Green fluorescent protein (GFP) was discovered from the jellyfish Aequorea victoria, and several improvements have been carried out to change its physicochemical properties. The resulting improved GFP variants have been used as reporter proteins for bioimaging techniques in various research fields including plant science. Almost all GFP variants were developed using Escherichia coli to improve fluorescence properties in mammalian cells, but the impact in other organisms such as plant cells remains to be determined. In this study, we performed comparative analysis of four improved GFP variants, GFP-S65T, eGFP, frGFP and sfGFP, with reference to the fluorescence intensity in Arabidopsis protoplasts, and found that sfGFP is the brightest. Using non-fluorescent fragments from the GFP variants, we also conducted bimolecular fluorescence complementation (BiFC) assays to find appropriate fragment pairs of GFP-based BiFC for visualization of protein-protein interactions in living plant cells. Our observations revealed that the brightest is the sfGFP-based BiFC. Further, as an evaluation method for the sfGFP-based BiFC, a BiFC competition assay was successfully completed for the first time in planta. The present study provides useful information for selection and improvement of the GFP molecule and its application to BiFC technology in plants.Key words: Arabidopsis, BiFC, BiFC competition, GFP, protein-protein interaction.Since the first attempt to employ wild-type green fluorescent protein (wtGFP) as a reporter protein, it has been developed for bioimaging techniques in various research fields including plant science (Chalfie et al. 1994;Heim et al. 1995). wtGFP is a typical β-barrel structure harboring a chromophore spontaneously formed by the three residues (S65-T66-G67), and thereby exhibits green fluorescence without any cofactor (Tsien 1998). To date, wtGFP has been improved by targeted or random mutations to increase its fluorescence intensity, and almost all of the improvements were performed using Escherichia coli for suitable expression in mammalian cells (Shaner et al. 2005). The first improvement was a single mutation of serine to threonine residue at position 65 (S65T) for the chromophore (Figure 1); the mutation shifted the excitation peak to 490 nm from 395 nm and 470 nm of wtGFP (Heim et al. 1995). Compared with wtGFP, this peak-shifted GFP (psGFP) gives 6-fold brighter Abbreviations: BiFC, bimolecular fluorescence complementation; eGC, the C-terminal fragment of eGFP; eGFP, enhanced GFP; eGN, the N-terminal fragment of eGFP; EYFP, enhanced yellow fluorescent protein; frGC, the C-terminal fragment of frGFP; frGFP, folding reporter GFP; frGN, the N-terminal fragment of frGFP; GFP, green fluorescent protein; MXMT, 7-methylxanthine methyltransferase; psGC, the C-terminal fragment of psGFP; psGFP, peak-shifted GFP; psGN, the N-terminal fragment of psGFP; PEG, polyethylene glycol; RC, the C-terminal fragment of DsRED monomer; sfGC, the C-terminal fragment of sfGFP; sfGFP, superfolder GFP; sfGN, the N-...