Changhui Sun

Mutations of genes in synthesis of the carotenoid precursors of ABA lead to pre-harvest sprouting and photo-oxidation in rice.

Pre-harvest sprouting (PHS) or vivipary in cereals is an important agronomic trait that results in significant economic loss. A considerable number of mutations that cause PHS have been identified in several species. However, relatively few viviparous mutants in rice (Oryza sativa L.) have been reported. To explore the mechanism of PHS in rice, we carried out an extensive genetic screening and identified 12 PHS mutants (phs). Based on their phenotypes, these phs mutants were classified into three groups. Here we characterize in detail one of these groups, which contains mutations in genes encoding major enzymes of the carotenoid biosynthesis pathway, including phytoene desaturase (OsPDS), ζ-carotene desaturase (OsZDS), carotenoid isomerase (OsCRTISO) and lycopene β-cyclase (β-OsLCY), which are essential for the biosynthesis of carotenoid precursors of ABA. As expected, the amount of ABA was reduced in all four phs mutants compared with that in the wild type. Chlorophyll fluorescence analysis revealed the occurrence of photoinhibition in the photosystem and decreased capacity for eliminating excess energy by thermal dissipation. The greatly increased activities of reactive oxygen species (ROS) scavenging enzymes, and reduced photosystem (PS) II core proteins CP43, CP47 and D1 in leaves of the Oscrtiso/phs3-1 mutant and OsLCY RNAi transgenic rice indicated that photo-oxidative damage occurred in PS II, consistent with the accumulation of ROS in these plants. These results suggest that the impairment of carotenoid biosynthesis causes photo-oxidation and ABA-deficiency phenotypes, of which the latter is a major factor controlling the PHS trait in rice.

The Histone Methyltransferase SDG724 Mediates H3K36me2/3 Deposition at MADS50 and RFT1 and Promotes Flowering in Rice

In rice, the two florigens Hd3a and RFT1 are separated by only 11.5 kb in the genome. This study reveals that rice SDG724 specifically affects the histone H3 lysine 36 me2/3 level of RFT1 but not its close paralog Hd3a. Therefore, RFT1 and Hd3a have functionally diverged to control flowering time under long-day and short-day conditions partly via a fine-tuned epigenetic mechanism. Chromatin modifications affect flowering time in the long-day plant Arabidopsis thaliana, but the role of histone methylation in flowering time regulation of rice (Oryza sativa), a short-day plant, remains to be elucidated. We identified a late-flowering long vegetative phase1 (lvp1) mutant in rice and used map-based cloning to reveal that lvp1 affects the SET domain group protein 724 (SDG724). SDG724 functions as a histone methyltransferase in vitro and contributes to a major fraction of global histone H3 lysine 36 (H3K36) methylation in vivo. Expression analyses of flowering time genes in wild-type and lvp1 mutants revealed that Early heading date1, but not Heading date1, are misregulated in lvp1 mutants. In addition, the double mutant of lvp1 with photoperiod sensitivity5 (se5) flowered later than the se5 single mutant, indicating that lvp1 delays flowering time irrespective of photoperiod. Chromatin immunoprecipitation assays showed that lvp1 had reduced levels of H3K36me2/3 at MADS50 and RFT1. This suggests that the divergent functions of paralogs RFT1 and Hd3a, and of MADS50 and MADS51, are in part due to differential H3K36me2/3 deposition, which also correlates with higher expression levels of MADS50 and RFT1 in flowering promotion in rice.

RLIN1, encoding a putative coproporphyrinogen III oxidase, is involved in lesion initiation in rice

Lesion mimic is necrotic lesions on plant leaf or stem in the absence of pathogenic infection, and its exact biological mechanism is varied. By a large-scale screening of our T-DNA mutant population, we identified a mutant rice lesion initiation 1 (rlin1), which was controlled by a single nuclear recessive gene. Map-based cloning revealed that RLIN1 encoded a putative coproporphyrinogen III oxidase in tetrapyrrole biosynthesis pathway. Sequencing results showed that a G to T substitution occurred in the second exon of RLIN1 and led to a missense mutation from Asp to Tyr. Ectopic expression of RLIN1 could rescue rlin1 lesion mimic phenotype. Histochemical analysis demonstrated that lesion formation in rlin1 was light-dependent accompanied by reactive oxygen species accumulated. These results suggest that tetrapyrrole participates in lesion formation in rice.

Understanding the genetic and epigenetic architecture in complex network of rice flowering pathways

ABSTRACTAlthough the molecular basis of flowering time control is well dissected in the long day (LD) plant Arabidopsis, it is still largely unknown in the short day (SD) plant rice. Rice flowering time (heading date) is an important agronomic trait for season adaption and grain yield, which is affected by both genetic and environmental factors. During the last decade, as the nature of florigen was identified, notable progress has been made on exploration how florigen gene expression is genetically controlled. In Arabidopsis expression of certain key flowering integrators such as FLOWERING LOCUS C (FLC) and FLOWERING LOCUS T (FT) are also epigenetically regulated by various chromatin modifications, however, very little is known in rice on this aspect until very recently. This review summarized the advances of both genetic networks and chromatin modifications in rice flowering time control, attempting to give a complete view of the genetic and epigenetic architecture in complex network of rice flowering pathways.

Molecular analysis of rice plants harboring a multi-functional T-DNA tagging system.

About 25,000 rice T-DNA insertional mutant lines were generated using the vector pCAS04 which has both promoter-trapping and activation-tagging function. Southern blot analysis revealed that about 40% of these mutants were single copy integration and the average T-DNA insertion number was 2.28. By extensive phenotyping in the field, quite a number of agronomically important mutants were obtained. Histochemical GUS assay with 4,310 primary mutants revealed that the GUS-staining frequency was higher than that of the previous reports in various tissues and especially high in flowers. The T-DNA flanking sequences of some mutants were isolated and the T-DNA insertion sites were mapped to the rice genome. The flanking sequence analysis demonstrated the different integration pattern of the right border and left border into rice genome. Compared with Arabidopsis and poplar, it is much varied in the T-DNA border junctions in rice.

Identification of a Geranylgeranyl reductase gene for chlorophyll synthesis in rice

Geranylgeranyl reductase (CHL P) catalyzes the reduction of geranylgeranyl diphosphate to phytyl diphosphate, and provides phytol for both Chlorophyll (Chl) and tocopherol synthesis. In this study, we isolated a yellow-green leaf mutant, 502ys, in rice (Oryza sativa). The mutant exhibited reduced level of Chls, arrested development of chloroplasts, and retarded growth rate. The phenotype of the 502ys mutant was controlled by by a recessive mutation in a nuclear gene on the long arm of rice chromosome 2. Map-based cloning of the mutant resulted in the identification of an OsChl P gene (LOC_Os02g51080). In the 502ys mutant, a single base pair mutation was detected at residue 1279 in DNA sequence of the gene, resulting in an amino acid change (Gly-206 to Ser) in the encoded protein. HPLC analysis of Chls indicated that the majority of Chl molecules are conjugated with an unsaturated geranylgeraniol side chain, in addition to small amount of normal Chls in the mutant. Furthermore, the mutant phenotype was complemented by transformation with the wild-type gene. Therefore, this study has confirmed the 502ys mutant resulted from a single base pair mutation in OsChl P gene.

Map-based cloning and characterization of the novel yellow-green leaf gene ys83 in rice (Oryza sativa)

Leaf-color mutants have been extensively studied in rice, and many corresponding genes have been identified up to now. However, leaf-color mutation mechanisms are diverse and still need further research through identification of novel genes. In the present paper, we isolated a leaf-color mutant, ys83, in rice (Oryza sativa). The mutant displayed a yellow-green leaf phenotype at seedling stage, and then slowly turned into light-green leaf from late tillering stage. In its yellow leaves, photosynthetic pigment contents significantly decreased and the chloroplast development was retarded. The mutant phenotype was controlled by a recessive mutation in a nuclear gene on the short arm of rice chromosome 2. Map-based cloning and sequencing analysis suggested that the candidate gene was YS83 (LOC_Os02g05890) encoding a protein containing 165 amino acid residues. Gene YS83 was expressed in a wide range of tissues, and its encoded protein was targeted to the chloroplast. In the mutant, a T-to-A substitution occurred in coding sequence of gene YS83, which caused a premature translation of its encoded product. By introduction of the wild-type gene, the ys83 mutant recovered to normal green-leaf phenotype. Taken together, we successfully identified a novel yellow-green leaf gene YS83. In addition, number of productive panicles per plant and number of spikelets per panicle only reduced by 6.7% and 7.6%, respectively, meanwhile its seed setting rate and 1000-grain weight (seed size) were not significantly affected in the mutant, so leaf-color mutant gene ys83 could be used as a trait marker gene in commercial hybrid rice production.

Mutation of FdC2 gene encoding a ferredoxin-like protein with C-terminal extension causes yellow-green leaf phenotype in rice.

Ferredoxins (Fds) are small iron-sulfur proteins that mediate electron transfer in a wide range of metabolic reactions. Besides Fds, there is a type of Fd-like proteins designated as FdC, which have conserved elements of Fds, but contain a significant C-terminal extension. So far, only two FdC genes of Arabidopsis (Arabidopsis thaliana) have been identified in higher plants and thus the functions of FdC proteins remain largely unknown. In this study, we isolated a yellow-green leaf mutant, 501ys, in rice (Oryza sativa). The mutant exhibited yellow-green leaf phenotype and reduced chlorophyll level. The phenotype of 501ys was caused by mutation of a gene on rice chromosome 3. Map-based cloning of this mutant resulted in identification of OsFdC2 gene (LOC_Os03g48040) showing high identity with Arabidopsis FdC2 gene (AT1G32550). OsFdC2 was expressed most abundantly in leaves and its encoded protein was targeted to the chloroplast. In 501ys mutant, a missense mutation was detected in DNA sequence of the gene, resulting in an amino acid change in the encoded protein. The mutant phenotype was rescued by introduction of the wild-type gene. Therefore, we successfully identified FdC2 gene via map-based cloning approach, and demonstrated that mutation of this gene caused yellow-green leaf phenotype in rice.

GRY79 encoding a putative metallo-β-lactamase-trihelix chimera is involved in chloroplast development at early seedling stage of rice.

Key messageThegreen-revertible yellow79mutant resulting from a single-base mutation suggested that theGRY79gene encoding a putative metallo-β-lactamase-trihelix chimera is involved in chloroplast development at early seedling stage of rice.AbstractFunctional studies of metallo-β-lactamases and trihelix transcription factors in higher plants remain very sparse. In this study, we isolated the green-revertible yellow79 (gry79) mutant in rice. The mutant developed yellow-green leaves before the three-leaf stage but recovered to normal green at the sixth-leaf stage. Meanwhile, the mutant exhibited reduced level of chlorophylls and arrested development of chloroplasts in the yellow leaves. Genetic analysis suggested that the mutant phenotype was controlled by a single recessive nuclear gene on rice chromosome 2. Map-based cloning revealed that the candidate gene was Os02g33610 encoding a putative metallo-β-lactamase-trihelix chimera. In the gry79 mutant, a single-base mutation occurred in coding region of the gene, resulting in an amino acid change in the encoded protein. Furthermore, the mutant phenotype was rescued by transformation with the wild-type gene. Therefore, we have confirmed that the gry79 mutant phenotype resulted from a single-base mutation in GRY79 (Os02g33610) gene, suggesting that the gene encoding a putative metallo-β-lactamase-trihelix chimera is involved in chloroplast development at early seedling stage of rice. In addition, we considered that the gry79 mutant gene could be applicable as a leaf-color marker gene for efficient identification and elimination of false hybrids in commercial hybrid rice production.

Fine mapping and candidate gene analysis of the dwarf gene d162(t) in rice (Oryza sativa L.)

In our previous study, d162(t), a single recessive gene, which caused rice dwarf mutant, had been mapped on the short arm of chromosome 3. In this study, the d162(t) gene was fine mapped to a confined region about 0.82 cM by RM14641 and RM3134, and co-segregated with InDel361-2, InDel361-3, InDel361-5, RM14645, RM1022 and RM14643, where no known gene involved in plant height has been identified. Based on the annotation results of TIGR, dozens of open reading frames (ORFs) were predicted in this region, among them, five ORFs were the most possible genes related to the phenotype. In these ORFs, Os03g13010, related to U-box domain containing protein, had a 62bp segment deletion in the coding region in 162d (mutant type, MT). The results of RT-PCR showed that the transcriptional level of Os03g13010 was significantly different between Shuhui162 (wild type, WT) and 162d (MT). Therefore, the gene (Os03g13010) encoding a U-box domain containing protein was considered as the candidate gene of d162(t).