Grapevine downy mildew, caused by the oomycete Plasmopara viticola, is one of the most considerable production challenges for the grape and wine industries. P. viticola injects a plethora of effectors into host cells to disrupt immune processes, but the mechanisms by which these effectors act at the molecular level have not been well characterized. Herein, we show that a candidate P. viticola Avirulence Homolog (Avh) RxLR effector gene, designated PvAvh77, was strongly up-regulated during the initial stages of P. viticola infection in Vitis vinifera. Further experiments demonstrated that PvAvh77 could trigger nonspecific cell death when expressed in the wild grapevine Vitis riparia and in tobacco (Nicotiana benthamiana and Nicotiana tabacum). We also found that a truncated form of PvAvh77, designated PvAvh77-M2, was more active in inducing cell death in N. benthamiana and V. riparia than full-length PvAvh77. Furthermore, PvAvh77-induced cell death required BAK1/SOBIR1, NDR1, SGT1, HSP90, and RAR1, but not EDS1 or MAPK pathway-related genes. Ectopic expression of PvAvh77 in Vitis vinifera cv. Thompson Seedless leaves neutralized host immunity and enhanced colonization by P. viticola, and the immune-inhibiting activity of PvAvh77 on susceptible Eurasian grapevine depended on its nuclear localization. Agrobacterium-mediated transient expression of PvAvh77 in N. benthamiana leaves also interfered with the immune response triggered by flg22. Using a yeast signal sequence trap approach, we showed that the signal peptide of PvAvh77 is functional in yeast. Moreover, PvAvh77 with a signal peptide stimulated plant immunity responses in the apoplast. Notably, application of exogenous purified PvAvh77-M2 effectively initiated defense responses in grapevines, as evidenced by increased accumulation of salicylic acid (SA) and H2O2, and reduced infection of inoculated P. viticola. In summary, we identified a novel effector, PvAvh77, from P. viticola, which has the potential to serve as a plant immunity inducer.

Grapevine downy mildew, caused by Plasmopara viticola, is one of the most devastating diseases in viticulture. Plasmopara viticola secretes RxLR effectors to modulate immune responses in grapevine. Here, we report an RxLR effector RxLR50253 from P. viticola that can interfere with plant immune response and thus promote pathogen colonization. RxLR50253 was induced at an early stage of P. viticola infection and could suppress elicitor (INF1 and Bax)-triggered cell death. RxLR50253 promote pathogen colonization in both tobacco and grapevine leaves. VpBPA1 was found to be the host target of RxLR50253 by yeast two-hybrid screening, and interaction between RxLR50253 and VpBPA1 was confirmed by multiple in vivo and in vitro assays. Further analysis revealed that VpBPA1 promoted pathogen colonization and decreased H 2 O 2 accumulation in transgenic tobacco and grapevine, while there was enhanced resistance and H 2 O 2 accumulation in NbBPA1-silenced Nicotiana benthamiana leaves. Moreover, transient expression of VpBPA1 in NbBPA1silenced N. benthamiana leaves could reduce the accumulation of H 2 O 2 . Experiments in vivo demonstrated that RxLR50253 inhibits degradation of VpBPA1. Taken together, our findings showed that RxLR50253 targets and stabilizes VpBPA1 to attenuate plant immunity through decreasing H 2 O 2 accumulation during pathogen infection.

Grapevine (Vitis spp.) is globally one of the most economically important fruit crops. China is the largest grapevine-growing country of the world and Shaanxi province is one of the major grapevine-growing provinces in the country. A survey of GLRaV-3 found it widespread, with 57–100% infection frequencies, in both wine and table grapevine cultivars of three grapevine-growing regions of Shaanxi province. The virus infection frequencies varied with cultivars and regions. In order to obtain the full genomic length of a new GLRaV-3 isolate, GLRaV-3-Sau (accession number MK988555), was sequenced. This isolate has a genome of 18026 nucleotides, and 14 open reading frames (ORFs). The full-genome of the isolate GLRaV-3-Sau shared 85.88% nucleotide identity to GLRaV-3-LN, another isolate found in China. Coat protein (CP) genes of GLRaV-3 isolates were identical (99%) to the Vitis vinifera isolate (accession number HQ185608.1) from the USA. Immunohistochemistry for virus localization found that distribution patterns were similar in red-berried cultivar ‘Cabernet Sauvignon’ and white-berried cultivar ‘Chardonnay’, and GLRaV-3 is restricted in phloem tissue of vascular bundles. Virus transmission by micrografting found virus transmission efficiency was higher in ‘Chardonnay’ and ‘Thompson Seedless’ than in ‘Hunan-1’, indicating that ‘Hunan-1’ was less sensitive to GLRaV-3. As far as we know, these are the most comprehensive comparisons on the genome and CP genes of GLRaV-3 worldwide and the first to have found that the grapevine ‘Hunan-1’ is less susceptible to GLRaV-3.