Differential Suppression of Nicotiana benthamiana Innate Immune Responses by Transiently Expressed Pseudomonas syringae Type III Effectors

Abstract

The plant pathogen Pseudomonas syringae injects about 30 different virulence proteins, so-called effectors, via a type III secretion system into plant cells to promote disease. Although some of these effectors are known to suppress either pattern-triggered immunity (PTI) or effector-triggered immunity (ETI), the mode of action of most of them remains unknown. Here, we used transient expression in Nicotiana benthamiana, to test the abilities of type III effectors of Pseudomonas syringae pv. tomato (Pto) DC3000 and Pseudomonas syringae pv. tabaci (Pta) 11528 to interfere with plant immunity. We monitored the sequential and rapid bursts of cytoplasmic Ca2+ and reactive oxygen species (ROS), the subsequent induction of defense gene expression, and promotion of cell death. We found that several effector proteins caused cell death, but independently of the known plant immune regulator NbSGT1, a gene essential for ETI. Furthermore, many effectors delayed or blocked the cell death-promoting activity of other effectors, thereby potentially contributing to pathogenesis. Secondly, a large number of effectors were able to suppress PAMP-induced defense responses. In the majority of cases, this resulted in suppression of all studied PAMP responses, suggesting that these effectors target common elements of PTI. However, effectors also targeted different steps within defense pathways and could be divided into three major groups based on their suppressive activities. Finally, the abilities of effectors of both Pto DC3000 and Pta 11528 to suppress plant immunity was conserved in most but not all cases. Overall, our data present a comprehensive picture of the mode of action of these effectors and indicate that most of them suppress plant defenses in various ways.

Characterization of Three Fusarium graminearum Effectors and Their Roles During Fusarium Head Blight

“…In contrast, we previously demonstrated that F. graminearum FgARB93B suppresses ROS induced by both flg22 and chitin (Hao et al, 2019). Similarly, previous reports have shown that some effectors from Pseudomonas syringae differently suppressed ROS induced by flg22 in comparison to chitin, whereas others suppress ROS induced by flg22 and chitin in a similar manner (Gimenez-Ibanez et al, 2018). However, it is unknown how the fungal and bacterial effectors evolve the specificity to suppress ROS triggered by different elicitors.…”

Ralstonia solanacearum Type III Effectors with Predicted Nuclear Localization Signal Localize to Various Cell Compartments and Modulate Immune Responses in Nicotiana spp.

“…Sup-pressor of G2 allele of skp1 (SGT1) is a major stabilizing factor of NLR proteins and is required for T3E-triggered HR (Kadota et al, 2010;Peart et al, 2002). To investigate the SGT1 requirement for RipA1-or RipE1-induced cell death, NbSGT1 expression was knocked-down using VIGS (Choi et al, 2017;Gimenez-Ibanez et al, 2018;Peart et al, 2002). As observed in non-silenced N. benthamiana plants, RipA1 or RipE1 expression triggered cell death in empty vector (EV)-silenced plants ( Fig.…”

“…Similar to RipA1, several Pseudomonas syringae T3Es induce cell death independent of NbSGT1 (Choi et al, 2017;Gimenez-Ibanez et al, 2018). Although SGT1 might not be required for every NLR function, it is possible that the cell death observed following expression of these T3Es is not related to their recognition by R protein but rather indicative of a form of cellular toxicity.…”

Comprehensive Identification of PTI Suppressors in Type III Effector Repertoire Reveals that Ralstonia solanacearum Activates Jasmonate Signaling at Two Different Steps

“…Similar comprehensive screenings for bacterial effectors that suppress PTI have been performed in Pto and Xanthomonas euvesicatoria . In Pto, seven out of the 22 effectors tested (32%) can suppress flg22-triggered ROS burst and expression of PTI marker genes in N. benthamiana [33]. In X. euvesicatoria , 17 out of the 33 effectors tested (52%) show the ability to suppress flg22-triggered PTI signaling in N. benthamiana [34].…”

“…For example, HopX1 from Pto DC3000 (HopX1 Pto ) does not show the protease activity in N. benthamiana [27]. For PTI suppression, HopX1 Pta and HopX1 Pto do not suppress flg22-induced ROS burst and expressions of PTI marker genes in N. benthamiana [33]. X. euvesicatoria possesses two HopX1 family effectors, XopE1 and XopE2 [35].…”