FLOWERING LOCUS C influences the timing of shoot maturation in Arabidopsis thaliana.

Many plant species undergo changes in leaf morphology as the vegetative shoot meristem matures (Poethig, 2003; Telfer et al., 1997). In Arabidopsis, juvenile leaves are produced before shoot maturation and are relatively round and lack trichomes on the abaxial side (underside) of the leaves. Adult leaves formed after the shoot meristem matures are more elliptical and have abaxial trichomes. An important difference between the juvenile‐vegetative and adult‐vegetative phases is that only during the adult‐ vegetative phase can the meristem become competent to respond to environmental and endogenous signals that promote flowering (Telfer et al., 1997). Genetic backgrounds and environmental conditions that delay flowering also delay shoot maturation (Telfer et al., 1997). For example, both flowering and shoot maturation occur later in the Columbia accession (Col) than in the Landsberg errecta accession (Ler) (Lee et al., 1993; Telfer et al., 1997). An important difference between these two accessions occurs at the FLC locus. FLC levels are slightly elevated in Col compared to Ler, which has a weak allele of FLC (Michaels et al., 2003). FLC encodes a MADS‐domain transcription factor that acts as a repressor of flowering (Michaels and Amasino, 1999a; Sheldon et al., 1999). Flowering is delayed under conditions or in genetic backgrounds in which FLC expression is elevated (Michaels and Amasino, 1999a). The observation that shoot maturation is delayed in Col relative to Ler suggests that FLC influences the rate of shoot maturation. However, there are many genetic differences between Col and Ler making it difficult to determine whether FLC expression is solely responsible for the delay in shoot maturation observed in Col. To further explore the potential role of FLC in the regulation of shoot maturation, we performed experiments in the Col background using mutations that eliminate FLC function, as well as mutations in known regulators of FLC. This allowed us to create a series of lines containing various levels of FLC expression that could be used to determine if there is a correlation between FLC expression and the timing of shoot maturation. The appearance of abaxial trichomes was used to determine when the transition from juvenile to adult leaves occurred (Telfer et al., 1997). The first leaf with four or more abaxial trichomes was scored as the first adult‐vegetative leaf. Leaves were photographed at the time of trichome counting, and those images confirm that the appearance of abaxial trichomes corresponds to leaf shape changes associated with shoot …