The GCR2 gene family is not required for ABA control of seed germination and early seedling development in Arabidopsis.
BACKGROUND
The plant hormone abscisic acid (ABA) regulates diverse processes of plant growth and development. It has recently been proposed that GCR2 functions as a G-protein-coupled receptor (GPCR) for ABA. However, the structural relationships and functionality of GCR2 have been challenged by several independent studies. A central question in this controversy is whether gcr2 mutants are insensitive to ABA, because gcr2 mutants were shown to display reduced sensitivity to ABA under one experimental condition (e.g. 22 degrees C, continuous white light with 150 micromol m(-2) s(-1)) but were shown to display wild-type sensitivity under another slightly different condition (e.g. 23 degrees C, 14/10 hr photoperiod with 120 micromol m(-2) s(-1)). It has been hypothesized that gcr2 appears only weakly insensitive to ABA because two other GCR2-like genes in Arabidopsis, GCL1 and GCL2, compensate for the loss of function of GCR2.


RESULTS
In order to test this hypothesis, we isolated a putative loss-of-function allele of GCL2, and then generated all possible combinations of mutations in each member of the GCR2 gene family. We found that all double mutants, including gcr2 gcl1, gcr2 gcl2, gcl1 gcl2, as well as the gcr2 gcl1 gcl2 triple mutant displayed wild-type sensitivity to ABA in seed germination and early seedling development assays, demonstrating that the GCR2 gene family is not required for ABA responses in these processes.


CONCLUSIONS
These results provide compelling genetic evidence that GCR2 is unlikely to act as a receptor for ABA in the context of either seed germination or early seedling development.
Guo
Jianjun
J
Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada.
Zeng
Qingning
Q
Emami
Mohammad
M
Ellis
Brian E
BE
Chen
Jin-Gui
JG
D000040:Abscisic Acid / Q000494:pharmacology*; D017360:Arabidopsis / Q000187:drug effects / Q000235:genetics* / Q000254:growth & development; D029681:Arabidopsis Proteins / Q000235:genetics*; D004269:DNA, Bacterial / Q000235:genetics; D018745:Genome, Plant; D018525:Germination / Q000187:drug effects / Q000502:physiology; D005810:Multigene Family; D016296:Mutagenesis; D043562:Receptors, G-Protein-Coupled / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D036226:Seedlings / Q000187:drug effects / Q000502:physiology; D012639:Seeds / Q000187:drug effects / Q000502:physiology*
D029681:Arabidopsis Proteins; D004269:DNA, Bacterial; C518578:GCR2 protein, Arabidopsis; D043562:Receptors, G-Protein-Coupled; C036483:T-DNA; D000040:Abscisic Acid