (Oryza sativa) proteins RGA4 and RGA5 and effectors from the rice blast fungal pathogen Magnaporthe oryzae. Intriguingly, both RGA4 and RGA5 are required for effector recognition, and, together, these two tightly linked genes correspond to the previously identified Pi-CO39 resistance locus in the indica rice line CO39. Indeed, rga4 mutants compromise Pi-CO39 resistance and transgenic lines expressing both RGA4 and RGA5 recapitulate similar resistance (see figure). Moreover, this R protein pair recognizes two effectors, the avirulence proteins (AVR) AVR1-CO39 and AVR-Pia (Okuyama et al., 2011). However, these two AVRs share no sequence similarity.