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Understanding DELLA-mediated regulation of plant growth

Reference	BB/D521665/1
Principal Investigator / Supervisor	Dr Margaret Boulton
Co-Investigators / Co-Supervisors	Professor Nicholas Harberd
Institution	John Innes Centre
Department	Cell and Develop Biology
Funding type	Research
Value (£)	205,545
Status	Completed
Type	Research Grant
Start date	12/12/2005
End date	11/12/2008
Duration	36 months

Abstract

This proposal describes experiments designed to advance understanding of how plant growth is regulated by DELLAs in response to phytohormonal (GA, auxin) and environmental (salt-stress) signals, focusing on four aspects of the relationship between the DELLAs and the SLY1 F-box component of the SCFSLY1 E3 ubiquitin ligase. First the specific domain of the DELLAs to which SLY1 binds will be identified. Plant lines expressing affinity-tagged DELLAs lacking specific conserved domains will be developed. Protein extracts from these plants will be used in immunoprecipitation (pull-down) experiments to determine which domain of the DELLA protein is recognised and bound by SLY1. Subsequent mutagenesis experiments will identify the specific amino acid residues of the DELLA protein that determine the DELLA-SLY1 interaction. Second, the possibility that GA, auxin or salt-stress regulate the levels of SLY1 transcripts (encoding the SLY1 subunit of the SCFSLY1 E3 ubiquitin ligase) will be tested. Previous experiments show that (transgenic) alterations in SLY1 transcript levels can alter rates of DELLA-destruction and plant growth. We will determine if alteration in SLY1 transcript levels contributes to growth-regulation in response to GA, auxin or salt-stress. Third, the effect of GA, auxin and salt-stress on the strength of the interaction between DELLAs and the SCFSLY1 E3 ubiquitin ligase will be determined. GA, auxin and salt-stress all regulate growth (at least in part) by controlling the rate of DELLA-destruction. Rate of DELLA-destruction is dependent on the DELLA-SCFSLY1 interaction. Thus we will directly determine if any of GA, auxin or salt-stress regulate the DELLA-SCFSLY1 interaction by controlling the strength of DELLA-SLY1 binding. These factors may alternatively modulate the affinity of DELLA for SLY1, or of SLY1 for DELLA (or both of these), and our experiments will distinguish between these possibilities. Fourth, changes in post-translation modification (of eitherDELLA or SLY1) in response to GA, auxin or salt-stress will be identified. These experiments will enable identification of those post-translational modifications that are responsible for the differential regulation of the strength of DELLA-SLY1 binding.

Summary

The growth of plants is controlled by a variety of factors. Some of these factors are environmental, some are molecules that signal between cells (hormones), and others are proteins from within cells. Recently, we have identified a class of proteins, the DELLAs, that are found in the nuclei of plant cells. The DELLAs are plant growth inhibitors. Accumulation of DELLAs in cell nuclei inhibits the cell expansion and proliferation that drives the growth of plants. In contrast, the plant growth hormone gibberellin (GA) stimulates the growth of plants. GA stimulates growth by promoting the destruction of the growth-inhibiting DELLAs in the proteasome (a protein-destroying enzyme complex that is found in the cells of all living organisms). Another plant hormone, auxin, also controls growth by controlling the level of DELLAs in the cell. Furthermore, adverse environmental conditions, such as when soils contain too much salt, cause accumulation of DELLAs and resultant growth inhibition. This proposal describes experiments that will further our understanding of the way in which the levels of DELLAs within the cell are controlled. A second enzyme complex marks DELLAs for destruction in the proteasome. Key to the action of this second complex is a protein component called SLY1, that recognises the DELLAs and binds to them. The proposal outlines four sets of experiments that will help us to understand how plant hormones and environmental factors control the affinity which DELLA and SLY1 have for one another, and thus regulate plant growth. First, we will determine the particular region of the DELLAs that SLY1 recognises. Second, we will test the possibility that plant hormones or environmental factors modulate the activity of SLY1 (and hence the destruction of DELLAs) by increasing (or decreasing) the levels of mRNA gene transcripts that encode it. It is also possible that plant hormones or environmental factors control plant growth by causing chemical modification of the DELLAs, or of SLY1. Such modifications are expected to affect the affinity with which DELLAs and SLY1 bind to one another, thus affecting the rate with which DELLAs are destroyed and ultimately affecting the rate of growth. The third set of experiments will therefore detect the effects of plant hormones and environmental factors on the affinity with which DELLAs and SLY1 bind to one another. The fourth set of experiments will specifically identify those modifications to DELLAs or SLY1 that affect the affinity of DELLA-SLY1 binding. In summary, the proposed experiments will enhance our knowledge of how the growth of plants is controlled. This improved knowledge will be an important advance in our understanding of how plants got to be the way they are. The DELLA system is an internal growth-controlling system that responds to environmental growth-controlling signals. Presumably, the DELLA system evolved because plants that could regulate their growth in response to changes in the environment had advantages over plants that could not. In addition this work may have important practical consequences. Environmental stress reduces both growth and yield of crop plants. Understanding how the DELLA system controls plant growth may help us to develop methods to reduce the effects of stress on agricultural productivity.

Committee	Closed Committee - Plant & Microbial Sciences (PMS)
Research Topics	Plant Science
Research Priority	X – Research Priority information not available
Research Initiative	X - not in an Initiative
Funding Scheme	X – not Funded via a specific Funding Scheme

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