Grain shape is an important agronomic trait affecting rice yield and quality, and its development is regulated by the three-dimensional morphology of grain (grain length, grain width, grain thickness). Identification and cloning of grain shape regulatory genes can enrich the molecular mechanism of rice grain development regulation, and provide theoretical basis and genetic resources for high-yield molecular design breeding of rice.

A stable inherited grain type mutant sgd13 (small grain and dwarf 13) was screened from the mutant library of Nanjing 9108 induced by ethyl methane sulfonate (EMS). The grain morphology, 1000-grain weight, seed setting rate, yield per plant, plant height, panicle length and other phenotypes of the mutants were statistically analyzed. Paraffin sections and scanning electron microscopy were used to analyze the changes in the number and size of glume and stem cells. The genetic analysis of sgd13 and Nanjing 9108 was carried out. The F₂ population constructed by sgd13 and Nanjing 9108 was used to locate the gene by BSA-seq technology. The SWISS-MODEL website was used to predict the three-dimensional structure of wild-type and mutant proteins.

The grains of sgd13 were significantly smaller and narrower, the grain length decreased by 19.98%, and the grain width decreased by 7.81%. Compared with WT, the plant height, spike length and yield per plant of sgd13 were significantly reduced. There was no significant difference in the number of internodes between sgd13 and WT, but the lengths of the first, second, third and sixth internodes were shorter. Cytological analysis showed that the glume and stem cells of sgd13 became smaller and less, indicating that sgd13 may affect organ development by regulating cell division and expansion. Genetic analysis confirmed that the trait was controlled by a single recessive nuclear gene. The candidate gene was mapped to LOC_Os01g52550 by BSA-seq, which encodes an ATP-binding cassette (ABC) transporter. The ABC transporter contains two typical core domains: A highly conserved nucleotide binding domain (NBD) and a less conserved transmembrane domain (TMD). In the sgd13 mutant, a single base substitution (T→A) occurred in the exon region of the gene, which was located in the NBD domain. This single base substitution directly causes the encoded amino acid to change from glutamic acid (E) to aspartic acid (D). Due to the differences in side chain structure and chemical properties between glutamic acid and aspartic acid, this change is likely to affect the spatial structure of SGD13 protein, thereby interfering with its normal function, and ultimately leading to a unique phenotype of the mutant sgd13. Genetic complementation experiments showed that the introduction of wild-type LOC_Os01g52550 could restore the grain shape of sgd13 to the wild-type level.

The sgd13 mutant phenotype was controlled by a single recessive nuclear gene, which was caused by the LOC_Os01g52550 mutation. The T→A mutation in the exon region of the gene causes the glutamic acid in the NBD domain to become aspartic acid, which affects the three-dimensional structure of the protein.

Rice panicle apical abortion affects yield. Identification and cloning of genes related to rice panicle apical abortion can enrich the molecular mechanism of rice panicle development regulation, and provide theoretical basis and genetic resources for rice high-yield molecular design breeding.

Here, a stably inherited panicle apical abortion 21 (paa21) mutant was screened from EMS mutant library of the japonica rice variety "Wuyunjing 30". Agronomic traits, such as ratio of degraded primary branches, degraded apical spikelets, grains per panicle, plant height, panicle length, and grain yield per plant, were statistically analyzed. Trypan blue and Evans blue staining were used to detect whether programmed cell death occurred in the apical spikelets. H₂O₂ content in young panicles at different development stages and different panicle parts of WT and paa21 was determined. Genetic analysis was carried out by reciprocal cross of paa21 with indica rice II-32B and 9311 respectively. The F₂ population constructed by crossing paa21 with indica rice II-32B was used for gene mapping and cloning. The three-dimensional structure of wild-type and paa21 proteins were predicted using SWISS-MODEL website. The expression levels of ROS response marker genes, programmed cell death related genes and catalase related genes were analyzed by RT-qPCR.

paa21 produced panicle apical abortion phenotype and the degenerated spikelets were mainly located on the primary branches at the apical panicle. The plant height, grain number per panicle, panicle length and grain yield per plant of paa21 were lower than those of WT. After observing the young panicles at different development stages, we found that the paa21 mutant had a panicle apical abortion phenotype when panicle developed to 12 cm. Trypan blue and Evans blue staining results showed that the apical spikelets of the paa21 mutant had programmed cell death. Stronger DAB staining was observed in the degenerated apical spikelets of paa21 than WT. The results of H₂O₂ content determination showed that higher level of ROS was accumulated in panicle of paa21 compared with WT. Genetic analysis suggested that paa21 mutant phenotype is controlled by a pair of recessive nuclear genes. The results of map-based cloning showed that a C to T mutation occurred in the second exon of Os02g0673100 in paa21, resulting in the mutation of alanine to valine. This gene encodes an aluminum activated malate transporter, ALMT7. The mutation site was located at the fourth transmembrane helix. SWISS-MODEL prediction results showed that the mutation site did not significantly affect the three-dimensional structure of the mutant protein. The expression level of ROS response marker genes Os01g0826400, Os05g0474800 and Os02g0181300 in paa21 was significantly higher than that in WT when the young spike developed to 10 cm. Compared with WT, the expression level of programmed cell death related genes VPE2 and VPE3 increased significantly in paa21. The expression level of CATA, CATB and CATC which encode catalase in 10 cm young panicle of paa21 was significantly higher than that of WT. The activity of CAT in paa21 10 cm young spikelet was significantly lower than that of WT.

paa21 accumulate excess ROS in the apical spikelet at late stage of panicle development, resulting in programmed cell death, which eventually leads to the degeneration of the apical spikelet. These results lay a good foundation for further enriching the genetic regulatory network of panicle development.