NONEXPRESSOR OF PATHOGENESIS-RELATED GENEScontrol Huanglongbing tolerance by regulating immune balance in citrus plants

Abstract

Huanglongbing (HLB) is a devastating citrus disease caused by the phloem-resident bacterial pathogen Candidatus liberibacter asiaticus (CLas). CLas infection of susceptible varieties triggers unbalanced immune responses, leading to overaccumulation of callose and reactive oxygen species (ROS), which in turn causes phloem plugging and HLB symptom development. Interestingly, some citrus relatives exhibit little or no symptoms in the presence of CLas, a phenomenon termed HLB tolerance. Moreover, overexpression of the Arabidopsis thaliana NPR1 (AtNPR1) gene in susceptible varieties has been shown to confer robust HLB tolerance. However, the mechanisms underlying HLB tolerance remain enigmatic. Here, we show that overexpression of AtNPR1 suppresses CLas- and Pseudomonas syringae pv. maculicola ES4326 (Psm)-induced overaccumulation of callose and ROS in citrus and Arabidopsis, respectively. Importantly, we found that knocking out of the Arabidopsis negative immune regulators, AtNPR3 and AtNPR4, and silencing of their Citrus sinensis ortholog CsNPR3, similarly suppress Psm- and CLas-induced callose and ROS overaccumulation, respectively, and that silencing of CsNPR3 also enhances HLB tolerance. These results reveal a conserved role of the NPR1/NPR3/NPR4-mediated signaling pathway in regulating plant immune balances and provide mechanistic support for overexpression of AtNPR1 or silencing of AtNPR3/AtNPR4 orthologs in citrus as a long-term solution to the HLB disease.

Homobrassinolide Delays Huanglongbing Progression in Newly Planted Citrus Trees

“…However, it has been discovered that overexpression of the AtNPR1 gene from Arabidopsis thaliana in susceptible varieties confers robust HLB tolerance [33]. The overexpression of AtNPR1 suppresses the overaccumulation of callose and ROS induced by CLas in citrus and Arabidopsis, respectively [34]. The function of NPR1 is centered on its interaction with SA, where NPR1 acts as an SA receptor, promoting redox changes that convert NPR1 from an oligomeric complex to monomers that move to the nucleus.…”

Plant volatile-triggered defense in citrus against biotic stressors

“…Furthermore, studying both reference genomes will allow us to determine which parental contributes more to the specific defensive response, highlighting the relevance of one over the other in activating defense mechanisms against herbivores and providing valuable data for future rootstock development programs. Furthermore, considering the increasing importance in recent years of limiting callose accumulation as a result of an over-immunity response in citrus ( Hijaz et al., 2020 ; Ma et al., 2022 ; Nehela and Killiny, 2023 ; Sarkar et al., 2024 ), we biochemically measured whether exposure to (Z)-3-HP influences callose deposition and, consequently, the activity of β-1,3-Glucanase. Lastly, we investigated whether exposure to (Z)-3-HP influences plant selection by pests and natural enemies and if it can reduce infestations by two citrus key pests.…”