Protection of citrus roots against infection by Phytophthora spp. by hypovirulent P. nicotianae is not related to induction of systemic acquired resistance
AIMS: Phytophthora root rot of citrus in Florida is caused by Phytophthora nicotianae and P. palmivora. In a field experiment, pre-inoculation of citrus fibrous roots before planting with a naturally occurring isolate of P. nicotianae (Pn117) reduced root rot caused by virulent P. nicotianae in an infested citrus orchard. On potted citrus rootstock seedlings, Pn117 caused less disease, but was not impaired in ability to infect and colonize roots compared to virulent Phytopththora spp. In a competition experiment, pre-inoculation of citrus rootstock seedlings with Pn117 followed by inoculation with virulent isolates of P. nicotianae (Pn198) and P. palmivora (Pp99) resulted in an increase in recovery of Pn117 and a decrease in recovery of virulent isolates compared to the co-inoculation treatment. METHODS: To investigate whether induction of systemic acquired resistance (SAR) by Pn117 is a mechanism for protection of roots against the virulent isolates, the chitinase gene CsPR4 was cloned from a differentially expressed cDNA library from Valencia sweet orange (Citrus sinensis). Sour orange (C. aurantium) seedling roots were exposed to zoospores of Pn117, Pn198, or 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. RESULTS: 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 implicating this gene as an inducible component of defense pathways in citrus. In the split root assay, infection by Pn117 failed to induce CsPR4 in distal roots. CONCLUSION: Lack of response of CsPR4 in distal roots to hypovirulent Pn117 did not confirm a role for SAR in protection of citrus roots, but reinforces field and greenhouse findings that pre-inoculation with the hypovirulent isolate Pn117 protects roots from virulent isolates by competitive exclusion through pre-emptive infection and reproduction on roots.
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