Biology subjects
Publications and source records attributed to Lamichhane, T..
Withdrawal StatementThis manuscript has been withdrawn by the authors as it was submitted and made public without the full consent of all the authors. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author.
Immunity is generally considered critical for plant health against pathogen infection. Citrus Huanglongbing (HLB) caused by the phloem colonizing bacterial pathogen Candidatus Liberibacter 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 of flavodoxin (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 genes CsCalS7a, CsCalS7b, or CsCalS7c demonstrated 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 of Eds1 and Pad4 but not RbohB, Bik1 and Sobir1 abolished ROS production, phloem callose deposition, cell death and disease damages caused by Lpsy. This study provides genetic evidence for Ca. Liberibacter-triggered immune disease and reveals that tuning plant immune responses converts Ca. Liberibacter into a benign endophyte, providing a promising strategy for precision breeding to enhance resistance/tolerance against HLB.
Agrobacterium-mediated transient expression (TE) is an important tool in plant genetics study and biotechnology. Citrus is one of the top tree crops, but Agrobacterium-mediated TE remains problematic in citrus. Previous study has shown that pretreatment of citrus leaves with Xanthomonas citri subsp. citri (Xcc), which causes citrus canker in almost all citrus cultivars, significantly improves the TE efficacy. Here we have shown that Xcc promotes Agrobacterium- mediated TE via improving Agrobacterium growth, suppressing plant immune response, triggering cell division, and upregulating plant cell wall degrading enzymes. We demonstrate that Xcc promotes Agrobacterium-mediated TE via PthA and mutation of pthA4 abolishes the promoting effect of Xcc. PthA4 is known to trigger cell division of citrus. We have demonstrated the causative relationship between cell division and Agrobacterium-mediated TE using cell division inhibitor mimosine. Importantly, the differential effects of mimosine and colchicine on Xcc promoting Agrobacterium-mediated TE indicate that S-phase is critical for agroinfiltration of citrus tissues. In addition, PthA4 is known to upregulate plant growth hormones auxin (IAA), gibberellin and cytokinin, as well as cell degrading enzymes (e.g., cellulase). Exogenous application of IAA, cytokinin, and cellulase but not gibberellin significantly improved Agrobacterium-mediated TE in leaves of sweet orange, pummelo, Meiwa kumquat, lucky bamboo and rose mallow. Our study provides a mechanistic understanding regarding how Xcc promotes Agrobacterium-mediated TE and identified two practical measures to improve Agrobacterium-mediated TE via pretreatment with plant hormones (i.e., auxin and cytokinin) and cellulase.