Functions of arabidopsis acyl-coenzyme A binding proteins in stress responses

In Arabidopsis thaliana, a gene family encodes acyl-CoA-binding proteins (ACBPs) conserved at the acyl-CoA-binding domain which facilitates the binding to acyl-CoA esters. These ACBPs, designated ACBP1 to ACBP6, range in size from 10.4 to 73.1 kD. Previous studies have shown that the the overexpression of ACBP1 or ACBP2 in Arabidopsis likely promotes repair of lipid membranes and result in enhanced tolerance to lead and cadmium, respectively. Microarray data (http://bar.utoronto.ca/) revealed that the expression of ACBP1 and ACBP2 is also regulated by other abiotic stresses, such as cold and drought, suggestive of their association with these environmental pressures. The aim of this study is to investigate and better understand the roles of ACBP1 and ACBP2 in different stress responses. It has been previously observed that the expression of both ACBP1 and ACBP4 is lead [Pb(II)]-inducible and recombinant ACBP1 and ACBP4 bind Pb(II) in vitro. In this study, ACBP1 and ACBP4 were overexpressed in Brassica juncea to test if these ACBPs could be extended for application in Pb(II) phytoremediation in transgenic B. juncea. On freezing (-12 to -8 °C) treatment, ACBP1-overexpressing Arabidopsis was freezing sensitive and accumulated more phosphatidic acid (PA), but less phosphatidylcholine (PC), in contrast to acbp1 mutant plants which were freezing tolerant and had reduced PA and elevated PC levels. Such changes in PC and PA were consistent with the expression of the mRNA encoding phospholipase D1 (PLD1), a major enzyme that promotes the hydrolysis of PC to PA. In contrast, the expression of phospholipase D (PLD), which plays a positive role in freezing tolerance, was up-regulated in acbp1 mutant plants and down-regulated in ACBP1-overexpressing plants. Reduced PLD1 expression and decreased hydrolysis of PC to PA may enhance membrane stability in the acbp1 mutant plants. Given that recombinant ACBP1 binds PA and acyl-CoA esters in vitro, the expression of PLD1 and PLD could be regulated by PA or acyl-CoAs maintained by ACBP1, if ACBP1 were to resemble the yeast 10-kD ACBP by its capability to modulate gene expression during stress responses. Interestingly, another membrane-associated ACBP, ACBP2, which shows high (76.9%) conservation in amino acid homology to ACBP1, did not appear to be affected by freezing treatment. Besides freezing stress, ACBP1, as well as ACBP2, have been observed to participate in abscisic acid (ABA) signaling. They both promote ABA signaling in seed germination and seedling development, while only ACBP2 is involved in the drought response. The overexpression of ACBP2 in Arabidopsis up-regulated reactive oxygen species (ROS) production culminating in reduction in stomatal aperture and water loss in guard cells, thereby enhancing drought tolerance. For tests in phytoremediation, B. juncea was selected for overexpression of ACBP1 and ACBP4 because it is fast-growing, has a higher biomass than Arabidopsis, and is known to be a good accumulator of Pb(II). However, results of Pb(II) treatment for two days showed that the overexpression of ACBP1 or ACBP4 in B. juncea did not significantly improve Pb(II) tolerance. Nevertheless, B. juncea overexpressing ACBP1 did accumulate Pb(II) in roots whereas ACBP4-overexpressing B. juncea lines accumulated Pb(II) in both shoots and roots. Given that B. juncea has a larger biomass than Arabidopsis, it is likely that the duration of Pb(II)-incubation tested in this study was not drastic enough for comparison, and the incubation time should be further extended for Pb(II) translocation. In addition, future studies on Arabidopsis should be conducted to better understand the mechanism of ACBP4-mediated Pb(II) accumulation using Arabidopsis acbp4 mutant and ACBP4-overexpressing plants.published_or_final_versionBiological SciencesDoctoralDoctor of Philosoph