Expression of an acyl-CoA-binding protein, ACBP6, in Arabidopsis phloem

Arabidopsis acyl-CoA binding proteins (AtACBPs) contain a conserved acyl-CoA-binding domain which facilitates the binding of long-chain acyl-CoA esters and they have been shown to functi on in plant stress responses and development. The 10-kDa AtACBP6 represents the smallest amongst the six in the AtACBP family and confers freezing tolerance in transgenic Arabidopsis rosett es and fl owers, possibly by its interacti on with phosphati dylcholine (PC), as recombinant AtACBP6 has been identi fi ed to bind acyl-CoA esters and PC. In eukaryotes, the 10-kDa isoform transports acyl-CoA esters, maintain an intracellular acyl-CoA pool and regulates lipid metabolism. AtACBP6 encodes a cytosolic protein expressed in fl oral organs besides other ti ssues in qRT-PCR analysis, and plays a combinatory role together with AtACBP4 and AtACBP5 in pollen and seed development. As AtACBP6 shares homology with the phloem ACBPs of rice and cucumber, its potenti al role in long-distance lipid transport was investi gated. Arabidopsis expressing AtACBP6pro::GUS exhibited strong GUS acti vity in the vascular ti ssues, fl owers, embryos as well as the stem cuti cle. Immunoelectron microscopy using anti -AtACBP6 anti bodies confi rmed AtACBP6 localizati on in the phloem, especially in the companion cells, sieve elements and plasmodesmata, indicati ng its potenti al in systemic traffi cking. Fatt y acid profi ling on the wild-type and acbp6 phloem exudates supported a lipid-associated role of AtACBP6 in the phloem.