Differential screening reveals genes differentially expressed in low- and high-virulence near-isogenic Phytophthora sojae lines

Introduction of the harpinXooc-encoding gene hrf2 in soybean enhances resistance against the oomycete pathogen Phytophthora sojae

Phytophthora root and stem rot (PRR) caused by an oomycete pathogen Phytophthora sojae is one of the most devastating and widespread diseases throughout soybean-producing regions worldwide. The diversity and variability of P. sojae races make effective control of the pathogen challenging. Here, we introduced an elicitor of plant defense response, the harpinXooc-encoding hrf2 gene from the rice bacterial pathogen Xanthomonas oryzae pv. oryzicola into soybean and evaluated resistance to P. sojae infection. Molecular analysis confirmed the integration and expression of hrf2 in the transgenic soybean. After inoculation with P. sojae, non-transformed control (NC) plants exhibited typical PRR symptoms, including necrotic and wilting leaves, and plant death, whereas most of the transgenic plants showed slightly chlorotic leaves and developed normally. Through T3 to T5 generations, the transgenic events displayed milder disease symptoms and had higher survival rates compared to NC plants, indicating enhanced and stable resistance to P. sojae infection, whereas without P. sojae inoculation, no significant differences in agronomic traits were observed between the transgenic and non-transformed plants. Moreover, after inoculation with P. sojae, significant upregulation of a set of plant defense-related genes, including salicylic acid- and jasmonic acid-dependent and hypersensitive response-related genes was observed in the transgenic plants. Our results indicate that hrf2 expression in transgenic soybean significantly enhanced resistance to P. sojae by eliciting multiple defense responses mediated by different signaling pathways. The potential functional role of the hrf2 gene in plant defense against P. sojae and other pathogens makes it a promising tool for broadening disease resistance in soybean.

Identification and analysis of Phytophthora cactorum genes up-regulated during cyst germination and strawberry infection

The oomycete Phytophthora cactorum can cause economically important diseases on numerous host plants worldwide, such as crown rot on strawberry. To explore the molecular mechanisms underlying the pathogenicity of P. cactorum on strawberry, transcriptional analysis of P. cactorum during strawberry infection and cyst germination was performed by applying suppression subtractive hybridization (SSH) and effector-specific differential display (ESDD) techniques. Two SSH cDNA libraries were generated, enriched for P. cactorum genes expressed during infection or during cyst germination, respectively, and 137 unique differentially expressed genes were identified. To specifically select RxLR effector genes from P. cactorum, ESDD was performed using RxLR and EER motif-based degenerate primers. Eight RxLR effector candidate genes as well as 67 other genes were identified out of 124 selected fragments. The expression levels of 20 putatively up-regulated genes were further analyzed using real-time RT-PCR, showing that, indeed 19 of these 20 genes were up-regulated during at least one of the studied developmental stages or during strawberry crown invasion, relative to the mycelium. This study provides a first overview of P. cactorum genes that are up-regulated immediately prior to or during strawberry infection and also provides a novel method for selecting RxLR effector genes from the unsequenced genome of P. cactorum.

Identification of Phytophthora sojae genes upregulated during the early stage of soybean infection

To explore the molecular mechanisms that are involved in the pathogenicity of Phytophthora sojae, a suppression subtractive hybridization method was developed to screen for P. sojae genes that are differentially expressed in the early stage of Glycine max (soybean) infection. A cDNA library enriched for upregulated parasite genes was generated; of the 73 genes that were found to be upregulated, 66 are significantly similar to sequences in the P. sojae genome, and seven have no significant similarities in the databases examined. These sequences are predicted to encode proteins involved in protein biosynthesis, energy production, cell signaling, cell-wall biogenesis, and transcription regulation. Virtual Northern assay of random selected seven genes revealed that they are all highly expressed in plant infection. Reverse transcriptase polymerase chain reaction was used to further examine the expression pattern of these genes during soybean infection. These results provide an important insight into the genes expressed during P. sojae infection of soybean, which may be involved in oomycete pathogenesis.