Genetic analysis and gene mapping of a new rolled-leaf mutant in rice (Oryza sativa L.)

Characterization of a Novel Rice Dynamic Narrow-Rolled Leaf Mutant with Deficiencies in Aromatic Amino Acids

The leaf blade is the main photosynthetic organ and its morphology is related to light energy capture and conversion efficiency. We isolated a novel rice Dynamic Narrow-Rolled Leaf 1 (dnrl1) mutant showing reduced width of leaf blades, rolled leaves and lower chlorophyll content. The narrow-rolled leaf phenotype resulted from the reduced number of small longitudinal veins per leaf, smaller size and irregular arrangement of bulliform cells compared with the wild-type. DNRL1 was mapped to chromosome 7 and encoded a putative 3-deoxy-7-phosphoheptulonate synthase (DAHPS) which catalyzes the conversion of phosphoenolpyruvate and D-erythrose 4-phosphate to DAHP and phosphate. Sequence analysis revealed that a single base substitution (T–A) was detected in dnrl1, leading to a single amino acid change (L376H) in the coding protein. The mutation led to a lower expression level of DNRL1 as well as the lower activity of DAHPS in the mutant compared with the wild type. Genetic complementation and over-expression of DNRL1 could rescue the narrow-rolled phenotype. DNRL1 was constitutively expressed in all tested organs and exhibited different expression patterns from other narrow-rolled leaf genes. DNRL1-GFP located to chloroplasts. The lower level of chlorophyll in dnrl1 was associated with the downregulation of the genes responsible for chlorophyll biosynthesis and photosynthesis. Furthermore, dnrl1 showed significantly reduced levels of aromatic amino acids including Trp, Phe and Tyr. We conclude that OsDAHPS, encoded by DNRL1, plays a critical role in leaf morphogenesis by mediating the biosynthesis of amino acids in rice.

REL2, A Gene Encoding An Unknown Function Protein which Contains DUF630 and DUF632 Domains Controls Leaf Rolling in Rice

BACKGROUND

Rice leaves are important energy source for the whole plant. An optimal structure will be beneficial for rice leaves to capture light energy and exchange gas, thus increasing the yield of rice. Moderate leaf rolling and relatively erect plant architecture may contribute to high yield of rice, but the relevant molecular mechanism remains unclear.

RESULTS

In this study, we identified and characterized a rolling and erect leaf mutant in rice and named it as rel2. Histological analysis showed that the rel2 mutant has increased number of bulliform cells and reduced size of middle bulliform cells. We firstly mapped REL2 to a 35-kb physical region of chromosome 10 by map-based cloning strategy. Further analysis revealed that REL2 encodes a protein containing DUF630 and DUF632 domains. In rel2 mutant, the mutation of two nucleotide substitutions in DUF630 domain led to the loss-of-function of REL2 locus and the function of REL2 could be confirmed by complementary expression of REL2 in rel2 mutant. Further studies showed that REL2 protein is mainly distributed along the plasma membrane of cells and the REL2 gene is relatively higher expressed in younger leaves of rice. The results from quantitative RT-PCR analysis indicated that REL2 functioning in the leaf shape formation might have functional linkage with many genes associated with the bulliform cells development, auxin synthesis and transport, etc.

CONCLUSIONS

REL2 is the DUF domains contained protein which involves in the control of leaf rolling in rice. It is the plasma membrane localization and its functions in the control of leaf morphology might involve in multiple biological processes such as bulliform cell development and auxin synthesis and transport.

The wheat MYB transcription factor TaMYB18 regulates leaf rolling in rice

Leaf rolling is an important agronomic trait in crop breeding. Moderate leaf rolling maintains the erectness of leaves and minimizes shadowing between leaves, leading to improved photosynthetic efficiency. Although some genes controlling leaf rolling have been isolated from rice and other plant species, few studies have examined leaf rolling in wheat. In the present study, the leaf rolling regulator gene, TaMYB18, was identified in a large-scale transgene project involving the transformation of 1455 wheat transcription factor genes into rice. Three homologous sequences of TaMYB18 were isolated from hexaploid wheat and localized to chromosomes 5A, 5B and 5D, respectively. TaMYB18, an R2R3-MYB transcription factor, localized to the nucleus. TaMYB18 overexpression induced leaf rolling in transgenic rice. Additionally, the three members of TaMYB18 exhibited functional redundancy in rice. Furthermore, the function of TaMYB18 in regulating leaf rolling in rice was a dose-dependent. Taken together, these results indicate that TaMYB18 may play an important role in the regulation of leaf development.

Semi-Rolled Leaf2 modulates rice leaf rolling by regulating abaxial side cell differentiation

Moderate leaf rolling maintains the erectness of leaves and minimizes the shadowing between leaves which is helpful to establish ideal plant architecture. Here, we describe asrl2(semi-rolled leaf2) rice mutant, which has incurved leaves due to the presence of defective sclerenchymatous cells on the abaxial side of the leaf and displays narrow leaves and reduced plant height. Map-based cloning revealed that SRL2 encodes a novel plant-specific protein of unknown biochemical function.SRL2 was mainly expressed in the vascular bundles of leaf blades, leaf sheaths, and roots, especially in their sclerenchymatous cells. The transcriptional activities of several leaf development-related YABBY genes were significantly altered in the srl2 mutant. Double mutant analysis suggested that SRL2 and SHALLOT-LIKE1(SLL1)/ROLLED LEAF9(RL9) function in distinct pathways that regulate abaxial-side leaf development. Hence, SRL2 plays an important role in regulating leaf development, particularly during sclerenchymatous cell differentiation.

Single base substitution in OsCDC48 is responsible for premature senescence and death phenotype in rice

A premature senescence and death 128 (psd128) mutant was isolated from an ethyl methane sulfonate-induced rice IR64 mutant bank. The premature senescence phenotype appeared at the six-leaf stage and the plant died at the early heading stage. psd128 exhibited impaired chloroplast development with significantly reduced photosynthetic ability, chlorophyll and carotenoid contents, root vigor, soluble protein content and increased malonaldehyde content. Furthermore, the expression of senescence-related genes was significantly altered in psd128. The mutant trait was controlled by a single recessive nuclear gene. Using map-based strategy, the mutation Oryza sativa cell division cycle 48 (OsCDC48) was isolated and predicted to encode a putative AAA-type ATPase with 809 amino-acid residuals. A single base substitution at position C2347T in psd128 resulted in a premature stop codon. Functional complementation could rescue the mutant phenotype. In addition, RNA interference resulted in the premature senescence and death phenotype. OsCDC48 was expressed constitutively in the root, stem, leaf and panicle. Subcellular analysis indicated that OsCDC48:YFP fusion proteins were located both in the cytoplasm and nucleus. OsCDC48 was highly conserved with more than 90% identity in the protein levels among plant species. Our results indicated that the impaired function of OsCDC48 was responsible for the premature senescence and death phenotype.

Oryza sativa Chloroplast Signal Recognition Particle 43 (OscpSRP43) Is Required for Chloroplast Development and Photosynthesis

A rice chlorophyll-deficient mutant w67 was isolated from an ethyl methane sulfonate (EMS)-induced IR64 (Oryza sativa L. ssp. indica) mutant bank. The mutant exhibited a distinct yellow-green leaf phenotype in the whole plant growth duration with significantly reduced levels of chlorophyll and carotenoid, impaired chloroplast development and lowered capacity of photosynthesis compared with the wild-type IR64. Expression of a number of genes associated with chlorophyll metabolism, chloroplast biogenesis and photosynthesis was significantly altered in the mutant. Genetic analysis indicated that the yellow-green phenotype was controlled by a single recessive nuclear gene located on the short arm of chromosome 3. Using map-based strategy, the mutation was isolated and predicted to encode a chloroplast signal recognition particle 43 KD protein (cpSRP43) with 388 amino acid residuals. A single base substitution from A to T at position 160 resulted in a premature stop codon. OscpSRP43 was constitutively expressed in various organs with the highest level in the leaf. Functional complementation could rescue the mutant phenotype and subcellular localization showed that the cpSRP43:GFP fusion protein was targeted to the chloroplast. The data suggested that Oryza sativa cpSRP43 (OscpSRP43) was required for the normal development of chloroplasts and photosynthesis in rice.

Increasing leaf vein density by mutagenesis: laying the foundations for C4 rice

A high leaf vein density is both an essential feature of C₄ photosynthesis and a foundation trait to C₄ evolution, ensuring the optimal proportion and proximity of mesophyll and bundle sheath cells for permitting the rapid exchange of photosynthates. Two rice mutant populations, a deletion mutant library with a cv. IR64 background (12,470 lines) and a T-DNA insertion mutant library with a cv. Tainung 67 background (10,830 lines), were screened for increases in vein density. A high throughput method with handheld microscopes was developed and its accuracy was supported by more rigorous microscopy analysis. Eight lines with significantly increased leaf vein densities were identified to be used as genetic stock for the global C₄ Rice Consortium. The candidate population was shown to include both shared and independent mutations and so more than one gene controlled the high vein density phenotype. The high vein density trait was found to be linked to a narrow leaf width trait but the linkage was incomplete. The more genetically robust narrow leaf width trait was proposed to be used as a reliable phenotypic marker for finding high vein density variants in rice in future screens.

Overexpression of OsZHD1, a zinc finger homeodomain class homeobox transcription factor, induces abaxially curled and drooping leaf in rice

Leaf rolling is receiving considerable attention as an important agronomic trait in rice (Oryza sativa L.). However, little has been known on the molecular mechanism of rice leaf rolling, especially the abaxial rolling. We identified a novel abaxially curled and drooping leaf-dominant mutant from a T₁ transgenic rice line. The abaxially curled leaf phenotypes, co-segregating with the inserted transferred DNA, were caused by overexpression of a zinc finger homeodomain class homeobox transcription factor (OsZHD1). OsZHD1 exhibited a constitutive expression pattern in wild-type plants and accumulated in the developing leaves and panicles. Artificial overexpression of OsZHD1 or its closest homolog OsZHD2 induced the abaxial leaf curling. Histological analysis indicated that both the increased number and the abnormal arrangement of bulliform cells in leaf were responsible for the abaxially curled leaves. We herein reported OsZHD1 with key roles in rice morphogenesis, especially in the modulating of leaf rolling, which provided a novel insight into the molecular mechanism of leaf development in rice.

Characterization and genetic analysis of a novel rice spotted-leaf mutant HM47 with broad-spectrum resistance to Xanthomonas oryzae pv. oryzae

A stable inherited rice spotted-leaf mutant HM47 derived from an EMS-induced IR64 mutant bank was identified. The mutant expressed hypersensitive response (HR)-like symptoms throughout its whole life from the first leaf to the flag leaf, without pathogen invasion. Initiation of the lesions was induced by light under natural summer field conditions. Expression of pathogenesis-related genes including PAL, PO-C1, POX22.3 and PBZ1 was enhanced significantly in association with cell death and accumulation of H2 O2 at and around the site of lesions in the mutant in contrast to that in the wild-type (WT). Disease reaction to Xanthomonas oryzae pv. oryzae from the Philippines and China showed that HM47 is a broad-spectrum disease-resistant mutant with enhanced resistance to multiple races of bacterial blight pathogens tested. An F2 progeny test showed that bacterial blight resistance to race HB-17 was co-segregated with the expression of lesions. Genetic analysis indicated that the spotted-leaf trait was controlled by a single recessive gene, tentatively named spl(HM47) , flanked by two insertion/deletion markers in a region of approximately 74 kb on the long arm of chromosome 4. Ten open reading frames are predicted, and all of them are expressed proteins. Isolation and validation of the putative genes are currently underway.

Semi-rolled leaf1 encodes a putative glycosylphosphatidylinositol-anchored protein and modulates rice leaf rolling by regulating the formation of bulliform cells

Leaf rolling is an important agronomic trait in rice (Oryza sativa) breeding and moderate leaf rolling maintains the erectness of leaves and minimizes shadowing between leaves, leading to improved photosynthetic efficiency and grain yields. Although a few rolled-leaf mutants have been identified and some genes controlling leaf rolling have been isolated, the molecular mechanisms of leaf rolling still need to be elucidated. Here we report the isolation and characterization of SEMI-ROLLED LEAF1 (SRL1), a gene involved in the regulation of leaf rolling. Mutants srl1-1 (point mutation) and srl1-2 (transferred DNA insertion) exhibit adaxially rolled leaves due to the increased numbers of bulliform cells at the adaxial cell layers, which could be rescued by complementary expression of SRL1. SRL1 is expressed in various tissues and is expressed at low levels in bulliform cells. SRL1 protein is located at the plasma membrane and predicted to be a putative glycosylphosphatidylinositol-anchored protein. Moreover, analysis of the gene expression profile of cells that will become epidermal cells in wild type but probably bulliform cells in srl1-1 by laser-captured microdissection revealed that the expression of genes encoding vacuolar H(+)-ATPase (subunits A, B, C, and D) and H(+)-pyrophosphatase, which are increased during the formation of bulliform cells, were up-regulated in srl1-1. These results provide the transcript profile of rice leaf cells that will become bulliform cells and demonstrate that SRL1 regulates leaf rolling through inhibiting the formation of bulliform cells by negatively regulating the expression of genes encoding vacuolar H(+)-ATPase subunits and H(+)-pyrophosphatase, which will help to understand the mechanism regulating leaf rolling.

Characterization and genetic analysis of a light- and temperature-sensitive spotted-leaf mutant in rice

A rice spotted-leaf mutant was isolated from an ethane methyl sulfonate (EMS) -induced IR64 mutant bank. The mutant, designated as spl30 (spotted-leaf30), displayed normal green leaf color under shade but exhibited red-brown lesions under natural summer field conditions. Initiation of the lesions was induced by light and the symptom was enhanced at 33 (°) C relative to 26 (°) C. Histochemical staining did not show cell death around the red-brown lesions. Chlorophyll contents in the mutant were significantly lower than those of the wild type while the ratio of chlorophyll a/b remained the same, indicating that spl30 was impaired in biosynthesis or degradation of chlorophyll. Disease reaction patterns of the mutant to Xanthomonas oryzae pv. oryzae were largely unchanged to most races tested except for a few strains. Genetic analysis showed that the mutation was controlled by a single recessive gene, tentatively named spl30(t), which co-segregated with RM15380 on chromosome 3, and was delimited to a 94 kb region between RM15380 and RM15383. Spl30(t) is likely a novel rice spotted-leaf gene since no other similar genes have been identified near the chromosomal region. The genetic data and recombination populations provided in this study will enable further fine-mapping and cloning of the gene.

Leaf rolling controlled by the homeodomain leucine zipper class IV gene Roc5 in rice

Leaf rolling is considered an important agronomic trait in rice (Oryza sativa) breeding. To understand the molecular mechanism controlling leaf rolling, we screened a rice T-DNA insertion population and isolated the outcurved leaf1 (oul1) mutant showing abaxial leaf rolling. The phenotypes were caused by knockout of Rice outermost cell-specific gene5 (Roc5), an ortholog of the Arabidopsis (Arabidopsis thaliana) homeodomain leucine zipper class IV gene GLABRA2. Interestingly, overexpression of Roc5 led to adaxially rolled leaves, whereas cosuppression of Roc5 resulted in abaxial leaf rolling. Bulliform cell number and size increased in oul1 and Roc5 cosuppression plants but were reduced in Roc5-overexpressing lines. The data indicate that Roc5 negatively regulates bulliform cell fate and development. Gene expression profiling, quantitative polymerase chain reaction, and RNA interference (RNAi) analyses revealed that Protodermal Factor Like (PFL) was probably down-regulated in oul1. The mRNA level of PFL was increased in Roc5-overexpressing lines, and PFL-RNAi transgenic plants exhibit reversely rolling leaves by reason of increases of bulliform cell number and size, indicating that Roc5 may have a conserved function. These are, to our knowledge, the first functional data for a gene encoding a homeodomain leucine zipper class IV transcriptional factor in rice that modulates leaf rolling.