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

Abstract

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 D1 (PLD1), 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 PLD1 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 PLD1 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.