Abstract
BackgroundDiverse plant pathogens deliver effectors into plant cells to alter host processes. Oomycete pathogen encodes a large number of putative RxLR effectors which are likely to play a role in manipulating plant defense responses. The secretome of Plasmopara viticola (downy mildew of grapevine) contains at least 162 candidate RxLR effectors discovered in our recent studies, but their roles in infection and pathogenicity remain to be determined. Here, we characterize in depth one of the putative RxLR effectors, PvRxLR16, which has been reported to induce cell death in Nicotiana benthamiana in our previous study.ResultsThe nuclear localization, W/Y/L motifs, and a putative N-glycosylation site in C-terminal of PvRxLR16 were essential for cell death-inducing activity. Suppressor of G-two allele of Skp1 (SGT1), heat shock protein 90 (HSP90) and required for Mla12 resistance (RAR1), but not somatic embryogenesis receptor-like kinase (SERK3), were required for the cell death response triggered by PvRxLR16 in N. benthamiana. Some mitogen-activated protein kinases and transcription factors were also involved in the perception of PvRxLR16 by N. benthamiana. PvRxLR16 could also significantly enhance plant resistance to Phytophthora capsici and the nuclear localization was required for this ability. However, some other PvRxLR effectors could suppress defense responses and disease resistance induced by PvRxLR16, suggesting that it may not trigger host cell death or immune responses during physiological infection under natural conditions.ConclusionThese data demonstrate that PvRxLR16 may be recognized by endogenous proteins in nucleus to trigger immune responses in N. benthamiana, which in turn can be suppressed by other PvRxLR effectors.
Highlights
Diverse plant pathogens deliver effectors into plant cells to alter host processes
Expression of the PvRxLR16 did not trigger cell death in these two plant species. These results demonstrated that PvRxLR16 may be recognized by an endogenous disease resistance protein, resulting in avirulence on N. benthamiana
Since P. viticola can hardly be genetically modified because of its typical obligate lifestyle, it is difficult to determine whether PvRxLR16 is essential for the pathogenicity of P. viticola during infection. This is the first study to report that a putative RxLR effector from P. viticola is recognized by the plant immune system and even may act as an avirulence gene in N. benthamiana
Summary
Diverse plant pathogens deliver effectors into plant cells to alter host processes. Oomycete pathogen encodes a large number of putative RxLR effectors which are likely to play a role in manipulating plant defense responses. All the Avr proteins from oomycetes, with one exception (ATR5), contain an N-terminal signal peptide which can direct the effector to the outside of the pathogen, followed by RxLR-EER motifs that are responsible for transporting effector into the interior of host cells [23, 25]. These effectors carrying RxLR motifs are defined as RxLR effectors which have been extensively studied in recent years [26]. The important discovery of RxLR effectors in oomycetes has expedited the identification process of avirulence genes [12]
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