The Direct Approach: Resistance Genes Team up to Recognize Different Fungal Effectors in Rice

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.

Both RGA4 and RGA5 are required for effector recognition. Disease symptoms after inoculation with M. oryzae carrying the AVR1-CO39 effector. Lack of symptoms in the plants expressing RGA4 and RGA5 indicates successful recognition and induction of disease resistance. (Reprinted from Césari et al. [2013], Figure 2B.)

R protein recognition can be direct or indirect, and known examples of recognition of multiple effectors usually involve indirect recognition, likely through effects on common cellular targets. By contrast, here, the authors show that both AVR1-CO39 and AVR-Pia interact directly with RGA5, using multiple methods including yeast two-hybrid, coimmunoprecipitation, and fluorescence resonance energy transfer–fluorescence lifetime imaging. Some natural variants of AVR-Pia fail to interact with RGA5 and also are unable to trigger activation of disease resistance, indicating that AVR recognition requires interaction with RGA5. Moreover, deletion analysis showed that the interaction with the two AVRs was not through the RGA5 nucleotide binding or leucine-rich repeat domains, but rather occurred through a small C-terminal region related to heavy metal–associated domains. This domain is also shared with another rice R protein, Pik-1, indicating that it may function in recognition of diverse effectors. Therefore, it seems that plants make use of a number of previously unexplored tricks to detect effectors, and examination of these mechanisms revealed a pair of R proteins with dual recognition specificity. The mechanism by which the novel protein domain in RGA4 and RGA5 recognizes two different (and indeed dissimilar) AVRs remains an open question.