Panicle Enclosure Research Articles

OsWRKY78 regulates panicle exsertion via gibberellin signaling pathway in rice.

Panicle exsertion is one of the crucial agronomic traits in rice (Oryza sativa). Shortening of panicle exsertion often leads to panicle enclosure and severely reduces seed production. Gibberellin (GA) plays important roles in regulating panicle exsertion. However, the underlying mechanism and the relative regulatory network remain elusive. Here, we characterized the oswrky78 mutant showing severe panicle enclosure, and found that the defect of oswrky78 is caused by decreased bioactive GA contents. Biochemical analysis demonstrates that OsWRKY78 can directly activate GA biosynthesis and indirectly suppress GA metabolism. Moreover, we found OsWRKY78 can interact with and be phosphorylated by mitogen-activated protein kinase (MAPK) kinase OsMAPK6, and this phosphorylation can enhance OsWRKY78 stability and is necessary for its biological function. Taken together, these results not only reveal the critical function of OsWRKY78, but also reveal its mechanism via mediating crosstalk between MAPK and the GA signaling pathway in regulating panicle exsertion.

Base Editing of EUI1 Improves the Elongation of the Uppermost Internode in Two-Line Male Sterile Rice Lines

The use of male sterile lines (MSLs) of rice is essential for heterosis utilization. However, MSLs have a common defect in the elongation of the uppermost internode, eventually leading to incomplete panicle exsertion, blocking pollination, and reducing the hybrid rice seed yield. Previously, the elongated uppermost internode 1 (EUI1) was identified as an active gibberellin-deactivating enzyme that plays a key role in panicle exsertion from the flag leaf sheath in rice (Oryza sativa L.). We used an adenine base editor to edit EUI1 and obtained two types of homozygous transgenic plants (eui1-1 and eui1-2). The transcription and translation levels of EUI1 in the two mutants were significantly lower than in the wild type, as was the oxidation activity of EUI1 to active gibberellins (GAs), which also decreased. The contents of the plant hormones GA1, GA3, and GA4 in eui1-1 (1.64, 1.55, and 0.92 ng/g) and eui1-2 (0.85, 0.64, and 0.65 ng/g) panicles were significantly higher than the wild type (0.70, 0.57, and 0.42 ng/g). The uppermost internode lengths of the mutant were 26.5 and 23.6 cm, which were significantly longer than that of the wild type (18.0 cm), and the cell lengths of the mutant were 161.10 and 157.19 μm, which were longer than that of the wild type (89.28 μm). Our results indicate that the adenine base editing system could increase the content of endogenous bioactive GAs in young panicles by fine-tuning EUI1 activity, reduce the defect of panicle enclosure in MSLs and increase the yield of hybrid rice seed production.

A new allele PEL9 GG identified by genome-wide association study increases panicle elongation length in rice (Oryza sativa L.).

Considering the male sterile line has the phenomenon of panicle enclosure, panicle elongation length (PEL) plays an important role in hybrid rice seed production. However, the molecular mechanism underlying this process is poorly understood. In this study, we investigated the PEL phenotypic values of 353 rice accessions across six environments, which shows abundant phenotypic variation. Combining the 1.3 million single-nucleotide polymorphisms, we performed a genome-wide association study on PEL. Three quantitative trait loci (QTL) qPEL4, qPEL6, and qPEL9 were identified as significantly associated with PEL, of which qPEL4 and qPEL6 were previously reported QTLs and qPEL9 was novel. One causal gene locus, PEL9, was identified and validated. The PEL of accessions carrying allele PEL9 GG was significantly longer than that of those carrying allele PEL9 TT. We also demonstrated that the outcrossing rate of female parents carrying allele PEL9 GG increased by 14.81% compared with that of the isogenic line carrying allele PEL9 TT in an F1 hybrid seed production field. The allele frequency of PEL9GG increased gradually with an increase in latitude in the Northern Hemisphere. Our results should facilitate the improvement of the PEL of the female parent of hybrid rice.

Inter-auricular distance: A morphological biomarker for ethylene production in the boot and coincidental grain loss of rice plant

Inter-auricular distance (IAD) has been often used for physical calibration of defects in panicle emergence of rice plants induced by stress-prone environments or cytoplasmic male sterility genetic factors slackening male gametogenesis. Information is scant on the physiological relationship inherent between IAD, panicle enclosure, and spikelet filling despite the wide phenotypic diversity of rice cultural types. In the present study, IAD extension and grain filling pattern in the panicle of the main shoot of rice cultivars contrasting for spikelet sterility were assessed concerning physiological factors like assimilate homeostasis, ethylene evolution, and expression of ethylene signal transducers genes at the anthesis stage of development. The panicle retention time was longer and the concentration and expression of ethylene and its transducer protein genes respectively, were higher in high sterile cultivars compared to low sterile ones. Spikelets subjected to a high concentration of ethylene for a longer time interval filled poorly because of infringement of starch synthesis in the post-anthesis stage of development. Alternatively, faster release of panicle by enhanced IAD extension shortened the exposure time and marginalized inhibitory effects of ethylene on grain filling. Rapid IAD extension and attenuation of boot ethylene production could be beneficial for male gametogenesis and grain filling.

水稻小粒不育系新组合卓两优141混播制种优势分析

<p id="C3">Traditional hybrid breeding is backward, low benefit, and high cost, which restricts the promotion and application in hybrid rice. However, Small Grain TMS lines are easy to be mechanically separated after mixed-sowing and mixed-harvesting. Their application will promote the realization of complete Mechanization of Hybrid Rice Seed Production, leading to the reduction of cost and the improvement of seed production efficiency. In this study, we investigated the agronomic characteristics and outcrossing rates suitable for Mechanization of Hybrid Rice Seed Production, using the Small Grain male sterile line Zhuo 201S, Large Grain restorer line R141, and their combination Zhuoliangyou 141 as materials for mixed-sowing and large-scale seed production practice. The results demonstrated that compared with the control TMS C815S, Zhuo 201S plants were shorter, exhibiting longer and more erect ear, fewer glume opening, lower percentage of panicle enclosure and germination on ears. Moreover, Zhuo 201S plants were resistant to smut disease but sensitive to “920” treatment and had weak seed-shattering characteristic. However, R141 plants were tall and insensitive to environmental temperature with large amount of pollen and a long florescence. Both Zhuo 201S and R141 had good outcrossing characteristics. The grain thickness of Zhuo 201S and R141 was 1.71 mm and 2.23 mm, respectively. The 1000-grain weight of Zhuo 201S and R141 was 14.00 g and 28.20 g, respectively. Due to the significant difference in grain sizes, the hybrid F₁ seeds could be easily separated with the male parent seeds through a special sieve with narrow and long apertures. The percentage of male parent seeds mixed in hybrid seeds was 0 while the hybrid seed loss rate was 2.31%, suggesting that the seed purity met the standards for Mechanization of Hybrid Rice Seed Production. Compared with the traditional seed production mode, the basic seedling of male parent was reduced by 85%, the capacity of female parent was increased by 20%, the seed production yield was increased by 21.37%, and the comprehensive benefit of seed production was increased by 31.4%. Zhuoliangyou 141 was endowed with the advantages suitable for mechanized production by mix-sowing, with excellently agronomic traits and outcrossing properties along with huge difference in grain size from its parents. Therefore, the whole process of Zhuoliangyou 141 mechanized seed production had a broad application prospect.

Metatranscriptomic Sequencing Suggests the Presence of Novel RNA Viruses in Rice Transmitted by Brown Planthopper.

Viral pathogens are a major threat to stable crop production. Using a backcross strategy, we find that integrating a dominant brown planthopper (BPH) resistance gene Bph3 into a high-yield and BPH-susceptible indica rice variety significantly enhances BPH resistance. However, when Bph3-carrying backcross lines are infested with BPH, these BPH-resistant lines exhibit sterile characteristics, displaying panicle enclosure and failure of seed production at their mature stage. As we suspected, BPH-mediated viral infections could cause the observed sterile symptoms, and we characterized rice-infecting viruses using deep metatranscriptomic sequencing. Our analyses revealed eight novel virus species and five known viruses, including a highly divergent virus clustered within a currently unclassified family. Additionally, we characterized rice plant antiviral responses using small RNA sequencing. The results revealed abundant virus-derived small interfering RNAs in sterile rice plants, providing evidence for Dicer-like and Argonaute-mediated immune responses in rice plants. Together, our results provide insights into the diversity of viruses in rice plants, and our findings suggest that multiple virus infections occur in rice plants.

Genome-wide association analysis of panicle exsertion and uppermost internode in rice (Oryza sativa L.)

BackgroundRice (Oryza sativa L.) yield is seriously influenced by panicle exsertion (PE) and the uppermost internode (UI) through panicle enclosure or energy transport during grain-filling stages. We evaluated the traits of PE and UI of 205 rice accessions in two independent environments and performed genome-wide association (GWAS) to explore the key genes controlling PE and UI, which could be used to improve panicle enclosure in rice breeding.ResultsIn this study, extensive genetic variation was found in both PE and UI among the 205 rice accessions, and 10.7% of accessions had panicle enclosure (PE/UI ≤ 0). Correlation analysis revealed that PE was significantly positively correlated with 1000-grain weight (1000-GW) but negatively correlated with heading date (HD), and UI was significantly positively correlated with HD but no significantly correlated with 1000-GW. A total of 22 and 24 quantitative trait loci (QTLs) were identified for PE and UI using GWAS, respectively. Eight loci for PE and nine loci for UI were simultaneously detected both in 2015 and in 2016, seven loci had adjacent physical positions between PE and UI, and ten loci for PE and seven loci for UI were located in previously reported QTLs. Further, we identified the CYP734A4 gene, encoding a cytochrome P450 monooxygenase, and the OsLIS-L1 gene, encoding a lissencephaly type-1-like protein, as causal genes for qPE14 and qUI14, and for qPE19, respectively. PE and UI were both significantly shorter in these two genes’ mutants than in WT. Allelic Hap.1/2/4 of CYP734A4 and Hap.1/2/4 of OsLIS-L1 increased PE, UI, PE/UI, and 1000-GW, but Hap.3 of CYP734A4 and Hap.3 of OsLIS-L1 reduced them. In addition, six candidate genes were also detected for four key novel loci, qPE16, qPE21, qUI1, and qUI18, that seemed to be related to PE and UI.ConclusionsOur results provide new information on the genetic architecture of PE and UI in rice, confirming that the CYP734A4 and OsLIS-L1 genes participate in PE and UI regulation, which could improve our understanding of the regulatory mechanism of PE and UI for rice breeding in the future.

Genome-wide association study of outcrossing in cytoplasmic male sterile lines of rice

Stigma exsertion and panicle enclosure of male sterile lines are two key determinants of outcrossing in hybrid rice seed production. Based on 43,394 single nucleotide polymorphism markers, 217 cytoplasmic male sterile lines were assigned into two subpopulations and a mixed-group where the linkage disequilibrium decay distances varied from 975 to 2,690 kb. Genome-wide association studies (GWAS) were performed for stigma exsertion rate (SE), panicle enclosure rate (PE) and seed-setting rate (SSR). A total of 154 significant association signals (P < 0.001) were identified. They were situated in 27 quantitative trait loci (QTLs), including 11 for SE, 6 for PE, and 10 for SSR. It was shown that six of the ten QTLs for SSR were tightly linked to QTLs for SE or/and PE with the expected allelic direction. These QTL clusters could be targeted to improve the outcrossing of female parents in hybrid rice breeding. Our study also indicates that GWAS-base QTL mapping can complement and enhance previous QTL information for understanding the genetic relationship between outcrossing and its related traits.

Rice HOX12 Regulates Panicle Exsertion by Directly Modulating the Expression of ELONGATED UPPERMOST INTERNODE1.

Bioactive gibberellins (GAs) are key endogenous regulators of plant growth. Previous work identified ELONGATED UPPERMOST INTERNODE1 (EUI1) as a GA-deactivating enzyme that plays an important role in panicle exsertion from the flag leaf sheath in rice (Oryza sativa). However, the mechanism that regulates EUI1 activity during development is still largely unexplored. In this study, we identified the dominant panicle enclosure mutant regulator of eui1 (ree1-D), whose phenotype is caused by the activation of the homeodomain-leucine zipper transcription factor HOX12. Diminished HOX12 expression by RNA interference enhanced panicle exsertion, mimicking the eui1 phenotype. HOX12 knockdown plants contain higher levels of the major biologically active GAs (such as GA1 and GA4) than the wild type. The expression of EUI1 is elevated in the ree1-D mutant but reduced in HOX12 knockdown plants. Interestingly, both HOX12 and EUI1 are predominantly expressed in panicles, where GA4 is highly accumulated. Yeast one-hybrid, electrophoretic mobility shift assay, and chromatin immunoprecipitation analyses showed that HOX12 physically interacts with the EUI1 promoter both in vitro and in vivo. Furthermore, plants overexpressing HOX12 in the eui1 mutant background retained the elongated uppermost internode phenotype. These results indicate that HOX12 acts directly through EUI1 to regulate panicle exsertion in rice.

Genetic Analysis and Mapping of an Enclosed Panicle Mutant Locus esp1 in Rice (Oryza sativa L.)

A mutant was isolated from the M2 of 60Co-γ ray mutagenized male-fertility restorer line Zao-R974 in rice. The mutant showed pleiotropic phenotypes including dwarfism, delayed heading time, short and partially enclosed panicles, short uppermost internode, decreased grain and secondary branch numbers per panicle, and partially degenerated spikelets. The mutant was named as esp1 (enclosed shorter panicle 1). Genetic analysis indicated that the mutant phenotype was controlled by a recessive locus. Spraying exogenous GA3 did not rescue the panicle enclosure. Using an F2 and a BC1 population of the cross between esp1 and a japonica cultivar Nipponbare, we mapped the ESP1 locus to a region of ∼260 kb on chromosome 11. This result provides a basis for further map-based cloning of the ESP1 locus.

Improving panicle exsertion of rice cytoplasmic male sterile line by combination of artificial microRNA and artificial target mimic

The adoption of hybrid rice caused the second leap in rice yield after the 'green revolution' and contributes substantially to food security of China and the world. However, almost all cytoplasmic male sterile lines (A lines) as females of hybrid rice have a natural deficiency of 'panicle enclosure', which blocks pollination between the A line and the fertility restorer line as the male (R line) of hybrid rice and decreases seed yield. In hybrid rice seed production, exogenous '920' (the active ingredient is gibberellin A3 ) must be applied to eliminate or alleviate panicle enclosure of the A line; however, this not only increases production cost and pollutes the environment, it also decreases seed quality. In this study, we designed a transgenic approach to improve plant height and panicle exsertion of the A line to facilitate hybrid rice production and maintain the semi-dwarf plant type of the hybrid. This approach comprising two components-artificial microRNA (amiRNA) and artificial target mimicry-can manipulate the differential expression of the endogenous Eui1 gene that is associated with rice internode elongation in the A line and the hybrid. amiRNA is a recently developed gene silencing method with high specificity, while target mimicry is a natural mechanism inhibiting the miRNA function that was also recently characterized. This approach provides a paradigm to tune the expression of endogenous genes to achieve the desired phenotype by combining amiRNA and artificial target mimicry technologies.

Elongation of the uppermost internode for Shuangdi Pei eS, TGMS rice with eui gene

Shuangdi Pei eS, a thermo-sensitive genic male sterile (TGMS) rice line with eui gene is derived from TGMS rice line Shuangdi S by irradiation with 350 Gy60Co γ-ray. To elucidate the uppermost internode elongation of TGMS line with eui gene, Shuangdi Pei eS and its original TGMS line Shuangdi S were used to study the effects of temperature on panicle exsertion. At 25°C, the uppermost internode of Shuangdi Pei eS had the quickest elongation from the 4th day before flowering to 0 day (flowering), being 2.1-fold as that of Shuangdi S, whereas it elongated slowly during the 12th day to the 4th day before flowering and the 1st to the 3rd day after flowering. The uppermost internode of Shuangdi Pei eS exserted from the flag leaf sheath at 23, 25 and 27°C, respectively, and the length of elongated uppermost internode increased with the decreasing temperatures. At 30°C, though the panicles of Shuangdi Pei eS were still enclosed in the leaf sheath, the degree of panicle enclosure was significantly lower compared with Shuangdi S. Cytological studies on Shuangdi Pei eS which showed that the uppermost internode elongation was attributed to the increase of cell number and cell elongation, and the latter was more significant. Moreover, the numbers of outermost and innermost parenchyma cells and the cell length of the uppermost internode reduced with the increasing temperatures. Key words: Rice (Oryza sativa L.), thermo-sensitive genic male sterility (TGMS), elongated uppermost internode gene (eui), panicle exsertion, temperature.

Downregulation of OsPK1, a cytosolic pyruvate kinase, by T-DNA insertion causes dwarfism and panicle enclosure in rice

Pyruvate kinase (PK) catalyzes the final step of glycolysis. There are few reports on the role of PK in rice. Here, we identified a novel rice dwarf mutant, designated as ospk1, showing dwarfism, panicle enclosure, reduced seed set, and outgrowth of axillary buds from culm nodes. Sequence analyses of 5'-RACE indicated that a single T-DNA was inserted in the transcriptional regulatory region of OsPK1 in ospk1. Quantitative RT-PCR result showed that OsPK1 expression was decreased by approximately 90% in ospk1 compared with that in WT. Enzyme assay and transient expression in protoplasts indicated that OsPK1 encodes a cytosolic PK (PK(c)). Complementation with OsPK1 demonstrated that OsPK1 is responsible for the phenotype of ospk1. Quantitative RT-PCR and GUS staining analyses exhibited that OsPK1 was expressed mainly in leaf mesophyll cells, phloem companion cells in stems, and cortical parenchyma cells in roots. The transcriptions of four other putative enzymes involved in the glycolysis/gluconeogenesis pathway were altered in ospk1. The amount of pyruvate is decreased in ospk1. We propose that OsPK1 plays an important role through affecting the glycolytic pathway. The contents of glucose and fructose were markedly accumulated in flag leaf blade and panicle of ospk1. The sucrose level in panicle of ospk1 was decreased by approximately 84%. These findings indicated that both monosaccharide metabolism and sugar transport are altered due to the decreased expression of OsPK1. Together, these results provide new insights into the role of PK(c) in plant morphological development, especially plant height.

Genetic analysis and fine mapping of an enclosed panicle mutant esp2 in rice (Oryza sativa L.)

The phenomenon of panicle enclosure in rice is mainly caused by the shortening of uppermost internode. Elucidating the molecular mechanism of panicle enclosure will be helpful for solving the problem of panicle enclosure in male sterile lines and creating new germplasms in rice. We acquired a monogenic recessive enclosed panicle mutant, named as esp2 (enclosed shorter panicle 2), from the tissue culture progeny of indica rice cultivar Minghui-86. In the mutant, panicles were entirely enclosed by flag leaf sheaths and the uppermost internode was almost completely degenerated, but the other internodes did not have obvious changes in length. Genetic analysis indicated that the mutant phenotype was controlled by a recessive gene, which could be steadily inherited and was not affected by genetic background. Apparently, ESP2 is a key gene for the development of uppermost internode in rice. Using an F2 population of a cross between esp2 and a japonica rice cultivar Xiushui-13 as well as SSR and InDel markers, we fine mapped ESP2 to a 14-kb region on the end of the short arm of chromosome 1. According to the rice genome sequence annotation, only one intact gene exists in this region, namely, a putative phosphatidylserine synthase gene. Sequencing analysis on the mutant and the wild type indicated that this gene was inserted by a 5287-bp retrotransposon sequence. Hence, we took this gene as a candidate of ESP2. The results of this study will facilitate the cloning and functional analysis of ESP2 gene.

Study on panicle enclosure characteristics and morphological and anatomical structure of uppermost internode in CMS line of rice

The cDNA library normalized by reassociation is a newly-developed and effective platform for EST acquisition and gene discovery.It decreases the prevalence of clones representing abundant transcripts and dramatically increases the efficiency of random sequencing and rare gene discovery.The principle,procedure and applications of normalized cDNA library were reviewed in this paper,which provides theoretical basis for the development of normalized cDNA library and discover more novel genes.

Elongation of the Uppermost Internode for Changxuan 3S, a Thermo-Sensitive Genic Male Sterile Rice Line

Changxuan 3S, a thermo-sensitive genic male sterile (TGMS) rice line with eui gene, is derived from the TGMS rice line Pei'ai 64S by irradiation with 350 Gy of 60Co γ-ray. To elucidate the uppermost internode elongation of the TGMS line with eui gene, Changxuan 3S and its parent Pei'ai 64S were used to study the effects of temperature on panicle exsertion. At 24°C, the uppermost internode of Changxuan 3S elongated the fastest from the 4 th day before flowering to 0 day (flowering), being 2.1-fold as that of Pei'ai 64S, whereas it elongated slowly during the 12 th day to the 4 th day before flowering and the 1 st to the 3 rd day after flowering. The uppermost internode of Changxuan 3S exserted from the flag leaf sheath at 22°C, 24°C and 26°C, and the length of elongated uppermost internode increased with the decreasing temperatures. At 28°C, though the panicles of Changxuan 3S were still enclosed in the leaf sheath, the degree of panicle enclosure was significantly lower compared with Pei'ai 64S. Cytological studies on Changxuan 3S showed that the uppermost internode elongation was attributed to the increase of cell number and cell elongation, and the latter was more significant. Moreover, the numbers of outermost and innermost parenchyma cells and the cell length of the uppermost internode reduced with the increasing temperatures.

Mapping of QTL for Three Panicle Exsertion-Related Traits in Rice Under Different Growing Environments

Panicle exclosure in cytoplasmic male sterility (CMS) line is highly correlated with outcrossing rate of CMS in hybrid rice ( Oryza sativa L.) seed production. Panicle enclosure occurs when flag leaf sheath length exceeds the uppermost internode length. To improve panicle exsertion, 98 backcross inbred lines (BILs), derived from a backcross of Nipponbare ( japonica)/Kasalath ( indica)//Nipponbare by the single seed descent method, were planted in 3 growing environments to disclose the genetic mechanism of flag leaf sheath length, the uppermost internode length, and panicle enclosure. At 5% of genome-wide type I error, QTLs of the 3 traits by Composite Interval Mapping (CIM) method were detected. Three of them, located on chromosomes 1, 3, and 4, were for flag leaf sheath length. They explained the phenotypic variances by 12.8–18.5%. The QTL, qFLL-1, was detected in all the 3 environments, whose positive allele was from Nipponbare. The QTLs, qFLL-3 and qFLL-4, were detected in a single environment with positive alleles from Kasalath. For the uppermost internode length, 3 QTLs were detected on chromosomes 1, 3, and 6. They explained 5.6–14.2% of the phenotypic variances. Among them, qUIL-6 was detected in all the 3 environments, and Nipponbare donated the positive allele in the locus. The other 2 QTLs were detected in 2 environments, and Kasalath carried the positive allele. Four QTLs for panicle enclosure length were located on chromosomes 1, 3, 5, and 10, which explained 6.8–17.8% of the phenotypic variances. Among them, qPEL-10 and qPEL-5 were detected in 3 and 2 environments, respectively, whereas the other 2 QTLs, qPEL-1 and qPEL-3, were only in a single environment. The positive alleles of qPEL-10 and qPEL-5 originated from Nipponbare, and the positive alleles of qPEL-1 and qPEL-3 were donated by Kasalath.

Interaction between the eui gene and thermo-sensitive genic male sterility in rice

Panicle enclosure is a typical phenotype of almost all male-sterile rice lines. An elongated uppermost internode (eui) mutant exhibited notably rapid elongation of the uppermost internode at the heading stage; this is considered as a potential mechanism to eliminate panicle enclosure. We developed thermo-sensitive genic male-sterile (TGMS) eui mutants that were characterized by notably elongated uppermost internodes. The elongation of the uppermost internode in the TGMS eui mutant Changxuan 3S (CX) is mainly attributed to an increase in cell number and cell elongation, the latter being the more significant process. Temperature treatments revealed that the effects of temperature on panicle exsertion were similar to those on fertility and that the most temperature-sensitive stage coincides with the period from the formation of the pollen mother cell to meiosis during panicle initiation. These results indicate that elongation increases as temperature decreases and that the expression of the eui gene is more efficient at low temperatures than at high temperatures. In hybrid rice seed production using the TGMS eui mutant, the temperature range should be optimized at 24–28°C in order to preserve the development of completely male-sterile pollen and to eliminate panicle enclosure. Consequently, by using TGMS eui rice lines, gibberellin application can be avoided, thereby reducing the cost of hybrid seed production.

Decreased panicle-derived indole-3-acetic acid reduces gibberellin A1 level in the uppermost internode, causing panicle enclosure in male sterile rice Zhenshan 97A

Cytoplasmic male sterile (CMS) rice Zhenshan 97A (ZS97A) has been widely used in hybrid rice production in China. However, ZS97A suffers from serious panicle enclosure, which blocks normal pollination and greatly reduces seed production of hybrid rice. Little is known about the cause of panicle closure in ZS97A. In this study, it was found that the occurrence of cytoplasmic male sterility caused a deficiency of indole-3-acetic acid (IAA) in ZS97A panicles, and less IAA was provided to the uppermost internode (UI). Further, it was found that the decreased panicle-derived IAA caused a gibberellin A(1) (GA(1)) deficiency in the UI by the down-regulation of OsGA3ox2 transcript level. Reduced GA(1) level in the UI led to decreases of both cell number and cell elongation, resulting in a shortened UI. The shortened UI was unable to push the panicle out of the flag leaf sheath that remained normal, which resulted in panicle enclosure in ZS97A. These findings suggest that decreased panicle-derived IAA reduces the GA(1) level in the UI, causing panicle enclosure in CMS rice ZS97A.

Identification of a 98-kb DNA segment containing the rice Eui gene controlling uppermost internode elongation, and construction of a TAC transgene sublibrary.

The recessive 'tall rice' phenotype associated with the mutation eui (elongated upper-most internode) is an important agronomic trait that has been introduced into hybrid rice to eliminate panicle enclosure in all types of male-sterile lines and produce good-quality seeds in high yield and at low cost. Based on our previous Eui mapping data, we conducted fine-structure mapping and positional cloning of the gene using an F2 population comprising more than 5000 individuals derived from a cross of the near-isogenic lines 307T (eui/eui) with the recurrent parent Zhenshan 97 (Eui/Eui). In total 45 CAPS (cleaved amplified polymorphic sequences) markers located within an interval of 14.5 cM were analyzed in the subpopulation of 1298 homozygous recessive plants. The resulting high-resolution map defined a 98-kb interval containing the Eui locus flanked by the markers M0387 and M01, and three markers were found to co-segregate with Eui. In order to facilitate the identification of the Eui gene, we used a transformation-competent artificial chromosome (TAC) vector to construct a set of contiguous TAC clones from the Nipponbare BACs (obtained from the Clemson University Genome Institute; CUGI) spanning this region. These clones can be used to streamline complementation testing. The markers tightly linked to the Eui locus can also be used in breeding male-sterile lines with the elongated uppermost internode.