The Cell Death Triggered by the Nuclear Localized RxLR Effector PITG_22798 from Phytophthora infestans Is Suppressed by the Effector AVR3b

Abstract

Phytopathogenic oomycetes, such as Phytophthora infestans, potentially secrete many RxLR effector proteins into plant cells to modulate plant immune responses and promote colonization. However, the molecular mechanisms by which these RxLR effectors suppress plant immune responses are largely unknown. Here we describe an RxLR effector PITG_22798 (Gene accession: XM_002998349) that was upregulated during early infection of potato by P. infestans. By employment of agroinfiltration, we observed that PITG_22798 triggers cell death in Nicotiana benthamiana. Confocal microscopic examination showed that PITG_22798-GFP (Green Fluorescent Protein) located in the host nucleus when expressed transiently in N. benthamiana leaves. A nuclear localization signal (NLS) domain of PITG_22798 is important for nuclear localization and cell death-inducing activity. Sequence alignment and transient expression showed that PITG_22798 from diverse P. infestans isolates are conserved, and transient expression of PITG_22798 enhances P. infestans colonization of N. benthamiana leaves, which suggests that PITG_22798 contributes to P. infestans infection. PITG_22798-triggered cell death is dependent on SGT1-mediated signaling and is suppressed by the P. infestans avirulence effector 3b (AVR3b). The present research provides a clue for further investigation of how P. infestans effector PITG_22798 associates with and modulates host immunity.

An RXLR effector secreted by Phytophthora parasitica is a virulence factor and triggers cell death in various plants

“…Cell death induced by a number of pathogen effectors is considered to be the outcome of recognition by the plant immune system, either PTI or ETI, involving a variety of receptors and signal transduction pathways (Lee et al ., ; Wang H et al., ; Wang Q et al ., ; Xiang et al ., ; Yang et al ., ; Yu et al ., ). To determine which signalling pathway is involved in PpE4 ‐induced cell death, virus‐induced gene silencing (VIGS) was used to silence a series of genes in N. benthamiana , including genes responsible for R protein function, such as HSP90 , SGT1 and RAR1 (Kanzaki et al ., ; Takahashi et al ., ; Zhang et al ., ), genes associated with the activation of the TIR‐NB‐LRR (Toll/interleukin‐1 receptor, nucleotide binding and leucine‐rich repeat) and CC‐NB‐LRR (coiled coil, nucleotide binding and leucine‐rich repeat) R proteins, EDS1 and NDR1 (Knepper et al ., ; Oh et al ., ), respectively, the receptor‐like kinases BAK1 and SOBIR1 (Chaparro‐Garcia et al ., ; Liebrand et al ., , ), the transcription factors MYB1 and WRKY3 , and the MAPK cascade genes NPK , MEK1 , MEK2 and SIPK (Jin et al ., ; Liu et al ., ).…”

“…However, these effectors could also induce HR when detected by corresponding R proteins or recognized by unknown mechanisms in plants. To eliminate the influence of cell death during the inoculation process, we reduced the concentration of the Agrobacterium tumefaciens suspension to an OD 600 value of 0.01 to delay and weaken cell death (Wang H et al ., ). As a result, PpE4 enhanced infection when transiently or stably expressed in plants, in spite of its cell death‐inducing activity (Figs and ).…”

“…Many RXLR effectors have been demonstrated to promote pathogen colonization when transiently expressed in plants, such as AVR1 (Du et al ., a,b), PexRD2 (King et al ., ), PITG_22798 (Wang H et al ., ), Avh241 (Yu et al ., ) and Avh238 (Yang et al ., ). However, these effectors could also induce HR when detected by corresponding R proteins or recognized by unknown mechanisms in plants.…”

“…Many genes involved in plant immune signalling are required for effector‐induced cell death. For example, MEK2 is required for Avh238 ‐triggered cell death (Yang et al ., ), MEK2 and WIPK are involved in Avh241 ‐induced cell death (Yu et al ., ), suppressor of G2 allele of skp1 ( SGT1 ) is required for the cell death activity of PexRD2 (Oh et al ., ) and PITG_22798 (Wang H et al ., ), a specific mitogen‐activated protein kinase (MAPK) cascade is responsible for Pi_23226 ‐induced cell death (Lee et al ., ), and SGT1 , HSP90 , RAR1 and MAPK cascades are required for PvRXLR16 ‐induced cell death (Xiang et al ., ).…”

“…Another classic example is the triggering of an R gene‐mediated hypersensitive response (HR) by AVR effectors that typically exert their virulence function on R gene‐absent plants (Kamoun, ). In addition to known AVR effectors, a few RXLR effectors, such as PsAvh241 (Yu et al ., ), PsAvh238 (Wang Q et al ., ; Yang et al ., ) and PITG_22798 (Wang H et al ., ), possess virulence functions even though they induce immune response‐related cell death in plants.…”