Plants have evolved annual and perennial life forms as alternative strategies to adapt reproduction and survival to environmental constraints. In isolated situations, such as islands, woody perennials have evolved repeatedly from annual ancestors 1 . Although the molecular basis of the rapid evolution of insular woodiness is unknown, the molecular difference between perennials and annuals might be rather small, and a change between these life strategies might not require major genetic innovations 2,3 . Developmental regulators can strongly affect evolutionary variation 4 and genes involved in meristem transitions are good candidates for a switch in growth habit. We found that the MADS box proteins SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and FRUITFULL (FUL) not only control flowering time, but also affect determinacy of all meristems. In addition, downregulation of both proteins established phenotypes common to the lifestyle of perennial plants, suggesting their involvement in the prevention of secondary growth and longevity in annual life forms.Plant growth originates from a small number of undifferentiated cells called meristems. Primary meristems are established during embryogenesis and form primary tissues from which all plant organs develop. Secondary meristems, such as axillary meristems and the cambium, originate within primary tissues. Meristems can be determinate-that is, consumed for the formation of an organ-or indeterminate, meaning that they are active throughout the life span of a plant. Upon floral induction in annual plants, vegetative shoot meristems undergo the transition to inflorescence meristems. These inflorescence meristems will remain indeterminate for some time to generate determinate floral meristems giving rise to flowers. Finally, all meristems are consumed and the plants die in the same growing season. In contrast, perennial plants have evolved more elaborate life strategies to survive harsh environmental conditions for many years by forming perennial structures such as overwintering buds, bulbs or tubers, which contain at least one indeterminate meristem for the outgrowth in the next season 2 . Often, perennial plants incorporate enormous amounts of long-lived and eventually dead biomass (wood) through cambial activity (secondary growth).Arabidopsis thaliana is a small annual herb in which floral induction is controlled by different flowering-time pathways. These pathways depend on environmental cues, such as day length (photoperiod) and vernalization (cold temperature), or on plant age. Arabidopsis is a facultative long-day plant that flowers much faster under long (16 h/day) than short (8 h/day) light periods. After perceiving flowering-inducing long days, the key regulator of the photoperiodic pathway, CONSTANS (CO), activates FT (FLOWERING LOCUS T) in the leaf vasculature. The FT protein is transported to apical meristems, where it triggers the floral transition 5 . SOC1 (AGL20) and FUL (AGL8) are MADS box genes acting downstream of FT in apical meristems, but they are ...

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.

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SummaryIn plants of Sinapis alba induced to¯ower by one long day, the MADS box gene, SaMADS A, is expressed initially in the central corpus (L3 cells) of the shoot apical meristem (SAM), about 1.5±2 days before initiation of the ®rst¯oral meristem. We have combined a physiological approach by testing the effects of three putative¯oral signals on SaMADS A expression in the SAM of S. alba plants with a transgenic approach using Arabidopsis thaliana plants. A single application of a low dose of a cytokinin or a gibberellin to the apex of vegetative S. alba plants is capable of mimicking perfectly the initial effect of the long day on SaMADS A transcription. A treatment combining the two hormones causes the same activation but seems to enhance the level of SaMADS A expression. A sucrose application to the apex of vegetative plants is, on the contrary, unable to activate SaMADS A expression. None of these chemicals, alone or combined, is capable of causing the¯oral shift at the SAM. Since the constitutive expression of SaMADS A leads to precocious¯owering in A. thaliana and antisense expression of a fragment of the A. thaliana homologue AGL20 leads to a delay in¯owering time, these results are consistent with SaMADS A activation being an intermediate event in a cytokinin-and/or gibberellin-triggered signal transduction pathway that is involved in the regulation of¯oral transition in S. alba.