To help learn how phytopathogens feed from their hosts, genes for nutrient transporters from the hemibiotrophic potato and tomato pest Phytophthora infestans were annotated. This identified 453 genes from 19 families. Comparisons with a necrotrophic oomycete, Pythium ultimum var. ultimum, and a hemibiotrophic fungus, Magnaporthe oryzae, revealed diversity in the size of some families although a similar fraction of genes encoded transporters.
RNA-seq of infected potato tubers, tomato leaves, and several artificial media revealed that 56 and 207 transporters from P. infestans were significantly up- or down-regulated, respectively, during early
infection timepoints of leaves or tubers versus media. About 17 were up-regulated >4-fold in both leaves and tubers compared to media and expressed primarily in the biotrophic stage. The transcription pattern of many genes was host-organ specific. For example, the
mRNA level of a
nitrate transporter (NRT) was about 100-fold higher during mid-
infection in leaves, which are
nitrate-rich, than in tubers and three types of artificial media, which are
nitrate-poor. The NRT gene is physically linked with genes encoding
nitrate reductase (NR) and
nitrite reductase (NiR), which mobilize
nitrate into
ammonium and
amino acids. All three genes were coregulated. For example, the three genes were expressed primarily at mid-stage
infection timepoints in both potato and tomato leaves, but showed little expression in potato tubers. Transformants down-regulated for all three genes were generated by
DNA-directed RNAi, with silencing spreading from the NR target to the flanking NRT and NiR genes. The silenced strains were nonpathogenic on leaves but colonized tubers. We propose that the
nitrate assimilation genes play roles both in obtaining
nitrogen for
amino acid biosynthesis and protecting P. infestans from natural or fertilization-induced
nitrate and
nitrite toxicity.