Modulating host homeostasis as a strategy in the plant-pathogen arms race

Abstract

In plant-pathogen interactions, pathogens aim to overcome host defence responses while plants employ a battery of responses to limit pathogen growth and thus disease. In this “arms race” between hosts and pathogens, horizontal gene transfer is a potent source of ‘pathogenic innovation’ for viruses and bacteria. However, bacteria rarely acquire ‘eukaryotic-like’ genes from their hosts, and where they appear to, evidence for a role of the acquired genes remains outstanding. We have recently reported experimental evidence that the citrus canker causing pathogen Xanthomonas axonopodis pv. citri contains a plant natriuretic peptide-like gene (XacPNP) that encodes a protein that modulates host homeostasis to its advantage. We argue that Xanthomonas PNP has been acquired in an ancient horizontal gene transfer, and given that plant and bacterial PNPs trigger a number of similar physiological responses, we make a case of molecular mimicry. Released XacPNP mimics host PNP and results in a suppressed host response, “improved” host tissue health and consequently better pathogen survival in the lesions. Finally, we propose that Xanthomonas axonopodis pv. citri host interactions can serve as model system to study the role of host homeostasis in plant defence against biotrophic pathogens.

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Figures and Tables

Figure 1 Zoospore encystment on the epidermis of onion. Zoospore suspension of wild-type strain and Gα silenced mutant were dropped on the surface of epidermis of onion, and incubated in 25°C. After 30 min, zoospore encystment was analyzed under microscope. (A) Zoospore encystment of wild-type strain on epidermis of onion. (B) Zoospore encystment of Gα silenced mutant on epidermis of onion.

Figure 2 Soybean leaves infected by P. sojae zoospores. (A) P. sojae germ tube penetrated through intercellular space of soybean leaf. (B) P. sojae germ tube penetrated through stoma of soybean leaf. The arrows point to the penetrating site of germ tubes.

Figure 3 Relative expression of PsRGS6 in P. sojae wild-type strain and Gα silenced mutants. WT, wild-type strain P6497, A2 (A27), Gα silenced mutant.

Figure 4 Relative expression of PsGPA1 in asexual development of P. sojae. MY, mycelia; SP, sporulating hyphae; ZO, zoosores; CY, cyst; GC, germinating cyst.

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