The grass spikelet is a unique inflorescence structure that determines grain size. Although many genetic factors have been well characterized for grain size and glume development, the underlying molecular mechanisms in rice are far from established. Here, we isolated rice gene, AGL1 that controlled grain size and determines the fate of the sterile lemma. Loss of function of AGL1 produced larger grains and reduced the size of the sterile lemma. Larger grains in the agl1 mutant were caused by a larger number of cells that were longer and wider than in the wild type. The sterile lemma in the mutant spikelet was converted to a rudimentary glume-like organ. Our findings showed that the AGL1 (also named LAX1) protein positively regulated G1 expression, and negatively regulated NSG1 expression, thereby affecting the fate of the sterile lemma. Taken together, our results revealed that AGL1 played a key role in negative regulation of grain size by controlling cell proliferation and expansion, and supported the opinion that rudimentary glume and sterile lemma in rice are homologous organs.

Increasing tiller number is a target of high-yield rice breeding. Identification of tiller-defect mutants and their corresponding genes is helpful for clarifying the molecular mechanism of rice tillering. Summarizing research progress on the two processes of rice tiller formation, namely the formation and growth of axillary meristem, this paper reviews the effects of genetic factors, endogenous hormones, and exogenous environment on rice tillering, finding that multiple molecular mechanisms and signal pathways regulating rice tillering cooperate rice tillering, and discusses future research objectives and application of its regulatory mechanism. Elucidation of theis mechanism will be helpful for breeding high-yielding rice cultivars with ideal plant type via molecular design breeding.

Lesion mimic mutants (LMMs) are advantageous materials for studying programmed cell death (PCD). Although some rice LMM genes have been cloned, the diversity of functions of these genes indicates that the mechanism of cell death regulation in LMMs needs further study. In this study, we identified a rice light-dependent leaf lesion mimic mutant 4 (llm4) that showed abnormal chloroplast structure, photoinhibition, reduced photosynthetic protein levels, massive accumulation of reactive oxygen species (ROS), and PCD. Map-based cloning and complementation testing revealed that LLM4 encodes zeaxanthin epoxidase (ZEP), an enzyme involved in the xanthophyll cycle, which functions in plant photoprotection, ROS scavenging, and carotenoid and abscisic acid (ABA) biosynthesis. The ABA content was decreased, and the contents of 24 carotenoids differed between the llm4 mutant and the wild type (WT). The llm4 mutant showed reduced dormancy and greater sensitive to ABA than the WT. We concluded that the mutation of LLM4 resulted in the failure of xanthophyll cycle, in turn causing ROS accumulation. The excessive ROS accumulation damaged chloroplast structure and induced PCD, leading eventually to the formation of lesion mimics.

Chlorophyll, a green pigment in photosynthetic organisms, is generated by two distinct biochemical pathways, the tetrapyrrole biosynthetic pathway (TBP) and the methylerythritol 4-phosphate (MEP) pathway. MEP is one of the pathways for isoprenoid synthesis in plants, with 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR) catalyzing its last step. In this study, we isolated a green-revertible yellow leaf mutant gry3 in rice and cloned the GRY3 gene, which encodes a HDR participating in geranylgeranyl diphosphate (GGPP) biosynthesis in chloroplast. A complementation experiment confirmed that a missense mutation (C to T) in the fourth exon of LOC_Os03g52170 causes the gry3 phenotype. Under high temperature and high light, transcript and protein abundances of GRY3 were reduced in the gry3 mutant. Transcriptional expression of chlorophyll biosynthesis, chloroplast development, and genes involved in photosynthesis were also affected. Excessive reactive oxygen species accumulation, cell death, and photosynthetic proteins degradation were occurred in the mutant. The content of GGPP was reduced in gry3 compared with Nipponbare, resulting in a stoichiometric imbalance of tetrapyrrolic chlorophyll precursors. These results shed light on the response of chloroplast biogenesis and maintenance in plants to high-temperature and high-light stress.

The development of rice cultivars with improved nitrogen use efficiency (NUE) is desirable for sustainable agriculture. Achieving this goal depends in part on understanding how rice responds to low soil nitrogen (N) and identifying causative genes underlying this trait. To identify quantitative trait loci (QTL) or genes associated with low N response, we conducted a genome-wide association study (GWAS) using a diverse panel of 230 rice accessions and performed a transcriptomic investigation of rice accessions with differential responses to low N stress at two N levels. We detected 411 GWAS-associated genes in 5 QTL and 2722 differentially expressed genes in response to low N, of which 24 were identified by both methods and ranked according to gene annotations, literature queries, gene expression, and genetic diversity analysis. The large-scale datasets obtained from this study reveal low N-responsive characteristics and provide insights towards understanding the regulatory mechanisms of N-deficiency tolerance in rice, and the candidate genes or QTL would be valuable resources for increasing rice NUE via molecular biotechnology.