The Genetic Analysis Of Two Floral Organ Mutants From Rice

Rice (Oryza sativa L.) is not only one of the most important food crops in the world, but also a model plant for study of the molecular developmental biology in monocots. The rice flower has an architecture different from those of the model dicot species. A rice floret consists of one pistil, six stamens, two lodicules, one palea and one lemma. Two empty glumes which are regarded as vestigial organs of two lower florets subtend the apical floret in an alternate arrangement. Together, these organs form a spikelet of rice. The acquisition and research on the mutants related to flower development play an important role in understanding the function and interaction of genes in reproductive process, especially in floral development. Research on rice reproductive mechanism is significant both in theory and in human agriculture.Although over 20 MADS-box genes were isolated from rice by using the conserved sequences of the MADS-box genes of dicots as probe screening the rice genomic or cDNA library, it is very difficult to determine the functions of these rice MADS-box genes without relative mutants. Furthermore, the study of the function and expression of genes using mutants is a trend in functional genomics. Rice production depends on the development of the floral organ. The study of molecular mechanism of floral organs development using rice flower mutants has drawn particular attention from biologists and becomes one of the focuses in plant molecular biology.We obtained two mutants from our rice mutant populations. One shows extra glume of most florets, the palea of the other is depressed. In this study, we have completed the investigation of the anatomical structure of floral organs, genetic analysis of mutant traits and fine mapping the genes of two mutants. The main results are summarized as follows: 1. The study of extra glume 1 (egl) mutant(1) Allelic test of 3 extra glume mutants. Allelic tests revealed that the three mutants are allelic and were designated as egl-1, egl-2 and egl-3, respectively. (2) Genetic analysis of extra glume mutant. During the heading stage, the individual plants in the F1 and F2 progenies from the crosses between egl-1 and each of Nipponbare and Zhefu802 were investigated. In the two Fl plants between the wild type and egl-1, all flowers exhibited wild-type phenotype, suggesting that the mutant trait is recessive. Second, in the two F2 populations, both the segregation rates of wild-type and extra glume plants are in accordance with a3:1 ratio. Therefore, the extra glume trait is controlled by one recessive gene.(3) Identification of egl floral organ number, anatomical structure. No detectable abnormalities were observed in the number of inflorescences and vegetative organs, phenotypic changes in the present mutants were restricted to flowers. Mutations in the egl locus causes an extra glume of most floral organs, also causes the number of stamens and pistils, lodicules into lemma- and palea-like structures, but a few spikelets have normal floret. However, the phenotypic severity differed significantly among the three mutants. In egl-2, the number of glumes increased one in all flowers examined, but there were less or no changes in the number of stamens, lodicules, pistils. The phenotype oiegl-3 was almost the same as egl-2 mutant. However, the egl-1 mutation affected all floral organs besides an extra glume. Decreases or increases in stamen number, varying from 0 to 8, were observed in egl-1 flowers. The pistil number varies from 0 to 4, and in the center of a few egl spikelets generating undifferentiated meristematic tissue. Homeotic transformation of lodicules into lemma- or palea-like structures was observed in about a fourth of egl-1 flowers. Very interestingly, at least 20% of the egl-1 spikelets carried one or more additional florets along with pedicel in the center of the floret (whorl 3, 4) composed of only lemma- and palea-like structures, occasionally 1-3 stamens. Moreover, some egl-1 spikelets have many reiterated lemma- and palea-like structure.(4) Fine mapping of EG1 gene. To elucidate the molecular function of EG1, the EG1 gene was fine mapped by a map-based approach. The EG1 locus was delimited in the 7.7 cM region between RM585 and RM217 on chromosome 6 by rough mapping. We further narrowed the EG I gene to a -31 kb interval by using 713 F2 plants. Only two open reading frame (ORF) was predicted within this region.2. The study of depressedpalea 2 (dp2) mutant( 1) Floral anatomical structure of mutant. Compared with normal rice spikelets, the dp2 mutant consists of one lemma, two lodicules, six stamens, one pistil, also has one palea which is depressed.(2) Genetic analysis of depressed palea mutant. During the heading stage, the individual plants in the Fl and F2 progenies from the crosses between dp2 and each of Nipponbare and Zhefu802 were investigated. In the two Fl plants between the wild type and dp2, all flowers exhibited wild-type phenotype, suggesting that the mutant trait is recessive. Second, in the two F2 populations, both the segregation rates of wild-type and depressed palea plants are in accordance with a 3:1 ratio. Therefore, the depressed palea trait is controlled by one recessive gene.(3) Fine mapping of DP2 gene. Using map-based approach, The DP2 locus was delimited in the 2.8 cM region between S45 and RM7633 on chromosome 8 by primary mapping. We further narrowed the DP2 gene to a 17 kb interval by using 2038 F2 mutant plants. Only one open reading frame (ORF) was predicted within this region.