Tomato fruit-weight 2.2 (FW2.2) was reported to control up to 30 % fruit weight. Recent studies demonstrated that FW2.2-like (FWL) genes also play important roles in plant growth and development. For instance, a maize homolog of FW2.2, named cell number regulator 1 (CNR1), negatively regulates plant and organ size. However, FWL genes in rice have not been characterized yet. In this study, eight FWL genes were identified in rice genome and designated as OsFWL1-8. The chromosome location, gene structure, protein motif, and phylogenetic relationship of OsFWL genes were analyzed. RT-PCR result and microarray data revealed that OsFWL genes exhibited diverse expression patterns and the detailed expression patterns of OsFWL5, 6, and 7 negatively correlated with leaf growth activity. Rice protoplast transient transformation experiment showed that most OsFWL proteins locate at cell membrane but OsFWL8 is present in the nucleus. In addition, the functions of OsFWL genes were investigated by analyzing two T-DNA insertion lines for OsFWL3 and 5. Compared with wild type, the grain weight of osfwl3 mutant and the plant height of osfwl5 mutant were increased by 5.3 and 12.5 %, respectively. We also found that the increase in grain length of osfwl3 mutant was due chiefly to incremental cell number, not cell size and the expression of OsFWL3 negatively correlated with glume growth activity. These results provide a comprehensive foundation for further study of OsFWL functions in rice.
Heterogeneous expression of the rice genes "fruit-weight 2.2-like" (OsFWL) affects Cd resistance in yeast, and OsFWL4 mediates the translocation of Cd from roots to shoots. Cadmium (Cd) induces chronic and toxic effects in humans. In a previous study (Xu et al. in Planta 238:643-655, 2013), we cloned the rice genes, designated OsFWL1-8, homologous to the tomato fruit-weight 2.2. Here, we show that expression of genes OsFWL3-7 in yeast confers resistance to Cd. The Cd contents of OsFWL3-, -4-, -6- and -7-transformed Cd(II)-sensitive yeast mutant ycf1 cells were strongly decreased compared with those of empty vector, with the strongest resistance to Cd observed in cells expressing OsFWL4. Evaluation of truncated and site-directed mutation derivatives revealed that the CCXXG motifs near the second transmembrane region of OsFWL4 are involved in Cd resistance in yeast. Real-time PCR analysis showed that OsFWL4 expression was induced by CdCl stress in rice seedlings. Compared with WT plants, the Cd contents in the shoots of RNAi mediated OsFWL4 knockdown plants were significantly decreased, and Cd translocation from roots to shoots was reduced. According to bimolecular fluorescence complementation, yeast two-hybrid and Western-blotting assays, the OsFWL4 protein forms homo-oligomers. These results suggest that OsFWL4 might act directly as a transporter and is involved in the translocation of Cd from roots to shoots in rice.
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