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_version / Biological Sciences / Doctoral / Doctor of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/208430 |
| Date | January 2011 |
| Creators | Du, Zhiyan, 杜志岩 |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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