AKSF1 Isolated From the Rice-Virulent Strain Acidovorax avenae K1 Is a Novel Effector That Suppresses PAMP-Triggered Immunity in Rice

Abstract

Microbial pathogens deliver effectors into plant cells to suppress plant immune responses and modulate host metabolism in order to support infection processes. We sought to determine if the Acidovorax avenae rice-virulent K1 strain can suppress pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) induced by flagellin isolated from the rice-avirulent N1141 strain. The flagellin-triggered PTI, including H2O2 generation, callose deposition, and expression of several immune-related genes were strongly suppressed in K1 pre-inoculated cultured rice cells in a Type III secretion system (T3SS)-dependent manner. By screening 4,562 transposon-tagged mutants based on their suppression ability, we found that 156 transposon-tagged K1 mutants lost the ability to suppress PTI induction. Mutant sequence analysis, comprehensive expression analysis using RNA-sequencing, and the prediction of secretion through T3SS showed that a protein named A. avenae K1 suppression factor 1 (AKSF1) suppresses flagellin-triggered PTI in rice. Translocation of AKSF1 protein into rice cells is dependent on T3SS during infection, an AKSF1-disruption mutant lost the ability to suppress PTI responses, and expression of AKSF1 in AKSF1-disruption mutant complemented the suppression activity. When AKSF1-disruption mutants were inoculated into the host rice plant, reduction of the disease symptoms and suppression of the bacterial growth were observed. Taken together, our results demonstrate that AKSF1 is a novel effector that can suppress the PTI in host rice plant.

Novel Effector RHIFs Identified From Acidovorax avenae Strains N1141 and K1 Play Different Roles in Host and Non-host Plants

“…Moreover, sequence analysis of regions flanking the transposons revealed that they were inserted into 68 different genes. In addition, multiple proteins were secreted by strains N1141 and K1 using their T3SS (Kondo et al, 2017;Kawaguchi et al, 2021). The presence of multiple effectors in A. avenae strains N1141 and K1 raises the question of whether NRHIF and KRHIF are the only ETI-inducible effectors for rice and finger millet, respectively.…”

“…When rice plants were inoculated with AKSF1-disruption mutants, disease symptom severity was reduced and bacterial growth was suppressed. These results indicate that AKSF1 is a novel effector that can suppress PTI in host rice plants (Kawaguchi et al, 2021). Interestingly, whole-genome sequencing of strain N1141 revealed that it lacks AKSF1.…”

“…We reported that inoculation of rice with the rice-avirulent strain N1141 of A. avenae caused ETI responses, including HR cell death, accompanied by loss of plasma membrane permeability, nuclear DNA fragmentation, and upregulation of ETI-related genes, including OsNAC4, which positively regulates HR cell death (Che et al, 1999;Kaneda et al, 2009). In contrast, the rice-virulent strain K1 can suppress PAMP-triggered immunity (PTI) caused by N1141 flagellin but cannot induce ETI responses in rice (Kawaguchi et al, 2021). Interestingly, a T3SS-deleted N1141 mutant (NΔT3SS) did not cause ETI responses, while the T3SS-deleted K1 mutant (KΔT3SS) was defective in PTI suppression (Kondo et al, 2012;Kawaguchi et al, 2021).…”

“…In contrast, the rice-virulent strain K1 can suppress PAMP-triggered immunity (PTI) caused by N1141 flagellin but cannot induce ETI responses in rice (Kawaguchi et al, 2021). Interestingly, a T3SS-deleted N1141 mutant (NΔT3SS) did not cause ETI responses, while the T3SS-deleted K1 mutant (KΔT3SS) was defective in PTI suppression (Kondo et al, 2012;Kawaguchi et al, 2021). These results indicate that strains N1141 and K1 have multiple effectors with different functions in rice.…”

Delivering the pain: an overview of the type III secretion system with special consideration for aquatic pathogens

“…Effectors from T3SS1 are generally detected earlier in the infection process and it has been suggested that they are more involved with the earlier stages of the vacuole maturation while effectors from T3SS2 are more involved with later stages [ 87 ]. Finally, experiments using mutants of Acidovorax avenae identified an effector protein ( A. avenae K1 suppression factor 1) that was associated with supressing the cellular response following recognition of flagellin associated molecular patterns [ 88 ].…”