An Engineered Distant Homolog of Pseudomonas syringae TTSS Effector From Physcomitrella patens Can Act as a Bacterial Virulence Factor

Abstract

Pseudomonas syringae pv. phaseolicola is the causative agent of halo blight in common bean (Phaseolus vulgaris). Similar to other pathogenic gram-negative bacteria, it secrets a set of type III effectors into host cells to subvert defense mechanisms. HopQ1 (for Hrp outer protein Q) is one of these type III effectors contributing to virulence of bacteria. Upon delivery into a plant cell, HopQ1 undergoes phosphorylation, binds host 14-3-3 proteins and suppresses defense-related signaling. Some plants however, evolved systems to recognize HopQ1 and respond to its presence and thus to prevent infection. HopQ1 shows homology to Nucleoside Hydrolases (NHs), but it contains a modified calcium binding motif not found in the canonical enzymes. CLuster ANalysis of Sequences (CLANS) revealed that HopQ1 and alike proteins make a distinct group of putative NHs located distantly from the classical enzymes. The HopQ1 – like protein (HLP) group comprises sequences from plant pathogenic bacteria, fungi, and lower plants. Our data suggest that the evolution of HopQ1 homologs in bacteria, fungi, and algae was independent. The location of moss HopQ1 homologs inside the fungal clade indicates a possibility of horizontal gene transfer (HGT) between those taxa. We identified a HLP in the moss Physcomitrella patens. Our experiments show that this protein (referred to as PpHLP) extended by a TTSS signal of HopQ1 promoted P. syringae growth in bean and was recognized by Nicotiana benthamiana immune system. Thus, despite the low sequence similarity to HopQ1 the engineered PpHLP acted as a bacterial virulence factor and displayed similar to HopQ1 virulence properties.

Are fungi‐derived genomic regions related to antagonism towards fungi in mosses?

“…On the other hand, some other fungi‐derived genes might have been recruited by mosses to regulate or counteract the activities of fungi or other microbes, in addition to their roles in other processes. This second possibility is in line with the role of the HLP gene in plant antimicrobial responses (Piechocki et al ., 2018) and supported by similar cases documented in the literature (Chou et al ., 2015; Di Lelio et al ., 2019). For example, endogenous viral DNAs are known to contribute significantly to the immunity of host animals against viral infection (Aswad & Katzourakis, 2012).…”

“…HLPs contain an inosine‐uridine nucleoside hydrolase domain and are homologous to the type III secretion system effector HopQ1 of bacterial pathogens (Li et al ., 2013). The HLP in P. patens only shares 24–30% sequence identity with bacterial HopQ1, but it was shown to induce plant immune responses when ectopically expressed in the bacterium Pseudomonas syringae (Piechocki et al ., 2018). This evidence suggests the HLP genes in fungi are probably involved in interactions with host plants.…”

Formation of HopQ1:14-3-3 complex in the host cytoplasm modulates nuclear import rate of Pseudomonas syringae effector in Nicotiana benthamiana cells

“…HopQ1 was shown to promote bacterial speck disease of tomato and halo blight of bean ( Wei et al., 2007 ; Ferrante et al., 2009 ; Li et al., 2013a ); but it is also produced by P. syringae strains identified as causal agents of recurring epidemics of chestnut or kiwifruit bleeding cankers ( Green et al., 2010 ; Choi et al., 2017 ). Interestingly, our studies ( Piechocki et al., 2018 ) revealed existence of putative HopQ1 homologs in other pathogenic bacteria and fungi but surprisingly also in lower plants and algae, including Aureococcus anophagefferens a dominant species responsible for destructive brown tide blooms ( Gobler and Sunda, 2012 ). HopQ1 – like proteins (HLPs) show overall homology to nucleoside hydrolases, but due to alterations in the predicted catalytic center compared to the consensus sequence were classified as a distinct family ( Piechocki et al., 2018 ).…”

“…Interestingly, our studies ( Piechocki et al., 2018 ) revealed existence of putative HopQ1 homologs in other pathogenic bacteria and fungi but surprisingly also in lower plants and algae, including Aureococcus anophagefferens a dominant species responsible for destructive brown tide blooms ( Gobler and Sunda, 2012 ). HopQ1 – like proteins (HLPs) show overall homology to nucleoside hydrolases, but due to alterations in the predicted catalytic center compared to the consensus sequence were classified as a distinct family ( Piechocki et al., 2018 ). HopQ1 was shown to alter purine metabolism and activate cytokinin signaling ( Li et al., 2013b ; Hann et al., 2014 ) whereas recent studies revealed that XopQ, a HopQ1 homolog from Xanthomonas euvesicatoria exerts 2’,3’-cAMP/cGMP phosphodiesterase activity, which is possibly employed by bacteria to hydrolyze signaling molecules produced by a TIR domain of TNL subclass of plant resistance receptors ( Yu et al., 2022 ).…”