A study on the FRI complex-mediated regulation of flowering and floral morphogenesis in Arabidopsis thaliana

Abstract

Flowering of winter annual Arabidopsis is prevented before winter, but the prolonged winter cold, called vernalization, accelerates flowering of the plants in response to environmental cues next spring. The key regulator of such flowering behavior is FLOWERING LOCUS C (FLC), which represses flowering but is suppressed by vernalization. The expression of FLC is transcriptionally activated by FRIGIDA-complex (FRI-C) which includes FRIGIDA LIKE1 (FRL1), FRIGIDA ESSENTIAL1 (FES1), FLC EXPRESSOR (FLX) and SUPPRESSOR OF FRIGIDA4 (SUF4). However, it is not known how vernalization affects the activity of FRI-C when FRI-C is not needed any longer. Here we show that the components of FRI-C are affected at neither transcriptional level nor translational level by vernalization. Instead, we show that the FRI-C structure is changed and FLC gene loop is disrupted by? vernalization. Before vernalization, FRI-C forms a large protein complex, ca 1 MDa size but after vernalization, a subcomplex containing SUF4, ca 200kDa size, is dissociated from the large complex. Additionally, we show that fri and suf4 mutant fails to produce FLC gene loop. Taken together, we propose that FRI and SUF4 are required for FLC looping and the disruption of the FLC gene loop caused by SUF4 subcomplex dissociation is a molecular basis of inability to activate FLC after vernalization.

Functions of FRI-C for FLC regulation is clearly elucidated, such that FRI-C activates FLC transcription with the general transcription factors and FRI-associated proteins. Among the components of FRI-C, SUF4 binds the promoter of FLC, FES1 and FLX provide the activity of transcriptional activation, and FRI acts as a scaffold protein to recruit not only all the components of FRI-C but also basal transcription factors and chromatin remodeling factors such as SWR1 complex. However, if the components of FRI-C have other function has not been addressed. In this study, we show that SUF4 and FES1, the two components of FRI-C, have functions in the development of flower, especially developing the right number of floral organs. When the tissue specificity of FRI-C were analyzed, we found that the components of FRI-C is highly expressed in flower tissues. Furthermore, flowers of suf4 fes1 double mutants produce extra floral organs, demonstrating that SUF4 and FES1 regulate floral organ number. Consistent with this, the expressions of genes regulating floral organ numbers such as CLAVATA (CLV), PERIANTHIA (PAN), ULTRAPETALA1 (ULT1) and PETAL LOSS (PTL) were altered in suf4 fes1 double mutants. Chromatin immunoprecipitation analysis shows that SUF4 is enriched at the?promoter region of CLV1, CLV3, PAN, ULT1 and PTL, suggesting that SUF4 directly regulates these genes. Taken together, we propose that the components of FRI-C regulates the number of floral organs by regulating the genes determining floral organ numbers.

In this study, we disclose that FRI-C regulates FLC expression by controlling gene loop. We also show that the components of FRI-C regulate floral organ numbers. From these results, we suggest that the FRI-C is a multi-functional protein regulating various developmental processes.