Identification, cloning and expression analysis of chloroplast TOC-TIC complex components in a C4 halophyte Suaeda aralocaspica without Kranz anatomy

The translocon at the outer/inner chloroplast envelope membrane (TOC/TIC) complexes constitute the core machinery mediating the recognition and transmembrane translocation of nucleus-encoded precursor proteins, fulfilling essential functions in chloroplast biogenesis and photosynthesis. However, the TOC-TIC system remains poorly characterized in extremophytes, particularly in single-cell C4 (SCC4) halophytes such as Suaeda aralocaspica, which possess unique anatomical and physiological adaptations to saline and arid environments. In this study, we performed a genome-wide identification and comprehensive analysis of TOC-TIC components in S. aralocaspica. A total of 7 SaTOC and 19 SaTIC genes were identified, belonging to 4 and 11 distinct subfamilies, respectively. These genes exhibited conserved gene structures, protein motifs, and phylogenetic relationships with known homologs. Expression profiling analysis revealed that all SaTOC and SaTIC genes exhibited tissue-specific expression patterns. Notably, SaToc159, SaToc75, SaTic56, and SaTic110 were markedly upregulated during early germination, implying a potential role in seedling establishment. Most SaTOC and SaTIC genes were induced by light, while SaToc132 and SaToc34 displayed constitutive expression patterns. Furthermore, SaToc159, SaToc75, and SaTic100 were significantly upregulated in response to salt stress in germinating seedlings. Subcellular localization experiments confirmed that SaToc34, SaToc75, SaTic20-I, and SaTic56 are localized in the chloroplast. These findings provide the first systematic overview of TOC-TIC complex in a SCC4 halophyte, establishing a foundation for elucidating chloroplast protein import mechanisms in plants adapted to extreme environments.