Abstract
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.
Highlights
IntroductionChloroplasts are for the most part composed of proteins encoded by the nuclear genome and synthesized as precursor proteins (pre-proteins) in the cytosol, with the
We showed that Western blot analysis using Tic55 specific antibody further depicted the complete absence of Tic55 protein in chloroplast of this knockout line, named as tic55-II
The absence of Tic55 in tic55-II knockout mutant inhibited expression of the senescence-related genes PEROXISOME DEFECTIVE 1 (PED1), BLUE COPPERBINDING PROTEIN (BCB), and SENESCENCE 1 (SEN1) at different stages of dark adaptation, and stimulation of RUBISCO SMALL SUBUNIT GENE 2B (RBCS2B) gene expression appeared at an early stage of dark response
Summary
Chloroplasts are for the most part composed of proteins encoded by the nuclear genome and synthesized as precursor proteins (pre-proteins) in the cytosol, with the. N-terminal transit peptide essential for targeting these pre-proteins to the appropriate membrane surfaces of chloroplasts. The N-terminal transit peptide of pre-proteins targeted to the chloroplasts via the Toc-Tic system is cleaved off by the stromal processing peptidase (SPP) and become mature proteins once pre-proteins enter the chloroplast stroma [1,2,3,4]. In addition to its possible role in chloroplast protein import, chloroplast inner membrane protein Tic may be involved in the senescence of Arabidopsis. It was reported previously that Arabidopsis-accelerated cell death gene ACD1, a potential Pheophorbide a Oxygenase (PaO)-encoding gene such as Tic, is implicated in the senescence of Arabidopsis [19]. Based on the amino acid alignment and structure analysis among distinct species, Tic contains three highly conserved regions, including Rieske iron–sulfur motif, mononuclear iron-binding motif, and motif C (CxxC)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
