Tiller angle and leaf angle are two important components of rice (Oryza sativa) plant architecture that play a crucial role in determining grain yield. Here, we report the cloning and characterization of the Loose Plant Architecture1 (LPA1) gene in rice, the functional ortholog of the AtIDD15/SHOOT GRAVITROPISM5 (SGR5) gene in Arabidopsis (Arabidopsis thaliana). LPA1 regulates tiller angle and leaf angle by controlling the adaxial growth of tiller node and lamina joint. LPA1 was also found to affect shoot gravitropism. Expression pattern analysis suggested that LPA1 influences plant architecture by affecting the gravitropism of leaf sheath pulvinus and lamina joint. However, LPA1 only influences gravity perception or signal transduction in coleoptile gravitropism by regulating the sedimentation rate of amyloplasts, distinct from the actions of LAZY1. LPA1 encodes a plant-specific INDETERMINATE DOMAIN protein and defines a novel subfamily of 28 INDETERMINATE DOMAIN proteins with several unique conserved features. LPA1 is localized in the nucleus and functions as an active transcriptional repressor, an activity mainly conferred by a conserved ethylene response factor-associated amphiphilic repression-like motif. Further analysis suggests that LPA1 participates in a complicated transcriptional and protein interaction network and has evolved novel functions distinct from SGR5. This study not only facilitates the understanding of gravitropism mechanisms but also generates a useful genetic material for rice breeding.Rice (Oryza sativa) is the staple food for more than half of the world's population. In the past 50 years, the green revolution and the use of heterosis have greatly improved rice yields (Peng et al., 2008). However, because of the increasing demand for rice production, food security can still be a serious problem. Thus, new elite varieties that can produce much higher grain yields need to be developed, and ideal plant architecture (ideotype) breeding has been shown to be the most promising strategy in tropical areas (Khush, 2005). Rice plant architecture is mainly determined by plant height and tiller and panicle morphology, of which tiller angle and leaf angle are two important agronomic traits.Tiller angle, defined as the angle between the main culm and its side tillers, has long attracted the attention of breeders due to the significant contribution of this trait to plant architecture .

Summary Panicle architecture is one of the most important agronomical traits that directly contribute to grain yield in rice (Oryza sativa L.). We report herein an in‐depth characterization of two allelic larger panicle (lp) mutants that show significantly increased panicle size as well as improved plant architecture. Morphological analyses reveal that panicles of two mutants produced more inflorescence branches, especially the primary branches, and contained more grains. Moreover, mutant plants also display more lodging resistance than the wild type. The grain yield per plant in mutants is also increased, suggesting that mutant plants have useful potential for high grain yield in rice breeding. Map‐based cloning reveals that LARGER PANICLE (LP) encodes a Kelch repeat‐containing F‐box protein. RNA in situ hybridization studies display that LP expression was enriched in the branch primordial region. Subcellular localization analyses demonstrate that LP is an endoplasmic reticulum (ER) localized protein, suggesting that LP might be involved in ER‐associated protein degradation (ERAD). Using yeast two‐hybrid assay and bimolecular fluorescence complementation analysis, we confirm that LP is an F‐box protein and could interact with rice SKP1‐like protein in an F‐box domain‐dependent manner. Quantitative real‐time PCR results show that OsCKX2, which encodes cytokinin oxidase/dehydrogenase, is down‐regulated evidently in mutants, implying that LP might be involved in modulating cytokinin level in plant tissues. These results suggest that LP plays an important role in regulating plant architecture, particularly in regulating panicle architecture, thereby representing promising targets for genetic improvement of grain production plants.