Guang-Hua He

The spikelet is a unique inflorescence structure of grass. The molecular mechanism that controls the development of the spikelet remains unclear. In this study, we identified a rice (Oryza sativa) spikelet mutant, multi-floret spikelet1 (mfs1), that showed delayed transformation of spikelet meristems to floral meristems, which resulted in an extra hull-like organ and an elongated rachilla. In addition, the sterile lemma was homeotically converted to the rudimentary glume and the body of the palea was degenerated in mfs1. These results suggest that the MULTI-FLORET SPIKELET1 (MFS1) gene plays an important role in the regulation of spikelet meristem determinacy and floral organ identity. MFS1 belongs to an unknown function clade in the APETALA2/ethylene-responsive factor (AP2/ERF) family. The MFS1-green fluorescent protein fusion protein is localized in the nucleus. MFS1 messenger RNA is expressed in various tissues, especially in the spikelet and floral meristems. Furthermore, our findings suggest that MFS1 positively regulates the expression of LONG STERILE LEMMA and the INDETERMINATE SPIKELET1 (IDS1)-like genes SUPERNUMERARY BRACT and OsIDS1.

The control of floral organ identity by homeotic MADS box genes is well established in eudicots. However, grasses have highly specialized outer floral organs, and the identities of the genes that regulate the highly specialized outer floral organs of grasses remain unclear. In this study, we characterized a MIKC-type MADS box gene, CHIMERIC FLORAL ORGANS (CFO1), which plays a key role in the regulation of floral organ identity in rice (Oryza sativa). The cfo1 mutant displayed defective marginal regions of the palea, chimeric floral organs, and ectopic floral organs. Map-based cloning demonstrated that CFO1 encoded the OsMADS32 protein. Phylogenetic analysis revealed that CFO1/OsMADS32 belonged to a monocot-specific clade in the MIKC-type MADS box gene family. The expression domains of CFO1 were mainly restricted to the marginal region of the palea and inner floral organs. The floral organ identity gene DROOPING LEAF (DL) was expressed ectopically in all defective organs of cfo1 flowers. Double mutant analysis revealed that loss of DL function mitigated some of the defects of floral organs in cfo1 flowers. We propose that the CFO1 gene plays a pivotal role in maintaining floral organ identity through negative regulation of DL expression.

Summary: As an important agronomic trait, leaf rolling in rice (Oryza sativa L.) has attracted much attention from plant biologists and breeders. Moderate leaf rolling increases the amount of photosynthesis in cultivars and hence raises grain yield. Here, we describe the map‐based cloning of the gene RL14, which was found to encode a 2OG‐Fe (II) oxygenase of unknown function. rl14 mutant plants had incurved leaves because of the shrinkage of bulliform cells on the adaxial side. In addition, rl14 mutant plants displayed smaller stomatal complexes and decreased transpiration rates, as compared with the wild type. Defective development could be rescued functionally by the expression of wild‐type RL14. RL14 was transcribed in sclerenchymatous cells in leaves that remained wrapped inside the sheath. In mature leaves, RL14 accumulated mainly in the mesophyll cells that surround the vasculature. Expression of genes related to secondary cell wall formation was affected in rl14‐1 mutants, and cellulose and lignin content were altered in rl14‐1 leaves. These results reveal that the RL14 gene affects water transport in leaves by affecting the composition of the secondary cell wall. This change in water transport results in water deficiency, which is the major reason for the abnormal shape of the bulliform cells.

A dynamically rolled leaf mutant (rl10) was identified from a spontaneous mutation in an Oryza sativa L. subsp. indica line, II-32B. The leaf chlorophyll content of rl10 is higher than that of the wild type. Genetic analysis using 3 F₂segregating populations derived from crosses between rl10 and the rice lines Mian5B, II-32B, and D62B, respectively, confirmed that the rolled leaf trait of rl10 is controlled by a single recessive gene. Of 719 SSR primer pairs that showed polymorphism between D62B and rl10, 151 were adopted to map the RL10(t) gene using an F₂segregating population of the cross rl10 × D62B, which contained 352 recessive plants. RL10(t) was primarily mapped on the long arm of chromosome 9, 5.09 cM from marker RM105 and 5.13 cM from marker RM3912. Using a novel set of 22 primer pairs between RM105 and RM3912, RL10(t) was further mapped between markers rlc3 (0.72 cM in distance) and rlc12 (0.1 cM in distance) using an F₂/F₃population containing 1172 recessive individuals. Mapped position analysis and homology analysis of the 20 genes within the 194-kb region between these 2 markers both indicated that a gene encoding a Myb-like domain transcription factor with homology to Arabidopsis KANADI (annotated in PAC clone AP005904) is the most probable candidate for RL10(t). This study enables further investigation of whether KANADI-like Myb genes are involved in leaf polarity modeling in monocots, as they are in dicots.