ions, a basic region and a helix-loop-helix. The former is required for DNA binding whereas the latter is needed for protein dimerization. Based on DNA-binding ability, the proteins are divided into two groups, 1) DNA-binding bHLH and 2) non-DNA-binding bHLH also known as atypical bHLH. It is 
likely that atypical bHLH function to inhibit bHLH from binding to DNA through heterodimerization. A typical bHLH protein, PIF3, encoded by Phytochrome Interaction Factors 3, can bind to the G-box motif in the promoter region of target genes and is involved in light signaling. In contrast, an atypical bHLH protein, HFR1, is unable to bind either phytochrome A or B proteins. However, HFR1 modulates phytochrome signaling through heterodimerization with PIF3. Recent studies have revealed antagonistic roles of HLH/bHLH proteins in various plant organ sizes. For instance, an antagonistic pair of bHLH proteins, Increased Leaf Inclination and ILI1 binding bHLH, controls cell length in the lamina joint and leaf bending in rice. Likewise, an Ili homolog of Arabidopsis, Paclobutrazol Resistance1, and AtIBH1 regulate cell elongation. Activation-tagged bri1 suppressor 1-Dominant and ATBS1 interaction factors regulate leaf cell size through a brassinosteroid signaling pathway. It is predicted that there are 167 bHLH genes in Arabidopsis, 177 in rice, 99 in poplar, 190 in moss and 13 from five algae species. Despite its vital role, the function of rice bHLH is poorly understood; so far, only,10% of genes have been characterized in rice, compared to 38% in Arabidopsis. In this study, we overexpressed an atypical bHLH gene named POSITIVE REGULATOR OF GRAIN LENGTH 1 in rice lemma/palea and found increases in the length and weight of the grain. We identified a typical bHLH protein named HLH/bHLH Pairs for Grain Length and Weight in Rice ANTAGONIST OF PGL1 as an interaction partner of PGL1 and the complex of them is localized in the nucleus. Silencing of APG by RNAi resulted in the same grain phenotype overexpression of PGL1. Our results suggest that PGL1 and APG antagonistically regulate rice grain length and weight by controlling cell elongation in lemma/palea through heterodimerization. Results Overexpression of an atypical PGL1 increases grain length and weight Os03g0171300 is an atypical bHLH gene that is not included in the predicted 177 bHLH genes of rice, and is reported to be homologous to tomato Style2.1, a gene that controls style cell length in tomato, at a level of 66% and 63% amino acid identity for the whole sequence and HLH domain, respectively. Analyses of a rice homolog of Os03g0171300 and Style2.1, Ili1, showed that it is involved in cell length in determining lamina joints of rice. Based on sequence homology, they detected six homologs of Ili1 in the rice genome, and called one of them, Os03g0171300, Ili6. Most members of the ILI family RO4929097 biological activity belong to subfamily 16 of the atypical bHLH protein family, however, the function of Os03g0171300/ ILI6 has not been elucidated yet. We named Os03g0171300 as POSITIVE REGULATOR OF GRAIN LENGTH 1, and analyzed PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22183349 its expression by RT-PCR. The result showed that it is expressed in the pistil, lemma/palea, young panicle, and predominantly in root but not leaf. To analyze the function of PGL1, we overexpressed the gene by using a rice chitinase promoter, which was reported to induce gene expression predominantly in rice florets especially in pistils . Twenty-four and nine independent T0 transgenic lines overexpressing PGL
