RTP1 And RTP2 In Arabidopsis Negatively Regulate Resistance Against Phytophthora Parasitica

Oomycetes, particularly Phytophthora species, infect various organisms and cause serious agricultural diseases, which lead to great economic losses across the world. As oomycetes are similar to fungi in terms of morphology but distant from fungi based on the phylogeny and physiological biochemistry, there may be unique genetic variation mechanisms. This might be exemplified by the potato late blight pathogen, P. infestans, which has overcome all the known plant disease resistance genes. Consequently, the control of late blight is highly dependent on agricultural chemicals.Plant-pathogen interactions are classified into compatible or incompatible interactions. Specifically, incompatible interactions result in resistance. In this process, a resistance (R) protein from the plant recognizes an avirulence (AVR) protein from the pathogen and activates a battery of plant defence responses. The recognition between pathogen AVRs and cognate host R proteins is an important area for research and in recent years numerous AVR-R pairs have been identified. In contrast, a compatible interaction occurs when the plant does not recognize the pathogen, resulting in susceptibility. The processes underpinning compatible interactions are important because elucidating these may help us to understand the molecular mechanisms of pathogenicity. However, to date these remain largely unexplored in Phytophthora species. Thus, it is necessary to identify key plant regulators involved in Phytophthora-plant compatible interactions.P. parasitica is emerging as a model pathogen for oomycetes as it represents most of the Phytophthora species characters such as soil-born and wide range of host plants. Based on the compatible interaction between P. parasitica and the reference plant, Arabidopsis thaliana, Arabidopsis mutant candidates that display resistance against P. parasitica infection were obtained by screening from a T-DNA insertion population. The loss-of-function mutants in this collection encode plant regulators required to support the Arabidopsis-P. parasitica compatible interaction.In this study, we focused on two mutants, namely rtp1(resistance to P. parasitica 1) and rtp2. We identified the loss-of-function genes in these mutants, RTP1 and RTP2 respectively, and examined their roles in Arabidopsis-pathogen interactions. The main findings are as follows:1, RTP1 negatively regulates resistance to P. parasitica. The rtp1-1 mutant shows both leaf and root resistance to P. parasitica based on the a pathogen growth assay. Transgenic plants expressing RTP1 are susceptible to P. parasitica, but RTP1 knockout plants exhibit a resistance phenotype. Further, rtp1-1 displays localized cell death, increased reactive oxygen species (ROS) production and accelerated PR1 expression, compared to the wild-type Col-0, in response to P. parasitica infection.2, RTP1 belongs to a nodulin-related MtN21 family in Arabidopsis and encodes an Endoplasmic Reticulum (ER) localized protein. Preliminarily analysis by particle bombardment-mediated transient expression in onion shows RTP1-GFP localized in the ER network. The ER localization of RTP1 is further confirmed by Agrobacterium mediated transient expression in tobacco combined with western blot technology.3, The rtpl-1 mutant shows broad resistance against Pseudomonas syringae pv. tomato (Pst) DC3000, Pst DC3000/avrRpml as well as Arabidopsis powdery mildew infection, all of which are biotrophic pathogens. Increased disease resistance, cell death and ROS production were also observed in rtp1-1 post Pst DC3000 or Pst DC3000/avrRpml infection. However, rpt1-1 does not exhibit resistance to the necrotrophic pathogen Botrytis cinerea.4, RTP2 is responsible for rtp2-mediated resistance and encodes a cell membrane localized protein. Based on southern blot, TAIL-PCR and sequencing analysis, rtp2 contains a single T-DNA insertion. RTP2 belongs to the NRT1/PTR transporter family (NPF) and encodes a cell membrane localized protein. Further, the rtp2 mutant exhibits resistance to P. parasitica, whereas overexpression of RTP2 in an rtp2 background compromises its resistance phenotype. Taken together, RTP2 negatively regulates Arabidopsis resistance to P. parasitica.5, Pathogenic assays showed rtp2 expresses increased resistance to Pst DC3000 and Pst DC3000/avrRpm1 compared with Col-0, but displays susceptibility to B. cinerea.6, RTP2 is response to nitrogen deficiency and re-supply and nitric oxide (NO) may be involved in Arabidopsis-Phytophthora interaction. qRT-PCR analyse indicates that RTP2 is up regulated post the nitrogen deficient treatment and re-supply. Histochemical staining shows P. parasitica infection enhanced NO production at 3 hours and peak at 6 hours in Col-0 and Ler, both of which showed susceptible to P. parasitica.