SummaryFlowering time in many plants is triggered by environmental factors that lead to uniform¯owering in plant populations, ensuring higher reproductive success. So far, several genes have been identi®ed that are involved in¯owering time control. AGL20 (AGAMOUS LIKE 20) is a MADS domain gene from Arabidopsis that is activated in shoot apical meristems during the transition to¯owering. By transposon tagging we have identi®ed late¯owering agl20 mutants, showing that AGL20 is involved in¯owering time control. In previously described late¯owering mutants of the long-day and constitutive pathways of¯oral induction the expression of AGL20 is down-regulated, demonstrating that AGL20 acts downstream to the mutated genes. Moreover, we can show that AGL20 is also regulated by the gibberellin (GA) pathway, indicating that AGL20 integrates signals of different pathways of¯oral induction and might be a central component for the induction of¯owering. In addition, the constitutive expression of AGL20 in Arabidopsis is suf®cient for photoperiod independent¯owering and the overexpression of the orthologous gene from mustard, MADSA, in the classical short-day tobacco Maryland Mammoth bypasses the strict photoperiodic control of¯owering.

SummaryDuring the transition to¯owering the FPF1 gene is expressed in the peripheral zone of apical meristems and in¯oral meristems of Arabidopsis. Constitutive expression of FPF1 causes early¯owering in Arabidopsis under both long-day and short-day conditions and leads to a shortened juvenile phase as measured by the trichome distribution on the abaxial leaf surface. In the classical late¯owering mutants, overexpression of FPF1 compensates partially for the late¯owering phenotype, indicating that FPF1 acts downstream or in a parallel pathway to the mutated genes. The co-overexpression of 35S::AP1 with 35S::FPF1 leads to a synergistic effect on the shortening of the time to¯owering under shortday conditions. The co-overexpression of 35S::FPF1 and 35S::LFY, however, shows only an additive reduction of owering time and the conversion of nearly every shoot meristem, except the in¯orescence meristem, to a¯oral meristem under the same light conditions. In addition, the constitutive expression of FPF1 attenuates the severe lfy-1 phenotype under short days and phenocopies to a great extent the lfy-1 mutant grown under long-day conditions. Thus, we assume that FPF1 modulates the competence to¯owering of apical meristems.

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