Abstract||Successful reproduction in plants is a highly-regulated process reliant on the integration of both endogenous and external cues. Different accessions of the model plant Arabidopsis thaliana have been collected, including those with a winter annual or rapid-cycling flowering habit. Natural variation and mutant screens have enabled many flowering time genes to be identified. A key regulator of flowering is FLOWERING LOCUS C (FLC). FLC is a repressor of flowering and is regulated by a number of genes, including those in the autonomous and FRIGIDA-mediated pathways.
Of particular interest are FRIGIDA (FRI) and FRIGIDA-LIKE 1 (FRL1) and the autonomous pathway members, FCA and FY. FRI and FRL 1 promote FLC expression making them dominant repressors of flowering. FRI is proposed to initiate chromatin remodelling at the FLC locus leading to increased FLC expression. Once elevated, FLC levels are maintained until plants undergo an extended period of cold, therefore flowering occurs in spring. In contrast, FCA and FY promote flowering by repressing FLC expression. FCA has also been identified as a receptor of the plant hormone abscisic acid (ABA). Upon binding to FCA, ABA is proposed to disrupt/inhibit the FCA:FY interaction which results in delayed flowering.
To characterise the FCA:ABA interaction and identify the ABA binding site, a number of truncated FCA proteins were utilised. Initially a FCA:FY GST-pulldown was used to identify the ABA binding site. However, when ABA failed to inhibit the FCA:FY interaction a direct binding assay using [³H]-ABA was employed. Another Arabidopsis ABA receptor, G-protein coupled receptor 2 (GCR2), was used as a positive control in these binding assays. Both FCA and GCR2 failed to bind [³H]-ABA suggesting a broader issue with the binding assay. The identification of FCA and GCR2 as ABA receptors can be attributed to the quality of the protein assayed, the sensitivity of the binding assay and the subsequent data analysis. This study resulted in the retraction of the original paper (Razem et at, 2006) reporting FCA as an ABA receptor.
To investigate the molecular mechanism by which FRI and FRL1 act as positive regulators of FLC expresion, a biochemical approach was taken. FRI and FRL1 have no known homology to any other protein or domain and the only method for assessing protein function is through plant complementation experiments. In the absence of sequence homology, or a timely functional assay, a classical approach was taken to produce soluble protein for analysis. Truncation of predicted regions of disorder and expression, solubility and stability screens produced soluble protein of reasonable purity. This allowed characterisation of the biochemical properties of FRI and FRL1. Interaction studies between FRI and FRL1, and the zinc finger protein SUPRESSOR OF FRIGIDA 4 (SUF4), were also carried out. Polyclonal antibodies against FRI and FRL1, made during this study, were useful for protein detection in these experiments. The interaction studies, together with plant complementation experiments, suggest that the C-terminus of FRI is essential for protein function, while the N-terminus improves FRI activity. These findings provide a better understanding of how the components of the proposed "FRI-complex" may interact to promote FLC expression.