Innate Resistance and Phosphite Treatment Affect Both the Pathogen's and Host's Transcriptomes in the Tanoak-Phytophthora ramorum Pathosystem.

Phosphites have been used to control Sudden Oak Death; however, their precise mode of action is not fully understood. To study the mechanism of action of phosphites, we conducted an inoculation experiment on two open-pollinated tanoak families, previously found to be partially resistant. Stems of treatment group individuals were sprayed with phosphite, and seven days later, distal leaves were inoculated with the Sudden Oak Death pathogen Phytophthora ramorum. Leaves from treated and untreated control plants were harvested before and seven days after inoculation, and transcriptomes of both host and pathogen were analyzed. We found that tanoak families differed in the presence of innate resistance (resistance displayed by untreated tanoak) and in the response to phosphite treatment. A set of expressed genes associated with innate resistance was found to overlap with an expressed gene set for phosphite-induced resistance. This observation may indicate that phosphite treatment increases the resistance of susceptible host plants. In addition, genes of the pathogen involved in detoxification were upregulated in phosphite-treated plants compared to phosphite-untreated plants. In summary, our RNA-Seq analysis supports a two-fold mode of action of phosphites, including a direct toxic effect on P. ramorum and an indirect enhancement of resistance in the tanoak host.

A common garden test of host-symbiont specificity supports a dominant role for soil type in determining AMF assemblage structure in Collinsia sparsiflora.

Specialization in plant host-symbiont-soil interactions may help mediate plant adaptation to edaphic stress. Our previous field study showed ecological evidence for host-symbiont specificity between serpentine and non-serpentine adapted ecotypes of Collinsia sparsiflora and arbuscular mycorrrhizal fungi (AMF). To test for adapted plant ecotype-AMF specificity between C. sparsiflora ecotypes and field AMF taxa, we conducted an AMF common garden greenhouse experiment. We grew C. sparsiflora ecotypes individually in a common pool of serpentine and non-serpentine AMF then identified the root AMF by amplifying rDNA, cloning, and sequencing and compared common garden AMF associates to serpentine and non-serpentine AMF controls. Mixing of serpentine and non-serpentine AMF soil inoculum resulted in an intermediate soil classified as non-serpentine soil type. Within this common garden both host ecotypes associated with AMF assemblages that resembled those seen in a non-serpentine soil. ANOSIM analysis and MDS ordination showed that common garden AMF assemblages differed significantly from those in the serpentine-only controls (R = 0.643, P<0.001), but were similar the non-serpentine-only control AMF assemblages (R = 0.081, P<0.31). There was no evidence of adapted host ecotype-AMF specificity. Instead soil type accounted for most of the variation AM fungi association patterns, and some differences between field and greenhouse behavior of individual AM fungi were found.