TheRalstonia solanacearumeffector RipAK suppresses plant hypersensitive response by inhibiting the activity of host catalases

Abstract

The destructive bacterial pathogen Ralstonia solanacearum delivers effector proteins via a type-III secretion system for its pathogenesis of plant hosts. However, the biochemical functions of most of these effectors remain unclear. RipAK of R. solanacearum GMI1000 is a type-III effector with unknown functions. Functional analysis demonstrated that in tobacco leaves, ripAK knockout bacteria produced an obvious hypersensitive response; also, infected tissues accumulated reactive oxygen species in a shorter period postinfection, compared with wild type. This strongly indicates that RipAK can inhibit hypersensitive response during infection. Further analysis showed that RipAK localizes to peroxisomes and interacts with host catalases (CATs) in plant cells. Truncation of 2 putative domains of RipAK caused it to fail to target the peroxisome and fail to interact with AtCATs, suggesting that RipAK localization depends on its interaction with CATs. Furthermore, heterologous expression of RipAK inhibited CAT activity in vivo and in vitro. Finally, compared with the ripAK mutant, infection with a bacterial strain overexpressing RipAK inhibited the transcription of many immunity-associated genes in infected tobacco leaves at 2- and 4-hr postinfection, although mRNA levels of NtCAT1 were upregulated. These data indicate that GMI1000 suppresses hypersensitive response by inhibiting host CATs through RipAK at early stages of infection.

Oh, the places they'll go! A survey of phytopathogen effectors and their host targets

“…Although underrepresented, the major targets of Ralstonia effectors identified to date are distinct from those of P. syringae and Xanthomonas and are associated with 'Metabolism' (52%), followed by 'Transcription' (19%), 'Signalling' (14%), and 'Protein Processing' (14%) (Figure 3d). The 'Metabolism' related targets in Ralstonia mainly represent 'Oxidoreductases' (48%), in particular h-type thioredoxins targeted by RipAY and catalases targeted by RipAK (CAT2 and CAT3), which act as ROS scavengers to degrade H 2 O 2 (Fujiwara et al, 2016;Sang et al, 2016;Sun et al, 2017).…”

Suppression of PAMP-triggered immunity (PTI) by effector proteins synthesized by phytopathogens and delivered into cells of infected plant

“…Produktami tej reakcji są dwa nowe koenzymy, a mianowicie 3'-NADP i 3'-NAADP, które przypuszczalnie blokują reakcje zależne od 2'-NAD(H) i 2'-NADP(H). Bakteria Ralstonia solanacearum syntetyzuje białko efektorowe RipAK, które w liściach tytoniu migruje do peroksysomów, gdzie hamuje aktywność katalazy, powodując hamowanie reakcji typu HR we wczesnej fazie infekcji [55]. Innym efektorem syntetyzowanym przez R. solanacearum jest białko RipAY z aktywnością γ-L-glutamylo cyklotransferazy (GGCT) metabolizującej glutation (GSH) [56].…”

Plant–microbe interactions: organelles and the cytoskeleton in action

“…Alternatively, other effectors are targeted to the nucleus, where they alter host gene expression. HopG1 localizes to mitochondria and interferes with unknown target(s) to inhibit plant immunity (Block et al, 2010), whereas the Ralstonia solanacearum effector RipAK targets catalase activity in the peroxisome (Sun et al, 2017). Effectors are indicated in red.…”

“…Peroxisomes play an important role during immunity. Specifically, peroxisome-localized catalases (CATs) scavenge ROS by degrading H 2 O 2 and thus enhance immunity (Zhang et al, 2015;Sun et al, 2017). Two effector proteins from the oomycete Phytophthora sojae, PsCRN63 and PsCRN115, interact with N. benthamiana CAT1 at peroxisomes to suppress host immune responses (Zhang et al, 2015).…”

“…The Ralstonia solanacearum effector RipAK is targeted to peroxisomes and interacts directly with CATs in Arabidopsis. This interaction is critical for the suppression of early defense responses (Sun et al, 2017).…”