Characteristics and Genetic Segregation of a Rolled Leaf Mutant in Rice

Kor. J. Breed. Sci. 43(4) : 181~185 (2011) Characteristics and Genetic Segregation of a Rolled Leaf Mutant in Rice Songyee Lee, Minseon Choi, Joohyun Lee, and Hee-Jong Koh* Department of Plant Science, Plant Genomics and Breeding Institute, and Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul 151-742, Korea Abstract : Leaf structure is one of the important agronomic traits. A rolled leaf mutant was induced from an ethyl methane sulfonate (EMS)-treated japonica rice, Koshihikari. The rolled leaf mutant showed phenotypes of reduced leaf width and leaf rolling. In addition, several abnormal morphological characteristics were observed, including dwarfism, defected panicle, delayed germination, and lower seed-setting. Microscopic analysis revealed that the number of small veins was decreased and the sizes of adaxial bulliform cells were reduced in the mutant leaves. The genetic study with two F 2 populations from the crosses of the rolled leaf mutant with Koshihikari and Milyang23 suggested that the mutant phenotype might be controlled by a single dominant gene. Key words : Rice (Oryza sativa L.), Rolled leaf, Genetic analysis INTRODUCTION Plant leaf is the main photosynthetic organ and the shape of leaf is important in light capture, carbon fixation, and gas exchange for photosynthesis (Govaerts, 1996). Moderate leaf rolling contributes to the erectness of leaves, which can increase the light transmission rate (Duncan, 1971). Thus, the study of rolled leaf mutant will be meaningful for breeding crops with improved photosynthetic efficiency and subsequently higher yield (Lang et al., 2004). Leaf rolling phenotypes are regulated by complicated developmental processes, including pattern formation, polarity establishment, and cell differentiation (Micol et al., 2003; Lang et al., 2004). Osmotic pressure affects leaf rolling between internal and external tissues, as well (Hsia et al., 1984). However, the mechanism underlying leaf rolling in monocots remains to be elucidated. In rice, many studies for the genes controlling leaf rolling have been reported. Up to the present, 11 rolled leaf (rl) mutants in rice have been characterized. Among them, six recessive genes of narrow or rolling leaf rl1, rl2, rl3, rl4, rl5, and rl6 were mapped on the rice chromosomes 1, 2, 12, 1, 3, and 7 by morphological markers, respectively (http:// *Corresponding author (E-mail: heejkoh@snu.ac.kr, Tel: +82-2- 880-4541, Fax: +82-2-873-2056) <Submitted June 7, 2011> www.gramene.org/). A dominant gene controlling the leaf rolling was reported by Liu et al. (1998). The QTLs controlling the leaf rolling were mapped by molecular markers. rl7 and rl8 located at different regions on chromosome 5 and rl9 (t) is located on chromosome 9 (Yan et al., 2005). The incomplete recessive rolled leaf mutant, rl (t), was fine-mapped to a 137-kb region on chromosome 2 (Shao et al., 2005). Much less is known about the molecular mechanism that control leaf shape in rice. Recently, several genes for leaf development were identified in rice. ROLLED LEAF (RL9) / SHALLOT-LIKE1 (SLL1) encodes a KANADI family protein and is involved in leaf abaxial cell development (Yan et al., 2008; Zhang et al., 2009). The environmental effects such as water stress were also involved in leaf rolling. CONSTITUTIVELY WILTED1 (COW1) / NARROW LEAF7 (NAL7) encodes a member of the YUCCA protein family. Mutation in NAL7 resulted in reduced leaf width and leaf rolling (Woo et al., 2007; Fujino et al., 2008). NARROW LEAF1 (NAL1) is a plant-specific protein with unknown biochemical function. NAL1 mutant shows narrow leaf phenotype because of decreasing number of longitudinal veins (Qi et al., 2008). ADAXIALIZED LEAF1 (ADL1) encodes a calpain-like cysteine protease that is associated with the maintenance of axis formation in 181

182 Kor. J. Breed. Sci. 43(4), (2011) leaf development (Hibara et al., 2009). Rice OsAFO7 gene encodes an argonaute (AGO) family protein. And the overexpression of OsAFO7 resulted in the upward curling of rice leaves (Shi et al., 2007). In the recent study, a narrow and rolled leaves mutant, narrow and rolled leaves 1 (NRL1), was mapped to the chromosome 12. The NRL1 gene encodes the cellulose synthase-like protein D4 (OsCslD4) that affected morphogenesis and vegetative development in rice (Jiang Hu et al., 2010). Here, we characterized the rolled leaf mutant which was induced by ethyl methane sulfonate space (EMS) and investigated the inheritance of this mutant phenotype. MATERIALS AND METHODS Plant materials A rolled leaf mutant was induced by the chemical mutagen EMS from a japonica cultivar, Koshihikari, in the Crop Molecular Breeding Laboratory of Department of Plant Science, Seoul National University (SNU). The rolled leaf mutant was crossed with Koshihikari and Milyang 23 respectively, to produce the F 2 population which was used for genetic analysis. All rice plants were grown in an experimental field at the SNU in Suwon, Korea during the natural growing season. The conventional cultivation method by Crop Molecular Breeding Lab was applied. The tissue samples were dehydrated in graded ethanol series to 100%, embedded in paraffin, cut ~20 μm sections using a microtome (HM340E, MICROM Lab), and adhered to glass slides. The sections were deparaffinized using xylene, rehydrated using a graded ethanol series, before staining with Safranin O (Sigma-Aldrich, St Louis, MO). Finally, cross sections of leaf were observed by optical microcopy at 50 and 100 x magnifications. Genetic analysis of rolled leaf mutant In summer of 2009, the rolled leaf mutant was crossed to the parental variety Koshihikari and Milyang 23 (tongil-type). The F 1 seeds were planted in the green house in the same year. In the spring of 2010, two F 2 populations derived from the cross between the mutant and two varieties: Kosihikari, Milyang23, respectively. A F 2 population from the cross between the mutant and Koshihikari was used for genetic analysis of the rolled leaf, and the other F 2 population from the cross between the rolled leaf mutant and Milyang23 was used for BSA (bulked segregant analysis). For BSA test, DNA bulks were made from 10 F 2 plants showing rolled leaf phenotype and 10 normal F 2 plants, respectively. A total of 106 STS markers of known chromosomal position covering all chromosomes evenly were used in BSA test. The 106 STS markers were designed in Crop Molecular Breeding Laboratory (Chin et al., 2000). Pollen fertility assessment Spikelets were harvested just before flowering, and then fixed in 3:1 ethanol : acetic acid (v/v) fluid for 1 hour. Pollens were placed on slides with 1% (w/v) iodine potassium iodide (I₂-KI) and 1% (w/v) acetocarmine solution, respectively (Chaudhary et al. 1981). The pollen fertility was examined under an optical microscope. Leaf sections and microscopic analysis Paraffin-embedded leaf tissue sections were prepared by the methods by Ji et al. (2006) with slight modifications. Leaves were harvested at the early stage and fixed in FAA solution (50% ethanol, 5% acetic acid, 3.7% formaldehyde). RESULTS & DISCUSSION The morphology of the mutant was compared with that of the Koshihikari. The mutant leaves were narrow and rolled upwardly. From the seedling stage, mutant plants can be distinguished from the Koshihikari by narrow and rolled leaf phenotypes. Apart from narrow and rolled leaves, other defects in the mutant were also observed. When compared with the Koshihikari, the plant height was significantly changed. At the seed setting stage, the mutant was determined to be shorter than Koshihikari (28.2 ± 3.5 cm compared with 91.3 ± 2.5 cm in the wild type; n=30, P<0.001) (Fig. 1A, B), which was approximately 31% of the height compared

Characteristics and Genetic Segregation of a Rolled Leaf Mutant in Rice 183 to the Koshihikari Plant height of the mutant was reduced were white and small (Fig. 2D). The pollen fertility was mainly due to the shortened the second and the third internode also lower than that of the Koshihikari (Fig. 2E, F). In elongation. Takeda (1977) categorized rice dwarf mutants seeds, both palea and lemma failed to close; the seeds were into six groups according to elongation patterns of the upper smaller and exposed (Grain length : 4.6 ± 0.03 cm compared five internodes. In d6-type mutant, the second and the third with 5.1 ± 0.01 cm in the wild type; Grain width : 2.2 ± internodes below the uppermost are shortened. Thus, the 0.05 cm compared with 3.0 ± 0.02 cm in the wild type; n=30, mutant could be grouped into d6-type according to the definition P<0.001) (Fig. 2G). by Takeda (Fig. 1C). In roots, the mutant showed defected The representative phenotype of this mutant is leaf rolling. growth. It has shortened primary roots (43.8% reduction) The anatomic analysis based on the study of transverse (Fig. 1D) and the numbers of lateral and adventitious roots sections revealed that the mutant leaves had fewer veins were reduced. Panicle length and the number of panicles and smaller bulliform cells comparing to those of Koshihikari. of mutant also were reduced. On the other hand, leaf length The mutant leaves were rolled while leaves in Koshihikari was 37% increased as compared to Koshihikari (Fig. 1E). plants were completely flat (Fig. 3A, B). Mutant had fewer The mutant plants showed decreased number of florets from large veins (lateral veins) and small veins (intermediate veins) the base to the peak of panicles. Frequently, the florets of than those of Koshihikari, resulting in the narrow-leaf phenotypes. the panicle did not develop completely (Fig. 2A). The mutant The analysis of cross-sections of mutant leaves showed reduced has defects in development of floral organ such as carpel, bulliform cells. The bulliform cells exist specifically in the stamen and lodicules. The lodicules were weaker than upper side of leaves in rice, and their number and density Koshihikari (Fig. 2B). The degenerated palea and lemma affects determination of the leaf rolling and stretching movement were unable to close, which resulted in the exposure of stamens (Li et al., 1979). Therefore, developmental defects of veins and stigmas. In this mutant, the pistil and stamens are and bulliform cells affected the leaf rolling morphology. malformed (Fig. 2C). Especially, the anthers of the mutant For the genetic analysis of the rolled leaf phenotype, the Fig. 1. Phenotypes of wild-type and rolled leaf mutant A. Plants of wild-type (left) and rolled leaf mutant (right) B. Plants of wild-type (left) and rolled leaf mutant (right) after heading C. Comparison of panicle length and length of upper six internodes (Ⅰ,Ⅱ,Ⅲ,Ⅳ,Ⅴ or Ⅵ) among wild-types (left) and rolled leaf mutants (right) D. Abnormal root development of the wild-types (left) and mutants (right) E. Traits of wild-types and rolled leaf mutants Fig. 2. Panicle and floral organ phenotypes of wild-type and rolled leaf mutant. A. Panicle of wild-type (left) and mutant (right) B. Spikelets of wild-type (left) and rolled leaf mutant (right) C. Floret organ of wild-types (left) and rolled leaf mutants (right) D. Stamen and carpel of wild-type (left) and mutant (right) E. Evaluation of pollen fertility by staining with acetocarmine between wild-type (left) and mutant (right) F. Evaluation of pollen fertility by staining with I₂-KI between wild-type (left) and mutant (right) G. Altered seed development of wild-type (left) and rolled leaf mutant (right)

184 Kor. J. Breed. Sci. 43(4), (2011) Fig. 3. Comparison of leaf microstructure of wild-type and rolled leaf mutant. A. Part of a flag leaf in wild-type (left) and mutant (right) B. Cross-section of leaves in wild-type (left) and mutant (right) Bar=1 mm. C. Cross-section of midrib in wild-type (left) and mutant (right). Bar=100 μm. D. Cross-section of leaf blade in wild-type (left) and mutant (right). Bar= 100 μm. E. Cross-section of large vein in wild-type (left) and mutant (right). Bar=100 μm. lv large vascular, sv small vascular, bc bulliform cells rolled leaf mutant was crossed to Milyang 23 (Tongil type) and Koshihikari (Table 1). Unexpectedly, the F 1 plants from the cross between the rolled leaf mutant and Milyang23 were segregated into 7 rolled leaf plants and 10 normal plants. The F 2 plants harvested from normal F 1 plants were all normal without segregation of rolled leaf plants, whereas F 2 plants derived from rolled leaf F 1 plants were segregated into normal and rolled leaf plants. In the seven F 2 populations from the rolled leaf F 1 plants, the segregation ratio didn t fit to clear-cut ratio, which could be explained by the distorted segregation phenomenon in the inter-subspecific hybrid (Lin et al., 1992). We also detected the segregation in the F 1 plants from the cross between the rolled leaf mutant and Koshihikari (15 rolled leaf plants and 14 normal plants) and the normal F 1 plant produced only normal plant in F 2, whereas the rolled leaf plants produced normal plants and rolled leaf plants in F 2. The F 2 populations derived from the rolled leaf F 1 plants derived from the cross of rolled leaf mutant / Koshihikari produced 114 rolled leaf plants and 33 normal plants, which showed the ratio of 3:1 (X 2 =0.51, X 2 0.05 = 0.48), implying that the mutant phenotype might be controlled by a single dominant gene. In addition, we found that 30 F 2 plants out of 114 rolled leaf F 2 plants showed abnormal growth so they died before producing seeds while 84 rolled leaf F 2 plants produced F 3 seeds. Segregation ratio between Table 1. Segregation ratio of the rolled leaf trait in F 1 and F 2 populations Cross combination Generation No. of plants Rolled leaf Normal Total X 2 (3:1) P Rolled leaf mutant Parent (790582-1) 12 8 20 Rolled leaf mutant Parent (790582-2) 17 10 27 790582-1/M.23 F 1 7 10 17 790582-2/Koshihikari F 1 15 14 29 790582-1/M.23 * F 2 Nor 0 100 100 F 2 Rolled 81 78 159 49.08 0.1 790582-2/Koshihikari F 2 Nor 0 100 100 F 2 Rolled 114(84:30) 33 147 0.51 0.48 * Nor and Rolled: F2 plants derived from normal and rolled leaf F1 plants, respectively

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