The SERRATE locus controls the formation of the early juvenile leaves and phase length in Arabidopsis.

The development of the shoot can be divided into a series of distinct developmental phases based on leaf character-istics and inflorescence architecture. The relationship between phase length, defined by the number of organs produced, and the timing of the floral induction (V3-I1 transition) is relatively ill defined. Characterization of the serrate mutant (CS3257; Arabidopsis Biological Research Center) revealed defects in both vegetative and inflores-cence phase lengths, the timing of phase transitions, leaf number, the leaf initiation rate, and phyllotaxy. The timing of floral induction, however, is the same as in wild-type in extended short days as well as in short days, whereas the flowering time response to photoperiod is unaffected. SERRATE is shown to be required for the development of early juvenile leaves (V1) and to promote late juvenile leaf development (V2), while suppressing adult leaf (V3) and inflorescence development (I1 and I2). The se mutation supports the hypothesis that the timing of floral induction is independent of vegetative and inflorescence phase lengths. The role of SERRATE in the regulation of phase length and leaf identity is discussed.