FRL1 is required for petal and sepal development in Arabidopsis

A novel flower mutant, frl1 (frill 1) was isolated in Arabidopsis thaliana. The frl1 mutant has serrated petals and sepals but the other floral and ve getative organs appear to be normal. To analyse the role of the FRL1 gene, morphological, cytological and double mutant analyses were carried out. The frl1 flower had broader petals and sepals as compared with the wild-type. The distal region of frl1 petals contained fewer epidermal cells but their size was variable and generally larger than that in the wild-type. However, no significant difference was found in the basal region. Observations of t he early petal development revealed that the morphology of the developing f rl1 petal was normal until the middle of stage 9, but the frl1 phenotype be came apparent in stages later than 10. Furthermore, larger nuclei with vari ed sizes were observed in the distal region of frl1 petals, but not in this region in wild-type petals. This strongly suggests that abnormal endo-redu plication had occurred. These observations indicate that the frl1 mutation affects the number of cell divisions and the subsequent cell expansion duri ng the late stage of petal lamina formation, and that FRL1 might be maintai ning the mitotic state or suppressing the transition to the endo-reduplicat ion cycle. Double mutants with the homeotic mutants apetala3-1 and agamous showed additive phenotypes. Ectopic petals in the third whorl of fr11 ag fl owers were serrated, indicating that the FRL1 gene acts in petal and sepal development in an organ-specific manner.