Transcriptional and hormonal analyses to elucidate the control of Arabidopsis seed dormancy

Sven Krishnan Nelson

Many land plants produce dormant seeds at maturity that cannot germinate, but gain the ability to germinate through the process of after-ripening during dry storage. The appropriate regulation of seed dormancy and germination is key to species survival and successful agriculture. The precise mechanisms regulating dormancy loss are one of biology's great mysteries. The hormone abscisic acid (ABA) promotes dormancy, whereas gibberellin (GA) is required for seed germination. This thesis elucidated the role of GA signaling in dormancy loss. GA stimulates germination by down-regulating DELLA repressors of seed germination by stimulating DELLA proteolysis via the SCFSLY1 E3 ubiquitin ligase. DELLA overaccumulation in GA-insensitive sly1-2 (sleepy1-2) mutants is associated with increased seed dormancy that is relieved either by long 1-2 years of after-ripening or by GA receptor GIBBERELIN INSENSITIVE DWARF1b overexpression (GID1b-OE). The fact that DELLAs are transcriptional regulators, suggests the sly1-2 dormancy phenotype results from altered transcription. Thus, the global pattern of gene expression associated with sly1-2 dormancy and dormancy loss was examined first in imbibing and then in dry seeds. Because of low metabolic activity in dry seeds, the 760 differentially regulated transcripts observed with after-ripening were more likely due preferential mRNA degradation rather than active transcription. Indeed, there was a strong correlation between transcript regulation and mRNA stability, such that more stable transcripts tended to be up-regulated and less stable tended to be down-regulated with dry after-ripening. Rescue of sly1-2 germination by after-ripening stimulated a large number of transcriptional changes, but rescue by GID1b-OE stimulated changes in only 26 genes. This suggests that either few transcriptional changes or non-transcriptional mechanisms can rescue sly1-2 seed germination. In both dry and imbibing seeds, a very large fraction of the genes differentially regulated with after-ripening were regulators of protein translation, suggesting that GA may be needed to stimulate protein translation during germination. Finally, changes in ABA and GA hormone levels associated seed after-ripening and germination were examined. GA levels increased before ABA levels decreased with after-ripening in sly1-2 and wild-type Arabidopsis. In fact ABA levels increase early in imbibition of after-ripened seeds, possibly serving as a checkpoint to prevent premature germination.

Transcriptional and hormonal analyses to elucidate the control of Arabidopsis seed dormancy

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