Publications - Faculty - Recently Submitted Faculty Publications 2010
<- Back
Graham - Plant and Soil Special Volume
Citrus roots are protected against infection by Phytophthora spp. by hypovirulent P. nicotianae due to competition not induction of systemic resistance
J. H. Graham, G. C. Colburn, K. R. Chung, and J. Cubero
Abstract.Phytophthora root rot of citrus in Florida is caused by Phytophthora nicotianae and P. palmivora. A naturally occurring isolate of P. nicotianae (Pn117) infected and colonized fibrous roots, but caused significantly less disease than virulent isolates P. nicotianae (Pn198) and P. palmivora (Pp99). Although Pn117 caused less disease, it was not impaired in ability to infect and colonize roots compared to the virulent isolates. In a competition study, pre-inoculation of citrus rootstock seedlings with Pn117 followed by inoculation with Pn198 or Pp99 resulted in an increase in recovery of Pn117 and a decrease in recovery of virulent isolates compared to the co-inoculation treatment. To investigate whether induction of systemic resistance by Pn117 is a mechanism for protection of roots against the virulent isolates, a novel citrus gene was cloned from a differentially expressed cDNA library cloned. CsPR4 gene was characterized as a class II chitinase and implicated in systemic resistance. Sour orange seedling roots were exposed to zoospores of Pn117, Pn198, and Pp99 for 2, 6, 24, and 48 h. Seedling roots were also treated with solutions of 100 µM salicylic acid (SA), benzothiadiazole (BTH), and jasmonic acid (JA). A split root system experiment was conducted to determine if CsPR4 is systemically induced by infection. One side of the root system was inoculated with zoospores of Pn117, Pn198, or Pp99 and the other side was placed in water. CsPR4 transcript accumulated at 2 h after inoculation with Pn117, whereas accumulation in response to Pp99 and Pn198 was delayed until 6 and 24 h, respectively. Systemic resistance-inducing SA, JA, or BTH, upregulated CsPR4 indicating that this gene is an inducible component of defense pathways. In the split root assay, infection by Pn117 failed to induce CsPR4 in distal roots. The split root assay confirmed that systemically induced defense is not mediating protection of roots by the hypovirulent isolate Pn117 against the virulent isolates Pn198 and Pp99.
