A Phytophthora infestans RXLR effector targets a potato ubiquitin-like domain-containing protein to inhibit the proteasome activity and hamper plant immunity.

Ubiquitin-like domain-containing proteins (UDPs) are involved in the ubiquitin-proteasome system because of their ability to interact with the 26S proteasome. Here, we identified potato StUDP as a target of the Phytophthora infestans RXLR effector Pi06432 (PITG_06432), which supresses the salicylic acid (SA)-related immune pathway. By overexpressing and silencing of StUDP in potato, we show that StUDP negatively regulates plant immunity against P. infestans. StUDP interacts with, and destabilizes, the 26S proteasome subunit that is referred to as REGULATORY PARTICLE TRIPLE-A ATP-ASE (RPT) subunit StRPT3b. This destabilization represses the proteasome activity. Proteomic analysis and Western blotting show that StUDP decreases the stability of the master transcription factor SYSTEMIC ACQUIRED RESISTANCE DEFICIENT 1 (SARD1) in SA biosynthesis. StUDP negatively regulates the SA signalling pathway by repressing the proteasome activity and destabilizing StSARD1, leading to a decreased expression of the SARD1-targeted gene ISOCHORISMATE SYNTHASE 1 and thereby a decrease in SA content. Pi06432 stabilizes StUDP, and it depends on StUDP to destabilize StRPT3b and thereby supress the proteasome activity. Our study reveals that the P. infestans effector Pi06432 targets StUDP to hamper the homeostasis of the proteasome by the degradation of the proteasome subunit StRPT3b and thereby suppresses SA-related immunity.

Potato protein tyrosine phosphatase StPTP1a is activated by StMKK1 to negatively regulate plant immunity.

Phytophthora infestans causes severe losses in potato production. The MAPK kinase StMKK1 was previously found to negatively regulate potato immunity to P. infestans. Our results showed that StMKK1 interacts with a protein tyrosine phosphatase, referred to as StPTP1a, and StMKK1 directly phosphorylates StPTP1a at residues Ser-99, Tyr-223 and Thr-290. StPTP1a is a functional phosphatase and the phosphorylation of StPTP1a at these three residues enhances its stability and catalytic activity. StPTP1a negatively regulates potato immunity and represses SA-related gene expression. Furthermore, StPTP1a interacts with, and dephosphorylates, the StMKK1 downstream signalling targets StMPK4 and -7 at their Tyr-203 residue resulting in the repression of salicylic acid (SA)-related immunity. Silencing of NbPTP1a + NbMPK4 or NbPTP1a + NbMPK7 abolished the plant immunity to P. infestans caused by NbPTP1a silencing, indicating that PTP1a functions upstream of NbMPK4 and NbMPK7. StMKK1 requires StPTP1a to negatively regulate SA-related immunity and StPTP1a is phosphorylated and stabilized during immune activation to promote the de-phosphorylation of StMPK4 and -7. Our results reveal that potato StMKK1 activates and stabilizes the tyrosine phosphatase StPTP1a that in its turn de-phosphorylates StMPK4 and -7, thereby repressing plant SA-related immunity.

Potato StMPK7 is a downstream component of StMKK1 and promotes resistance to the oomycete pathogen Phytophthora infestans.

A cascade formed by phosphorylation events of mitogen-activated protein kinases (MAPKs) takes part in plant stress responses. However, the roles of these MAPKs in resistance of potato (Solanum tuberosum) against Phytophthora pathogens is not well studied. Our previous work showed that a Phytophthora infestans RXLR effector targets and stabilizes the negative regulator of MAPK kinase 1 of potato (StMKK1). Because in Arabidopsis thaliana the AtMPK4 is the downstream phosphorylation target of AtMKK1, we performed a phylogenetic analysis and found that potato StMPK4/6/7 are closely related and are orthologs of AtMPK4/5/11/12. Overexpression of StMPK4/7 enhances plant resistance to P. infestans and P. parasitica. Yeast two-hybrid analysis revealed that StMPK7 interacts with StMKK1, and StMPK7 is phosphorylated on flg22 treatment and by expressing constitutively active StMKK1 (CA-StMKK1), indicating that StMPK7 is a direct downstream signalling partner of StMKK1. Overexpression of StMPK7 in potato enhances potato resistance to P. infestans. Constitutively active StMPK7 (CA-StMPK7; StMPK7D198G, E202A ) was found to promote immunity to Phytophthora pathogens and to trigger host cell death when overexpressed in Nicotiana benthamiana leaves. Cell death triggered by CA-StMPK7 is SGT1/RAR1-dependent. Furthermore, cell death triggered by CA-StMPK7 is suppressed on coexpression with the salicylate hydroxylase NahG, and StMPK7 activation promotes salicylic acid (SA)-responsive gene expression. We conclude that potato StMPK7 is a downstream signalling component of the phosphorelay cascade involving StMKK1 and StMPK7 plays a role in immunity to Phytophthora pathogens via an SA-dependent signalling pathway.

Phytophthora infestans RXLR effector PITG20303 targets a potato MKK1 protein to suppress plant immunity.

Pathogens secret a plethora of effectors into the host cell to modulate plant immunity. Analysing the role of effectors in altering the function of their host target proteins will reveal critical components of the plant immune system. Here we show that Phytophthora infestans RXLR effector PITG20303, a virulent variant of AVRblb2 (PITG20300) that escapes recognition by the resistance protein Rpi-blb2, suppresses PAMP-triggered immunity (PTI) and promotes pathogen colonization by targeting and stabilizing a potato MAPK cascade protein, StMKK1. Both PITG20300 and PITG20303 target StMKK1, as confirmed by multiple in vivo and in vitro assays, and StMKK1 was shown to be a negative regulator of plant immunity, as determined by overexpression and gene silencing. StMKK1 is a negative regulator of plant PTI, and the kinase activities of StMKK1 are required for its suppression of PTI and effector interaction. PITG20303 depends partially on MKK1, PITG20300 does not depend on MKK1 for suppression of PTI-induced reactive oxygen species burst, while the full virulence activities of nuclear targeted PITG20303 and PITG20300 are dependent on MKK1. Our results show that PITG20303 and PITG20300 target and stabilize the plant MAPK cascade signalling protein StMKK1 to negatively regulate plant PTI response.

[Detection of Late Blight Disease on Potato Leaves Using Hyperspectral Imaging Technique].

Hyperspectral imaging feature on potato leaves stressed by late blight was studied in the present paper. The experiment used 60 potato leaves. Among those 60 potato leaves, 48 leaves were vitro inoculated with pathogen of potato late blight, the rest 12 leaves were used as control samples. The leaves were observed for 7 continuous days before and after inoculated and samples including healthy and infested were acquired. Hyperspectral data of healthy and infected potato samples of different disease severity were obtained by the hyperspectral imaging system from 374 to 1,018 nm and then extract spectral data of region of interest (ROI) from those hyperspectral data by the ENVI software. In order to improve the signal-to-noise ratio, the spectral data were preprocessed using different pretreatment methods such as moving average smoothing, normalization, derivative, baseline etc. The least squares-support vector machine(LS-SVM) models were developed based on the raw and those preprocessed data. Among the nine models, the model that used the raw data and the data after the spectroscopic transformation performed best with the discrimination of 94.87%. It was demonstrated that it is realized to determine the potato late blight disease of different disease severity using hyperspectral imaging technique.

Population Structure of the Late Blight Pathogen Phytophthora infestans in a Potato Germplasm Nursery in Two Consecutive Years.

As the causal agent of late blight on potato, Phytophthora infestans is one of the most destructive plant pathogens worldwide and widely known as the Irish potato famine pathogen. Understanding the genetic structure of P. infestans populations is important both for breeding and deployment of resistant varieties and for development of disease control strategies. Here, we investigate the population genetic structure of P. infestans in a potato germplasm nursery in northwestern China. In total, 279 isolates were recovered from 63 potato varieties or lines in 2010 and 2011, and were genotyped by mitochondrial DNA haplotypes and a set of nine simple-sequence repeat markers. Selected isolates were further examined for virulence on a set of differential lines containing each resistance (R) gene (R1 to R11). The overall P. infestans population was characterized as having a low level of genetic diversity and resistance to metalaxyl, and containing a high percentage of individuals that virulent to all 11 R genes. Both A1 and A2 mating types as well as self-fertile P. infestans isolates were present but there was no evidence of sexual reproduction. The low level of genetic differentiation in P. infestans populations is probably due to the action of relatively high levels of migration as supported by analysis of molecular variance (P < 0.01). Migration and asexual reproduction were the predominant mechanisms influencing the P. infestans population structure in the germplasm nursery. Therefore, it is important to ensure the production of pathogen-free potato seed tubers to aid sustainable production of potato in northwestern China.