Proteasome-associated HECT-type ubiquitin ligase activity is required for plant immunity

Abstract

Regulated degradation of proteins by the 26S proteasome plays important roles in maintenance and signalling in eukaryotic cells. Proteins are marked for degradation by the action of E3 ligases that site-specifically modify their substrates by adding chains of ubiquitin. Innate immune signalling in plants is deeply reliant on the ubiquitin-26S proteasome system. While progress has been made in understanding substrate ubiquitination during plant immunity, how these substrates are processed upon arrival at the proteasome remains unclear. Here we show that specific members of the HECT domain-containing family of ubiquitin protein ligases (UPL) play important roles in proteasomal substrate processing during plant immunity. Mutations in UPL1, UPL3 and UPL5 significantly diminished immune responses activated by the immune hormone salicylic acid (SA). In depth analyses of upl3 mutants indicated that these plants were impaired in reprogramming of nearly the entire SA-induced transcriptome and failed to establish immunity against a hemi-biotrophic pathogen. UPL3 was found to physically interact with the regulatory particle of the proteasome and with other ubiquitin-26S proteasome pathway components. In agreement, we demonstrate that UPL3 enabled proteasomes to form polyubiquitin chains, thereby regulating total cellular polyubiquitination levels. Taken together, our findings suggest that proteasome-associated ubiquitin ligase activity of UPL3 promotes proteasomal processivity and is indispensable for development of plant immunity.

The effector GpRbp‐1 of Globodera pallida targets a nuclear HECT E3 ubiquitin ligase to modulate gene expression in the host

“…Our data show that although AtUPL3 may not have a significant effect on the establishment of nematode infection in Arabidopsis , female growth and syncytium size, it has a major impact on the transcriptome in nematode‐infected roots of Arabidopsis . Interestingly, a parallel can be drawn with the findings of Furniss et al () where there is a disparity between the relatively small effect of UPL3 on the development of disease symptoms induced by P. syringae pv. maculicola and the major impact on UPL3‐dependent transcriptomic modulation by the exogenous application of SA to Arabidopsis plants.…”

“…Our results may therefore point towards a previously undescribed strategy to manipulate plant cells by nematodes and other plant pathogens. Recently, UPL3 has been shown to function as a proteasome‐associated amplifier of immune responses activated by salicylic acid (SA) (Furniss et al , ). Interaction of GpRbp‐1 with StUPL3 may therefore interfere with proteasome‐dependent ubiquitination to suppress plant immunity.…”

“…In conclusion, our results suggest that nematode effector GpRbp‐1 may manipulate the ubiquitin‐proteasome machinery of the host to modulate plant immune responses. On SA treatment, UPL3 has a large impact on total cellular polyubiquitination, suggesting that it may function as a E4 ubiquitin ligase by ‘promiscuous’ extension of polyubiquitination with low substrate specificity (Furniss et al , ). Further characterization of the substrate(s) for ubiquitination by UPL3, their subcellular localization and their roles in plant–nematode interactions will lead to additional knowledge on how sedentary nematodes manipulate the (nuclear) ubiquitination machinery of the host to promote susceptibility.…”

Comprehensive Genomic Survey, Characterization and Expression Analysis of the HECT Gene Family in Brassica rapa L. and Brassica oleracea L.

“…Five and 28 HECT genes have been identified in the genomes of Saccharomyces cerevisiae and human, respectively [13]. In Arabidopsis , seven HECT genes ( UPL1 – UPL7 ) were identified [14], of which UPL3 was shown to be involved in trichome development [14] and genome endoreduplication [15], UPL5 in leaf senescence [16], and UPL1 , UPL2 and UPL5 in plant immunity [17]. These seven Arabidopsis thaliana HECT genes were classified into four subfamilies according to both HECT domain sequence similarity and the presence of additional domains [14].…”

Ubiquitylome Analysis Reveals a Central Role for the Ubiquitin-Proteasome System in Plant Innate Immunity

“…Agrobacterial infection in tomato (Liu et al, 2012b), which might explain the connection between CUL4 and immune elicitation, while UPL5 is required for SA-and NONEXPRESSER OF PR GENES 1 (NPR1)-mediated plant immunity (Furniss et al, 2018c).…”