Chloroplast Protein Tic55 Involved in Dark-Induced Senescence through AtbHLH/AtWRKY-ANAC003 Controlling Pathway of Arabidopsis thaliana

The chloroplast comprises the outer and inner membranes that are composed of the translocon protein complexes Toc and Tic (translocon at the outer/inner envelope membrane of chloroplasts), respectively. Tic55, a chloroplast Tic protein member, was shown to be not vital for functional protein import in Arabidopsis from previous studies. Instead, Tic55 was revealed to be a dark-induced senescence-related protein in our earlier study. To explore whether Tic55 elicits other biological functions, a tic55-II knockout mutant (SALK_086048) was characterized under different stress treatments. Abiotic stress conditions, such as cold, heat, and high osmotic pressure, did not cause visible effects on tic55-II mutant plant, when compared to the wild type (WT). In contrast, senescence was induced in the individually darkened leaves (IDLs), resulting in the differential expression of the senescence-related genes PEROXISOME DEFECTIVE 1 (PED1), BLUE COPPER-BINDING PROTEIN (BCB), SENESCENCE 1 (SEN1), and RUBISCO SMALL SUBUNIT GENE 2B (RBCS2B). The absence of Tic55 in tic55-II knockout mutant inhibited expression of the senescence-related genes PED1, BCB, and SEN1 at different stages of dark adaptation, while causing stimulation of RBCS2B gene expression at an early stage of dark response. Finally, yeast one-hybrid assays located the ANAC003 promoter region with cis-acting elements are responsible for binding to the different AtbHLH proteins, thereby causing the transactivation of an HIS3 reporter gene. ANAC003 was shown previously as a senescence-related protein and its activation would lead to expression of senescence-associated genes (SAGs), resulting in plant senescence. Thus, we propose a hypothetical model in which three signaling pathways may be involved in controlling the expression of ANAC003, followed by expression of SAGs that in turn leads to leaf senescence in Arabidopsis by this study and previous data.