Figure 5: Four AtABCGs mediate ABA transport in Arabidopsis seeds.

(a,b) The endosperms and testa of atabcg mutants release less ABA than do those of the wild type (WT). Endosperms attached to testa were dissected from 500 WT, atabcg31, atabcg25 and atabcg31 atabcg25 (g31/g25-1, g31/g25-2) seeds 24 h after imbibition on medium containing 10 μM PAC. The tissues were then incubated on ½ MS liquid medium for 24 h before ABA content was assayed using an ELISA. The aqueous medium was collected for assay of secreted ABA (a), and the remaining tissues were washed three times and extracted to assay the retained ABA (b). (c) atabcg30, atabcg40 and two different lines of double mutant embryos (g30/g40-1 and g30/g40-2) accumulated less ABA than the corresponding WT. Accumulation of ³H-ABA by embryos dissected from WT and mutant seeds during a 30-min period. Embryos were incubated in a solution of 12.5 nM ³H-ABA (1.63 Tbq mmol⁻¹) and 17.5 pM ¹⁴C-glycerol (5.40 GBq mmol⁻¹) at pH 6.5. The radioactivity uptake was normalized to the ¹⁴C-glycerol d.p.m. (disintegrations per min) value. (d) Time-dependent loading assay of ³H-ABA to yeast cells expressing AtABCG31 or AtABCG30, or transformed with the empty vector (EV). Yeast cells were incubated in SG-URA medium containing 50 nM ³H-ABA (7.4 kBq, 1.63 Tba mmol⁻¹) at pH 6.5. Data are means±s.e.m. of n=12 from three independent experiments (N=3, n=4). (e) Four AtABCGs participate in the delivery of ABA from the endosperm to the embryo in Arabidopsis seeds. AtABCG31 and AtABCG25 are expressed in the endosperm and secrete ABA from the endosperm to the embryo. AtABCG30 and AtABCG40 are ABA uptake transporters located in the embryo. To inhibit embryo germination, de novo biosynthesized ABA is transported from the endosperm to the embryo by the four transporters. Solid arrows indicate the direction of AtABCG-mediated ABA transportation. Data are means±s.e.m. of n=6 (a,b) or n=4 (c) from three or four independent experiments. (*P<0.05; **P<0.01 compared with the WT by Student’s t-test and ^##P<0.01 compared with the single mutants by Student’s t-test).