HPE21, a liberibacter effector modulating H2O2 accumulation in tomato plants

' Candidatus Liberibacter solanacearum' is a phloem-limited Gram-negative bacterium transmitted by psyllids. Several haplotypes have been reported worldwide infecting different plants including solanaceous and apiaceous crops. In the United States of America, haplotypes A and B severely impact tomato and potato production. While the molecular mechanisms at play during ' Ca . L. solanacearum' infection remain unclear, this pathogen might manipulate its host plants by secreting effector proteins. Effectors that induce or suppress plant immunity can provide insights into the pathogen-host plant interactions leading to plant infection, but to date, ' Ca . L. solanacearum' effectors inducing immunity remain poorly studied. HPE21 is a ' Ca . L. solanacearum' effector previously reported as inducing reactive oxygen species in plants. Here, it is confirmed that HPE21 induced H 2 O 2 accumulation in Nicotiana benthamiana. It was also discovered that this haplotype B-specific effector induced cell death in N. benthamiana and interacted with tomato aldo-keto reductase 1 (SlAKR1). Furthermore, the expression of SlAKR1 decreased HPE21-induced H 2 O 2 accumulation in N . benthamiana . Finally, it was demonstrated that H 2 O 2 accumulates in a haplotype-dependent manner in ' Ca . L. solanacearum'-infected plants. These results suggest that oxidative stress might play a key role in disease progression in ' Ca . L. solanacearum'-infected plants, and the interaction between HPE21 and SlAKR1 can be involved in the pathogenicity differences between the ' Ca . L. solanacearum' haplotypes A and B. • HPE21, 'Candidatus Liberibacter solanacearum' haplotype B–specific effector, induces H 2 O 2 and cell death in Nicotiana benthamiana. • HPE21 interacts with tomato aldo-keto reductase 1 (SlAKR1). • Coexpression of HPE21 and SlAKR1 reduces HPE21-induced plant immunity. • Lso haplotypes lead to distinct H 2 O 2 accumulation, linking to different pathogenicity between 'Candidatus Liberibacter solanacearum' haplotypes A and B.