Allelic polymorphism ofGIGANTEAis responsible for naturally occurring variation in circadian period inBrassica rapa

GIGANTEA(GI) was originally identified by a late-flowering mutant inArabidopsis,but subsequently has been shown to act in circadian period determination, light inhibition of hypocotyl elongation, and responses to multiple abiotic stresses, including tolerance to high salt and cold (freezing) temperature. Genetic mapping and analysis of families of heterogeneous inbred lines showed that natural variation inGIis responsible for a major quantitative trait locus in circadian period inBrassica rapa.We confirmed this conclusion by transgenic rescue of anArabidopsis gi-201loss of function mutant. The twoB. rapa GIalleles each fully rescued the delayed flowering ofArabidopsis gi-201but showed differential rescue of perturbations in red light inhibition of hypocotyl elongation and altered cold and salt tolerance. TheB. rapaR500GIallele, which failed to rescue the hypocotyl and abiotic stress phenotypes, disrupted circadian period determination inArabidopsis.Analysis of chimericB. rapa GIalleles identified the causal nucleotide polymorphism, which results in an amino acid substitution (S264A) between the two GI proteins. This polymorphism underlies variation in circadian period, cold and salt tolerance, and red light inhibition of hypocotyl elongation. Loss-of-function mutations ofB. rapa GIconfer delayed flowering, perturbed circadian rhythms in leaf movement, and increased freezing and increased salt tolerance, consistent with effects of similar mutations inArabidopsis.Collectively, these data suggest that allelic variation ofGI—and possibly of clock genes in general—offers an attractive target for molecular breeding for enhanced stress tolerance and potentially for improved crop yield.