Chloroplast-Associated ROS Signaling Shapes Tissue-Specific Pathogenicity Mechanisms of 'Candidatus Liberibacter asiaticus' in Leaf and Root.

Abstract

The phloem-colonizing bacterial pathogen 'Candidatus Liberibacter asiaticus' (CLas) causes the devastating citrus huanglongbing (HLB). Despite the important roles of roots, how CLas affects roots remains largely unknown. Here, we dissected host responses in leaf and root at different time points after CLas inoculation. Our data showed that CLas triggers less reactive oxygen species (ROS) accumulation and phloem callose deposition, which are known to contribute to HLB symptom development in leaves, in roots than in leaves. Because photosynthetically active chloroplasts are abundant in leaf tissues, but absent in roots, and serve as major hubs of pathogen-induced ROS production, elevated chloroplast-associated ROS likely contributes to the greater ROS accumulation observed in leaves during CLas infection. qRT-PCR analyses in roots and leaves, together with root RNA-seq analysis and comparison with previously reported leaf transcriptomic datasets, showed global induction of immune-related pathways in both roots and leaves, whereas photosynthesis-related pathways were only downregulated in leaves, but not in roots, by CLas infection. The differential expression pattern related to immunity and photosynthesis in the leaf and root tissues suggests that chloroplasts might be involved in the differences in ROS accumulation and callose deposition. Root cell death was observed in HLB-symptomatic trees regardless of the CLas infection status of roots. Significantly more starch accumulation was first observed in CLas-positive leaves than in healthy controls, followed by starch depletion in CLas-positive roots later, suggesting a causative relationship between them. Taken together, root cell death caused by CLas infection may be caused by carbon starvation rather than via a similar mechanism as in leaves. Our data demonstrate that chloroplast-associated ROS accumulation plays a critical role in the tissue-specific host responses to CLas infection and provide a framework for future investigations into the mechanisms underlying HLB pathogenesis. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.