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In Arabidopsis thaliana, a family of six genes (ACBP1 to ACBP6) encodes acyl-CoA binding proteins (ACBPs). Investigations on ACBP3 reported here show its upregulation upon dark treatment and in senescing rosettes. Transgenic Arabidopsis overexpressing ACBP3 (ACBP3-OEs) displayed accelerated leaf senescence, whereas an acbp3 T-DNA insertional mutant and ACBP3 RNA interference transgenic Arabidopsis lines were delayed in dark-induced leaf senescence. Acyl-CoA and lipid profiling revealed that the overexpression of ACBP3 led to an increase in acyl-CoA and phosphatidylethanolamine (PE) levels, whereas ACBP3 downregulation reduced PE content. Moreover, significant losses in phosphatidylcholine (PC) and phosphatidylinositol, and gains in phosphatidic acid (PA), lysophospholipids, and oxylipin-containing galactolipids (arabidopsides) were evident in 3-week-old dark-treated and 6-week-old premature senescing ACBP3-OEs. Such accumulation of PA and arabidopsides (A, B, D, E, and G) resulting from lipid peroxidation in ACBP3-OEs likely promoted leaf senescence. The N-terminal signal sequence/transmembrane domain in ACBP3 was shown to be essential in ACBP3-green fluorescent protein targeting and in promoting senescence. Observations that recombinant ACBP3 binds PC, PE, and unsaturated acyl-CoAs in vitro and that ACBP3 overexpression enhances degradation of the autophagy (ATG)-related protein ATG8 and disrupts autophagosome formation suggest a role for ACBP3 as a phospholipid binding protein involved in the regulation of leaf senescence by modulating membrane phospholipid metabolism and ATG8 stability in Arabidopsis. Accelerated senescence in ACBP3-OEs is dependent on salicylic acid but not jasmonic acid signaling.

Summary• Arabidopsis thaliana acyl-CoA-binding protein 2 (ACBP2) was observed to interact with farnesylated protein 6 (AtFP6), which has a metal-binding motif (M/LXCXXC). Their interaction and expression in response to heavy metals were investigated.• Yeast two-hybrid analysis and in vitro assays showed that an ACBP2 derivative lacking ankyrin repeats did not interact with AtFP6, indicating that the ankyrin repeats mediate protein-protein interaction. Autofluorescence-tagged ACBP2 and AtFP6 transiently co-expressed in tobacco (Nicotiana tabacum) were both targeted to the plasma membrane.• Reverse transcriptase polymerase chain reaction and northern blot analyses revealed that AtFP6 mRNA was induced by cadmium (Cd(II)) in A. thaliana roots. Assays using metal-chelate affinity chromatography demonstrated that in vitro translated ACBP2 and AtFP6 bound lead (Pb(II)), Cd(II) and copper (Cu(II)). Consistently, assays using fluorescence analysis confirmed that (His) 6 -AtFP6 bound Pb(II), like (His) 6 -ACBP2.• Arabidopsis thaliana plants overexpressing ACBP2 or AtFP6 were more tolerant to Cd(II) than wild-type plants. Plasma membrane-localized ACBP2 and AtFP6 probably mediate Pb(II), Cd(II) and Cu(II) transport in A. thaliana roots. Also, (His) 6 -ACBP2 binds [ 14 C]linoleoyl-CoA and [ 14 C]linolenoyl-CoA, the precursors for phospholipid repair following lipid peroxidation under heavy metal stress at the plasma membrane. ACBP2-overexpressing plants were more tolerant to hydrogen peroxide than wild-type plants, further supporting a role for ACBP2 in post-stress membrane repair.

Small 10-kD acyl-coenzyme A-binding proteins (ACBPs) are highly conserved proteins that are prevalent in eukaryotes. In Arabidopsis (Arabidopsis thaliana), other than the 10-kD ACBP homolog (designated Arabidopsis ACBP6), there are five larger forms of ACBPs ranging from 37.5 to 73.1 kD. In this study, the cytosolic subcellular localization of Arabidopsis ACBP6 was confirmed by analyses of transgenic Arabidopsis expressing autofluorescence-tagged ACBP6 and western-blot analysis of subcellular fractions using ACBP6-specific antibodies. The expression of Arabidopsis ACBP6 was noticeably induced at 48 h after 4°C treatment by northern-blot analysis and western-blot analysis. Furthermore, an acbp6 T-DNA insertional mutant that lacked ACBP6 mRNA and protein displayed increased sensitivity to freezing temperature (28°C), while ACBP6-overexpressing transgenic Arabidopsis plants were conferred enhanced freezing tolerance. Northern-blot analysis indicated that ACBP6-associated freezing tolerance was not dependent on the induction of cold-regulated COLD-RESPONSIVE gene expression. Instead, ACBP6 overexpressors showed increased expression of mRNA encoding phospholipase Dd. Lipid profiling analyses of rosettes from cold-acclimated, freezingtreated (28°C) transgenic Arabidopsis plants overexpressing ACBP6 showed a decline in phosphatidylcholine (236% and 246%) and an elevation of phosphatidic acid (73% and 67%) in comparison with wild-type plants. From our comparison, the gain in freezing tolerance in ACBP6 overexpressors that was accompanied by decreases in phosphatidylcholine and an accumulation of phosphatidic acid is consistent with previous findings on phospholipase Dd-overexpressing transgenic Arabidopsis. In vitro filterbinding assays indicating that histidine-tagged ACBP6 binds phosphatidylcholine, but not phosphatidic acid or lysophosphatidylcholine, further imply a role for ACBP6 in phospholipid metabolism in Arabidopsis, including the possibility of ACBP6 in the cytosolic trafficking of phosphatidylcholine.