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Abstract
BackgroundThe transcription factor DOF AFFECTING GERMINATION1 (DAG1) is a repressor of seed germination acting downstream of the master repressor PHYTOCROME INTERACTING FACTOR3-LIKE 5 (PIL5). Among others, PIL5 induces the expression of the genes encoding the two DELLA proteins GA INSENSITIVE 1 (GAI) and REPRESSOR OF ga1-3 (RGA).ResultsBased on the properties of gai-t6 and rga28 mutant seeds, we show here that the absence of RGA severely increases dormancy, while lack of GAI only partially compensates RGA inactivation. In addition, the germination properties of the dag1rga28 double mutant are different from those of the dag1 and rga28 single mutants, suggesting that RGA and DAG1 act in independent branches of the PIL5-controlled germination pathway. Surprisingly, the dag1gai-t6 double mutant proved embryo-lethal, suggesting an unexpected involvement of (a possible complex between) DAG1 and GAI in embryo development.ConclusionsRather than overlapping functions as previously suggested, we show that RGA and GAI play distinct roles in seed germination, and that GAI interacts with DAG1 in embryo development.
Highlights
The transcription factor DOF AFFECTING GERMINATION1 (DAG1) is a repressor of seed germination acting downstream of the master repressor PHYTOCROME INTERACTING FACTOR3-LIKE 5 (PIL5)
The gai-t6 and rga28 mutant alleles show different seed germination phenotypes It has been reported that concurrent inactivation of both GA INSENSITIVE 1 (GAI) and REPRESSOR OF ga1-3 (RGA) increases the seed germination potential: gai-t6rga28 double mutant seeds require less R light fluences than wild-type ones to germinate [7] - a phenotype that is reminiscent of dag1 mutant seeds, which need a fluence rate six times lower than wild-type to germinate [20]
Under white light the only substantial difference in the germination rate of stratified seeds was observed at 24 hours between rga28 and gai-t6rga28 mutant seeds compared to wild-type ones (56%, 64% and 85% respectively), and in all cases 100% germination was attained in 72 hours (Figure 1B)
Summary
The transcription factor DOF AFFECTING GERMINATION1 (DAG1) is a repressor of seed germination acting downstream of the master repressor PHYTOCROME INTERACTING FACTOR3-LIKE 5 (PIL5). PIL5 induces the expression of the genes encoding the two DELLA proteins GA INSENSITIVE 1 (GAI) and REPRESSOR OF ga (RGA). Several physical factors affect seed germination, such as light, negative control by GA and the proteasome [11]. An insertional mutagenesis approach enabled cloning of Arabidopsis GAI by isolation of a Ds transposon-mutated gai-t6 allele [13], while RGA was identified by loss-of-function mutations [14] and shown to encode a protein closely related to GAI [15]. GAI and RGA were shown to have overlapping functions in repressing many growth processes, such as leaf expansion, stem elongation, floral initiation and seed germination [16,17]. Double mutant seeds have a higher germination rate than the wild-type ones in response to increasing Red (R) light fluences [7]
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