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SIB1-SEC23A undergo ER to chloroplast relocalization to mediate immunity in Arabidopsis thaliana

ABSTRACT: Inter-organellar communication has emerged as a critical factor in maintaining cellular homeostasis under stress conditions. Chloroplasts function not only as central organelles for energy production but are also increasingly recognized as stress sensors and signal integrators. SIGMA FACTOR-BINDING PROTEIN 1 (SIB1), encoded by a nuclear gene, has been identified as a positive regulator of plant immunity, with localization in both chloroplasts and the nucleus. In this study, we identified Arabidopsis SEC23A, a COPII complex component known to mediate membrane trafficking between the endoplasmic reticulum (ER) and the Golgi apparatus, as a novel interactor of SIB1. Our findings reveal that under normal conditions, both SIB1 and SEC23A localize to the ER, while SIB1 is also localized in the nucleus. Upon ER stress and/or treatment with the immunity inducer salicylate, both proteins are relocated from the ER to the chloroplasts. Notably, SEC23A, similar to SIB1, also functions as a positive regulator of disease resistance. In response to pathogen infection, SIB1 and SEC23A downregulate expression of chloroplast- and nucleus-encoded genes associated with photosynthesis while enhancing expression of defense-related genes. Together, our findings reveal a previously uncharacterized pathway of ER–chloroplast communication mediated by SIB1 and SEC23A during plant stress responses and immunity, providing novel insights into the intricate regulatory networks that govern inter-organellar communication under stress in plants.

ORGANISM(S): Arabidopsis thaliana

PROVIDER: GSE300417 | GEO | 2026/04/13

REPOSITORIES: GEO

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Project description:Aim: To identify regulatory factors that control: (1) chloroplast protein importand (2) chloroplast-to-nucleus signalling. This project is a joint proposal from the Jarvis lab which is interested in chloroplast protein import [1] and the Moller lab which is interested in plastid-to-nucleus signalling [2]. Background: The majority of chloroplast proteins are encoded in the nucleus and imported post-translationally into chloroplasts. The abundance of chloroplast proteins may therefore be regulated at multiple levels. It is well documented that the nuclear gene expression is responsive to (largely unknown) signals from the chloroplast [23] and evidence is now emerging that protein import is also a regulated process [1]. Protein import into chloroplasts is mediated by protein complexes in the outer and inner envelope membranes called Toc and Tic respectively. Biochemical studies of pea chloroplasts identified several Toc/Tic components. These proteins are mechanistic or structural components of the import apparatus. Arabidopsis homologues of the pea Toc/Tic proteins were identified by the AGI. Pea Toc34 is represented in Arabidopsis by two genes "Toc33 and Toc34" and pea Toc75 is represented by three genes. These different Tocs have different expression patterns and are proposed to have different precursor protein recognition specificities. The factors that regulate Toc expression in concert with the needs of plastids in developmentally different cells are unknown. Proposal: Two Arabidopsis mutants will be analysed. The ppi1 mutant is null for the putative precursor protein receptor Toc33 [1]and the ppi3 mutant is null for a putative component of the protein import channel Toc75-IV (on chromosome IV). ppi1 plants are yellow-green in appearance but remarkably healthy and grow only slightly more slowly than wild type. By contrast ppi3 plants are indistinguishable from wild type by eye although analysis of the mutant's chloroplast proteome is beginning to reveal some differences (K. Lilley personal communication). Gene expression changes in ppi1 are likely to be quite extensive. Retardation of chloroplast development in ppi1 will activate retrograde signalling pathways so that many nuclear photosynthetic genes are down-regulated. Changes in the expression of photosynthetic genes and of the genes responsible for mediating these responses may therefore be observed. Any regulatory and signalling genes identified will be of interest to the Moller lab. The expression of factors that regulate Toc/Tic gene expression may also be altered in ppi1. It should be possible to distinguish these factors from those involved in the general control of chloroplast gene expression by comparing the results from the two mutants. Genes affected in both mutants are more likely to be involved in regulating chloroplast import since it is unlikely that widespread changes in gene expression will be observed in ppi3. Changes in the expression of factors that regulate import post-translationallyand of the Toc/Tic genes themselves (many are on the RNA) may also be observed. References: 1. Jarvis P. et al. (1998) Science 282: 100-103. 2. Moller S.G. et al. (2001) Genes Dev. 15:90-103. 3. Jarvis P. (2001) Curr. Biol. 11: R307-R310.

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SIB1-SEC23A undergo ER to chloroplast relocalization to mediate immunity in Arabidopsis thaliana

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