March 11, 2009
Researchers identify a process that regulates seed germination
WEST LAFAYETTE, Ind. - |
Mike Hasegawa, the Bruno C. Moser Distinguished Professor of Horticulture and Landscape Architecture, and Kenji Miura, a former Purdue postdoctoral researcher and now an assistant professor at Tsukuba University in Japan, discovered the step involved in keeping seeds from germinating in adverse conditions such as freezing temperatures or drought, a factor in the survival of plant species.
The work, which was published Wednesday (March 11) in the early online version of the Proceedings of the National Academy of Sciences, is part of ongoing research that has uncovered that similar processes affects a plant's freeze tolerance and absorption of phosphate.
"We've found the process, called sumoylation, is involved in the regulation of some major agricultural traits," Hasegawa said. "It is fundamental, basic research like this that allows us to understand how plants respond to hormones and environmental conditions."
Seeds produce a hormone called abscisic acid, or ABA, that prevents germination. When environmental factors such as temperature are not optimal for seed germination, ABA levels are high, which causes production of higher levels of a protein called ABI5. When the ABI5 protein is active, it switches on genes that prevent germination.
Hasegawa's research showed that when a SUMO peptide is attached to the ABI5 protein, the protein becomes inactive, switching off the genes that prevent germination and seedling development.
"A single stimulus such as ABA affects transcription factors, which are major controllers of genes involved in complex processes such as seed germination," Hasegawa said. "Sumoylation seems to be an important process in the control of significant plant characteristics."
Hasegawa said that the ABI5 protein can become active again, halting germination and seedling development if condition are no longer optimal. When conditions change to make plant development possible, the protein can once again be deactivated.
The National Science Foundation and the U.S. Department of Agriculture have funded the research in Hasegawa's laboratory. Hasegawa's next step is to determine how the sumoylation process leads to gene suppression and expression.
Writer: Brian Wallheimer, 765-496-2050, bwallhei@purdue.edu
Source: Mike Hasegawa, 765-494-1315, paul.m.hasegawa.1@purdue.edu
Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
Agriculture News Page
PHOTO CAPTION:
Mike Hasegawa discovered a process that regulates the genes in seeds that control germination and seedling development. When a peptide is attached to a particular protein in the seed, a process can begin that turns off the genes that are prohibiting germination. (Purdue Agricultural Communications photo/Tom Campbell)
Sumoylation of ABI5 by the Arabidopsis SUMO E3 Ligase SIZ1 Negatively Regulates Abscisic Acid Signaling
SUMO (small ubiquitin-related modi�?er) conjugation (i.e., sumoylation) to protein substrates is a reversible posttranslational modi�?cation that regulates signaling by modulating transcription factor activity. This paper presents evidence that the SUMO E3 ligase SIZ1 negatively regulates abscisic acid (ABA) signaling, which is dependent on the bZIP transcription factor ABI5. Loss-of-function T-DNA insertion siz1–2 and siz1–3 mutations caused ABA hypersensitivity for seed germination arrest and seedling primary root growth inhibition. Furthermore, expression of genes that are ABA-responsive through ABI5-dependent signaling (e.g., RD29A, Rd29B, AtEm6, RAB18, ADH1) was hyperinduced by the hormone in siz1 seedlings. abi5– 4 suppressed ABA hypersensitivity caused by siz1 (siz1–2 abi5– 4), demonstrating an epistatic genetic interaction between SIZ1 and ABI5. A K391R substitution in ABI5 [ABI5(K391R)] blocked SIZ1-mediated sumoylation of the transcription factor in vitro and in Arabidopsis protoplasts, indicating that ABI5 is sumoylated through SIZ1 and that K391 is the principal site for SUMO conjugation. In abi5– 4 plants, ABI5(K391R) expression caused greater ABA hypersensitivity (gene expression, seed germination arrest and primary root growth inhibition) compared with ABI5 expression. Together, these results establish that SIZ1-dependent sumoylation of ABI5 attenuates ABA signaling. The double mutant siz1–2 afp-1 exhibited even greater ABA sensitivity than the single mutant siz1, suggesting that SIZ1 represses ABI5 signaling function independent of AFP1.
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