Mechanistic dissection ofCandidatusLiberibacter Triggered Chronic Immune Disease

Immunity is generally considered critical for plant health against pathogen infection. Citrus Huanglongbing (HLB) caused by the phloem colonizing bacterial pathogenCandidatusLiberibacter asiaticus (CLas) was suggested to be a pathogen triggered chronic immune disease. However, the genetic evidence and mechanistic understanding for such a disease model is lacking. Here, we show CLas triggers phloem cell death, reactive oxygen species (ROS) production and callose deposition in photosynthesis tissues, but little or none in non-photosynthesis tissues of citrus. We further demonstrate that CLas triggers ROS production in chloroplasts. Overexpression offlavodoxin(Fld), an electron shuttle, in chloroplasts reduced ROS production, cell death and HLB symptoms, but not phloem callose deposition induced by CLas. Knockout, silencing, and overexpression of phloem callose synthase genesCsCalS7a,CsCalS7b, orCsCalS7cdemonstrated that phloem callose deposition also caused phloem cell death. Trunk injection with 2-deoxy-d-glucose, a callose deposition inhibitor, reduced fruit drop and increased fruit yield of HLB symptomatic trees in field trials. Using tomato-Ca. L. psyllaurous (Lpsy) as a model, knockout ofEds1andPad4but notRbohB,Bik1andSobir1abolished ROS production, phloem callose deposition, cell death and disease damages caused by Lpsy. This study provides genetic evidence forCa. Liberibacter-triggered immune disease and reveals that tuning plant immune responses convertsCa. Liberibacter into a benign endophyte, providing a promising strategy for precision breeding to enhance resistance/tolerance against HLB.