The reproductive phase of flowering plants involves at least three initial step s : transition to flowering, initiation of individual flowers and fl oral patterning (Ma et a!. , 1 994). The transition to flowering involves a switch from the vegetative phase, during which shoots and leaves are p roduced, to the reproductive phase, where flowers are i nitiated. I n general, the transition to flowering i s influenced b y developmental programs and pathways that respond to environmental cues . Four pathways are known to influence flowering time; these are the photoperiod pathway, vernalization promotion pathway, the gibberellic acid promotion pathway, and the autonomous pathway (Levy and Dean, 1 99 8 ; B lazquez, 2000; F igure 1 .4). The photoperiod and vernalization p ro motion pathways mediate signals from the environment, light and temperature respectively. The gibberel l in pathway is responsive to GA biosynthesi s whereas the autonomous p athway controls flowering irrespective of environmental conditions (Wilson et al. , 1 992; Blazquez e t a!. , 1 998; Pineiro and Coupland, 1 99 8 ) .
1 . 4 . 2
M ADS-box genes regulate the switch to reproductive phase
There are at least five MADS-box genes invol ved in the regulation o f flowering time in A rahidopsis: SUPPRESSOR OF O VEREXPRESSION OF CO i (SOC1 ), SHOR T VEGE TA TiVE PHASE (SVP), FLO WERfJVG LOCUS C (FL C) , FL O WERiNG L OC US M (FUvf) and FR UITFULL (FUL). These genes either promote or repress /lo weri ng. S()Cl and FUL act to promote floweri ng ( B on h o m me et al. , 2000; S amach et (If. . 200 0) whereas FLC FLM and S VP are inhibitors o f fl o weri ng (Sheldon et 0[ , 2 0 0 0 ; S c ortecci ef oi , 200 1).
5,'()C l acts as an integrator o f the autonomOllS, photoperiod and vernalization f10ral prom o tion pathways in i!rahiLiopsis (Araki, 200 1 ; Lee cl 01. , 2000; Figure I A). S()Cl expre s s I on responds positively to long-day photopcriods. Loss or ,SDC! fun c tion suppresses early fl owering phenotype o f i!ra/;ic/opsis plants overcxpres sing the C()l\/,)'7�-1NS gene, which normally promotes t10wering under long days ( S amach et aI , 2000). Also, SOCl expression is downregulated by a m u tation in FC'A , a gene t hat is
involved in both the vemalisation and autonomous flowering pathways , suggesting that FCA positively regulates sac 1 (Samach et a!. , 2000; Lee et a!. , 2 0 0 0). Thes e observations support the suggestion that SOC1 i ntegrates the photoperiod, vernalisatio n and autonomous floral pro motion pathways. I nterestingly, the G A pathway may also regulate the expression o f SOC1 . GA3 app l ication on wil d type A rabidopsis p lants caused an increase in SOC1 expression (Bomer et a!. , 2000).
FIC is involved in the vemalisation and autonomous pathways In Arabidopsis. M utations i n FIC result i n eady flowering, thus indicating that the role of the wild type FIC i s to repres s flowering (Michaels and Amasino, 1 999). Vernalisation or cold treatment promotes flowering i n late-flowering Arabidopsis plants, possibly through a direct effect on FIC transcripts and protein. This i s supported by a strong negative correlation between FIC transcripts and vernalisation (Sheldon et a!. , 2000). However, vernalisatio n may not affect flowering solely through FIe. This is because fie nul l mutants stil l respond to cold treatment by flowering early suggesting the presence o f FIC-dependent and FIe-independent vernalisation pathways i n A rabidopsis (Michaels and Amasi no, 200 I ) .
S'VP and FIAf are recently identified MADS-box genes tl1at repress flowering i n A rabidopsis b y acti ng independent o f environmental factors (Hartman n et a!. , 2000;
S co rtecci et a!" 200 1 ). FUv! is 7 0% identical to FIC and also controls fl owering i n a
dosage-dependent m an ner as FIe. However, unlike FIC the auto no mous or
vernal i sati o n pathway does not i nfluence its expression. Both S VP and FIAl have s i m i l ar expression patterns but the pathways i n whi c h they repress flowering i s not yet c lear. S i nce M AD S-box proteins form functional DNA bindi ng complexes with other
proteins i t i s suggested that S VP, f L M and f L C prote i ns may i nteract to c on trol flowering (Sc ortecci et of , 200 1 ). MA OS-box genes are therefore i m portant p l ayers i n the genetic pat!nvays regulating the transition t o ilowering i n plants.
Vernalizat ion pathway
v
Autonomous pathway FCA1
Photoperiod pathway FLC colX!
SOC 11AGL20 FT LFY
Gibberellin pathway
Figure 1 .4. A model shows the integrative role of AGL20/S0Cl and the interaction of
flowering pathways in Arabidopsis . [Figure adapted from Araki, T (200 1 )] . The
horizontal line represents the vegetative (V) to reproductive (R) transition. Arrows
indicate promotion, and T-bars indicate repression. In the autonomous pathway, FCA
represses FLC, and FLC represses A GL20/S0CI . AGL20/S0CI acts as a floral activator.
Vernalization also promotes flowering by activating A GL20 expression through the
repression of FLC. Photoperiod pathway gene CONSTANS (CO) promote floweri ng by
acti vating SOC] and also through other factor(s).