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SPINDLY and GIGANTEA Interact and Act in Arabidopsis thaliana Pathways Involved in Light Responses, Flowering, and Rhythms in Cotyledon Movements

^a Department of Plant Biology and Plant Molecular Genetics Institute, University of Minnesota, St. Paul, Minnesota 55108
^b Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755

² To whom correspondence should be addressed. E-mail neil{at}biosci.cbs.umn.edu; fax 612-625-1738.

SPINDLY (SPY) is a negative regulator of gibberellin signaling in Arabidopsis thaliana that also functions in previously undefined pathways. The N terminus of SPY contains a protein–protein interaction domain consisting of 10 tetratricopeptide repeats (TPRs). GIGANTEA (GI) was recovered from a yeast two-hybrid screen for proteins that interact with the TPR domain. GI and SPY also interacted in Escherichia coli and in vitro pull-down assays. The phenotypes of spy and spy-4 gi-2 plants support the hypothesis that SPY functions with GI in pathways controlling flowering, circadian cotyledon movements, and hypocotyl elongation. GI acts in the long-day flowering pathway upstream of CONSTANS (CO) and FLOWERING LOCUS T (FT). Loss of GI function causes late flowering and reduces CO and FT RNA levels. Consistent with SPY functioning in the long-day flowering pathway upstream of CO, spy-4 partially suppressed the reduced abundance of CO and FT RNA and the late flowering of gi-2 plants. Like gi, spy affects the free-running period of cotyledon movements. The free-running period was lengthened in spy-4 mutants and shortened in plants that overexpress SPY under the control of the 35S promoter of Cauliflower mosaic virus. When grown under red light, gi-2 plants have a long hypocotyl. This hypocotyl phenotype was suppressed in spy-4 gi-2 double mutants. Additionally, dark-grown and far-red-light–grown spy-4 seedlings were found to have short and long hypocotyls, respectively. The different hypocotyl length phenotypes of spy-4 seedlings grown under different light conditions are consistent with SPY acting in the GA pathway to inhibit hypocotyl elongation and also acting as a light-regulated promoter of elongation.

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