The Flowering Time Gene SOC1 (AGL20)

The SOC1 gene, which consists of seven exons and introns, encodes a MADS-domain protein with a molecular weight of 24.5 kDa, which shows 96% amino acid similarity to the mustard homologue MADSA, and 50.6% overall similarity to the rice OsMADS50/OsSOC1 protein. In the MADS domain region the level of similarity between SOC1 and OsMADS50/OsSOC1 is 97%. (Borner et al., 2000; Tadege et al., 2003; Lee et al., 2004) . Additionally SOC1-like sequences have been identified in Sinapis alba, maize (ZmMADS1), tobacco (NtMADSA) and petunia (PhFBP28) (Bonhomme et al., 2000; Tadege et al., 2003) .

SOC1 promotes flowering of Arabidopsis,

(Koornneef et al., 1998b; Lee et al., 2000; Samach et al., 2000) . In short-day grown ArabidopsisSOC1 mRNA is not present in the SAM and leaf primordia, however it is upregulated in both the SAM and leaf primordia after a single inductive long-day photoperiod (Lee et al., 2000; Samach et al., 2000) . The expression pattern of SOC1 mRNA in floral primordia shows similarity to that of AGAMOUS (Yanofsky et al., 1990) , since its mRNA is present in the inflorescence meristem but not in young floral primordia and the mRNA reappears in stage 2 or 3 flowers (Samach et al., 2000) . Studies of RNA expression by Northern blot suggested an almost ubiquitous expression pattern (Lee et al., 2000) .

The abundance of SOC1 mRNA is ten-fold higher in 35S::CO plants in comparison to wild-type Ler ecotype (Samach et al., 2000) . In 35S::CO:GR plants SOC1 mRNA increases three-fold upon the application of dexamethasone (DEX) (which allows fusion proteins containing the GR domain to be imported into the nucleus). The application of the translational inhibitor cycloheximide (CYC) determined that SOC1 is an immediate target of CO, because SOC1 mRNA levels responded similarly to DEX or DEX plus CYC.

soc1 mutant is a suppressor of flowering

The recessive soc1 mutation was identified by Onouchi et al., (2000) , as a suppressor of the early flowering caused by 35S::CO plants. Also, in wild-type backgrounds, soc1 causes a delay in flowering under both long and short day photoperiods. This suggests that the autonomous, as well as photoperiod pathways play a role in the regulation of SOC1 (Samach et al., 2000) . In agreement with this a dominant mutation that causes overexpression of SOC1 was isolated as a suppressor of FRI FLC (Lee et al., 2000) . In these plants, overexpression of SOC1 promotes early flowering but does not reduce FLC levels, and high FLC reduces SOC1 expression (Lee et al., 2000) . These and related experiments confirm that SOC1 acts downstream of FLC and is repressed by FLC (Michaels and Amasino, 2001).

The SOC1 homologue SaMADSA from Sinapis alba is activated by gibberellin (Bonhomme et al., 2000) . Borner et al (2000) have suggested that this is also true for the Arabidopsis SOC1 gene, by showing that there is an increase in the abundance of SOC1 in the primary shoot of plants treated with the gibberellin GA 3. Similarly, the soc1-2 null mutant shows reduced sensitivity to GA in triggering flowering (Moon et al., 2003) .

SOC1 in Rice

In rice, expression of OsMADS50/OsSOC1 is upregulated during the floral transition, and its overexpression leads to extreme early flowering during regeneration of transgenic plants in tissue culture (Lee et al., 2004) . Reducing expression of the gene by RNAi resulted in later flowering (Lee et al., 2004) . Overexpression of the rice gene in Arabidopsis caused earlier flowering. Interestingly, although no FLC homologue has been found in rice, expression of ArabidopsisFLC in rice reduced the expression of OsMADS50/OsSOC1 (Tadege et al., 2003).

Forward to The flowering-time gene FLOWERING LOCUS T (FT)