The molecular and cellular responses of the soybean pathogen, Phytophthora sojae, to calcium, isoflavones, and host tissues

This dissertation has examined several aspects of the interaction between the oomycete pathogen Phytophthora sojae and its host soybean. These investigations included: (1) The effects of soybean isoflavones on calcium stores and developmental fate of P. sojae cysts; (2) The effects of abscisic acid (ABA) on the behavior and physiology of P. sojae, and attempts to provide evidence for ABA biosynthesis by P. sojae; and (3) The cloning and partial characterization of the ABC transporter, PsABC1, from P. sojae. The importance of soybean isoflavones in host recognition and infection by P. sojae, and the subsequent finding that calcium plays a role in determining the developmental fate of P. sojae encysted zoospores, led us to investigate the effects of soybean isoflavones on intracellular calcium stores following zoospore encystment. Confirmation that isoflavones trigger a net influx of calcium into the cell was demonstrated by X-ray microanalysis of individual encysted zoospores. The levels of internal calcium stores at the time of encystment were found to be a key determining factor controlling the developmental fate of cysts. Cysts with higher calcium reserves produced hyphal germlings, while cysts with low levels of calcium released a second zoospore.;Because levels of the plant hormone ABA are elevated in compatible P. sojae-soybean interactions, and some members of the Kingdom Stramenopiles produce ABA, we looked for evidence of ABA biosynthesis in P. sojae . However no evidence, direct or indirect, was found to indicate that P. sojae synthesized or was responsive to ABA.;A degenerate primer based on a short peptide sequence in combination with a 3' arbitrary primer was found to amplify a 1320 bp fragment with significant homology to TUR2, a member of the Pleiotropic Drug Resistance family of ATP-binding-cassette transporters from the aquatic weed Spirodela polyrrhiza. A P. sojae BAC genomic library was screened to gain the full sequence of the gene named PsABC1. Expression analysis by RT-PCR indicated that this gene was expressed at high levels in zoospores, at low to moderate levels in hyphal germlings and in planta, but was not detected in hyphae supported by synthetic or complex media.