RETARDED PALEA1 controls palea development and floral zygomorphy in rice

Poaceae, one of the largest flowering plant families in angiosperms, evolved distinct inflorescence and flower morphology diverging from eudicots and other monocots. However, the mechanism underlying the specification of flower morphology in grasses remains unclear. Here we show that floral zygomorphy along the lemma-palea axis in rice (Oryza sativa) is partially or indirectly determined by the CYCLOIDEA (CYC)-like homolog RETARDED PALEA1 (REP1), which regulates palea identity and development. The REP1 gene is only expressed in palea primordium during early flower development, but during later floral stages is radially dispersed in stamens and the vascular bundles of the lemma and palea. The development of palea is significantly retarded in the rep1 mutant and its palea has five vascular bundles, which is similar to the vascular pattern of the wild-type lemma. Furthermore, ectopic expression of REP1 caused the asymmetrical overdifferentiation of the palea cells, altering their floral asymmetry. This work therefore extends the function of the TCP gene family members in defining the diversification of floral morphology in grasses and suggests that a common conserved mechanism controlling floral zygomorphy by CYC-like genes exists in both eudicots and the grasses.     

Identification and fine mapping of a mutant gene for palealess spikelet in rice

In grass, the evolutionary relationship between lemma and palea, and their relationship to the flower organs in dicots have been variously interpreted and wildely debated. In the present study, we carried out morphological and genetic analysis of a palealess mutant (pal) from rice (Oryza sativa L.), and fine mapping the gene responsible for the mutated trait. Together, our findings indicate that the palea is replaced by two leaf-like structures in the pal flowers, and this trait is controlled by one recessive gene, termed palealess1 (pal1). With a large F2 segregating population, the pal1 gene was finally mapped into a physical region of 35 kb. Our results also suggest that the lemma and palea of rice are not homologous organs, palea is likely evolutionarily equivalent to the eudicot sepal, and the pal1 should be an A function gene for rice floral organ identity.     

MOSAIC FLORAL ORGANS1, an AGL6-Like MADS Box Gene,Regulates Floral Organ Identity and Meristem Fate in Rice

Floral organ identity and meristem determinacy in plants are controlled by combinations of activities mediated by MADS box genes. AGAMOUS-LIKE6 (AGL6)-like genes are MADS box genes expressed in floral tissues, but their biological functions are mostly unknown. Here, we describe an AGL6-like gene in rice (Oryza sativa), MOSAIC FLORAL ORGANS1 (MFO1/MADS6), that regulates floral organ identity and floral meristem determinacy. In the flower of mfo1 mutants, the identities of palea and lodicule are disturbed, and mosaic organs were observed. Furthermore, the determinacy of the floral meristem was lost, and extra carpels or spikelets developed in mfo1 florets. The expression patterns of floral MADS box genes were disturbed in the mutant florets. Suppression of another rice AGL6-like gene, MADS17, caused no morphological abnormalities in the wild-type background, but it enhanced the phenotype in the mfo1 background, indicating that MADS17 has a minor but redundant function with that of MFO1. Whereas single mutants in either MFO1 or the SEPALLATA-like gene LHS1 showed moderate phenotypes, the mfo1 lhs1 double mutant showed a severe phenotype, including the loss of spikelet meristem determinacy. We propose that rice AGL6-like genes help to control floral organ identity and the establishment and determinacy of the floral meristem redundantly with LHS1.     

Diversification of three <Emphasis Type="Italic">APETALA1/FRUITFULL</Emphasis>-like genes in wheat

The genomes of grass family species have three paralogs of APETALA1/FRUITFULL (AP1/FUL)-like genes (FUL1, FUL2 and FUL3) that are derived from the FUL lineage. In this study, we focus on the different roles of the wheat AP1/FUL-like genes, WFUL1 (identical to VRN1), WFUL2 and WFUL3, during the transition from vegetative to reproductive growth. Sequence analysis indicated that there was a high level of variability in the amino acid sequence of the C-domain among three WFUL genes. Expression analyses using the spring wheat cultivar Chinese Spring indicated that WFUL1/VRN1 was expressed in leaves as well as spike primordia of non-vernalized plants at the vegetative stage just before phase transition, while WFUL2 and WFUL3 were not expressed in leaves. This result indicates that WFUL1/VRN1 performs a distinct role in leaves before phase transition. In young spikes, WFUL1/VRN1 and WFUL3 were expressed in all developing floral organs, whereas WFUL2 expression was restricted in the floral organs to the lemma and palea. Furthermore, yeast two-hybrid and three-hybrid analyses revealed that WFUL2, but not WFUL1/VRN1 or WFUL3, interacted with class B and class E proteins. These results suggest that WFUL2 of wheat has class A functions in specifying the identities of floral meristems and outer floral organs (lemma and palea) through collaboration with class B and class E genes.     

BEAK-SHAPED GRAIN 1/TRIANGULAR HULL 1, a DUF640 gene, is associated with grain shape, size and weight in rice

Grain shape and size both determine grain weight and therefore crop yield. However, the molecular mechanisms controlling grain shape and size are still largely unknown. Here, we isolated a rice mutant, beak-shaped grain1 (bsg1), which produced beak-shaped grains of decreased width, thickness and weight with a loosely interlocked lemma and palea that were unable to close tightly. Starch granules were also irregularly packaged in the bsg1 grains. Consistent with the lemma and palea shapes, the outer parenchyma cell layers of these bsg1 tissues developed fewer cells with decreased size. Map-based cloning revealed that BSG1 encoded a DUF640 domain protein, TRIANGULAR HULL 1, of unknown function. Quantitative PCR and GUS fusion reporter assays showed that BSG1 was expressed mainly in the young panicle and elongating stem. The BSG1 mutation affected the expression of genes potentially involved in the cell cycle and GW2, an important regulator of grain size in rice. Our results suggest that BSG1 determines grain shape and size probably by modifying cell division and expansion in the grain hull.     

Functional characterization of the stunt lemma palea 1 mutant allele in rice

The rice stunted lemma/palea 1 (slp1) mutant displays a dwarf, shortened panicle length, degenerated lemma and palea, and sterility. A previous study suggested that a missense mutation at the sixth amino acid of the OsSPL16 protein was likely to be responsible for the slp1 mutant phenotype. The current study shows that the overexpression of the wild-type OsSPL16 allele in slp1/slp1 and Slp1/slp1 mutants was unable to convert the slp1 mutant phenotype to normal. However, the introduction of the mutant OsSPL16 allele into a normal rice cultivar led to the slp1 mutant phenotype in transgenic plants. These results indicated that the missense mutation in OsSPL16 creates a neomorphic allele, which affects plant height and the development of the inflorescence and spikelet.     

Bacterial leaf streak 1 encoding a mitogen-activated protein kinase confers resistance to bacterial leaf streak in rice

Bacterial leaf streak (BLS) is a major bacterial disease of rice. Utilization of host genetic resistance has become one of the most important strategies for controlling BLS. However, only a few resistance genes have been characterized. Previously, a recessive BLS resistance gene bls1 was roughly mapped on chromosome 6. Here, we further delineated bls1 to a 21 kb region spanning four genes. Genetic analysis confirmed that the gene encoding a mitogen-activated protein kinase (OsMAPK6) is the target of the allelic genes BLS1 and bls1. Overexpression of BLS1 weakened resistance to the specific Xanthomonas oryzae pv. oryzicola (Xoc) strain JZ-8, while low expression of bls1 increased resistance. However, both overexpression of BLS1 and low expression of bls1 could increase no-race-specific broad-spectrum resistance. These results indicate that BLS1 and bls1 negatively regulate race-specific resistance to Xoc strain JZ-8 but positively and negatively control broad-spectrum resistance, respectively. Subcellular localization demonstrated that OsMAPK6 was localized in the nucleus. RGA4, which is known to mediate resistance to Xoc, is the potential target of OsMAPK6. Overexpression of BLS1 and low expression of bls1 showed increase in salicylic acid and induced expression of defense-related genes, simultaneously increasing broad-spectrum resistance. Moreover, low expression of bls1 showed increase an in jasmonic acid and abscisic acid, in company with an increase in resistance to Xoc strain JZ-8. Collectively, our study provides new insights into the understanding of BLS resistance and facilitates the development of rice host-resistant cultivars.     

Ectopic expression of <Emphasis Type="Italic">OsMADS1</Emphasis> caused dwarfism and spikelet alteration in rice

In rice, an E-class gene, OsMADS1, acts to specify the identities of the lemma and palea. In this study, the OsMADS1 gene with a CaMV35S promoter was transformed into a japonica cultivar, Zhonghua 11. All transgenic plants successfully showed similar phenotypes, including dwarfism, distorted panicles, decreased numbers of branches and spikelets, and elongated sterile lemma. Histological analysis showed that the elongated sterile lemma developed with silicified epidermal and sclerenchymal cells, which were lacking in the wild-type sterile lemma, suggesting that the elongated sterile lemma had assumed the identity of the lemma or palea. Some marker genes were subjected to a detailed analysis of the distribution of their expression among the lemma, palea and sterile lemma. DROOPING LEAF (DL) and OsMADS6 genes were only expressed in the normal lemma or palea, respectively. In the elongated sterile lemma, a high level of DL gene expression was detected, while no expression of OsMADS6 was found, implying that the sterile lemma transformed into the lemma but not the palea. These results provide clues to elucidate the mechanism of evolution from lemma to sterile lemma in rice. qPCR analysis also suggested that the ectopic expression of OsMADS1 induced abnormal brassinosteroid and gibberellin acid activation, and then resulted in developmental defects in the stem and panicle.     

Reproductive biology of Setaria magna Griseb. (Poaceae: Panicoideae: Paniceae)

The genus Setaria is economically important because many species are cultivated for grains or forage. Setaria magna is an American species, native to North America and introduced in South America. The morphological aspect of this species is similar to S. italica (foxtail millet), suggesting its potential value as a crop. The purpose of this work was to understand the breeding system (self-pollination vs. open pollination) of S. magna; additionally, the floral development was described. The results of the breeding system analysis indicated that S. magna is mainly autogamous and does not appear to have a self-incompatibility mechanism. The floral development observed was similar to that described for other Paniceae; in most of the spikelets only the upper anthecium developed fruit, but a small number of spikelets presented bisexual lower florets. These spikelets produced two caryopses per spikelet. Thus, S. magna can produced two types of dispersal units; a few whitish naked caryopses from the lower anthecium that fall without their lemma and palea at maturity; and a high number of brown caryopses protected by the lemma and palea from the upper anthecium. The presence of a normal embryo sac in the upper anthecium suggests that S. magna would present normal sexual reproduction, although we can not reject the formation of apomitic seeds in the lower anthecium.     

一个水稻内颖退化突变体的形态特征及基因的精确定位

水稻产量和品质受花器官发育的直接影响,因此对水稻颖花发育机理的研究将有助于水稻产量提高和品质的改良。文章利用60Coγ射线辐照亲本8PW33(籼稻背景)获得一个性状能稳定遗传的内颖退化突变体(编号:MU102),并对其农艺性状和花器官进行了观察和分析。结果显示,相对于野生型,该突变体的株高、每穗总粒数及剑叶宽均显著增加,而结实率则显著降低,差异均达显著水平。解剖镜下观察表明,该突变体内颖退化,外颖弯曲呈现镰刀状,其余器官与野生型表型基本一致。扫描电镜观察显示,突变体与野生型叶片维管束的结构组成以及外颖表皮细胞组成、排列均正常,没有明显差异;与野生型相比,突变体内颖表皮细胞排列较为紧密,推测可能是内颖收缩退化导致的。遗传分析显示该突变性状是由隐性单基因控制,并命名为pd2。利用实验室现有的SSR分子标记将PD2基因定位于水稻第9号染色体上,通过进一步扩大群体和开发新的Indel标记,将PD2基因定位在2个Indel标记之间,两者间的物理距离大约是82 kb。在该物理区间内有一个已经克隆的内颖发育基因REP1,经过测序和比对分析,推测REP1与PD2为等位基因。     

Molecular mapping of split rice spikelet mutantsrs-1 and analysis of its homeotic function in rice

srs-1, a new floral organ identity gene in rice, was mapped using RAPD and RFLP markers. Firstly, the cross was made between "ZhaiYeQing 8" (ZYQ8, indica) and split rice spikelet (SRS, japonica) mutant. The ratio of wild-type individuals and mutant plants in F2 population is 3:1, which indicates that the mutant characteristics are controlled by single recessive gene, srs-1. Consequently, BSA method was adopted and 520 random 10-mer primers were used to screen polymorphic bands between two bulks. Six primers could amplify polymorphic bands, of which S465 generates the most stable RAPD patterns. Then, S465 that cosegregates in F2 population has been converted into an RFLP marker successfully. Furthermore, srs-1 gene was mapped on chromosome 3 using DH mapping population. The effect of srs-1 gene results in the mutant of split rice spikelet. The mutant has longer and softer palea/lemma than those of wild-type, and two small palea/lemma-like organs between palea and lemma. In addition, there is a flower wit